Development of active low-angle normal fault systems during
orogenic collapse: Insight from Tibet
Paul Kapp* Department of Geosciences, University of Arizona, 1040 E 4th Street, Tucson, Arizona 85721, USA
Michael Taylor
Department of Geology, University of Kansas, 1475 Jayhawk Boulevard, Lawrence, Kansas 66045, USA
Daniel Stockli
Lin Ding Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100029, China
Fig. 1), where the southeast-dipping (22°–37°)
Nyainqentanglha detachment has exhumed
mylonitic gneisses and granitoids as young as
late Miocene in the footwall (Pan and Kidd,
1992; Harrison et al., 1995; Kapp et al., 2005)
and is spatially associated with a prominent
intrabasin drainage divide (Fig. 2B).
In this paper, we document a rift in westcentral Tibet (the Lunggar Rift; Fig. 1) that is at
an intermediate stage of development between
the ubiquitous nascent rifts and the more evolved
central part of the Yadong-Gulu Rift. We suggest that along-strike variability in the Lunggar
Rift provides a snapshot in space and time of
how detachment systems and rift basins develop
during extension of hot, thick crust.
ABSTRACT
Active north-trending rifts in Tibet vary significantly in their character as a function of
extension magnitude. Most rifts are characterized by internally drained basins bounded by
high-angle normal faults with Paleogene or older rocks in the footwall. However, the central
part of the Yadong-Gulu Rift (near Lhasa) and the newly documented Lunggar Rift in westcentral Tibet are bounded by low-angle normal faults (detachments) with mylonitic rocks
and Miocene granites in the footwall, and exhibit active basin incision and intrabasin topographic highs in areas of inferred maximum extension. We suggest that Tibetan rifts initiate
as high-angle normal fault and half-graben or graben basin systems and evolve in response to
increasing extension and footwall isostatic rebound into detachment systems that are active at
uppermost crustal levels and above which rift basin fill is being uplifted and eroded.
Keywords: Tibet, extension, rift basin development, low-angle normal faulting, metamorphic core
complex.
LUNGGAR RIFT
The Lunggar Rift valley is 60 km long and
5–10 km wide (Fig. 3). Its central part is bounded
on its western flank by a <40°E dipping normal
fault (the Lunggar detachment) that juxtaposes
mylonitic granitoids and orthogneiss in the
North Rift
80°E
90°E
Qiangtang
terrane
Fig. 3
lt
fau
*E-mail: [email protected].
crustal assemblages and undeformed granitoids
in the footwall, and internally drained graben or
half-graben basins show minimum elevations
(Fig. 2A) in the central parts of the rifts (e.g.,
Armijo et al., 1986; Taylor et al., 2003; Pan
et al., 2004). The only previously documented
exception within the plateau is the central part of
the Yadong-Gulu Rift (e.g., Armijo et al., 1986;
m
ra
ko
ra
Ka
INTRODUCTION
It has long been speculated (Coney and
Harms, 1984) and predicted from geodynamic
models (e.g., Lachenbruch and Morgan, 1990;
Buck, 1991) that the development of continental
low-angle normal fault (detachment) systems
and metamorphic core complexes is favored in
regions characterized by hot, thick crust. Furthermore, the presence of a weak mid-crustal
layer, capable of flow, has been invoked to
explain geometric aspects of core complexes and
activation of slip on detachments in the upper
crust (e.g., Yin, 1989; Block and Royden, 1990;
Wernicke, 1990). However, supporting geological documentation from actively extending orogens with these characteristics remains scarce
(e.g., Cordillera Blanca detachment system in
the Peruvian Andes; Doser, 1987; McNulty and
Farber, 2002). An ideal locality to investigate
orogenic collapse in action is Tibet, which is
actively extending east-west at a rate of one-half
the total convergence rate between India and
Asia (Wang et al., 2001; Zhang et al., 2004) and
is underlain by crust as much as 85 km thick that
is arguably flowing at mid-crustal levels (e.g.,
Clark and Royden, 2000; Klemperer, 2006).
Tibetan rifts (Fig. 1) are characterized by
narrow-width rift flank uplifts (30–40 km) and
basins (<10–15 km), consistent with isostatic
compensation occurring in a low-viscosity midcrust (Masek et al., 1994). Other characteristics
of Tibetan rifts are in general quite ordinary:
moderate extension has been accommodated
(a few kilometers), range-bounding high-angle
normal faults exhume Paleogene or older supra-
BS
Pung Co
Rift
Lunggar Rift
Lhasa
terrane
30°N
Yadong
Gulu
Rift
IIY
YS
Suture zone
Thrust fault
Normal fault
Strike-slip fault
80°E
0
30°N
Lhasa
Hima
layan
thrus
t belt
200
400 km
90°E
Figure 1. Color shaded relief map of Tibet with color palette to emphasize active faulting.
