Seismotectonic range-front segmentation and mountain-belt growth in the
Pamir-Alai region, Kyrgyzstan (India-Eurasia collision zone)
J R. Arrowsmith* Department of Geology, Arizona State University, Tempe, Arizona 85287
M. R. Strecker†
Institut für Geowissenschaften, Universität Potsdam, Postfach 60 15 53, D-14415 Potsdam, Germany
ABSTRACT
INTRODUCTION
The Trans Alai Range along the northern
perimeter of the Pamir region of Kyrgyzstan
is at the northern edge of the India-Eurasia
collision zone. The range defines a section of
Main Pamir thrust, which is divided into eastern, central, and western segments that record
differential absorption of plate convergence.
The 50-km-long, east-west–striking, central
fault segment of the Main Pamir thrust has
dip-slip thrust fault offsets (with a minimum
Holocene dip-slip rate of ~6 mm/yr) and is
linked to the other segments via northweststriking dextral transfer. The western transfer faults are well characterized geomorphically, and the westernmost records the
predominant transition to north-vergent
thrusting along the western segment via
north-vergent, low-angle thrust faults rooted
in a steeply south-dipping dextral shear
zone. In contrast, the eastern segment deformation is widely distributed and geomorphically less evident, but the transfer also takes
place in a structurally complex zone. Asymmetric offset of a regionally correlated terrace, geomorphic zonation, and the distribution of historic earthquakes suggest that the
central segment is mechanically linked with
the western segment, but not with the eastern segment. The progressive closure of the
Alai Valley by the northward advance of
thrusting exemplifies the annihilation of an
intramontane basin. If the high surface uplift rates implied by these geologic observations have been maintained for the last few
million years, they attest to the high level of
tectonic activity and the recency of construction of the Trans Alai Range.
Deformation in orogenic belts is typically localized into separate zones in which activity
varies over time and space in style as well as in
magnitude (e.g., Noblet et al., 1996). A consequence of such variability is the complex structural relationship and superposition of structural
styles preserved in all mountain belts, which
poses a major problem in the assessment of the
importance of individual orogenic processes
(Burchfiel et al., 1992; Molnar and Lyon-Caen,
1988). However, in contrast to older and erosionally more altered regions, the structures and tectonically dominated landscapes in actively developing orogens such as the Pamir Mountains of
Central Asia offer the possibility to better characterize the temporal variation of fault kinematics,
the migration of deformation, and the relation between tectonism and climate in shaping mountainous terrains. Furthermore, by studying the deformation of landforms and Quaternary deposits,
we can bridge the gap between rates and geometries incompletely defined by geodesy and seismicity and those generally defined over the
longer time scale of the geologic record of
orogeny. The Pamir Mountains are well suited for
studies on spatial and temporal changes in the behavior of range-bounding faults in a collisional
orogen. The Pamir orogen is characterized by coeval major zones of thrusting, strike-slip, and normal faulting (Burtman and Molnar, 1993; Frisch
et al., 1994; Strecker et al., 1995). It records sustained late Cenozoic deformation in an array of
tectonically controlled landforms, migration of
fault activity, and asymmetric slip offset distribution of important faults along segmented mountain fronts. In this study, we use detailed geologic
and geomorphic observations from the Main
Pamir thrust along the Trans Alai Range front,
Kyrgyzstan, at the northwestern sector of the India-Eurasia collision zone to illustrate how convergence between two continental plates is partitioned, and how active mountain fronts in a
*E-mail: [email protected].
†E-mail: [email protected].
GSA Bulletin; November 1999; v. 111; no. 11; p. 1665–1683; 14 figures.
1665
collisional setting are segmented. Our results
from the central sector of the Main Pamir thrust
show that it is a key element of the collision that
localizes deformation along a narrow zone. Transitions to the lateral sectors of the range front are
accommodated by complex areas in which convergence is kinematically transferred via obliquely
slipping transfer faults and thrust systems. This
large-scale structural arrangement is also reflected in morphotectonic zonation defined by
systematic landform responses to the deformation, such as uplifted pediment and terrace surfaces, and areas of important landsliding. In addition, the segmentation is documented in the
distribution and rupture characteristics of historic
earthquakes. On the basis of the offset history of
a fluvial terrace, we determined a preliminary
minimum slip rate along the Main Pamir thrust of
~6 mm/yr, which implies that this narrow sector
accommodates a minimum of 10% of ongoing
shortening associated with the India-Eurasia
collision.
Regional Tectonic Setting
With elevations of more than 7000 m and
Paleozoic to Quaternary rocks documenting ongoing deformation since Paleogene time, the
Trans Alai Range and the adjacent Alai Valley of
Kyrgyzstan are ideal locations to document the
geometry, rate, and spatial structural development
within a young mountain belt (Figs. 1–4). The
Trans Alai Range defines the northern perimeter
of the convex-northward Pamir Mountains, where
Eurasian continental crust is overthrust by the
Pamir block (e.g., Burtman and Molnar, 1993).
The Pamir Mountains consist of amalgamated Paleozoic and Mesozoic terranes that were sutured
to southern Eurasia and further deformed and displaced northward in the course of the Cenozoic
collision processes (Burtman and Molnar, 1993,
and references therein). Truncation of Cretaceous
and Paleogene facies boundaries, systematic
anomalies in paleomagnetic declination, and the
ARROWSMITH AND STRECKER
Figure 1. Major late Cenozoic faults of the Pamir and adjacent areas. Topography of the Pamir region in Central Asia illustrates that the Pamir
block is encroaching on the southern Tien Shan, but has not yet closed the intramontane Alai Valley. Detailed mapping of active structures and
Quaternary landforms and deposits along the southern edge of the Alai Valley provides constraints on the distribution of slip in space and time
on the faults in the center of the Pamir–Tien Shan convergence zone. Digital topographic data were provided by M. Hamburger; topographic
gradients are enhanced by artificial illumination from a northwestern azimuth. Dashed box in inset shows area covered by digital topography
and major faults of the Himalaya-Tibet-Pamir region. Rectangle in the Alai Valley depicts approximate location of Figure 2 and the location of
Figure 3. Modified from Armijo et al. (1986), Frisch et al. (1994), and Strecker et al. (1995).
pronounced northward deflection of a Paleozoic
suture imply that the northern rim of the Pamir
has been overthrust onto southern Eurasia by
~300 km (e.g., Bazhenov et al., 1994; Burtman
and Molnar, 1993).
The northern limit of the Trans Alai Range coincides with the updip projection of the southward-dipping zone of seismicity in the collision
zone and is bounded by the north-vergent Main
Pamir thrust, which is also called the Trans Alai
thrust in this area (Burtman and Molnar, 1993;
Hamburger et al., 1992). To the east and west, the
orogen is bounded by important dextral and sinistral strike-slip faults, respectively (Fig. 1). The
Main Pamir thrust separates the Trans Alai Range
from the intramontane Alai Valley; to the north the
Alai Valley is bounded by the Alai Range of the
Tien Shan Mountains, which constitute a Variscan
orogen reactivated and uplifted to elevations
higher than 5000 m in the course of the Cenozoic
India-Eurasia collision (Burtman and Molnar,
1666
1993). Burtman and Molnar (1993) inferred that
the faults along the Trans Alai Range are part of a
thrust system soling into a detachment horizon
that also accommodates a major thrust at the
northern boundary of the Tien Shan. The 70-kmthick crust of the overriding Pamir indicates that
along with underthrusting of Eurasian crust, the
collision has been accommodated by significant
crustal thickening (Burtman and Molnar, 1993).
Active deformation quantified by repeated global
positioning system (GPS) measurements between
the Tadjik Pamir highland south of the Trans Alai,
the Kyrgiz Tien Shan Mountains, and the foreland
regions (Fergana Valley) demonstrate active
shortening. This accounts for about 15–30 mm/yr
convergence between India and Eurasia, which
amounts to one- to two-thirds of the total plate
motion (Burtman and Molnar, 1993; Michel et al.,
1997; Larson et al., 1999). Pegler and Das (1998)
analyzed regional seismicity and concluded that
the seismicity distribution was more consistent
with overturning of the eastward continuation of
the northward subducting Hindu Kush slab, rather
than the down-dip projection of a subducting
Eurasian slab below the Pamir. Our geologic and
geomorphic analyses indicate high contraction
rates localized along the Main Pamir thrust. If that
contraction is sustained, significant underthrusting of Eurasia is required.
STRATIGRAPHIC AND STRUCTURAL
FRAMEWORK
Permian-Carboniferous metasedimentary, volcanic, and volcaniclastic rocks are the oldest rocks
preserved in the Trans Alai Range (Kyrgize Geologic Agency, 1979). These units are overlain by
~800 m of Jurassic red shales and sandstones with
interbedded limestone and gypsum (Davidzon
et al., 1982). Mesozoic rocks include ~1000-mthick Lower Cretaceous red beds and mudstone
with layers of marl and gypsum, which are over-
Geological Society of America Bulletin, November 1999
MOUNTAIN-BELT GROWTH, PAMIR-ALAI REGION, KYRGYZSTAN
lain by 60–100 m of red beds, siltstone, and conglomerate, and about 400 m of limestone and
clay, with layers of marl, red clay and gypsum.
