Stratigraphy and Structure of the Mendenhall Gneiss,
South-Central San Gabriel Mountains, California
By Jeffrey T. DeLand
Undergraduate Senior Thesis presented to the faculty of the Geological Sciences
Deparetment, California State Polytechnic University, Pomona, California
May 22, 2003
i
Abstract
STRATIGRAPHY AND STRUCTURE OF
THE MENDENHALL GNEISS
By Jeffrey T. DeLand
Department of Geological Sciences
California State Polytechnic University, Pomona
An undergraduate thesis presented on the Stratigraphy and Structure of the
Mendenhall Gneiss including structural and lithologic data colleted in remote
regions of the Angeles National Forest, southern California. Geologic field
mapping of basement rocks in the south central San Gabnel Mountains has
yielded data enabling correlation of the largest continuous section of
metamorphic rock with displaced rock bodies located on opposing sides of
strike-slip fault zones. Palinspastic reconstructions provide insights that can be
used for correlation with mountain ranges such as the Chocolate and Orocopia
Mountains located more than 200 km south, and on the north side of the San
Andreas Fault. New data supports tighter constraints on piercing points along
the San Gabriel Fault with 218 ± 3 Ma Triassic Mt. Lowe intrusive rocks in
contact with 1700 Ma Paleoproterozoic Mendenhall Gneiss. Large and small
scale structural features provide information which can be used to aid in
reconstruction of late Cenozoic brittle deformation events, as well as help in the
determination of which land mass (Antarctica, Australia, Siberia or China) rifted
from the western Laurentian margin during the 750 Ma break up of Rodinia.
ii
TABLE OF CONTENTS
Abstract……………………………………………………………………………………..ii
List of Figures………………………………………………………………………………iv
Introduction…………………………………………………………………………………1
Previous Studies……………………………………………………………………………5
Rock Units and Field Relations
Paleoproterozoic Fine Grained Banded Gneiss………………………………………9
Paleoproterozoic Granite Augen Gneiss .....................................................................15
(Mz?) or (pC?) Foliated Diorite or Hornblende Diorite..............................................17
Triassic Plutonic and Intrusive Rocks.........................................................................19
Cretaceous Plutonic and Intrusive Rocks....................................................................22
Structural Analysis
Plot Sets 1-8…………………………………………………………………...26-32
Methods…………………………………………………………………………...24
Foliation…………………………………………………………………………..33
Lineation……………………………………………………………………….....34
Faults……………………………………………………………………………...35
Regional Implications / Interpretations / Significance……………………………...…….36
Conclusions……………………………………………………………………………….47
Future Studies……………………………………………………………………………..49
Acknowledgments………………………………………………………………………...50
Bibliography…………………………………………………………………………........51
Plates
Plate 1 - Geologic Map of the Study Area………………………………………..52
Plate 2 – Cross Section A-A’…………………………………………………..…53
Plate 3 – Cross Section B-B’……………………………………………………..54
Plate 4 – Cross Section C-C’……………………………………………………..55
Plate 5 – Cross Section D-D’……………………………………………………..56
Appendices
Structural Data…………………………………………………………………57-58
iii
LIST OF FIGURES
1. Geologic Map of the San Gabriel Mountains…………………………………………….3
2. Location Map of the Eastern Transverse Ranges showing Study Area .............................3
3. Simplified Geologic Map of Southern California showing Study Area.............................4
4. Metamorphic Facies Diagram showing Type Metamorphism............................................6
5. Index Map of Central San Gabriel Mountains showing Study Area..................................7
6. Photograph of Mendenhall Gneiss showing texture...........................................................8
7. Photograph of Mendenhall Gneiss showing foliation and fault........................................10
8. Photograph of Mendenhall Gneiss showing folding.........................................................11
9. Photograph of Mendenhall Gneiss showing folding.........................................................12
10. Geologic Map showing localities of Mendenhall Gneiss………………………………13
11. Photograph of granite augen gneiss showing foliation…………………………………16
12. Photograph of gneissic diorite showing cross cutting relationships ...............................17
13. Photograph of gneissic diorite xenolilth in Cretaceous Granite.......................................l8
14. Photograph of Mt. Lowe intrusive rocks with diorite dike……………………………..19
15. Photograph of gneissic diorite in Mt. Lowe…………………………………………….20
16. Distribution of Dating Localities for Triassic Mt. Lowe.................................................20
17. Concordia Diagram showing age of Josephine Mountain rocks.....................................23
18. Stereographic Plot of Mendenhall Gneiss.......................................................................33
19. Stereographic Plot showing metamorphic lineation........................................................34
20. Middle Miocene palinspastic reconstruction of San Gabriel Mtns.................................39
22. Paleoproterozoic reconstruction of Laurentia (Siberia Connection) ..............................45
23. Neoproterozoic reconstruction of Laurentia (Cathaysia)................................................46
24. Diagram showing possible connection of China and Laurentia (Cathysia) ...................47
iv
I. Introduction
The Mendenhall Gneiss was first studied while early California geologists began
mapping an adjacent anorthosite - gabbro - syenite intrusive sequence during early
exploration of the western San Gabriel Mountains (Miller, 1934, 1946), but was not
studied in any detail until Oakeshott (1958) described the type locality around
Mendenhall Peak (Barth, et al., 1995). This type locality has been the focus of many
studies, (Barth, et al., 2001, see also figure 1) while other localities of the Mendenhall
Gneiss remain largely unexplored. The purpose of this project is to map the stratigraphy
and structure of one of these locations on the north side of the San Gabriel Fault. Using
brunton compasses and topographic basemaps, data was taken on rock foliations and
lineations, fault orientations along with slip direction lineation when available, as well as
axial planar surfaces to construct a basement rock geologic map. The research area
includes the largest unfaulted, continuous section of Paleoproterozoic metamorphic rock
in the San Gabriel Mountains that can be compared and correlated with other areas.
Late Cenozoic strike slip faulting along the San Andreas, San Jacinto and San
Gabriel Faults have given way to the increasing study of structural relationships for the
purpose of correlation. Various lithologic units may offer piercing points along certain
faults which when palinspastic reconstructions are made show distinct patterns that can
be useful in determination of the amount of movement on lateral faults as well as
formulating the tectonic and/or depositional history of a region (Ehlig, 1981; Nourse,
2002, see figure 2). Detailed mapping of the San Gabriel Mountains basement terrain
may also offer evidence as to which present-day continent existed on the western
continental margin of Laurentia prior to Neoproterozoic rifting of the supercontinent
1
Rodinia. The San Gabriel Mountains offer a distinct challenge to the geologist in that
multiple rock units of varying ages may be associated along multiple fault systems, and
record many separate tectonic events.
The present-day San Gabriel Mountains are believed to have been formed by Late
Cenozoic thrust faulting along the south boundaries, for example the Sierra Madre and
Cucamonga fault systems. These reverse faulting events may have been caused by the
left lateral transform movement that offset the San Gabriel fault in the “Big Bend” region
(Ehlig, 1981). This type of scenario combines transform movement with compression of
the adjacent tectonic plates to form a rotational structural environment known as
transpression and transrotation. Subsequent erosion and brittle deformation of the
basement terrain, along with 160 – 240 km of movement laterally by the San Andreas
Fault (Matti and Morton, 1993; Dillon and Ehlig, 1993; Powell, 1993), have shaped the
crystalline rocks into what we see today.
Figure 1 shows a location map including the major lithologic units and faults in
the study area. The study area is marked in a red and black rectangle, and it can be seen
on the Chileo Flats USGS 7.5 minute topographic quadrangle. It is located along
Angeles Crest Highway (Highway 2) in the Angeles National Forest, just north of the San
Gabriel Fault in the central San Gabriel Mountains. Figure 2 shows the location of the
San Gabriel Mountains relative to the San Andreas Fault and San Bernardino Mountains;
also possible correlative units are located in mountain ranges throughout Southern
California (see figure 3).
2
Figure 1 Geologic Map of the San Gabriel Mountains modified from Ehlig (1981) showing
approximate location of study area. Inset Map shows terranes of the San Gabriel Mountains; abbreviations
are SGT = San Gabriel terrane; CT = Cucamonga terrane; SAT San Antonio terrane; PS = Pelona Schist.