White dashed lines indicate profile locations in Figure 2. After Kapp and Guynn (2004).
BS—Bangong suture; IYS-Indus—Yarlung suture.
© 2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
GEOLOGY,
January
2008
Geology,
January
2008;
v. 36; no. 1; p. 7–10; doi: 10.1130/G24054A.1; 4 figures; Data Repository item 2008006.
7
6.5
S
6.0
North Rift
(nascent)
5.5
5.0
N
Figure 2. Topographic
swath profiles of rift
ranges and basins generated from three-arc
second Shuttle Radar
Topog raphy
Mission
digital elevation model
(SRTM DEM, 90 m resolution). Vertical dashed
line indicates region of
inferred maximum extension. A: Basin profiles for
nascent rifts are flat in
the central part, reflecting local base level. B:
Basin profiles for rifts
bounded by detachments
show a topographic high
in their central part. Profile locations are shown
in Figure 1.
6.0
5.5
Elevation (km)
5.0
4.5
Pung Co Rift
(nascent)
6.5
6.0
5.5
5.0
Lunggar Rift
4.5
7.0
Nyainqentanglha Rift
6.5
6.0
5.5
5.0
4.5
4.0
–60
–40
–20
0
20
Horizontal Distance (km)
40
footwall against Paleozoic–Neogene strata in
the hanging wall. At the two localities investigated in detail (yellow circles in Fig. 3), the
proximal footwall includes chloritized breccia,
cataclasite, mylonitic gneiss, and leucogranite.
It shows foliations subparallel to the detachment and east-plunging stretching lineations
(Fig. 3). Chatter marks on the detachment
surface, and S-C fabrics and decimeter-scale
offsets on detachment-parallel ductile normal
faults in the footwall, indicate a top-to-the east
sense of shear. Zircons from three samples of
mylonitic leucogranite (67052, 67053, and
611051B; Fig. 3) yielded weighted mean U-Pb
ages of 8.9 ± 0.2 Ma (n = 11; mean square of
weighted deviates, MSWD = 1.4), 9.0 ± 0.2 Ma
(n = 9; MSWD = 0.6) and 15.2 ± 0.4 Ma (n = 19;
MSWD = 1.2), respectively. Apatite (U-Th)/He
ages from two of the same samples (67052 and
67053) are in the 0.4–0.7 Ma range, whereas
those from a footwall orthogneiss (611051A;
Fig. 3) are in the 1.7–1.8 Ma range (see GSA
Data Repository1 for description of analytical
methods and Tables DR1 and DR2).
Neogene gravels in the hanging wall of
the Lunggar detachment record slip along
the detachment and unroofing of the Lunggar
Range as indicated by growth strata with clasts
of both footwall and hanging-wall lithologies.
The detachment is currently inactive at the
surface, as indicated by undisturbed moraines
1
GSA Data Repository item 2008006, analytical
methods, Figure DR1 (fault scarp profile), and Tables
DR1 and DR2 (U-Pb zircon and U-Th-He apatite
data, respectively), is available online at www.
geosociety.org/pubs/ft2008.htm, or on request from
[email protected] or Documents Secretary,
GSA, P.O. Box 9140, Boulder, CO 80301, USA.
8
60
80
and alluvial fans that unconformably overlie
it (Fig. 3). However, 2–4 km east of the inactive range front, active east-dipping normal
faults cut Quaternary alluvium within the rift
basin (Fig. 3). We used high-precision topographic measurements obtained by a real-time
kinematic global positioning system survey to
estimate the throws and heaves on the active
faults (see Fig. DR1 for scarp profile and Fig. 3
for profile location). The throws range from
~1 m for the youngest surfaces cut to >35 m
for the oldest cumulative fault scarps. Net
throw and heave are ~75 m and 140 m, respectively, consistent with the active faults soling
into a low-angle (~28°) master detachment
(see Axen et al., 1999, for a similar approach).
An intriguing observation is that normal fault
scarps cutting Quaternary deposits in the
northern and southern part of the Lunggar Rift
are located along or within hundreds of meters
of the range front, in contrast to the central
part of the range where fault scarps are located
kilometers from the range front (Fig. 3).