Cretaceous rocks dominate the intermediate elevations of the Trans Alai Range at about 3000 m
(Figs. 2, 3, and 5). Paleogene rocks in the Trans
Alai Range change upward from marine to nonmarine (total thickness of 600–1400 m) and are
typically well bedded, fine- to medium-grained
sandstones, siltstones, and shales with lenses of
gypsum and conglomerate. The Paleogene sedimentary basin was apparently continuous from
the Tadjik Depression in the west across the Alai
region to the Tarim Basin to the east (Davidzon
et al., 1982). Late Neogene (here designated N2)
deposits are siltstones, sandstones, and mainly
gravel to boulder conglomerates. These deposits
include large brecciated carbonate blocks interpreted as rock-avalanche deposits derived from
the northward advancing thrust front of Paleozoic
and Mesozoic rocks at that time (Nikonov et al.,
1983). The N2 rocks are interpreted as a molasse
deposited in front of these thrust systems. The
present Trans Alai piedmont is an analogous depositional environment for the Neogene molasse.
The overall geologic structure of the Trans Alai
Range is a stack of gently southward-dipping
thrust sheets with the Paleozoic rocks defining the
highest topography; they overlie in thrust contact
Cretaceous tan-gray carbonates and red-gray clastic rocks that are separated into at least two thrust
sheets with occasionally well-preserved, typically
asymmetric tight east-west–trending folds with
moderately to steeply south-dipping axial planes.
Immediately north of Peak Lenin (7134 m), we
noted a 50-m-wide and 30-m-tall box fold in
basal Cretaceous carbonates apparently just
above a thrust that emplaced the carbonates over
Cretaceous red beds. The lowermost thrust fault
bounding Cretaceous rocks locally places carbonates over the red Paleogene rocks. In the central
Trans Alai Range, Cretaceous and Paleogene
rocks are overthrust onto Neogene rocks. Isolated
klippen of Cretaceous–Paleogene rocks (Burtman
and Molnar, 1993) imply at least 8 km of thrust
offset (only the largest is shown in Fig. 3b). The
base of the klippen occurs to the north of and
about 200–300 m lower than the thrust outcrops.
Thus the thrust surface is gently folded, and the
current exposure of these rocks indicates localized erosional retreat of the paleo–range front by
8 km. Alternatively, the blocks could be interpreted as large olistostromes similar to extensive
mass-movement deposits in the western Pamir
(e.g., Schwab et al., 1980); the magnitude of
thrust displacement of the Cretaceous–Paleogene
rocks is not constrained; and the embayment in
the Trans Alai Range front was developed prior to
northward propagation of the currently active
central portion of the Main Pamir thrust. Neo-
gene rocks that are exposed along the central portion of the Trans Alai Range are folded and cut by
the Main Pamir thrust at the range front (Fig. 2).
The folds in the Neogene deposits are typically
gentle and upright; for example, between the Komansu and Adziksu Rivers is a 1–2-km-wide
syncline, which is paired to the north with an
asymmetric tight anticline, about 1 km wide, that
is truncated at the range front by the active thrust
(Fig. 3B). The fold axes trend about east-west,
approximately parallel with the thrust faults of
the Trans Alai Range. The axial planes of the anticlines usually dip steeply to the south, and the
northern limbs are vertical or overturned and
south dipping (Fig. 3B). We infer that these structures developed when fault-propagation folds as
the Main Pamir thrust fault stepped northward
and cut through the Neogene conglomerates. Uplifted fluvial terraces that cross these structures
are not folded, indicating that presently deformation is localized in a narrow sector along the
thrust fault.
QUATERNARY DEPOSITS AND
LANDFORMS
The Quaternary deposits in the Trans Alai
Range and the Alai Valley region are mainly
coarse pediment-cover gravels, glaciofluvial terrace and alluvial-fan gravels, as well as glacial tills
and landslide deposits. The chronology that we
have developed permits us to constrain the spatial
and temporal distribution of late Quaternary deformation along the Trans Alai Range front.
Pediment Cover Gravels
No absolute ages can be assigned to these oldest Pliocene(?)–Pleistocene deposits. Pedimentcover gravels, 4–6 m thick, lie unconformably on
the uplifted and deformed Neogene conglomerates (N2) and are associated with erosional remnants of formerly contiguous pediment surfaces,
which now terminate abruptly at the Main Pamir
thrust. In some cases, as in the region between the
Minjar and Komansu Rivers, the pediments are
covered by glacial till of unknown age. In any
case, these glacial deposits on top of the pediments belong to former piedmont glaciers and
document older events than the most recent three
glaciations, whose deposits are found in valleys
cut into the pediments. The composition of the
pediment-cover gravels reflects the local Neogene, Paleogene, and Cretaceous rocks. Gravelcovered ridges at elevations of about 3600 m
form accordant summits typically 200–400 m
above the modern trunk streams and about 600 m
above the range front, especially in the central
segment, and attest to the wide lateral extent of
these landforms and deposits.
Landslide Deposits
Landslide deposits are ubiquitous and are associated with other deposits spanning the entire
Quaternary period. The oldest landslide deposits
are found on an old pediment surface at the
Tashkungey River. These 20–40-m-thick deposits mainly constitute pervasively fractured Paleozoic turbidites and a minor fraction of Cretaceous
limestone blocks in a fine matrix of angular clasts
with the same composition. The brecciated nature of the rocks and their exotic composition in
an area dominated by Neogene and Quaternary
gravel suggests a rock-avalanche origin. Pronounced relief contrasts in the Trans Alai Range
would have been necessary to permit generation
of such a large landslide; also, a smooth pediment
or alluvial-fan surface in the adjacent undeformed foreland must have existed at the time in
order to allow a long runout distance of 5–6 km
away from the mountain front. Such inferences
are consistent with the early existence of a structural embayment in the area of the central Trans
Alai Range. The majority of landslide deposits,
however, is late Pleistocene or Holocene in age
and either consists of chaotic masses of Neogene
sandstones and argillites, rotational slump blocks
with apparently intact stratigraphy but significant
internal shearing, or masses of brecciated Neogene and Quaternary gravel. In either case, these
deposits are easily identified because they have a
pronounced hummocky topography, corresponding arcuate break-off scars at the top of hillsides,
or a surface morphology indicative of flowage.
The best developed examples occur in the gypsiferous and argillic Neogene deposits west of the
Altyndara River (Figs. 2 and 3).
Terrace Gravels
Terrace gravels in conjunction with glacial deposits are typical for all valleys that are connected
with the principal Trans Alai Range. However,
valleys with smaller catchment areas and originating within the Neogene deposits yield terrace
deposits that are less coarse and consist of sandy
gravel or sandy silt. There are a total of five welldeveloped terrace levels (Qt1–5) in most valleys of
the central segment. Many older terrace remnants, however, characterize the uplifted sections
of the frontal range along present river courses or
at the intersections of river valleys with the
mountain front. Due to their isolated occurrence
and indistinguishable clast compositions, these
remnants cannot be correlated any further.
An excellent example for the correlation of terraces between different valleys is represented by
the third oldest terrace (Qt3), which is associated
with moraines of the penultimate glaciation of
the region, the deposits of which are denoted as
Geological Society of America Bulletin, November 1999
1667
in the eastern portion of the image is indicated in white. Arrow depicts the sharp depositional contact between Qm2 and Qm3 in the area of Adziktash. No definite evidence for
thrust-related deformation is evident north of the range front. Patterns on the Alai Valley
floor correspond to irrigated and nonirrigated fields. The area covered by the satellite image is marked by four corner marks in Figure 3A.
Figure 2. Landsat TM scene (RGB bands 5, 4, and 2) of the southern Alai Valley and
Trans Alai Range foothills. Pronounced erosion due to numerous landslides is evident in
easily erodable Paleogene sedimentary rocks outcropping along the western Trans Alai
range front west of the Altyndara River. Figure 3B shows the extent of the Paleogene
rocks. Hummocky topography in the eastern portion of the Alai Valley corresponds to
dissected moraines and outwash fans. A zone of springs along the front of the moraines
0
20 km
N
ARROWSMITH AND STRECKER
1668
Qm3 (Fig. 6). Moraine units are denoted by the
same subscript as the coeval terrace deposits (see
the following discussion). We noted that when
these units are in contact (Qt3 and Qm3), they interfinger in the lower portions of exposures. The
terrace deposits (Qt3) ultimately bury the uppermost portions of the moraines (Qm3). This relationship was noted in an outcrop in a drainage between the Kokkik and Altyndara drainages and in
a portion of the Komansu drainage. In the more
easterly of the major drainages in Figure 6, Qm3
interfingers with the Qt3 deposits, whereas in the
more westerly drainage an erosional contact between Qm2 and Qt3 is evident; note how the
moraine has been trimmed by the fluvial erosion
prior to Qt3 deposition (see following paragraph).
We did not note a significant difference among
the clast compositions of the moraine units, so
these correlations depend on the consistent outcrop distribution along the range front and relative ages of Qt3, Qm2, and Qm3. This terrace constitutes an ~20-m-thick gray gravel overlain by
~20 cm of tan silt with occasional pebble clasts,
which are capped by ~20 cm of red-brown silt.
The upper ~40 cm of the Qt3 gravel contains interstitial carbonate and thin carbonate coatings on
clasts (<1 mm), indicating a stage II soil carbonate development (e.g., Birkeland, 1984). The Qt3
gravel overlies and regrades or cuts into Qm2
(older) moraine deposits (Komansu, Adziksu,
and Tashkunguey Rivers); in the downstream
portions it may overlie a tan-gray coarse cobble
conglomerate, ~15 m thick, which in turn supersedes a red coarse cobble conglomerate with an
irregular upper surface, again indicating the cutand-fill nature of the upper gravel deposits (e.g.,
Minjar and Syrinadjar Rivers; Fig. 3).