Abbreviations for Cretaceous Plutons in San Gabriel terrane are JMI = Josephine Mountain intrusion; VMS
= Vetter Mountain stock; WMB = Waterman Mountain/Mt. Wilson batholith.
Figure 2 Location map of the central and eastern Transverse Ranges of southern California,
showing outcrops of the Mesoproterozoic San Gabriel anorthosite complex (pink) and the Mendenhall and
Augustine gneisses (black). Study area marked in red rectangle. Taken from (Barth et al., 2001).
3
4
zones, as well as the distribution of the Pelona-Orocopia-Chocolate Mountains schist. Modified from Ehlig (1993).
Figure 3 Simplified geologic map of Southern California showing present day location of San Gabriel Mountains, major strike slip fault
II. Previous Studies
Many ambitious researchers have provided a vast amount of data on the
Mendenhall Gneiss itself, the other surrounding metamorphic rocks, local faults, and the
intrusive rocks native to the study area. As for particular samples taken in the study area,
at this time, the author is not aware of any analysis other than thin section investigation to
have taken place with samples taken directly in the area of study. Due to the high relief
of the mountains, poison oak abundance, seasonal fire danger, temperature extremes,
scarcity of easily traveled roads, and dense vegetation mapping was mostly done on main
roads, or fire roads. The only published geologic map is the USGS 1:250,000 Los
Angeles sheet mapped by Tom Dibblee (Dibblee, 1969) which does not have the small
scale detail that is required for lithologic subdivision and correlation. In fact, on the LA
sheet, in this particular area of study there is only one single unit colored brown which is
labeled undifferentiated metamorphic rock. Perry Ehlig (1981) began to analyze the
tectonic history having to do with the central and eastern San Gabriel Mountains in which
he recognized the Precambrian gneiss-amphibolite complex located on the upper plate of
the Vincent Thrust. He also characterized the rocks adjacent to the South Branch of the
San Gabriel Fault as well as mapping much of the high country in the eastern and central
parts of the range (Nourse, 2002). In relation to this study, he also proposed a 22 km
offset of the eastern contact of Mendenhall gneiss with the Mt. Lowe intrusion along the
San Gabriel Fault.
Leon Silver (Silver, et al., 1963; Silver, 1971) gathered information on the ages
and correlation of basement units including the ca 1700 Ma Mendenhall Gneiss, the 1200
±15 Ma anorthosite – syenite - gabbro intrusive complex, which lies along strike of the
5
San Gabriel Fault from the study area, the 220 ± 10 Ma Mt. Lowe - Parker Mountain
granodiorite, as well as the occurrences of augen gneisses using U/Pb dating techniques.
He began to correlate the crystalline rocks in the San Gabriel Mountains with rocks that
lie 200 – 250 km south, and on the opposite side of the San Andreas Fault in the
Orocopia and Chocolate Mountains (Nourse, 2002; see also figures 2 and 3). Using
isotopic, geochemical, and geochronological tools, other scientists including Andrew
Barth et al., (1995, 2001) were able to characterize the different petrologic identities of
the Mt. Lowe intrusion as well as obtain a more precise age of 218 ± 3 Ma. Barth
classified the metamorphic and plutonic history in the western San Gabriel Mountains,
including pyroxene and feldspar solvus geothermometry in the Mendenhall Gneiss which
yielded metamorphic crystallization temperatures and pressures that are 900° - 950°C,
and 0.6 GPa, respectively, granulite facies metamorphism (Barth, et al., 2001; see also
figure 4).
Figure 4
Metamorphic facies
diagram showing
temperatures and
pressures of
metamorphosed
rocks. Black
rectangle indicates
type Mendenhall
Gneiss granulite
facies
Metamorphism at
6kb and ~950C.
Figure courtesy of
Professor David
Jessey; California
Polytechnic
University Pomona,
Department of
Geological
Sciences.
6
7
topographic quadrangle locations, study areas of; Ehlig (1958), Jennings and Strand (1969), Morton (1973), and Bordugo and Spittler (1986). Portion in
orange is the general location of the study area for this paper. Modified from Nourse (2002).
Figure 5 Index map of the central San Gabriel Mountains, showing geographic features such as mountain peaks, major rivers, freeways, color coded
Figure 6 A Mendenhall Gneiss exposure in Lady Bug Canyon polished by stream action, notice
vertical dip and small scale compositional banding alternating from quartzofelspathic to biotite rich gneiss.
Student sitting in photo is approximately 6 feet tall, 3 feet tall sitting.
8
This work showed that felsic gneisses with a probable detrital zircon component as old
as1800 Ma and granite augen gneiss 1680 ± 40 Ma were metamorphosed during
emplacement of the 1196 ± 5 Ma anorthosite – syenite – gabbro complex.
III. Rock Units and Field Relations
Many different types of rocks have been found in the area of study. The section
below gives rock definitions, map classifications, field occurrence, regional significance,
as well as a tectonic background and geologic history of each rock unit. Photographs and
figures will be utilized whenever available.
Paleoproterozoic Fine Grained Banded Gneisses
(Map units p? qfgn, p? gn, p? bgn , p? g +bgn, p? a, colored brownish red on Plates)
The oldest unit on the map is the Paleoproterozoic fine grained gneisses (p? gn,
see figure 6) which range in composition from a light colored quartzofelspathic gneiss
(quartz and feldspar rich; p? qfgn), a biotite rich gneiss (p? bgn), to a dark colored fine
grained foliated amphibolite (p? a). These rock units along with interlayered bodies of
granitic augen gneiss are collectively called the Mendenhall Gneiss, or in some cases are
referred to as the San Gabriel Gneiss (see figures 6, 7, 8). The Mendenhall Gneiss lies
structurally on the upper plate of the Vincent Thrust and is the country rock for younger
plutonic intrusions. It is characterized by millimeter to 20 centimeter scale banding
throughout the unit, and relatively simple mineral assemblages. Heating that caused
ductile deformation has resulted in tight to isoclinal folding (see figures 7 and 8), while
outcrops have been compressed and shortened along with boudenage that stretches
parallel to the foliation, and biotite rich units occasionally are garnet bearing (Barth, et
9
al., 2001; Nourse, 2002). Dating zircon cores from the type Mendenhall Gneiss (figure
10) give very discordant ages between ~1.7 and ~1.2 Ga (Barth, et al., 2001). This
discordancy is due to the 1.19 Ga intrusion of the nearby anorthosite – syenite – gabbro
assemblage and subsequent metamorphic events that led to an amphibolite facies
metamorphism. The extreme heat and low pressure re-sets or “zones” the zircons used
for age dating such that 1.2 Ga rims surround older cores.
Figure 7 Photograph of Mendenhall Gneiss with right – lateral strike slip fault cutting the
outcrop. Notice characteristic fine banding of the Mendenhall Gneiss, along with alternating light and dark
layers. Rock hammer is approx 14 cm long.
In looking at a possible protolith for this rock, it is important to think about the
depositional, tectonic, and thermal environments that this rock unit has been subject to in
its 1.7 billion years. In Paleoproterozoic time, on the western edge of the continent
10
known as Rodinia, there was a depositional environment that was fairly complex. Before
the right lateral strike slip motion of the San Andreas Fault carried these rocks some 250
km northward, they originated somewhere near the present – day Salton Sea. At that
time and in that area, the collision of the Mojavia and Yavapai tectonic plates brought
about the Ivanpah Orogeny that took place 1700 – 1710 million years ago (Barth, et al.,
2000; Wooden and Miller 1991). This orogeny created a vast amount of volcanic
sediments from the fore – arc, as well as a large quantity of terrestrial sediments running
off of the Laurentian Craton that collected in basins located to the southwest of the
Yavapai Province.
Figure 8 Photograph taken of the Mendenhall Gneiss showing fine banding as well as isoclinal
folding. Outcrop dipping steeply into the page, photo taken looking down. Rock unit truncated in middle
by right lateral fault. Pencil is approx 10 cm long.
11
Figure 9 Photograph of Mendenhall Gneiss showing inclusions of foliated diorite, isoclinal
folding and fine scale banding associated with the unit. Notice how the diorite is also isoclinally folded.
Pencil for scale is approximately 10cm.