The central part of the Lunggar Rift basin is
bounded on its eastern margin by a west-dipping
high-angle normal fault with Cretaceous rocks in
the footwall (Fig. 3). Gently to moderately eastdipping rift basin fill is exposed in the hanging
wall due to recent incision. It consists of >100 m
of lacustrine mudstone capped by granite- and
gneiss-clast alluvial fan conglomerates derived
from the Lunggar Range. Rivers flow north
and south from a drainage divide in the central
part of the Lunggar Rift basin into lake basins
located near the northern and southern terminations of the rift (Figs. 2B and 3). Collectively,
these observations indicate that the central part
of the Lunggar Rift basin underwent a transi-
tion from sediment accumulation in a lacustrine
basin to denudation during rift development.
The strike length of the Lunggar detachment
is no more than 25 km. To the north and south,
the detachment bifurcates into systems of eastdipping high-angle normal faults over a distance
of <10 km (Fig. 3). In the north, footwall rocks
include Paleozoic strata intruded by a granite
body <1 km in diameter. The granite yielded a
U-Pb zircon age of 152 ± 2 Ma (69052, n = 39,
MSWD = 1.7; Table DR1) and a (U-Th)-He
apatite age of 5.1 ± 0.2 Ma (Fig. 3; Table
DR2). In the south, footwall rocks consist of
hornblende-bearing orthogneiss that shows
north-south stretching lineations and crosscutting, undeformed biotite granite (Fig. 3). A
sample of the granite yielded a U-Pb zircon age
of 21.7 ± 0.6 Ma (615053, n = 21, MSWD = 0.6;
Table DR1), which overlaps in age with a suite
of undeformed granites that core the Nyainqentanglha Range (Kapp et al., 2005) and provides a
minimum age for the orthogneiss. We infer that
the gneissic fabric formed during north-south
shortening prior to late Cenozoic extension.
SIMILARITIES BETWEEN THE
LUNGGAR AND YADONG-GULU RIFTS
The detachments and overlying basins in the
central parts of the Lunggar (this study) and
Yadong-Gulu (Armijo et al., 1986; Pan and Kidd,
1992; Cogan et al., 1998; Stockli et al., 2002;
Kapp et al., 2005) rifts show notable similarities. Footwall mylonitic shear zones and Miocene granitoids underwent significant and rapid
exhumation during the Pliocene–Pleistocene.
The detachments are the only obvious structures along which this exhumation could have
occurred, and they have in no place been observed
to be cut by normal faults with displacements of
more than tens of meters. Whereas the detachments are in direct fault contact with variably
tilted rift basin fill, they are currently inactive
at the surface. Instead, active surficial extension
is manifested by Quaternary fault scarps within
the rift basin. Collectively, these observations
can be explained by slip along active normal
faults within the rift basins soling into the downdip extension of the detachments at very shallow
structural levels in the upper crust. This explanation is further supported by analysis of seismic
reflection profiles across the Yadong-Gulu Rift
(Cogan et al., 1998) and the fault scarp profile
in the Lunggar Rift. Most noteworthy about
the supradetachment basins is that they exhibit
intrabasin topographic highs that are actively
undergoing incision and that correspond to
areas of maximum extension as inferred from
their central location and range topography
(Fig. 2B). This contrasts with high-angle normal
fault systems where intrabasin highs correspond
to accommodation zones between overlapping
fault segments (e.g., Anders and Schlische,
GEOLOGY, January 2008
Figure 3. Geological map of the Lunggar Rift. Blue
numbers are U-Pb zircon ages (in Ma). Red numbers are (U-Th)-He apatite ages (in Ma). Pz-Mz—
Paleozoic to Mesozoic strata, undivided; K—Cretaceous strata; ogn—orthogneiss; gr/mgn—variably
deformed granite, mylonitic granite, and gneiss;
N-Q—Neogene–Quaternary deposits; Qo—older
Quaternary deposits; Qy—younger Quaternary
deposits; Qg—Quaternary glacial deposits.
Normal fault scarp
69052
152 5.1
Inactive normal fault
Detachment fault
Dip
River
Pz-Mz
abandoned
breakaways?