The Qt3 deposit is always unconformable with
respect to the extensive Qm2 deposits. Figure 6 illustrates the eroded edges and unconformable relation between Qt3 and Qm2 just north of the
range front at the Komansu River and in the upper portions of the eastern Minjar drainage. This
distinct relationship permits us to correlate the
Qt3 surface along the range front. We infer that
the Qt3 deposit represents a part of a formerly extensive outwash fluvial braidplain associated
with the melting of the glaciers that carried the
Qm3 materials. The capping fine-grained sediments are probably eolian loess-like deposits developed after this extensive phase of fluvial deposition and are interpreted to be reworked glacial
rock flour. These deposits also indicate that the
Qt3 surface has not been regraded or eroded since
it was first abandoned. The Qt3 surface thus represents the maximum level of Quaternary aggradation after its formation. The formation of this
surface was followed by ongoing thrust faulting
and range uplift as well as regional incision of
20–40 m; this trend was punctuated by occa-
Geological Society of America Bulletin, November 1999
MOUNTAIN-BELT GROWTH, PAMIR-ALAI REGION, KYRGYZSTAN
sional aggradation or lateral erosion resulting in
the inset lower terrace surfaces (Qt4–5). Figure 7
illustrates the morphologic relationships among
the Qm2 moraine deposit, the Qt2–4 surfaces, the
Main Pamir thrust, and the present position of the
incised trunk stream at the Komansu River. Similar relationships are also evident elsewhere, for
example at the Tashkunguey and Minjar Rivers
(Figs. 2 and 3).
At the Syrinadjar River, accelerator mass spectrometry (AMS) radiocarbon dating of intercalated organic material within faulted gravel of terrace Qt3 yields a calibrated 14C age of 4467 to
4367 yr B.C. (2σ error). The younger terraces
Qt4–5 occur as insets within the Qt3 terrace, are
consequently younger than about 4400 yr B.C.,
and provide an impressive record of combined
aggradation, incision, and regrading in late Pleistocene–Holocene time (Figs. 5–7). In the small
valley between the Syrinadjar and Adziksu
Rivers (Fig. 2), a charcoal sample collected 2.1 m
below a broad terrace surface similar in height
and degree of degradation to the correlated Qt3
surface yields a calibrated 14C age of 5213 to
4991 yr B.C. for the brown-red clayey silt. This
valley does not drain the higher Trans Alai Range
and probably has not had significant glaciation in
its upper reaches. The formation of the surface
was probably coeval with that of Qt3 elsewhere,
and thus the date corroborates the age from the
Syrinadjar River. To date, the age of terraces
Qt1–2 cannot be quantified from field work, but
they clearly predate the glacial deposits pertaining to the maximum late Pleistocene glaciation of
the Trans Alai Range. All terrace remnants are
characterized by smooth surfaces and gentle gradients between 2° and 4°; the regraded terrace
surfaces continue out into the piedmont region
where they finally converge with the axial trunk
stream Kyzilsu (Figs. 2 and 3).
Quaternary Tills
Glacial tills are numerous and widespread in the
Alai region. However, on the basis of crosscutting
relationships, superposition, and morphologic appearance, four separate glacial tills (Qm1–4) and
associated moraines can be distinguished in the
narrow valleys within the Trans Alai Range and in
the Alai piedmont. Moraines of the oldest two
glaciations (Qm1–2) belonged to major coalesced
piedmont glaciers that advanced northward to
cross the Alai Valley onto the southern piedmont of
the Tien Shan, whence major glaciers did not advance southward. The moraines related to these
earlier ice advances are undated. In contrast, the
glacial advance associated with the Qm3 moraines
did not reach the piedmont (Fig. 6); an exception
is the ice advance documented in the Adziktash
Valley (Figs. 2 and 3B). Here, the penultimate ad-
vance is documented at 3400 m elevation, and the
terminal moraine occurs 2.5 km away from the
mountain front, where it rests on Qm2 tills and
abuts Qt3 deposits. The sharp topographic break
between the terrace surface and the moraine is easily mistaken as a fault contact when viewed on
satellite images (see arrow on Fig. 2); however, it
is purely depositional with an elliptical plan shape,
continues into the Qm2/Qm3 contact, and attests to
the youthfulness of this glacial advance. Morphologically, Qm1 tills can be distinguished from
Qm2–3 tills because of a smoother and more subdued topography related to postdepositional erosion and eolian processes, while the younger Qm2
moraines have a pristine morphology of rounded
hills and kettle holes. The Qm3 tills are difficult to
distinguish from Qm2 deposits; however, the associated distinct glacial landforms are better preserved, are less extensive, and cut the Qm2 deposits. The relation between the glaciofluvial terrace
Qt3 and the penultimate moraine deposits of Qm3
shows that glacial tills of this moraine generation
must be older than 4400 yr B.C. (Fig. 6). The deposits of the youngest advance (Qm4) are unvegetated with irregular surfaces adjacent to the active
glaciers in the upper portions of the valleys of the
Trans Alai Range.
GEOLOGIC STRUCTURE ALONG THE
TRANS ALAI RANGE FRONT
Eastern Segment
The eastern transfer zone ends at the intersection of the principal faults that bound the folded
Neogene conglomerates west of 73°00′E and the
fault that defines the thrust contact of the Cretaceous rocks with Quaternary alluvial fans in the
Alai piedmont (Fig. 3B). Although evidence for
major Quaternary deformation is seen in the region, important Quaternary offsets along the
range-bounding faults to the east of the Adziktash
River are scarce. The region east of the transfer
zone is dominated by extensive tills pertaining to
the Qm1–3 units observed in other sectors of the
piedmont. However, only minor faults could be
verified along this sector of the Trans Alai mountain front. This assessment is supported by
Nikonov et al. (1983), who also observed only
minor faults with Holocene surface offsets.
The effects of the 1974 Markansu M 7.0 earthquake are also consistent with these observations
because no important tectonic ruptures were
found in the eastern Alai front (Fan et al., 1994;
Fig. 4). Instead, earthquake-related deformation
appears to have been more widely distributed
within the Trans Alai Range and is manifested in
the form of secondary effects such as landsliding
and discontinuous nonsystematic surface fracturing. The mechanism of the Markansu event was
difficult to determine despite the fact that it was
extensively studied (e.g., Jackson et al., 1979; Ni,
1978). Fan et al. (1994) favored a thrust event
with a small strike-slip component at a shallow
depth. The aftershock focal mechanisms varied
from thrusts with east-west–striking P-axes to
strike-slip events with a normal component of dip
slip. As the aftershock sequence developed,
strike-slip focal mechanisms with nearly northsouth–oriented P-axes along a northeast-southwest–oriented belt dominated.
In contrast to the central segment of the Alai
region, features in the eastern segment, located
several kilometers from the mountain front, suggest blind thrusting and associated folding of alluvial fan strata. For example, an aligned set of
springs along an east-west–trending linear feature in the middle portion of the Kyzilagin River
suggests fault-controlled spring activity and surface deformation in the piedmont (Figs. 2 and 3).
Ponding of fine-grained material against this linear structure after deposition of the Qm2 till corroborates the interpretation of Quaternary tectonic activity in this sector.
Central Segment
The ~50-km-long central segment of the Main
Pamir thrust is comprised of the region between
the Adziktash River in the east and the Kokkik
River in the west (Fig. 3). The unifying characteristic of this mountain-front sector is a pronounced, segmented, occasionally left-stepping
thrust-fault system, which is readily seen on
satellite imagery and which produces a pronounced break in the topography of the Trans
Alai mountain front (Figs. 2–5). In a few localities the trace is covered by landslides of inferred
late Holocene age, which are often faulted. The
pronounced surface break is related to offset of
the Qt3 terrace and shows variable amounts of
displacement along strike (Figs. 6–8). Linear features are not offset horizontally across the fault,
indicating that fault motion is predominantly dipslip. Vertical offset of this surface along the central segment is 0 m at the Kokkik River, and increases to 13 m about 4 km to the east at the
Adziksu River, and then increases rapidly to a
maximum of 20 m at the mouth of the Syrinadjar
River (8 km east of Kokkik). The peak offset continues to the east for 5 km, but begins to diminish
to about 10 m about 19 km farther east at the
Minjar River, where a second scarp 200 m south
of the range front adds another 3 m of offset. The
offset increases to 15 m at the Komansu River (20
km east of Kokkik) before dropping to 5 m at the
Tashkunguey River (32 km east of Kokkik). The
amount of displacement further diminishes to 3
m in the area that is covered by the Qm3 tills 1 km
west of the Adziktash River (40 km east of
Geological Society of America Bulletin, November 1999
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Geological Society of America Bulletin, November 1999
Figure 3. (A) Topographic map of the Alai Valley and adjacent regions with important
late Cenozoic structures. Contour interval is 100 m. Original map scale was 1:200 000. Dark
line in the south is the international border between Kyrgyzstan and Tadjikistan. Modified
A
Kokkik), and tapers to 0 m a few kilometers east
of the Tuyuksu River (48 km east of Kokkik;
Figs. 3 and 8). Because the offset distribution is
defined by the relative elevations of the Qt3 surface remnants on either side of the fault trace,
variations in deposition rates do not influence its
determination.
The incision of the modern drainage channels
between 20 and 40 m exposes the near-surface
geometry of the principal mountain-bounding
fault (Figs. 9 and 10A). The fault zone is characterized by a thrust fault, approximately eastwest–striking and dipping southward between
30° and 45° (see the following discussion). The
exposure of the fault zone is often subtle, but reveals folded gravel beds in the hanging walls and
footwalls, as well as small shear zones parallel to
the main trace, emphasized by sheared gravels
parallel to the fault (Figs. 9 and 10).