12
Figure 10 Simplified geologic map of the Mendenhall Peak area, western San Gabriel
Mountains, with data from P.L. Ehlig (unpublished mapping 1958), Carter (1980), and Barth, et al., (1995).
Circles are localities sampled for geochronology, triangles and squares are mafic granulite and felsic gneiss
localities, respectively, used for geothermometry (Barth, et al., 1995).
Although no sedimentary mineral assemblages are present in the outcrops studied, the
wide scatter of zircon ages suggests a detrital origin for the Mendenhall Gneiss (Barth, et
al., 2001). Any pre-existing depositional fabric was soon to be cooked as the heat of the
orogenic belt itself gave the Mendenhall Gneiss its first thermal event and destroyed any
relict bedding, sedimentary structures, if indeed it was a sedimentary protolith, and any
igneous flow foliation it might have had if the protolith was volcanic. The upper
intercept of the U/Pb dating of zircons is believed to indicate the date at which an igneous
protolith was originally crystallized, at 1789 Ma (Barth, et al., 2001). Detritus shed from
this protolith into a basin, possibly an aulacogen, which may have been metamorphosed
during the late Paleoproterozoic, and then metamorphosed again at 1196 Ma during
formation of the anorthosite – syenite – gabbro complex. Some discrepancy has risen
concerning another granulite facies metamorphic event that occurred about 1440 million
years ago which, according to Silver (1963), is the cause of the discordant age in zircons
13
dated from a granulite pegmatite (Ehlig, 1981). But as Barth states in his 2001 paper in
the Journal of Geology, the lack of these ages on other types of rocks may suggest that
these rocks were not formed on the edge of the Laurentian Craton, but rather were put
there by some continental collision event and left behind following the breakup of
Rodinia.
The next thermal event was the emplacement of the anorthosite – syenite –
gabbro complex at 1196 Ma that intruded adjacent to the rock body (Barth, et al., 1995,
2001). The areas that have been dated (see figure 10) radiometrically show the zircon
dating clock reset due to the high temperatures that are experienced during
metamorphism. Although nobody has tried dating samples taken from my study area, it
seems that the gneisses in this research area are in a better condition for dating due to the
fact that this area resides farther away from the heating event that occurred ~ 1.2 Ga,
hence the rocks farther away from the thermal event show less deformation. No large
bodies of this intrusive complex were mapped in the study area, although some dikes that
have a composition and texture representative of this intrusion have been seen in some
localities.
The area experienced a long period of solitude in which the rock unit exhibits no
clues as to a thermal or tectonic event taking place until the emplacement of the Mt.
Lowe intrusive complex in late Triassic (Ehlig, 1981; Barth, et al., 1991). This intrusive
event is followed by the intrusion of the Mt. Wilson and Mt. Waterman plutons in late
Cretaceous time, as well as the Josephine Mountain intrusion (Barth, et al., 1995). These
events emplaced large amounts of plutonic rocks in the area, probably causing thermal
overprint. Subsequent erosion and brittle deformation events displaced the rocks from
14
their origin and shape them into steep cliff faces, landslide debris, and the general
alluvium around the study area.
Paleoproterozoic Augen Gneiss
(Map unit pCagn, pCbagn, colored red on Plates)
Throughout the San Gabriel Mountains occurs a medium to coarse grained biotite
augen gneiss. This rock is one of the most distinctive rocks in the San Gabriel
Mountains. Augen gneiss occurs as concordant intrusions which share the same foliation
and trends as the fine grained gneisses. It is characterized by a coarse grained leucocratic
type, as well as a more abundant darker – more biotite rich version. The leucocratic
version is characteristic of having augens up to 8 cm long, of which make up around 60%
of the total rock volume (Nourse, 2001). The more common, biotite rich version
commonly contains 0.5 to 2 cm long augens composed of potassium feldspar.
Similarities between the coarse grained augen gneiss and the finer grained gneisses found
in the study area relate thermal events that have recrystallized the rock mass as a whole.
Small scale isoclinal folding seen in outcrop and large map scale structures are
continuous throughout both units as shown on the geologic map and cross sections (refer
to Plates 1-5). Structural similarities shared between these two rock units may be
attributed to synchronous post emplacement thermal events that have deformed the
separate units simultaneously.
Dating zircons in the rock gives ages of 1670 +/- 20 Ma (Silver, 1971) in rocks
taken from the San Gabriel Mountains, as well as north of the San Andreas Fault in the
Orocopia and Chocolate Mountains. A layer contained within the Mendenhall gneiss
contain zircons which indicate un upper intercept age of 1679 +/- 22 Ma (Barth, et al.,
15
2001) in the zoned cores, while the outer rims indicate an age of 1172 +/- 22 Ma ( Barth,
et al., 2001) which corresponds with the intrusion of the anorthosite – syenite – gabbro
complex.
Figure 11 An outcrop
of Paleoproterozoic
augen gneiss along the
West Fork San Gabriel
River is admired by
fellow students Shawn
Wilkins, and Seth
Brodie.
The distinctive
texture of this rock gives
some idea as to what it
may originally have been.
It appears that sometime
shortly after the
accumulation of the fine
grained Mendenhall
gneiss protoliths, an
intrusive event
predominantly composed
of porphyritic granite
and/or porphyritic quartz monzonite occurred (Silver 1971). The dates given for such an
event are revealed in the inner zones of the dated zircons from augen gneiss. The
essentially porphyritic granitic rock was subsequently deformed along with the
surrounding rocks in the sequence of heating and brittle deformation events. In the
16
Mendenhall Peak area this deformational fabric is intruded by the 1196 ± 5 Ma
anorthosite – syenite – gabbro complex.
Mz (?) or pC (?) gneissic or foliated diorite or quartz dirorite
(Map units bhqd, bhdi, hdi, hbdi, colored purple on Plates)
Figure 12 Gneissic diorite in contact with Triassic Mt. Lowe intrusive rocks, and cross cutting
granitic dikes. Rock hammer approx 14 cm long.
An enigmatic unit, of which not much is known, appears in great quantity in the
study area (see figures 12 and 13). This unit is a foliated diorite, ranging from a highly
foliated, biotite rich, almost schistose texture, to a coarse grained foliated hornblende
diorite or quartz diorite. Age of this unit is not known, nor has it been dated or mapped
in any significant quantity. Field relationships suggest that the emplacement of this unit
occurred after the formation of the augen gneiss, but before the Triassic intrusive events,
which leaves quite a bit of time for speculation. Xenoliths of this foliated diorite have
been seen in outcrops of Triassic and Cretaceous intrusions which demonstrate a late
17
Triassic emplacement age. Although a roughly hornblende rich mafic intrusive series
accompanies the Triassic intrusive events (Nourse, 2002), field relations and samples
indicate no relation, while no subsequent geochemical studies have been done. General
weathering of this unit appears to be very high, and the look of the rock shows that it
does not seem to be in the same condition of younger units.
Figure 13 Photograph showing a gneissic diorite xenolith fully enclosed within a Cretaceous
monzogranite. Rock hammer is approximately 14 cm long.
18
Triassic Plutonic and Intrusive Rocks
(Map units TRdi, TRpbqm, TRlbqm, TRhqmzd, colored light blue on Plates)
Early Mesozoic time offered a variety of thermal intrusive events in the study
area. The most major of these incidents is referred to as the 218 ± 3 Ma Mt. Lowe
intrusion which intrudes the area with the Mendenhall Gneiss as the country rocks. Many
studies have been done on this rock unit for use in correlation with rocks of similar age
and composition in the mountains that lie to the south, and on the opposite side of the San
Andreas Fault (Ehlig, 1981; Barth, et al., 1991; Nourse, 2002). The vertical contact at the
north east edge of the study area between the Mt. Lowe intrusive rocks and the
Mendenhall Gneiss projects south east into the West Fork San Gabriel River (below
Cogswell Reservoir) and meets up with the San Gabriel Fault. Here is what Ehlig used as
his classic piercing point on the north branch of the San Gabriel Fault.
Figure 14 Mt. Lowe (TRhqmzd) intrusive rock outcrop in lower Shortcut Canyon with a layer
of Triassic diorite (TRdi) cutting across. Rock hammer is approx 14 cm long.