Qo
1994; Faulds and Varga, 1998). A final observation is that magma generation was occurring
before and during the early stages of extension, and is likely ongoing at depth today (e.g.,
Gaillard et al., 2004; Kapp et al., 2005).
gr/mgn
Qg
yellow circled
localities
GEOLOGY, January 2008
K
N-Q
Qg
31°40′N
Lunggar
detachment
Qy
Qo
ice/snow
N-Q growth
strata
Qg
31
611051B/A
15 1.8
67052
ice/snow
9 0.64
33
Qo
intrabasin
drainage
divide
N
67053
9 0.48
ogn
Qo
gr/mgn
Pz-Mz
31°30′N ice
615053
snow
22
Qo
0
2
Qo
Qo
Qo
ogn
Qy
K
Qy
to lake
basin
83°30′E
4
6
(A) Cross Section
“breakaway 1”
half-graben basin fill
8
10 km
(A) Map View
lake
shorelines
contour interval = 10 m
83°40′E
(C) Cross Section
(C) Map View
eroded breakaways
and basin fill
supradetachment
basin
active
breakaway
abandoned
breakaway
lake
lake
mylonitic
front
brittle-ductile
transition
mylonitic
shear zone
lake
detachment fault
active in upper crust
detachment
fault
(B) Cross Section
“breakaway 1” abandoned
mylonitic
front
initiation of “breakaway 2”
lake
Figure 4. Schematic cross sections and map
patterns illustrating how Tibetan rifts may
evolve during progressive (A–C) extension.
scarp
profile
Qo
gr/mgn
d
ne
do ays?
an
ab akaw
bre
DISCUSSION AND CONCLUSIONS
Our study of Tibetan rift evolution supports
previous suggestions that the core-complex
mode of extension is favored in extending regions
characterized by a hot, thick crust that is capable
of flowing at mid-crustal levels. Tibetan rifts
reveal how detachments and associated basins
Qy
Lake shorelines
foliation and
lineation at
KINEMATIC DEVELOPMENT OF
TIBETAN RIFT SYSTEMS
We propose a kinematic model for Tibetan rift
development that is based on the premise that
rift segments with varying magnitudes of extension provide snapshots of a regionally applicable extension process through time. The rifts
initiate as half-graben (or graben) basins (e.g.,
Leeder and Gawthorpe, 1987), bounded by a
master high-angle normal fault that soles into a
subhorizontal ductile shear zone (Fig. 4A). With
increasing extension, tectonic unloading results
in isostatic rebound, back-rotation, and eventual
abandonment of the updip portion of the rangebounding fault (breakaway 1), and upwarping of
the normal fault in the mid-upper crust (Fig. 4B).
Kinematically, this is similar to rolling-hinge
models (e.g., Wernicke, 1992). Progressive,
basinward-stepping development of breakaway
normal faults and upwarping of the master normal fault at depth during continued slip result
in a detachment that is actively slipping in the
uppermost crust. In the process of breakaway
development, portions of the rift basin are continuously being cannibalized, i.e., captured into
the footwall, uplifted, and eroded (Fig. 4C).
Furthermore, isostatic rebound in areas of maximum extension results in basin inversion and the
development of an intrabasin topographic high
and drainage divide (Figs. 4B, 4C).
to lake
basin
Qo
abandoned
breakaway
progressive upwarping
due to isostatic rebound
magma emplacement and crustal flow in mid-crust
(B) Map View
9
develop in action, and provide fresh insight into
this topic in general. They cut obliquely across
older east-west–striking contractional structures in the region, demonstrating that detachments can develop in the absence of preexisting
faults of similar orientation. The footwalls of
the Tibetan detachments are not corrugated
like those of metamorphic core complexes with
bounding detachments that have accommodated
more displacement. This suggests that corrugations develop during progressive extension and
are not cast into the shape of arcuate breakaway
brittle normal faults (Spencer, 1999). Both the
primary breakaway zones and supradetachment
basins in Tibet have low potential for preservation in the geological record because they
are continuously being uplifted and eroded. If
extension were to continue in the manner proposed, metamorphic core complexes would be
produced in which the primary breakaway zone,
most if not all of the supradetachment basin
fill, and evidence for significant hanging-wall
extension are not preserved due to erosion. This
erosion is ultimately tied to footwall isostatic
rebound, amplified by emplacement of granitic
melts and inflow of weak middle crust beneath
the detachments.
ACKNOWLEDGMENTS
This research was supported by grants from the
U.S. National Science Foundation (EAR-0438115 to
Kapp; EAR-0309976 and EAR-0414817 to Stockli),
the University of Kansas (Taylor), Chinese Ministry
of Science and Technology (2002CB412602 to Ding),
and the National Natural Science Foundation of China
(40625008 to Ding). We thank Shundong He and Jen
McGraw for assistance with U-Pb data collection,
John Lee and Zhang Liyun for field assistance, Gary
Axen, Roger Buck, Eric Cowgill, Jerome Guynn,
John Volkmer, Doug Walker, and An Yin for insightful
discussions, and Jon Spencer and Brian Wernicke for
critical reviews.
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Revised manuscript received 13 August 2007
Manuscript accepted 17 August 2007
Printed in USA
GEOLOGY, January 2008