The well-exposed fault zone at the Syrinadjar
River is exemplary in this respect because it is
comprised of numerous small lensoid shear bodies and oblique fault strands (Fig. 10B). Slip
along this fault caused the hanging wall to override organic material that is part of a sheared
body in the lower portion of the exposed fault
zone; the organic material yields a calibrated 14C
age of about 4400 yr B.C. (see preceding discussion in section on terrace gravels). Based upon
our topographic surveying, and inferring that the
organic materials were deposited in front of a
fault scarp that was active during the accumulation of the Qt3 gravels, we can determine the slip
rate of this fault. We assume that the deposition of
the gravels was rapid. Therefore, the date represents a maximum age for the Qt3 surface. The
vertical offset of this surface is 18 m, and with a
27° dip of the fault, a total slip of 40 m can be obtained, resulting in a minimum Holocene dip-slip
rate of the Main Pamir thrust on the order of 6
mm/yr (40 m/6.4 k.y.). This slip rate is broadly
compatible with the only other determination for
the area of a minimum of 2–4 mm/yr, which is
based on the overthrusting of Stone Age artifacts
farther west (Nikonov et al., 1983; Burtman and
Molnar, 1993).
The deformation observed in all terraces is always confined to a narrow zone, usually 2–4 m
wide, but occasionally drag-folded gravels along
the fault broaden the zone of deformation to a
width of 10 m (note the gravels in Figs. 9 and 10).
This important conclusion of localized deformation is corroborated by the smooth and constant
sloping surface of the Qt3 terrace, which maintains a gentle gradient downstream until it intersects the Main Pamir thrust (Figs. 11 and 12). Figure 11 shows a base map and topographic profiles
for three sites along the Main Pamir thrust depicting the sharp topographic break in the geomorphic surfaces. The profiles are perpendicular to
after Kyrgize Geodetic and Cartographic Agency (1991). The area covered by the satellite
image of Figure 2 is marked by four corner marks.
ARROWSMITH AND STRECKER
Ta
Pik
Sverdlov
15
3560
14
5
md
i
45
dar a
tyn
Al
Geological Society of America Bulletin, November 1999
Pik Lenin
4969
g in
3087
Eastern Segment
Transfer fault
Thrust fault
10 km
Tadjikistan
Kyrgyzstan
major faults along the southern edge of the Tien Shan. This map was prepared from new geologic mapping and compilation from Kyrgize Geologic Agency (1979); Nikonov et al.
(1983); Burtman and Molnar (1993); Strecker et al. (1995). The location of Figure 6 is indicated by the rectangle along the range front in the central segment.
Neogene and early Quaternary conglomerates
Adzik s u
Undifferentiated Pz Tien Shan rocks
7134
30
40
89
Spot elevations in meters
4309
12
ansu
Kom
Undifferentiated late Quaternary deposits
5654
10
75
Figure 6
Ko
ru
Tu yuksu
Undifferentiated Cretaceous and older rocks
11
ar
i
Paleogene mud- and siltstones
4625
5654
j ar
2842
inj
la
73 00'
Eastern Transfer
rt
Figure 3. (Continued). (B) Geologic map of central and western portions of the Alai Valley, Trans Alai Range, and southern Tien Shan region depicting the active range bounding
Main Pamir thrust and the adjacent Alai Valley foredeep. The segments and transfer zones
along the bounding Main Pamir thrust are indicated along the top of the map. The lack of
certainty of the edges of the eastern transfer zone are indicated by dashing. Note the lack of
Berk-Sum
Mnts.
ad
3229
Sy r i n
3140
Ko
kk
ik
M
2884
Ta
y
ge
un
k
sh
45
ta
2647
30'
sh
zikta
Ad
2594
Kyzilsu
72
Central Segment
z
Ky
30
rA
Ta
ka
4058
Western Transfer
il a
4073
4289
Western Segment
39
40'
B
z
Ky
1671
100 km
A
40°N
38°N
5250
4750
4250
3750
36°N
68°E
70°E
72°E
74°E
76°E
78°E
3250
2750
B
50 km
Markansu
14
Za Alai
2250
15
1750
13
40°N
1250
750
250
39°N
71°E
72°E
73°E
74°E
75°E
Figure 4. Seismicity overlain on digital topography of the Pamir region. (A) Regional seismicity and focal mechanisms for the Pamirs are consistent with the tectonic regions of the orogen (Fig. 1). Strike-slip events predominate in the west and east, while normal faulting is manifest in the
central Pamir and thrusting is clearly evident along the northern periphery. The large cluster to the southwest is part of the Hindu Kush seismic
zone that extends to depths of 200–300 km (e.g., Roecker et al., 1980; Pegler and Das, 1998). Note that the seismicity is color-coded with depth
(black is 0–100 km and red is 100–300 km). The black rectangle indicates the area covered by B. Strecker et al. (1995), and Pegler and Das (1998)
present further recent analysis of the regional seismicity. (B) The Alai region seismicity is concentrated along the Trans Alai. The blue focal mechanisms are from Burtman and Molnar (1993) and represent the Za Alai earthquake and the Markansu earthquake and its three prominent aftershocks. Note the leaders that connect the locations to mechanisms for clarity. Both events occurred in the transfer zones at either end of the
Trans Alai central segment. Black focal mechanisms are from the Harvard CMT Catalog from the beginning of 1976 to the present. Seismicity is
from the Council of the National Seismic System earthquake catalog with events from 1963 to the present. Digital topography is the GTOPO30 1 km
data set from the U.S. Geological Survey. Black lines are international borders.
1672
Geological Society of America Bulletin, November 1999
MOUNTAIN-BELT GROWTH, PAMIR-ALAI REGION, KYRGYZSTAN
Figure 5. Photo take from north side of Alai Valley looking southeast at the Trans Alai Range and Peak Lenin (7134 m). The base of range front
is 3000 m elevation. The range is composed of a series of thrust sheets that place progressively older rocks over younger with Paleozoic rocks making up the higher portions of the range. Cretaceous rocks are in the middle portion, and Paleogene–Neogene are in the foothills, with the active
thrust at the base of the range placing Tertiary to Pleistocene conglomerates over Holocene rocks.
the average local strike of the fault. There are no
other breaks in the topographic profiles aside
from where they cross the fault, indicating that the
deformation is localized along a narrow frontal
zone. Thus, with the exception of a smaller subsidiary dextral oblique thrust fault at the Minjar
River, bedding-plane slip and small-scale backthrusting in N2 gravels at the Tashkungey River
and between the Syrinadjar and Adziksu Rivers,
and some left-stepping <3.5-m-high surface
breaks in the upper Syrinadjar drainage, this style
of localized deformation is characteristic of the
entire central segment (Figs. 9, 10A, and 12).
Although the Quaternary displacements typically rupture coarse terrace gravels, in several
places (Komansu, Minjar, and Tashkunguey
Rivers) we noted wedge-shaped deposits of matrix-supported fine silt and sand with coarse pebbles to cobbles that were both massive and stratified. These deposits extend from a sharp wedge
boundary about 2 m below the upper portion of
the fault scarp upward and grade into the soil profiles of the hanging walls and footwalls. We infer
that these features formed in a major surface rupture when the overlying loess cover was incorporated into the fault zone and overridden by the
older gravels. The reverse slip in the last event
was at least 1.5 m on the basis of the observations
at Minjar River, where a steeper slope segment
coincides with the surface rupture (Fig. 13). Examination of these deposits at the Komansu River
showed that the (Fig. 9) degree of lithification,
composition, and soil development of the entire
wedge is of the same age.
Western Transfer
On the basis of deformation features observed
in the 1978 M 6.5 Za Alai earthquake epicentral
area (Fig. 4B), Nikonov et al. (1983) mapped a
dextral Quaternary strike-slip fault between the
Altyndara and Tar Ata Rivers at about 72°15′E
(Fig. 3). We interpret this fault as an integral part
of a complex 20-km-wide zone characterized by
northeast-striking thrust faults and northweststriking transfer faults with indicators of oblique
dextral sense of slip, between the Kokkik and the
Tar Ata Rivers (Figs. 2 and 3). The eastern end of
this region (at Kokkik) also defines the western
termination of the central segment of the Main
Pamir thrust. The northwest-striking transfer
faults have tens of meters of dextral offset of Qua-
ternary landforms and deposits and terminate in
northeast-striking thrust faults that vertically offset Quaternary piedmont surfaces about 10 m
(Figs. 2, 3, and 11). The northwest-trending range
fronts are linear and have aligned springs where
bounded by transfer faults, in contrast to the more
sinuous range fronts limited by thrust faults.
At the Taka River, striations in faulted Cretaceous sandstones document the dextrally oblique
nature of the western transfer fault system. This
is further corroborated by dextral offsets in an
east-southeast–striking fold in Cretaceous limestones (Fig. 3B). In the area between the Taka
and Altyndara Rivers, the geometry of alternating
northwest-striking and steeply dipping bodies of
gypsiferous sandstones and limestone is consistent with dextral shearing in this zone. Finally,
northwest-southeast–oriented gypsum fibers in
these deformation zones attest to this most recent
deformation style.