19
Figure 15 Xenolith of diorite (TRdi) in Mt. Lowe (TRhqmzd) intrusive rocks. Photo taken in
lower Shortcut Canyon. Rock hammer is approximately 14 cm long.
Figure 16 Graphic shows
distribution and radiometric
data for Triassic plutonic and
volcanic rocks in the
Southwestern U.S. Solid
circles are radiometrically
dated localities with ages in
Ma. All dates by U/Pb zircon
geochronology. Inverted V
symbols are outcrops of
Triassic and Triassic (?)
volcanic rocks. Double solid
lines mark inferences to the
independence dike swarm.
Figure from Barth et al.,
1990.
20
The Triassic arc (see figure 16) follows a roughly north-northwestern trend
corresponding to the back arc volcanism and plutonism of the era (Barth, et al., 1991).
The Mt. Lowe intrusion forms a compositionally zoned pluton ranging in composition
due to fractional crystallization, in which the pluton crystallized from the bottom up
(Ehlig, 1981). It is broken down into many different zones according to main mineral
assemblages which are either present or lacking in certain localities. As a whole, the
intrusion is characterized by high feldspar content, ranging from 60 – 95%, and on
average, low quartz content, around 10% (Ehlig, 1981). The stratigraphy of this pluton
from the bottom – up consists of a medium grained hornblende diorite on the bottom,
grading to a coarser grained quartz diorite with hornblende phenocrysts in a
predominantly sodic feldspar matrix. Further upward the pluton zones into a coarser
“dalmation” phase with larger hornblende phenocrysts. Around 1.5 km above the base of
the pluton begins a zone in which the rock begins to bear garnets. These garnets appear
as crystals as much as 2 cm across as well as fill in cracks that appear to be mending
seams during residual fluid crystallization. The final stage of crystallation occurs in the
upper zone of the intrusion. It is characterized by conspicuous Orthoclase phenocrysts
that have been referred to as “pigeon eggs” due to their roughly egg-shape as well as
similarities in size (Ehlig, 1981).
Magma generation for the emplacement of these plutons is generally indicative of
being derived from the partial melt of pre-existing crustal rocks due to high silica content
in the rocks themselves (Barth, et al., 1995). Analysis of the Mt. Lowe Intrusion has
yielded an isotopic signature which corresponds with a parent magma of roughly highaluminum basaltic composition (Barth, et al., 1995). This data supports the conclusion
21
that these magmas were derived by a combination of partial melt of subducting balsaltic
crust, as well as hints at a high pressure partial melting event of the lower continental
crustal material (Barth, et al., 1995; Silver 1971).
Subsequent to emplacement of the pluton, these rocks have been subjected to
various thermal and deformation events. The general foliation of the pluton remains
somewhat parallel to the bottom, which indicates an igneous flow foliation (Ehlig, 1981).
The rock unit has been metamorphosed near upper amphibolite facies near granitic rocks
of the central San Gabriel Mountains, while also subjected to lower amphibolite facies
near the western part of the range near the Soledad Basin (Ehlig, 1981). These
metamorphic events have enhanced the original foliation as well as replaced some of the
hornblende by epidote in some localities. Radiometric dating techniques employed on
this rock unit have yielded ages of 220 ± 10 Ma using U/Pb zircon methods, as well as
208 ± 7 Ma using whole – rock methods (Ehlig, 1981; Silver, 1971).
Cretaceous Plutonic and Intrusive Rocks
(Map units Kbmgr, Klbmgr, colored light green on Plates)
Intrusive events in the Late Mesozoic are characterized by granitic sills that
extend into the study area. These intrusions project eastward from the Late Cretaceous
Josephine Mountain Intrusion (Barth, et al., 1995). They regularly occur as concordant
sills that intrude parallel to the local foliation in the metamorphic rocks of the
Mendenhall Gneiss, as well as the Mt. Lowe intrusive rocks. Small occurrences of the
Mt. Wilson batholith crop up as a diorite to quartz diorite found in the southern part of
the study area, south of the San Gabriel Fault. These rocks show very little to no
foliation in outcrop, and are usually in very good shape due to a good resistance of
weathering. More common in the study area are sills related to the Josephine Mountain
22
Intrusion. This rock suite is a mostly calc-alkaline series showing very little to no
foliation in outcrop. Truncation of the rock unit on the southern side of the study area
limits study possibilities, but gives a potential piercing point along the San Gabriel Fault.
The rock units in the study area that correspond to this intrusion are a porphyritic biotite
monzogranite (Kbmgr), as well as a more leucocratic biotite monzogranite (Klbmgr).
Granites from the intrusion of the Josephine Mountain complex yield U/Pb zircon
ages of Late Cretaceous origin from 88 ± 3 Ma, to the discordant age of 78 ± 8 Ma
(Nourse 2002, Barth et al., 1995). Zircon analysis shows some discordance due to
contamination from the gneisses in which it intrudes. These rocks also crystallized
Figure 17
Concordia
diagram of
zircon from a
tonolite sample
from the
Josephine
Mountain
Intrusion
Taken from
Barth et al.,
1995.
under oxidizing conditions, while it contains an assemblage of quartz, biotite, almandine
garnet, muscovite, and magnetite in the same sub assemblage (Barth, et al., 1995).
Cretaceous granitic rocks are believed to have come from partial melting of crustal rocks
that pre-existed in the area of formation. The fine grained equivalents of the phaneritic
23
rocks include basalt to basaltic andesite, to a high silica rhyolite (Barth, et al., 1995).
Differences in composition of adjacent plutons have been attributed to the mixing of the
parent rocks with the magmas, along with fractional crystallization (Barth, et al., 1995).
Quaternary sediments
(Map units Qal, Qcv, Qls, all colored yellow on Plates)
Brittle deformation, general uplift of the mountains, and exposure to the elements
has caused deposition of various units marked on the map in yellow. Quaternary
alluvium, map unit Qal, is restricted to active stream channels. Quaternary colluvium,
map unit Qcv, is the result of the steep terrane of the San Gabriel Mountains, and occurs
as cobble to boulder size angular fragments of rock bodies located on steep slopes. Rapid
uplift of the area has caused landslides to occur, as well as formed alluvial terraces on the
south side of the San Gabriel Fault. The landslides are marked as Qls, and occur as large
block slides that displace conformable rock units, and alluvial terraces are denoted as
Qoa.
IV. Structural Analysis
Methods
Structural analysis of the study area is accomplished using statistical
methods to identify trends in the various rock units, as well as the map area as a whole.
Stereographic projections can represent a 3-dimensional orientation in space, in 2
dimensions on paper, in this case using equal area stereonets, and a stereographic plotting
computer program called GEOrient. The program allows many different types of plots to
be analyzed different ways, as well as produces some good quality graphics. Cross
sections are utilized for depicting what may be happening at depth, calculating unit
thickness, and give visual representations of structural relationships (refer to Plates 1-5).
24
The following plot sets (1 through 8) represent the data taken and recorded during field
investigation of the study area.
Plot Set 1
2
1
3
2
4
Stereographic projections of the biotite rich member of the Mendenhall Gneiss. (1) Planes to 83
foliations taken throughout the study area. (2) Poles to planes showing two best fit orientations related to
the two clusters of poles, as well as fold axis. (3) Contour plot showing point density, best fit orientations
and fold axis. (4) Contour, and density dot plot showing point density, best fit planes and fold axis. Best
fit plane orientations (dotted lines); S74°E, 80° SW; N59°W, 72° NE. Trend and plunge of fold axis;
(arrow) S69°E 31°.
25
Plot Set 2
1
3
2
4
Stereographic projections of the quartzofelspathic member of the Mendenhall Gneiss. (1) Planes
to foliations of 116 data points taken throughout the field area. (2) Poles to foliations showing 2 best fit
representative planes and trend and plunge of fold axis. (3) Contour plot color coded to show point
density, showing 2 representative planes, and fold axis. (4) Contour and color coded dot plot showing
point density, 2 best fit planes, and fold axis. Best fit planes (dotted lines); S58°E, 82° SW; N52°W, 82°
NE. Trend and plunge of fold axis (arrow); S56°E 24°.
26
Plot Set 3
1
2
3
4
Stereographic projections of the amphibolite unit in the Mendenhall Gneiss. (1) Planes to 22
foliations, (2) poles to those 22 planes, two best fit planes and fold axis. (3) Contour plot colored to show
point density. (4) Contour and dot density plot to show point density, two mean planes to foliation and
fold axis. Best fit plane orientations (dotted lines); S65°E, 82° SW; N81°W, 77° NE. Trend and plunge of
fold axis (arrow); N71°W 39°.
27
Plot Set 4
1
2
3
4
Stereographic nets plotted for foliations of the biotite augen gneiss. (1) Plot showing planes to 56
foliations, (2) Plot showing poles to same 56 foliations, best fit planes to the two pole clusters, and fold
axis. (3) Contour plot showing color coded pole density, best fit planes and fold axis. (4) Contour and
color coded dot density plot showing two best fit planes and fold axis. Best fit plane orientations (dotted
lines); S65°E, 69° SW, N75°W, 83° NE. Trend and plunge of fold axis (arrow); N72°W 22°.
28
Plot Set 5
1
3
2
4
Stereographic plots of the foliated diorite unit exposed in the study area. (1) Planes to 16
metamorphic foliations of the foliated diorite unit, (2) poles to the 16 foliations taken, best fit planes and
fold axis. (3) Contour plot showing color coded point density, best fit planes and fold axis. (4) Color
coded contour dot plot showing point density, best fit planes and fold axis. Best fit plane orientations
(dotted lines); S70°E, 50° SW; S82W°, 69° NW. Trend and plunge of fold axis (arrow); N90°W, 24°.
29
Plot Set 6
1
2
3
4
Stereographic plots representing the Mt. Lowe intrusive complex. (1) Planes to 52 metamorphic
foliations taken in the study area, (2) Poles to those 52 planes, as well as a best fit mean plane. (3)
Contour plot representing a color coded pole density with location of the best fit plane. (4) Color coded
contour dot plot showing point density as well as best fit plane. Best fit fold axis plane orientation; S40°E,
47° SW.
30
Plot Set 7
1
2
3
4
5
Stereographic plots representing fault
trends measured in the study area. (1)
Planes to 30 fault orientations, (2) Poles
to the same 30 measurements, as well as
best fit planes from the 3 pole clusters,
(3) Contour plot color coded to show
density and 3 best fit planes, (4) Contour
dot plot color coded to show density and
best fit planes, (5) Planes to faults
showing lineation, triangles denote
lineation. Best fit plane
orientations;S52°W, 85°NE; S79°E,
84°SW; S13°,85°SW.
31
Plot Set 8
1
2
3
4
Stereographic projections showing foliation with lineation for lithologies in the study area. (1)
Planes to foliation and lineation of data collected on the quartzofelspathic unit in the Mendenhall Gneiss;
(2) Planes to foliation and lineation of the biotite rich member of the Mendenhall Gneiss; (3) Planes to
foliation and lineation of the coarse grained granite augen gneiss; (4) Planes to foliation and lineation of
the Mesozoic intrusive rocks measured in the study area. Curves show fault planes, triangles denote trend
and plunge of striated rocks.
32
Figure 18 Stereographic
projection of metamorphic rock
units (277 points) which make up
the Mendenhall Gneiss. Includes
all data collected on biotite rich
quartzofelspathic, granite augen,
and amphibolite units gneiss
units. Plot shows color coded
point density plot, two best fit
planes (dashed curves), fold axis
(arrow), and interlimb angle
(shorter arrow). (Best fit plane
orientations; S66°E 84° SW,
N64°W 79° NE. Fold axis trend
and plunge S66°E 8°. Inter limb
angle ~10°-15°)
Foliation
Plot sets 1 through 6 represent the results of measurements on metamorphic
foliation. Notice how some of the plots have multiple foliation pole clusters. These are
the general trends of the individual rock units in accordance to their respective
stereographic plots. Lines with arrows denote axis trend and plunge of the best fit fold
axes, assuming that the foliations have been cylindrically folded during postmetamorphic deformation, which correspond in direction to the red and blue symbols
denoted on the geologic map (see Plate 1). The fold axes are large scale structures that
extend continuously throughout the Mendenhall Gneiss. Fold trends are roughly parallel,
which is expressed in the stereographic projections (see plot sets 1-4 and figure 18), and
can be seen on the geologic map (see Plate 1). Foliation trends within the main
33
Mendenhall Gneiss unit (see figure 18) appear to be very similar in orientation. The two
main pole clusters give way to a nearly 180° separation in metamorphic foliation.
A tectonic event seems to have affected the area of study after foliation
development, as these features are seen in all Paleoproterozoic lithologic units. Gently
plunging antiform and synform structures indicate a compressional event in which the
main stress direction was oriented from the NNE and SSW. Given the steepness of the
fold limbs, and an inter limb angle of 10°-15°, this event must have shortened these rocks
significantly (see figures 8 and 9, refer to Plates 2-5). Cross section analysis gives a
consistent percentage shortening of ~40%, which indicates that these rocks have been
shortened ~40% relative to their initial length. Measurements taken of the small-scale
isoclinal fold hinges show a population of orientations which are closely similar to large
scale structural features seen on the geologic map and cross sections (refer to Plates 1-5).
Figure 19 Stereographic plot
revealing poles to lineation that
are color coded to show point
density. Data plotted on the
quartzofelspathic, biotite rich,
augen, and amphibolite gneiss
units in the study area.
Lineation
Metamorphic lineation is
common among most rock units
exposed in the study area. This
fabric is caused by shearing of the
rock mass at depth while the rock
is very hot and under intense pressure. Tectonic stress is then applied to the rock unit and
34
instead of deforming in a brittle state as it would on the surface, the rock tends to smear
and form a feature called lineation. This feature is helpful in identifying stress regimes
that may have occurred in the past. The main feature of the study area is the fact that the
lineations have steep plunges that are almost down-dip (although some are oblique).
With one main pole cluster and two adjacent lesser dense pole clusters show that if you
rotate the folded foliations back to horizontal you get a NNE – SSW original trend of
lineation (see plot set 8 and figure 19). This idea makes it difficult to determine whether
these lineations predated the folding or formed during the folding.
Plot set 8 shows characteristic lineation in all rock units in which the structure
was found. They all share the same general lineation trend which indicates that at some
time in the past, an event occurred which (in localized areas) sheared these rocks in a
ductile state. Appearance of these lineation features indicates that these rocks were
subjected to a stress event when the rock was at depth and in a relatively ductile state.
This heating and shearing of the rocks forms lineation features with S and C planes that
show direction and magnitude of shear, note that these features occur in almost every
rock unit which suggests that this event affected all rock units before the intrusion of the
Cretaceous rocks. Data points to formation of lineation to have taken place synchronous
with the foliation or tight folding event.
Faults
Brittle deformation events occur in the Cenozoic that cut all units in the area. As
shown by plot set 8 these faults have two distinct trends. One trend is correlative to that
of the San Gabriel Fault. Its S70°W orientation is shown in one of the best fit planes
shown in the stereographic projection. This analysis suggests that the stress regime in the
35
area that caused the rupture of the San Gabriel fault had a maximum stress direction
coming in from the northeast and the southwest. Using lineation of these faults which
contain hematite and chlorite staining on fault surfaces indicate a nearly pure strike slip
orientation with very little normal or reverse faulting component. The other two best fit
planes represent the conjugate set of faults that occur as smaller stress faults. These
faults offset all units as well with a right lateral component that also has little or no dip
slip component.
This data gives way to ideas about the San Gabriel Fault system as a whole (see
figures 1 and 3). This fault system is a largely southwest striking right lateral strike slip
fault zone which cuts through the San Gabriel Mountains. This fault indicates some of
the first movements along a stress regime that ultimately created the San Andreas Fault
system. Movement along this fault zone cut the Mendenhall Gneiss in a few areas,
displacing some of the outcrops more than 30 km from their original location.
Palinspastic reconstructions of the area show this by correlating the rock units and
recovering the slip on these faults to reveal what these rock units may have looked like at
certain times in the geologic past (see figure 20).