The wide en échelon transfer zone between the
Kokkik and Tar Ata Rivers is a geologic feature
that is manifested at various spatial and temporal
scales. For example, a 10-m-tall dextral striated
fault zone in Cretaceous limestones in the lower
Altyndara River was reactivated several times
Geological Society of America Bulletin, November 1999
1673
ARROWSMITH AND STRECKER
B
A
39 28'30'' N
72 42'30'' E
N
72 42'30'' E
39 31'45'' N
r
nja
Mi
72 36'30'' E
1 km
72 36'30'' E
39 31'45'' N
39 28'30'' N
Fault, dashed where approximate
Lineament, usually parallel to bedding
Young channel alluvium
Qt5
Qt4
Qt3
}
Qm2
Alluvial Terraces
Qm3
}
Moraines
Neogene gravels
Active drainage
Figure 6. Relationships between landforms, deposits, and structures along the Trans Alai Range front near the Komansu and Minjar Rivers
(see Fig. 3B for location). (B) The Declassified Intelligence Satellite photograph on the right (May 1959) shows the relationships that are mapped
in (A), the Quaternary geologic map shown on the left. The map documents the erosional contact between the Qt3 materials and the Qm2
moraines, and the apparently coeval timing of deposition of Qt3 and Qm3.
during Quaternary time, as clearly shown by a 4
m vertical offset (115°–120° strike, south side
up) of Quaternary terrace gravels. To the northwest of this outcrop, Nikonov et al. (1983) documented sets of fractures oriented northwestsoutheast that show oblique dextral-thrust slip.
Late Pleistocene moraines and terraces are affected by these faults. The transfer between the
thrust fault bounding the Dzolsu-Tyor Mountains
and the Berk-Suu Mountains (Fig. 3, A and B)
further west is characterized by left-stepping fault
traces in the Qt3 fluvial terrace. Pressure ridges
also indicative of strike-slip deformation have developed in the stepovers. To the south, older Quaternary offsets occur along a more northerly striking dextral shear zone (Fig. 3). Similar to the fault
1674
bounding the Dzolsu-Tyor Mountains, this fault
kinematically links the thrust fault bounding
Sverdlov Peak to the south with the regions farther northwest that define the Neogene–Quaternary thrust front that continues west to Tadjikistan (Fig. 3B).
The rupture zone that traverses the Altyndara
River continues northwestward into older moraine
deposits (Qm1) where it is clearly visible as a topographic break, 4–6 m high. Erosional cuts reveal
that the fault zone is cut by 130°–140° striking,
smaller shear zones with associated brecciated
gravels. Continuing northwest, the zone of deformation bifurcates into two subparallel sectors
(Fig. 3). The southwestern sector traverses a large
Holocene landslide composed of gypsiferous
sand, mud, and siltstones. The irregular hummocky topography of the landslide and postlandslide mass wasting make verification of the fault
trace through the slide materials conjectural. However, a clearly defined scarp exists along strike immediately to the northwest (Fig. 3), where the fault
transforms into an oblique thrust fault that places
Paleogene conglomerate over Neogene gypsiferous argillite (Figs. 2 and 3).
The second, more northeasterly located fault
zone also provides clear evidence for Quaternary
strike-slip offset (Fig. 14). Detailed topographic
mapping indicates that the small ephemeral
drainage channels have dextral offsets of at least
25 m. The magnitude of offset of beheaded channels at the site is equivocal, but suggests dis-
Geological Society of America Bulletin, November 1999
MOUNTAIN-BELT GROWTH, PAMIR-ALAI REGION, KYRGYZSTAN
Qm2
0
North
2
Elevation (m)
Qt3
Qm2
30
4
Qt4
Qt5
20
5
6
10
0
iver
nsu R
Koma
–200
200
10
24
–100
100
Easting (m)
Qt4
Northing (m)
0
Qt3
12
0
Qm2
28
32
16 20
14
Explanation
Moraine deposit (Qm2)
Qt5
Thrust fault, barbs
on hanging wall
QT3-5
Terrace surfaces
Minor Contour interval = 0.5 m
Major Contour interval = 2 m
Survey points
0
100 m
Figure 7. Contour map and shaded relief maps of landforms at Komansu River (see Figs. 3 and 6 for location) illustrating the geomorphic and
erosional relationships among the Qm2 moraine deposit, the Qt3 and lower terraces, and the active thrust fault (line with barbs on the hanging
wall). The Qt3 surface is cut into the moraine. This relationship is consistent along the Trans Alai Range front. The thrust fault offsets all of the
terrace surfaces from the correlative surface on the north (Qt3 is vertically offset ~18 m, Qt4 is vertically offset ~8 m, and Qt5 is vertically offset
~6 m). The modern Komansu River has incised 30 m. Note that some minor contours are removed for clarity in the steeper slopes of Qm2 on the
contour map. Major contours are labeled in meters relative to an arbitrary datum. The heavier lines are the geologic contacts.
placement of up to 60 m. A preliminary estimate
of the age of the drainage channels is based on
the assumption of an early Holocene age (10 ka)
for the young moraine and alluvial-fan deposits
into which the drainage channels are cut. Therefore, the Quaternary slip rate for this zone may be
at least 2.5 mm/yr, and could be as high as 6
mm/yr. These values are consistent with the better constrained slip rate along the central segment
(assuming that slip is efficiently transferred along
this zone). The transfer zone continues to the
northwest and changes into an oblique thrust
fault. The transition is well documented at the Tar
Ata River, where landslide and gravel-covered
Quaternary deposits overthrust young coarse
Quaternary terrace gravel by at least 14 m.
The northwestern portion of the transfer-fault
zone, in particular at the Tar Ata River, comprises north-vergent, low-angle thrusts (090°,
27°S) rooted in a steeply dipping dextral shear
zone (100°–120°, 55°–70°S) of the transfer
fault. The combination of the low-angle thrusts
and numerous landslides mainly along northeast-facing interfluves has resulted in a sinuous
mountain front, where the actual thrust-faulted
range front is covered by the toes of landslip
masses, which in turn are often offset by recurrent thrusting activity. This peculiar tectonic
morphology is characteristic for at least several
kilometers along the northwestern Trans Alai
Range and records the close relation between
tectonic style, rocks, and climate with respect to
landsliding (Figs. 2 and 3).
Not only the structural style, but also the
range-scale morphology of drainage basins reflects the western structural transition along the
Trans Alai mountain front (Figs. 2 and 3A). Most
of the drainage basins of the central segment are
elongate north-south and drain from the Trans
Alai Range crest to the Alai Valley. However, in
the transfer zone, the drainage basins are elongate
east-west, generally smaller in area than those of
the central segment, and do not systematically
drain the Trans Alai Range crest. Furthermore, a
major drainage, the Altyndara River, flows
through the western portion of this transfer zone
and drains the southern side of the Trans Alai
Range, analogous to transfer fault drainage systems in rift zones (e.g., Lambiase and Bosworth,
1995). The reflection of tectonic style in gross
physiography thus indicates that the segmentation of the structures has been sustained.
DISCUSSION
Seismotectonic Segmentation
The present segmentation of the Trans Alai
mountain front is a structural phenomenon that
has been maintained at least throughout Quaternary time and perhaps since late Neogene time.
The segmentation is defined by the relatively simple central segment with uniform north-vergent
thrust faulting, and the adjacent thrust fronts of
the eastern and western segments, which are
linked to the central segment via complex transfer zones with dextral strike-slip and dextrally
oblique thrust faults. The offset of Quaternary
landforms in all sectors along major faults demonstrates that deformation on a regional scale is not
pervasive but is instead accommodated by thrust
and strike-slip faults active along the range front
at all temporal scales.
Limited fracturing and offsets due to faulting
away from the Main Pamir thrust exist but do not
Geological Society of America Bulletin, November 1999
1675
1676
Tuyuksu
Adziktash
Tashkunguey
Minjar (+2nd scarp)
Komansu
Syrinadjar
Adziksu
20
15
Vertical offset
(m)
contradict the conclusion of important localized
deformation. Left-stepping surface ruptures of
apparent thrusting origin with up to 3.5 m vertical offset were observed in Holocene colluvium
close to the thrust-faulted contact between Cretaceous and Paleogene units in the upper Syrinadjar River area. Furthermore, the steeply dipping
rocks of the N2 deposits at the Tashkunguey
River and between the Syrinadjar and Adziksu
Rivers have accommodated Holocene beddingplane slip or small-scale backthrusting in the order of 1 m (Figs. 2, 3, and 6). However, the
smoothness and straightness of the terrace surfaces (Figs. 11 and 12) indicate that most of the
deformation occurs at the range front only, and
the minimum slip rate of ~6 mm/yr determined at
Syrinadjar represents a large portion of the overall Quaternary deformation along the central segment when ~1 m of Holocene offset is accounted
for within the range. Furthermore, the lack of
sharp offsets or curvature (folding) of the terrace
surfaces implies that the principal thrust fault is
relatively planar or only gently curving downdip
for at least several kilometers. This interpretation
is based upon simple models of landscape development and active deformation (Arrowsmith et al.,
1996) that indicate a direct relation between terrace curvature and the scale of discontinuities or
sharp changes in dip of range-front thrust faults.
For the tectonic situation at the Trans Alai Range
front this means that the length scale of deformation of the fluvial terraces will be directly related
to the depth scale of the deformation source. With
the exception of minor faulting in the upstream
area of the Minjar River, folding or secondary
breaks in the terrace profiles are absent for at
least 7 km upstream perpendicular to the strike of
the range-bounding fault (Fig. 12).
The interpretation of one principal fault being
responsible for most of the deformation in this region is underscored by the multiple flights of fluvial terraces and pediment surfaces that terminate
abruptly at the precipitous mountain front. These
terraces document that the deformational style
observed on the historic and Holocene time
scales has prevailed throughout Quaternary time.
The presence of multiple terraces in the hanging
wall and their absence in the gently sloping Alai
piedmont clearly links their development to recurrent tectonism. However, the connection of
some of the better-preserved terraces with terminal moraine systems upstream and the cut and fill
nature of the corresponding gravels in the graded
piedmont also indicate an important climatedriven influence; the stratigraphic relationships
show that aggradation occurred during glacial or
finiglacial conditions when more sediments were
available relative to the fluvial transport capacity
(Figs. 5 and 6).