V. Regional Implications/Interpretations/Significance
The folded map scale stratigraphy shows unit thickness in cross section (see main
plate) as well as interaction between the different rock units. Cross section analysis
shows that unit thicknesses vary depending on location, and thickness is dependant upon
the amount of intrusive rocks that may have displaced the country rocks. The
quartzofeslpathic and biotite rich units are mapped together and give a combined
thickness of ~160m (525 ft) to ~750+m (2500ft), on average the unit thickness is ~215m
36
(700ft). Granite augen gneiss varies drastically depending upon which area the cross
section is located with thicknesses ranging from as small as a few meters, to about 300m
(1000ft), with an average thickness of ~120m (400ft). The foliated diorite unit also has a
varying thickness from a few meters to about 330m (1100ft), with an average thickness
of 50m (170ft) (refer to Plates 1-5).
As the units of the study area seem to be deformed as a whole, the relationships
between these rocks can be used to correlate through palinspastic reconstructions, fault
movements and rates to other areas (see figure 20). Both the large scale folds, as well as
small scale features such as isoclinal folding and lineation data can be used for detailed
comparison to rocks that are located on opposite sides of fault zones. Antiform and
synform structures come into contact with fault zones which cut the area and can be used
for comparison to correlative units either on the south side of the San Gabriel Fault or on
the north side of the San Andreas Fault. In looking at the palinspastic reconstruction in
figure 20, you can see possible correlation with rock units located in the Chocolate and
Orocopia Mountains. These rock units display similar characteristics in comparison to
the Mendenhall Gneiss located in the San Gabriel Mountains. Other structures such as
the Vincent Thrust also make the data more convincing as to what rocks correlate with
others located on opposite sides of fault zones.
The northwest – southeast trend indicative to the San Gabriel fault can also be
seen roughly parallel to the map scale structures. Similarity in trends of the brittle and
ductile deformation events may or may not be a coincidence. Chronology suggests that
the events which compressed the Mendenhall Gneiss ductally do not correspond directly
with brittle deformation events occurring in the Cenozoic. Orientations of both ductile
37
and brittle deformation stresses are similar in that the maximum stress directions come in
from the northeast and southwest. This can mean two different scenarios; these ideas
hypothesize around stresses that caused the ductile deformation and brittle deformation
events. The creation of structures in the study area may have occurred simultaneously, or
nearly simultaneously, giving gross foliation to the rock mass while the uplifting, and
cooling rocks began to act brittle. Another idea is that these two events took place totally
independent of each other, and the similarity in trends is attributed to the fact that rocks
tend to break at their weakest points. The weakest points in this particular rock body may
have been on the axis of these map scale features. Map scale structures were created in
the first event, folding the rock into its current orientation, while a second event, a more
recent event, occurred subsequent to tectonic uplift of the area. This uplifted area had
cooled and was subject to brittle deformation events caused by stresses similar to what is
happening today.
The many ductile and brittle deformation events that have taken place throughout
the history of the Mendenhall Gneiss cause problems because one event may mask
another. Field relations taken from other areas in which the Mendenhall Gneiss is present
indicate formation of isoclinal folding to have occurred at or near the same time as a
partial melting event; evidence is given by metamorphic mineral assemblages throughout
the rock unit which have replaced its original crystal structure (Barth, et al., 1995). These
mineral assemblages record multiple deformation events, having occurred at the time in
which the protolith first formed, intrusion of the anorthosite – syenite – gabbro complex,
38
39
of rock units and their relationships to the respective faults. This particular model restores 240 km of slip along the San Andres Fault zone, 22 km
of lateral slip along the north branch of the San Gabriel Fault and 38 km along the south branch of he San Gabriel Fault. Taken from Nourse
2002.
Figure 20 Middle Miocene palinspastic reconstruction of the San Gabriel Mountains along major strikd slip fault zones. Notice the location
plutonic events in the Mesozoic, and others that may have taken place any time in
between (Barth, et al., 1995). Many periods of deformation may be masked by events
that have taken place after that time. When heat and pressure is applied to a body of
rock, changes occur within the rock that as long as no other chemical component is added
to the rock mass, will only alter the physical composition of the rock. The chemical
composition of the rock mass as a whole does not change, only the occurrence of
different minerals.
Plot set 6 shows metamorphic foliation as shown in the Mt. Lowe intrusive
complex. The Triassic foliations in the Mt. Lowe rocks show comparatively the same
general trends as the older Paleoproterozoic rocks, but only in one direction. These rocks
do not show the two separate orientations that reveal fold axis orientation which suggests
that these rock bodies are not as deformed in this area as the older rocks. An event of
deformation must have occurred before the emplacement of the Cretaceous rocks that
affected the Triassic and older rocks. As these Triassic and Cretaceous rocks were
intruded into the host rocks, they were emplaced parallel to the local foliation into areas
in which were the easiest to intrude. This is shown in the cross sections (see Plates 2-5)
as intrusive dikes and sills that have orientated themselves parallel to foliation.
These data also give opportunities to address questions relating to which
Neoproterozoic rocks were connected to the western margin of Laurentia before the 750
Ma rifting event. Aside from the Achaean terrane located near the center of the
Laurentian craton, the oldest rocks exposed on the western Cordillera occur today in the
San Gabriel Mountains and the San Bernardino Mountains (Barth, et al., 2000, 2001).
These rocks include the Mendenhall Gneiss, and data suggests that since no older rocks
40
are seen to the west of these units after restoration of Cenozoic faulting, these rocks mark
part of the western margin of Laurentia. Many hypotheses have been given about how to
correlate lithologic units on the present southwestern United States to another continent
which would have resided adjacent to these rocks by looking at the depositional, igneous,
tectonic and metamorphic events that shaped the area (Unrug, 1997). Different theories
have been developed to explain the complexity of such an undertaking. The SWEAT,
AUSWUS (see figure 21), Siberia connection (see figure 22), and Cathaysia hypotheses
explain Proterozoic rifting, and state different models as to which continent appears to
have resided prior to separation.
Throughout the last few years, researchers have
collected large amounts of data to either support or disprove these theories.
One of the earliest such hypothesis for this late Precambrian fit of continents,
named SWEAT for Southwestern United States-East Antarctica, was proposed by E.M.
Moores (1991). He suggested that the rocks located in the southwestern U.S. correlated
best with those of eastern Antarctica, and rocks located in the northwest U.S. correlate
with rocks found in Australia (see figure 21). He reasoned that the evidence for such a
correlation is based on the age and stratigraphy of two thick sedimentary rock sequences
along the western margin of the continental United States. These rocks range in locality
from Canada and Montana, all of the way down to southwestern California (Moores,
1991). He stated that the late pre- Cambrian Gondwana rocks prior to rifting can be
related to similar rock units located on Antarctica. Implications of this hypothesis include
trends of the Greenville province orogenies continue onto rocks that have been found in
east Antarctica, orogenies occurring to the north of Greenville belts extend onto
southeastern Australia, other orogenies that become truncated before the continental
41
margin do not show up in correlated units, and evidence of similar rocks that occur in the
Yavapai, Mazatzal provinces and the Belt Purcell groups can be seen in Antarctica
(Moores, 1991). These implications are very thought provoking and speculative to say
the least, but offered the first insight into what may have happened in a time that seemed
to have been erased in the rock record until this point.
Karl Karlstrom (1999) expanded on this work by placing Australia adjacent to
southwestern Laurentia, and placing Antarctica further to the south (see figure 20). This
proposal is called AUSWUS for Australia-Southwest U.S. He provides evidence of this
hypothesis by citing examples of how the SWEAT proposal does not meet certain criteria
to correlate, and argues that the AUSWUS hypothesis provides an explanation of
geologic and paleomagnetic data that is better constrained. Relationships of rock units
exposed in southwest U.S. and Australia appear to be similar in age, composition and
tectonic setting. These units display the same mid-crustal shortening events, according to
composition may have formed from juvenile volcanic arc assemblages, and display
correlative matches between major ore assemblages (Karlstrom, 1999). Paleoproterozoic
and Mesoproterozoic rock units exposed in Australia may correlate to rocks such as the
Mendenhall Gneiss and the granite augen gneiss found in the study area, and may display
similar structural characteristics.
Subsequent to the proposition of the ideas stated previously, James W. Sears and
Raymond A. Price (2000) revised one of their hypothesis related to the placement of the
Siberian platform adjacent to southwestern Laurentia (see figure 21). This idea negates
the whole concept of having Australia or Antarctica located where they were previously
42
43
Reconstruction of
Laurentia for 1.7 to
0.8 Ga showing both
locations of the
Australian continent
supported by the
SWEAT and
AUSWUS
hypotheses., and
locations of tectonic
provinces. Location
of the Australian
continent and rock
correlations are based
on older rock units.