Aggradation was followed by incision and ter-
Kokkik
ARROWSMITH AND STRECKER
10
5
west
east
0
0
10
20
30
Distance along range front
(km)
40
50
Figure 8. Vertical offset distribution of the Qt3 surface at range front along the central segment of the Trans Alai Range. The asymmetry in the offset distribution suggests that the central
segment interacts mechanically with the segments to the west, but not with those to the east. Error bars indicate field estimate of uncertainty in offset determination.
race development probably due to a combination
of greater transport capacities and less available
material, as well as continuing uplift at the range
front. In such a scenario, the climatically controlled downcutting was enhanced by the tectonically induced local base-level changes at the
mountain front; in contrast, the dynamics of the
piedmont streams and the trunk stream Kyzilsu
were balanced and resulted in a smooth piedmont
morphology without terraces. The alternation of
aggradation and incision processes was restricted
to the hanging wall, and continued uplift provided new accommodation space for subsequent
glacial advances. The uplift pattern of the well
documented Qt3 terrace and older landforms in
the central Trans Alai Range versus the nonuniform distribution of terraces with respect to the
entire Trans Alai mountain front thus documents
that the central sector of the range front is a rupture segment with its own uplift history.
On the historic time scale, the deformation
style of the central segment and the neighboring
western transfer zone is reflected in the rupture
patterns of earthquakes such as the M 6.5 Za Alai
earthquake in 1978 (Fig. 4), mapped by Nikonov
et al. (1983). Figure 42 of Nikonov et al. (1983)
documents the prominent Holocene paleoseismic
dislocations along the central segment and their
continuation in the western transfer zone. With a
few exceptions, these ruptures generally coincide
with the range-front fault responsible for the observed Quaternary deformation. In contrast, the
eastern part of the Trans Alai Range front only
exhibits isolated and minor ruptures. Using limited radiocarbon dating and interpretation of the
ages of Quaternary landforms, Nikonov et al.
(1983) suggested that surface rupture has occurred along these faults in the past 7 k.y. However, the data cannot always be confidently interpreted because the chronology of faulting was
partly derived by means of lichenometry, and the
degree of preservation of mass-movement deposits that were inferred to be seismically driven.
The 1978 M 6.5 Za Alai earthquake sequence was reanalyzed by Fan et al. (1994).
The epicenter of the main shock was located at
lat 39.34°N, long 72.56°E, about 16 km south
of the range front at the upper drainage divide
of the Minjar River (Figs. 3 and 4); the earthquake therefore occurred at the western termination of the central segment. The Harvard
CMT solution included an ad hoc depth of 15
km (Fan et al., 1994). However, using the dip of
the range-bounding fault (30° south), the epicentral location would indicate a downdip rupture depth of about 10 km. Assuming that the
geologic constraints can be used to select the
south-dipping focal plane, the strike of this
mechanism was 103° with a dip of 58° south
and a slip vector of 163°, indicating a dextral
Geological Society of America Bulletin, November 1999
MOUNTAIN-BELT GROWTH, PAMIR-ALAI REGION, KYRGYZSTAN
Sketch of important
features:
Overridden
colluvial material
Person
Deformed gravels
Komansu River
Figure 9. Komansu fault zone and scarp (see Figs. 3 and 6 for location). The fault zone distorts the gravels on both the hanging walls and footwalls. View is to the west from the western edge of topographic map in Figure 7. Note person for scale at top of scarp.
strike-slip component, but the main shock was
not adequately modeled by a point source. Numerous aftershocks with strike-slip and dextrally oblique thrust solutions were noted; all
were apparently shallower than the main shock,
and occurred closer to the range front, suggesting that they too may have been located along
the Main Pamir thrust. Ruptures in the Za Alai
event probably propagated into the central segment and westward into the transfer zone
where, according to Nikonov et al. (1983), offsets and shaking were greatest. The overall
structural character, the style of rupture propagation, and the varied focal-mechanism solutions thus identify the western transfer zone as
a small semi-independent rupture segment between the other segments, which are relatively
simple thrust-dominated regions.
Interaction among Thrust-Fault Segments
One of the compelling observations from our
field work along the Main Pamir thrust is the
asymmetric vertical offset distribution of the Qt3
surface along the central segment (Fig. 8). Based
upon the mechanical consequences of the interaction between en echelon fault segments for slip
distributions (see the following discussion), we
suggest that the asymmetry in the Qt3 surface offset indicates that the central and western segments, via the western transfer zone, mechanically interact.
Discontinuities along faults affect the slip distribution along individual segments of a fault array. The degree of mechanical interaction between fault segments is controlled by the material
properties of the offset material, the mechanical
properties of the faults themselves (in particular
their strength distribution), and the fault geometries, including size and proximity of fault segments (e.g., Segall and Pollard, 1980; Bürgmann
et al., 1994; Willemse et al., 1996). Fracturemechanics models for individual faults with uniform strength and embedded in a homogeneous
and isotropic material driven by constant remote
stress predict an elliptical slip distribution (Pollard
and Segall, 1987; Willemse et al., 1996). However,
models for two interacting fault segments of constant geometry arrayed along strike show that the
slip distributions become asymmetric with the
peak slip closer to the center of the array; furthermore, the maximum slip increases relative to that
expected for a mechanically isolated segment
(Willemse et al., 1996). This results from a perturbation in stress due to slip along one fault that enhances fault slip of a similar sense on faults along
strike. The degree of interaction is enhanced with
decreasing distance between the fault segments of
an array. The analysis by Willemse et al. (1996) indicated that the region of influence for individual
fault segments scales with their shortest spatial dimension (length or down-dip width). Even for infinitely tall dip-slip faults, interaction is negligible
with spacings greater than the fault half-lengths.
Interaction is further enhanced with fault-segment
overlap, but slip in the overlap zone may actually be decreased, because near the interior of a
fault segment the stress perturbation tends to reduce shear stress, while around the peripheries
it is enhanced.
In this context, the rupture histories and style
of segmentation of the central, western transfer,
and western segments of the Main Pamir thrust
may be explained in terms of a mechanically interacting fault array. Due to the en echelon kine-
Geological Society of America Bulletin, November 1999
1677
A
B
Upper material is massive sand
with gravels (to cobbles) Light gray
dd
grav
Crus
hed
Be
d
he
us
c
Cr
bri
Fa
Covered
ing
Strong fabric
development
el
Dark gypsumstained zone
Little sand,
lots of fabric
Coarse
gravel
with sand
matrix
1m
x
1m
Gravel with
fabric
parallel to fault
zone
Covered
Explanation:
Sand layers, dots indicate bedding
Coarse gravel, modest sand matrix
Sand with gravel (fine to coarse) clasts (massive)
Sheared gravels along fault zone.
Good fabric development.
Prominent discontinuity with offset
Sample A4-1
4535 to 4350 BC
(sheared soil horizon?)
Gray and red sands with
0.5 cm carbonate nodules and
a few disseminated pebbles
Figure 10. View of Syrinadjar fault zone and scarp (see Fig. 3 for location). (A) From the west side of the Syrinadjar River looking toward the
east-southeast, showing the fault scarp that has developed in the Qt3 surface and the river cut that exposed the fault zone. Note person for scale
in channel bottom and vehicle and tents in background at scarp base. Arrow indicates location of B. (B) Detailed cross-section view of a fault zone
exposed in a river channel cut along the active mountain front at the Syrinadjar River. Deformed terrace gravels have overridden an inferred paleosol. A corrected 14C age of about 4500 yr B.C. indicates that the deformation at the mountain front is young and amounts to a maximum vertical offset of 20 m in Holocene time.
1678
Geological Society of America Bulletin, November 1999
A) Terrace profiles at Adziksu
Distance north-south (m)
Map of observation
locations and projection
planes
Channel and terrace
projection plane
400
0 Cross-valley
projection
plane
-400
Explanation:
-800
-1000
-500
0
Distance east-west (m)
Active channel profile
500
Qt3 terrace profile
Relative
elevation (m)
100
Other terrace profile
Approximate local fault trace
0
200
400
600
800
1000
1200
Cross-valley topographic profile
60 m
0
200
400
600
800
1000
Distance along profile (m)
B) Terrace profiles at Syrinadjar
C) Terrace profiles at Komansu
Channel
and terrace
projection
plane
100
Distance north-south (m)
Distance north-south (m)
500
0
-500
-1000
-1500
-400
Channel and terrace
projection plane
0
400
Distance east-west (m)
-300
800
-400
-100
Distance east-west (m)
Relative elevation (m)
40
100
0
500
1000
1500
0
Distance along profile (m)
400
Distance along profile (m)
Figure 11. Map and topographic profiles for three sites along the Trans Alai Range front depicting the sharp topographic break in the geomorphic surfaces. Because the profiles were projected to a single plane and the thrust fault trace was not straight, successive profiles do not always plot with the scarp in the same position. The open circles and thinner lines indicate the profiles of the active channels; black dots and lines
indicate profiles of the correlated Qt3 surface, and the gray dots and lines indicate other surveyed surfaces. Topographic profiles are not vertically
exaggerated. These data were gathered using an electronic total station. See Figure 3, A and B, for locations of these sites. (A) Profiles at Adziksu
document the progressively higher and older terraces. The cross-valley topographic profile illustrates the extent of one terrace surface incised by
the active channel. (B) Terrace profiles at Syrinadjar illustrate the abrupt offset at the range-bounding thrust fault. (C) Terrace profiles at Komansu, along with those at Syrinadjar, show the straight modern channel incised 20–40 m below the offset Holocene terrace surfaces. The profiles are generally perpendicular to the local strike of the range-front bounding thrust fault, the local trace of which is shown by a line with teeth.