Figure modified from
Karlstrom, Williams,
McLelland,
Geissman, and Ahall,
Figure 21
thought to be before the break up of Laurentia, and the idea that Siberia was located on
the northeastern margin of Laurentia (see figure 23). Using isotope analysis they initially
related Sr/Sr isotope ratios in Siberia to those of similar ratios located in western
Laurentia, and by using general rock trends were able to correlate these two continents.
This hypothesis also uses some of the same techniques observed in other ideas to validate
the assumption that it was in fact Siberia and not Antarctica or Australia. The BeltChurchill basin is a linear feature exposed in western Laurentia and when it is placed
adjacent to the Taimyr trough, a long depressional feature located in northern Siberia that
displays a poorly understood Mesoproterozoic stratigraphic section, it forms one
continuous truncated basin that was rifted apart (Sears and Price, 2000). This feature is
an accumulation of sediment that may have been transported from one side of the
continent to the other and back again before the area rifted apart. If this is indeed what
happened, the sediment transported to the area could be a possible protolith for the
Mendenhall Gneiss.
Another idea proposed in the 1990’s by Zheng-Xiang Li states that he may have
found a connection between the Yangtze block of South China and the western margin of
Laurentia (Li, 1995, 2002). Using SHRIMP (sensitive high-resolution ion microbe)
analysis, he was able to conclude that a Grenvillian continental collision between south
China and western Laurentia happened by relating metamorphic events that affected both
the Yangtze and Cathaysia blocks and by looking at locations of sedimentary basins on
the Yangtze block that may have come from the Cathaysia block (Li, 1995, 2002; see
figure 24).
44
Figure 22 Paleoproterozoic reconstruction of Laurentia showing locations of rifted margins and
tectonic provinces related to Sears and Price’s Siberian Connection model. Shows trends associated with
correlation of separate continents. Modified from Sears and Price, 2000.
45
Figure 23 Neoproterozoic reconstruction of supercontinent Rodinia showing locations of rifted
continents. This is one of the first figures showing location of continents. Does not relate to the SWEAT,
AUSWUS, or Siberian connection models. A-F=Albany-Fraser mobile belt, M=Musgrave block,
TL=Tasman line, TT=Thelon-Taltson line, BB=Belt basin. Modified from Hoffman, 1991.
46
Figure 24 Diagram showing the possible
position of the Yangtze block adjacent to
western Laurentia. Y=Yangtze block,
T=Western Tazmania, B=Belt Basin,
BC=British Colombia, M=Mackenzie Mt.,
NC=NW Canada. Modified from Li, 2002.
Using the appearance of 1800 Ma
basement rocks and Archean protoliths,
the correlation of the Cathaysia block and
the Yavapai and Mazatzal blocks can be
made. When this puzzle is all put
together, located between the eastern Antarctica block and the western margin of
Laurentia is a separate small continent composed of the Yangtze and Cathaysia blocks
(Li, 2002; see figures 23, 24).
These hypothesis all offer differing ideas about what may have happened 750 Ma
ago. Using basically the same techniques of analysis, many conclusions are formulated
but can not be positively proven. Detailed mapping projects similar to this paper offer
large amounts of data that can be used to help solve problems like these. Much more data
needs to be collected before any ideas can be absolutely proven.
VI. Conclusions
Geologic mapping of the Mendenhall Gneiss and surrounding rocks have
illustrated correlation possibilities associated with many different events. Detailed
mapping of structures in the Mendenhall Gneiss can be correlated with lithologic units
located on opposite sides of Cenozoic fault zones located throughout Southern California.
Restoration and palinspastic reconstructions of these faults show characteristics that are
47
very similar, and present on both sides. Areas in the San Gabriel Mountains on the north
side of the San Gabriel fault contain rocks that correlate with mountain ranges located
around 250 km to the south and on the north side of the San Andreas fault, in the
Chocolate and Orocopia Mountains. These rock units display the same characteristics
and generally have the same geochemistry in both localities. Although protoliths and
environment of deposition remain in doubt at this time due to multiple brittle and ductile
deformation events, subsequent dating and chronological evidence offer insight into the
history of this rock unit.
Events that shaped the history of this rock are being established in order to relate
these units to past continental collision and rifting events that have taken place in the
Proterozoic. Essentially, since these gneisses are the oldest rocks exposed in western
U.S.A., detailed mapping is a necessity in order to resolve correlation problems with the
rifting of western Laurentia. Determination of the mystery continent which resided there
has been a problem that many geologists have tried to resolve, citing evidence based
upon speculative as well as concrete ideas. As the techniques for correlating lithologic
units become more and more complex, it will all come down to projects like these to
close up any doubts and misgivings about conclusions. The more data that is collected in
an area of study, the more the geologic community can speculate about the history of
events that shaped the planet as we see it today.
The Mendenhall Gneiss, as it is seen in the area of study, has been dated fairly
well at ~1.7 Ga, and has been exposed to a multitude of deformation events (Barth et al.,
2001). Showing very small scale foliation, large map-scale folding structures and
multiple intrusion events, this rock shows a very complicated history which makes
48
studying the unit a difficult task. The rocks exposed demonstrate features that enable the
correlation of the largest continuous section of metamorphic rock in the San Gabriel
Mountains to other areas located on opposite sides of large strike-slip faults. Mapping of
the area by taking rock orientations whenever available gives hundreds of data points in
which to characterize its behavior, and predict what may be happening at depth. Studies
of this type are necessary in order to define characteristics used in correlation and
reconstruction of the south-central San Gabriel Mountains and its correlative units
located in Southern California, and around the globe.
VII. Future Studies
Possible future studies in the area not mapped may help to define characteristics
of certain rock units in more detail. One idea may be to map canyons and other roads
located to the north of Angeles Crest Highway to clarify the occurrence of the granite
augen gneiss that outcrops in many localities in the study area. Mapping of Ladybug
Canyon has yielded evidence that more of this rock type exists to the north that may help
in defining structural relationships related to the environment of emplacement of its
protolith, and data that may help define events that characterize the eye catching augen
gneiss. Other rock types may be seen on the north side of the map that has not yet been
mapped in any significant quantity. The Triassic and Cretaceous intrusive rocks that
occur in the areas already mapped may be in this unmapped area in significant quantity.
These rocks do not show up in any published geologic map of the area, and may be
helpful in determining dates of deformation events. The gneisses exposed in the study
area also may outcrop in this area and also need to be studied in detail because rocks in
49
this area are located farther away from the 1190 Ma intrusive anorthosite-syenite-gabbro
complex, and may show less deformation that would help in dating zircons in the rock.
Generally, more mapping in unmapped areas is necessary to help constrain rock types for
the purpose of correlating the Mendenhall Gneiss to rocks located on other continents,
and Southern California, the more data collected, the better the results.
VIII. Acknowledgments
A very big thank you goes to Professor Jon Nourse for his patience and guidance
on this long arduous journey, without him, none of this would have been possible. The
American Association of State Geologists (AASG) provided a grant for $3300.00 that
made the start of this mapping project possible, I thank the AASG Student Mentorship
Program for enabling me to obtain guidance and experience in field mapping. I would
also like to thank those incorrigible fellow students that helped with field mapping, data
collection and advice. They include; Terry Watkins, Shawn Wilkins, Meredith Staley,
Seth Brodie, Jennifer Beal, Julie Parra and Cami Anderson. Thank you all for your help
and support. I would also like to thank my parents for paying for this whole college
endeavor, without them; also, this would not have been possible.
50
BIBLIOGRAPHY
Barth, Andrew P., et al., Joseph L. Wooden, Drew S. Coleman, 2001 “Shrimp-RG U-Pb
Zircon Geochronology of Mesoproterozoic Metamorphism and Plutonism in the
Southwestern United States.” The Journal of Geology vol. 109, p 319-325.
Barth, Andrew P., et al., Joseph L. Wooden, R.M. Tosda, Jean Morrison, D.L. Dawson and
B.M. Henry, 1995 “Origin of gneisses in the aureole of the San Gabriel anorthosite
complex and implications for the Proterozoic crustal evolution of southern California.”