There are no other breaks in the topographic profiles, aside from where they cross the fault indicating that deformation is localized at the range
front. The map shows the plane(s) to which the data were projected to prepare the profiles.
Geological Society of America Bulletin, November 1999
1679
West
Explanation:
Active channel profile
Adiksu
Qt3 terrace profile
Ichaksu
Other terrace profile
Active range bounding thrust
fault, dip is schematic
Sirinadjar
Minjar
Komansu
Tashkunguey
Relative elevation (m)
300
–3000
–2000
East
–1000
0
Distance along profile
(m)
1000
2000
3000
4000
Figure 12. Topographic profiles of terrace surfaces and active channels along the Trans Alai Range front (see Fig. 3, A and B, for locations). This
expanded synoptic view (relative to Fig. 11) shows a lack of deformation of the terraces far upstream, suggesting that faulting is localized at the
range front and that the thrust fault is planar downdip for at least 7 km. The Tashkunguey Qt3 terrace profile crosses a Qm2 moraine to produce
the bump in the upper portion of the profile. Profiles have no vertical exaggeration. These data were gathered using an electronic total station.
Silt and sand
Figure 13. Minjar fault scarp and
wedge of colluvial material overridden by thrust slip along the fault (see
Figs. 3 and 6 for location). Note also
the break in slope coincident with
the offset in the exposure. This view
provides evidence for the most recent surface rupturing earthquake
along the range front and indicates
that it was sufficiently large to offset
the surface by ~1.5 m. View toward
the west. Boy on left is 1.4 m tall.
1680
Geological Society of America Bulletin, November 1999
MOUNTAIN-BELT GROWTH, PAMIR-ALAI REGION, KYRGYZSTAN
A
U
D
Explanation
D
U
Major channels
Fault zone defined by offset
landforms. Arrows indicate
sense of offset. U means
this side up, and D means
this side down.
Beheaded?
Offset
North
0
100 m
Minor contour interval 1 m
Major contour interval 5 m
B
Offset
100
Relative
Elevation (m)
40
Beheaded
-500
–400
Northing (m)
–200
–400 Easting (m)
–300
Figure 14. (A) Detailed topographic map of the dextral transfer zone across the Altyndara
River (see Fig. 3 for location). Dextral offsets suggest 2.5–5.7 mm/yr Holocene slip rate.
(B) Shaded relief map of the Altyndara River region (same area as A) showing beheaded and
offset ephemeral streams in a zone of pronounced Quaternary faulting. This region defines a
structural transfer zone in which deformation is transferred from the Trans Alai mountain
front in the east to a thrust-fault segment farther west.
matic transfer between the central and western
segments, the western transfer zone is structurally complicated and lacks a simple surface
offset distribution. Nevertheless, the existence of
the transfer zone effectively diminishes the distance between the central and western segments,
thus enhancing the potential interaction between
them. If these faults also mechanically interacted
with the eastern transfer zone and other structures
farther east, it would not be expected to see the
westward asymmetry in the offset distribution of
the central segment (Fig. 8).
Cumulative Effects of Convergence in the
Central Sector of the Collision Zone
The progressive closure of the Alai Valley by
the northward advance of thrusting in the Trans
Alai exemplifies the annihilation of an intramontane basin by collision tectonics. The continuity of
the Paleogene sedimentary basin from the Tadjik
Depression in the west across the Alai region to
the Tarim Basin to the east (Davidzon et al., 1982)
was ended by north-vergent and northward propagating thrust faulting (Burtman and Molnar,
1993), which juxtaposed the Tien Shan and Pamir
Mountains. Thrust faulting along the central segment of the Trans Alai thrust fault stepped northward from the fault, which places Paleogene over
Neogene units to develop the active range bounding Main Pamir thrust, which in turn places Neogene over Quaternary. This event transferred a
section of the Neogene basin fill to the hanging
wall of the Main Pamir thrust system (Figs. 2 and
3). The structural embayment of these Neogene
rocks coincides with the central segment of the
Main Pamir thrust defined here based upon Quaternary geologic relationships. If the 4 mm/yr vertical displacement rate along the central segment
has been sustained since the latest fault was initiated, it would take only 150 k.y. to generate the
<600 m of relief and relative elevation of ridges of
Neogene rocks and their now isolated early Quaternary pediment deposits.
Due to high sediment yields of the Trans Alai
piedmont rivers, the Kyzilsu River is forced to
migrate to the northern margin of the Alai Valley
(Figs. 2 and 3). The position of its thalweg and
the broad northward-sloping Trans Alai piedmont indicate that deposition of sediments derived from the Trans Alai Range has created a
wedge of sediment that fills the Alai Valley and
onlaps the southern side of the Tien Shan. This is
seen on north-south–oriented seismic reflection
profiles, which show a pronounced wedge geometry with facies onlapping northward (Kyrgize
Institute of Seismology, unpublished data). The
width is reduced to a narrow valley separating the
thrust front from the Tien Shan in the area of the
western segment of the Main Pamir thrust, im-
Geological Society of America Bulletin, November 1999
1681
ARROWSMITH AND STRECKER
mediately west of the Tar Ata River. Landsliding
removes much of the material that is uplifted
along the thrust and delivers it to the Kyzilsu
River, analogous to a lateral conveyor belt that removes material delivered to the deformation front
by thrusting (Pavlis et al., 1997). The river therefore facilitates advance of the thrust front by constantly removing the mass that would otherwise
be deposited as molasse in a foredeep such as the
Alai Valley proper. However, further to the west,
thrusting has completely closed this intramontane basin; the Kyzilsu River flows across the
hanging wall, and backthrusts place Tien Shan
basement rocks over the Cretaceous–Neogene
sequence (Hamburger et al., 1992; Leith and
Alvarez, 1985; Pavlis et al., 1997). Thus, the progressive closure of this intermontane basin is
documented by the change from foredeep environment to active thrusting and landsliding to
complete intramontane basin annihilation.
On a regional scale, Burtman and Molnar
(1993) suggested that the 44 ± 5 mm/yr convergence between India and Eurasia in the greater
Pamir region is partitioned into northward underthrusting of India below the Salt Range of Pakistan, distributed deformation within the Hindu
Kush–Pamir, and southward underthrusting of
Eurasia below the Pamir. Convergence and underthrusting in the Pamir area are accommodated
at the surface not only along the Main Pamir
thrust, but also within the Tien Shan and adjacent
regions to the north (Burtman and Molnar, 1993),
and by strike-slip faulting in the Pamir farther
south (Strecker et al., 1995).
Michel et al. (1997) determined that the overall shortening between the central Pamir and the
Kazakh platform (Eurasia) is about 3 cm/yr.
Southward tilting of the Alai Valley basement and
deformation along the southern margin of the
Tien Shan as seen on seismic reflection profiles
(Kyrgize Institute of Seismology, unpublished
data), as well as the geodetically determined velocity distribution (Michel et al., 1997; Larson
et al., 1999) all indicate that shortening may be
accommodated within the Tien Shan and adjacent regions. Estimates of the magnitude of active shortening in the Pamir region relative to the
slip rate determined in this study for the central
segment of the Trans Alai thrust fault show that
the Main Pamir thrust alone accommodates more
than 10% of the total India-Eurasia relative motion at this longitude.
One of the implications of the high slip rate
along the Main Pamir thrust, coupled with the
lack of basement rocks exposed in the Trans Alai
Range, is that the range may have started to uplift only a few million years ago. As discussed in
the preceding section, the currently active central
segment with pediment remnants elevated to 600
m above the mountain front may have begun to
1682
slip about 150 ka, assuming that the vertical uplift rate of 4 mm/yr has remained constant. If this
surface uplift rate is considered to be in the typical range for long-term uplift rates along the
Trans Alai thrust-fault system it would take
about 1 m.y. to create the current relief (4 km)
with no erosion of the ridge lines. If one assumes
that thrusting did not significantly thicken the
cover rocks, one can use the correlative sedimentary basin fill of the adjacent Tadjik Depression (10 km; Bazhenov et al., 1994) to derive
first-order estimates of uplift and denudation.
The 10 km of cover rocks minus 4 km of relief
with no basement exposed provides an estimate
of 6 km for the possible maximum exhumation
relative to the peaks in the Trans Alai Range. The
thickness of the Neogene sediments of the Alai
Valley is about 3 km adjacent to the Trans Alai
Range. Therefore, an estimate of the Neogene to
Quaternary vertical offset along the Trans Alai
thrust fault involves 6 km of denudation, plus 4
km of relief and an additional 3 km of foredeep
sediments, equaling 13 km. Inferring a minimum vertical offset rate of 4 mm/yr, the thrust
systems and the related relief in the Trans Alai
Range could have been initiated at about 4 Ma,
or more recently if the exhumation is less than 6
km or the average vertical offset rate is greater
than 4 mm/yr. This rapid construction of a major
mountain range is in line with the general conclusions of Molnar and Lyon-Caen (1988) regarding the spatial and temporal variability of
deformation in mountain belts, and is consistent
with the results of Abdrakhmatov et al. (1996),
who extrapolated high geodetically determined
deformation rates to geologic time scales to explain the adjacent eastern Kyrgize Tien Shan.
These authors concluded that the Tien Shan may
have developed in the past 10 m.y. In conclusion,
the greater Pamir region is an extraordinary
area characterized by extreme relief contrasts
that exemplifies the partitioning of ongoing deformation and the recency of uplift, as well as
the combined influence of active tectonics and
Pleistocene climate change on the evolution of
orogens.
ACKNOWLEDGMENTS
Arrowsmith acknowledges support from National Science Foundation grant EAR-9805319.
Strecker acknowledges financial support from the
German Research Council (D.F.G.) and funding
by Universität Tübingen granted to W. Frisch.
Discussions with P. Blisniuk, W. Frisch, S. Gao,
M. Hamburger, K. Haselton, J. Hernlund, G.
Hilley, M. Kornilov, S. McManus, G. Michel,
G. Pavlis, T. Pavlis, L. Ratschbacher, C. Rubin,
and M. Schwab have been helpful. J. Johnson,
S. Selkirk, and S. Wood (Arizona State Univer-
sity), and H. Gehrighausen, N. Stahlberg, and
R. Thiede (Universität Potsdam) helped with
research tasks. Figures 1 and 4 were plotted using GMT (Wessel and Smith, 1995). We thank
H. Kaufmann (GFZ-Potsdam) for providing the
Landsat TM-scene (Fig. 2). We appreciate thorough reviews by D. Yule, S. Roecker, and
J. A. Stock.
REFERENCES CITED
Abdrakhmatov, K. Y., Aldazhanov, S. A., Hager, B. H., Hamburger, M. W., Herring, T. A., Kalabaev, K. B. K. K. B.,
Makarov, V. I., Molnar, P., Panasyuk, S. V., Prilepin,
M. T., Reilinger, R. E., Sadybakasov, I. S., Souter, B. J.,
Trapeznikov, Y. A., Tsurkov, V. Y., and Zubovich, A. V.,
1996, Relatively recent construction of the Tien Shan inferred from GPS: Nature, v. 384, p. 450–453.
Armijo, R., Tapponnier, P., Mercier, J. L., and Han, T. L., 1986,
Quaternary extension in southern Tibet: Field observations and tectonic implications: Journal of Geophysical
Research, v. 91, p. 13803–13872.
Arrowsmith, R., Pollard, D. D., and Rhodes, D. D., 1996, Hillslope development in areas of active tectonics: Journal of
Geophysical Research, v. 101, p. 6255–6275.
Bazhenov, M. L., Perroud, H., Chauvin, A., Burtman, V. S., and
Thomas, J. C., 1994, Paleomagnetism of Cretaceous red
beds from Tadzhikistan and Cenozoic deformation due to
India-Eurasia collision: Earth and Planetary Science Letters, v. 124, p. 1–18.
Birkeland, P. W., 1984, Soils and geomorphology: New York,
Oxford University Press, 372 p.
Burchfiel, B. C., Cowan, D. S., and Davis, G. A., 1992, Tectonic overview of the Cordilleran orogen in the western
United States, in Burchfiel, B. C., Lipman, P. W., and
Zoback, M. L., eds., The Cordilleran orogen: Conterminous U.S.: Boulder, Colorado, Geological Society of
America, Geology of North America, v. G-3, p. 407–479.
Bürgmann, R., Pollard, D. D., and Martel, S. M., 1994, Slip
distributions on faults: Effects of stress gradients, inelastic deformation, heterogeneous host-rock stiffness, and
fault interaction: Journal of Structural Geology, v. 16,
p. 1675–1690.
Burtman, V. S., and Molnar, P., 1993, Geological and geophysical evidence for deep subduction of continental crust beneath the Pamir: Geological Society of America Special
Paper 281, 76 p.
Davidzon, R. M., Kraidenkov, G. P., and Salibaev, G. K., 1982,
Stratigraphy of Paleogene deposits of the Tadjik Depression and adjacent territories (in Russian): Dushanbe, Tadjikistan, Donish, 151 p.
Fan, G., Ni, J. F., and Wallace, T. C., 1994, Active tectonics of
the Pamirs and the Karakorum: Journal of Geophysical
Research, v. 99, p. 7131–7160.
Frisch, W., Ratschbacher, L., Strecker, M., Waldhoer, M., Klishevich, V., Kornilov, M., Semiletkin, S., and Zamoruyev,
A., 1994, Tertiary and Quaternary structures in the eastern
Pamir: Journal of Nepal Geological Society, v. 10,
p. 48–50.
Hamburger, M. W., Sarewitz, D. R., Pavlis, T. L., and Popandopulo, G. A., 1992, Structural and seismic evidence for
intracontinental subduction in the Peter the First Range,
Central Asia: Geological Society of America Bulletin,
v. 104, p. 397–408.
Jackson, J., Molnar, P., Patton, H., and Fitch, T., 1979, Seismotectonic aspects of the Markansu Valley, Tadjikistan,
earthquake of August 11, 1974: Journal of Geophysical
Research, v. 84, p. 6157–6167.
Kyrgize Geodetic and Cartographic Agency, 1991, Topographic map of Kyrgyzstan, sheet Pik Lenina: Bishkek,
scale 1:200 000.
Kyrgize Geologic Agency, 1979, Tectonic map of Kyrgyzstan,
sheet J-43-A: Bishkek, scale 1:500 000.
Lambiase, J. J., and Bosworth, W., 1995, Structural controls on
sedimentation in continental rifts, in Lambiase, J. J., ed.,
Hydrocarbon habitat in rift basins: Geological Society
[London] Special Publications, p. 117–144.
Larson, K. M., Bürgmann, R., Bilham, R., and Freymuller, J. T.,
Geological Society of America Bulletin, November 1999
MOUNTAIN-BELT GROWTH, PAMIR-ALAI REGION, KYRGYZSTAN
1999, Kinematics of the India-Eurasia collision zone
from GPS measurements: Journal of Geophysical Research, v. 104, p. 1077–1093.
Leith, W., and Alvarez, W., 1985, Structure of the Vakhsh foldand-thrust belt, Tadjik SSR: Geologic mapping: Geological Society of America Bulletin, v. 96, p. 875–885.
Michel, G. W., Reigber, C., Angermann, D., Klotz, J., CATSTeam, and GEODYSSEA-Team, 1997, Ongoing and Recent deformation in Central and SE-Asia [abs.]: Eos
(Transactions, American Geophysical Union), v. 78,
p. F172.
Molnar, P., and Lyon-Caen, H., 1988, Some simple physical aspects of the support, structure, and evolution of mountain
belts, in Clark, S. P., Jr., Burchfiel, B. C., and Suppe, J.,
eds., Processes in continental lithospheric deformation:
Geological Society of America Special Paper 218,
p. 179–207.
Ni, J., 1978, Contemporary tectonics in the Tien Shan region:
Earth and Planetary Science Letters, v. 41, p. 347–354.
Nikonov, A. A., Vakov, A. V., and Veselov, I. A., 1983, Seismotectonics and earthquakes in the convergent zone between
the Pamir and the Tien Shan (in Russian): Moscow,
Nauka, 240 p.
Noblet, C., Lavenu, A., Marocco, R., Molnar, P., and LyonCaen, H., 1996, Concept of continuum as opposed to periodic tectonism in the Andes: Tectonophysics, v. 255,
p. 65–78.
Pavlis, T., Hamburger, M. W., and Pavlis, G., 1997, Erosional
processes as a control on the structural evolution of an actively deforming fold and thrust belt: An example from
the Pamir–Tien Shan region, Central Asia: Tectonics,
v. 16, p. 810–822.
Pegler, G., and Das, S., 1998, An enhanced image of the
Pamir–Hindu Kush seismic zone from relocated earthquake hypocentres: Geophysical Journal International,
v. 134, p. 573–595.
Pollard, D. D., and Segall, P., 1987, Theoretical displacements
and stresses near fractures in rock: With applications to
faults, joints, veins, dikes, and solution surfaces, in Atkinson, B. K., ed., Fracture mechanics of rock: San Diego,
Academic Press, p. 277–350.
Roecker, S. W., Soboleva, V., Nersesov, I. L., Lukk, A. A.,
Hatzfield, D., Chatelain, J. L., and Molnar, P., 1980, Seismicity and fault plane solutions of intermediate depth
earthquakes in the Pamir–Hindu Kush region: Journal of
Geophysical Research, v. 85, p. 1358–1364.
Schwab, G., Katzung, G., Ludwig, A., and Lützner, H., 1980,
Neogene molasse sedimentation in the Tadzhik Depression (in German): Zeitschrift fuer Angewandte Geologie,
v. 26, p. 225–237.
Segall, P., and Pollard, D. D., 1980, The mechanics of discontinuous faults: Journal of Geophysical Research, v. 85,
p. 4337–4350.
Strecker, M. R., Frisch, W., Hamburger, M. W., Ratschbacher,
L., Semiletkin, S., Zamoruyev, A., and Sturchio, N., 1995,
Quaternary deformation in the eastern Pamirs, Tadzhikistan and Kyrgyzstan: Tectonics, v. 14, p. 1061–1079.
Wessel, P., and Smith, W. H. F., 1995, New version of the
Generic Mapping Tools released: Eos (Transactions,
American Geophysical Union), v. 76, p. 329.
Willemse, E. J. M., Pollard, D. D., and Aydin, A., 1996, Threedimensional analyses of slip distributions on normal fault
arrays with consequences for fault scaling: Journal of
Structural Geology, v. 18, p. 295–309.
MANUSCRIPT RECEIVED BY THE SOCIETY APRIL 16, 1998
REVISED MANUSCRIPT RECEIVED FEBRUARY 1, 1999
MANUSCRIPT ACCEPTED MARCH 18, 1999
Printed in U.S.A.
Geological Society of America Bulletin, November 1999
1683