Tectonics vol. 14, no. 3, p. 736-750.
Barth, Andrew P., et al., J.L. Wooden, R.M. Tosdal, J. Morrison, 1995 “Crustal contamination
in the petrogenisis of a calc-alkalic rock series: Josephine Mountain Intrusion, California”
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Barth, Andrew P., et al., 1997 “Triassic plutonism in southern California: Southward younging
of arc initiation along a truncated continental margin.” Tectonics vol. 16, no. 2, p. 290-304
Dalziel, Ian W.D., 1991 “Pacific margins of Laurentia and East Antarctica – Australia as a
conjugate rift pair: Evidence and Implications for an Eocambrian supercontinent.” Geology
vol. 19, p. 598-601.
Dibblee, T.W., Jr., 1998, Geologic map of the Mt. Wilson/Azusa quadrangles, southern
California, Map No. DF 67, Dibblee Geological Foundation.
Ehlig, Perry, 1981 “Origin and Tectonic History of the San Gabriel Mountains.” p. 258-266.
Silver, Leon T., 1971 “Problems with crystalline rocks of the Transverse Ranges.” Geological
Society of America abstracts with programs, Cordilleran Section, Riverside, CA., p. 193-194.
Karlstrom, Karl E., Michael L. Williams, Stephen S. Harlan, James McLelland, John W.
Geissman, Karl-Inge Ahall, 1999 “Refining Rodinia: Geologic Evidence for the AustraliaWestern U.S. connection in the Proterozoic.” GSA Today vol. 9, no. 10, p. 1-6.
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collision in South China: New SHRIMP U-Pb zircon results and implications for the
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Moores, E.M., 1991 “Southwest U.S.-East Antarctic (SWEAT) connection: A hypothesis.”
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Nourse, Jonathan A. 2002 “Middle Miocene reconstruction of the central and eastern San
Gabriel Mountains, southern California, with implications for evolution of the San Gabriel
fault and Los Angleles basin.” Geological Society of America Special Paper 365, p. 161185.
Sears, James W., Raymond A. Price, 2000 “New look and the Siberian connection: No
SWEAT.” Geology vol. 28, no. 5., p. 423-426.
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Assembly.” GSA Today vol. 7, no. 1, p. 1-4.
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56
Amphibolite
Gneiss
S41E,79,NW
S59E,71,SW
S35E,85,SW
S66E,45,SW
S41E,84,NE
S77E,59,SW
S65E,85,SW
S70E,60,SW
S54E,65,NE
S81E,64,SW
S40E,90,SW
S55E,80,NE
S58E,85,SW
S76E,62,NE
S80E,59,NE
S79E,76,NE
S84E,90,SW
N61W,43,NE
N64W,80,SW
N79W,78,NE
N50W,75,NE
N71W,81,SW
Foliated Diorite
S26E,52,SW
S85E,80,SW
S60E,76,NE
S80E,84,SW
S74E,49,SW
S69E,56,SW
S69E,46,SW
S84W,79,NW
S70W,84,NW
N90W,61,NE
S31W,71,NW
N15W,74,SW
N49W,60,NE
S80W,63,NW
N50W,45,NE
N80W,90,SW
Biotite Gneiss
S71W,54,SE
S21W,73,NW
N88E,48,SE
S71E,79,SW
N24W,83,NE
N31W,54,SW
S06W,54,NW
N70W,81,SW
N88E,76,SE
S82E,87,SW
N66W,52,SW
S47E,75,NE
N59W,79,NE
N52E,48,SE
N69E,49,SE
N85E,85,SE
S50E,83,SW
N80E,46,SE
N64E,54,SE
N54E,62,SE
S79W,41,SE
S434E,74,SW
S84E,79,SW
S62E,85,SW
N70W,90,SW
N66W,72,NE
N75W,81,NE
N59W,77,SW
N46,W,49,SW
S40E,41,SW
S54W,44,SE
N71E,75,SE
N90W,66,SW
S77E,80,SW
S78E,86,SW
S81E,77,NE
N39W,50,NE
N52W,55,NE
N90E,75,NW
S44E,34,SW
S57E,68,NE
S31E,48,NE
N50W,48,NE
N50W,48,NE
S59E,69,NE
S54E,76,NE
S58E,78,NE
Quartzofelspathic Gneiss
S85E,71,SW
S60E,15,SW
S35E,49,SW
N84W,75,SW
N54W,69,SW
S39E,64,SW
S65E,90,SW
S56E,90,SW
S80W,85,SE
N49W,81,SW
N04E,70,SE
N10E,65,SE
N39W,81,NE
N84E,56,SE
N74E,65,SE
S59E,80,NE
N80W,90,NE
S39E,86,SW
S52E,54,SW
S34E,51,SW
N60W,66,SW
S76E,59,SW
S63E,44,SW
N86W,52,SW
S85W,60,NW
S90E,59,NE
N59W,54,NE
S76W,39,NW
N82E,19,NW
N73W,65,NE
S64E,76,SW
S48E,64,SW
S19E,55,SW
S68E,75,NE
S64E,52,NE
S42E,45,NE
S47E,61,NE
N66W,79,SW
N73W,90,SW
N68W,81,SE
N11W,59,NE
S74W,80,SE
S41W,59,SE
S76E,43,SW
N46W,84,NE
S73W,59,SE
N63E,18,SE
S26E,74,NE
S39E,82,SW
S60E,78,SW
S51E,65,SW
S34E,64,SW
N88E,46,SE
S54E,41,SW
N39E,39,SE
N90W,37,SW
N81E,44,SE
S41W,79,NW
S36E,29,SW
N64W,81,NE
S38W,85,SE
S30E,76,SW
S40E,75,NE
S59E,71,SW
S05E,85,SW
S35E,85,SW
S29E,49,SW
S11E,58,SW
S29E,69,SW
S20E,39,SW
S60E,53,SW
S49E,65,SW
S86E,49,SW
S54E,59,SW
N90W,32,SW
N56E,30,SE
S84E,40,SW
S66E,45,SW
S41E,84,NE
S77E,57,SW
S71E,56,SW
S65E,85,SW
S70E,60,SW
S54E,65,NE
S81E,64,SW
S40E,90,SW
S55E,86,NE
S51E,75,NE
S76E,61,SW
S60E,90,SW
S59E,90,SW
S59E,90,SW
S62W,60,SE
S59E,76,NE
S23E,34,SW
N44E,39,SE
N90W,S4,SW
S58E,85,SW
S75E,80,NE
N81W,63,NE
Triassic
Intrusive
N13E,65,NW
S46E,34,SW
N15E,74,SE
S22E,55,SW
N60W,72,SW
N74W,53,SW
S49E,83,SW
N37W,72,SW
S26E,61,SW
N53W,79,SW
S75E,73,SW
N56W,80,NE
S52E,77,SW
S61E,62,SW
S44E,55,NE
S24E,61,SW
N65W,90,NE
S35E,43,SW
S73E,80,SW
S67E,66,SW
N39W,51,NE
S39E,43,SW
N81E,55,SE
S77E,41,SW
N32E,54,SE
N75W,49,SW
S83E,46,SW
S75E,37,SW
S21E,66,SW
N74W,46,SW
N63W,79,NE
S05E,51,SW
S72E,76,SW
S00E,40,W
N85W,84,NE
N06E,32,NW
S86W,79,NW
S12W,26,NW
N70W,46,SW
S46W,29,NW
S71W,53,SE
S31W,30,NW
S70E,46,SW
S90W,24,SE
N71W,79,SW
S72W,35,NW
N72E,45,SE
N54E,34,NW
N90W,44,SW
S36W,36,NW
N77W,60,SW
S20E,71,SW
S85E,85,SW
S22E,62,SW
S11E,85,SW
S26W,17,SW
S46E,75,SW
S47W,29,NW
S65E,82,SW
N32W,36,SW
S65E,56,SW
S55E,61,SW
N59W,82,SW
N52E,46,SE
S74E,76,NE
N16W,16,NE
N60E,85,SE
S29E,51,SW
N76E,71,SE
S82E,34,SW
N85E,75,SE
S56E,48,SW
N65E,73,SE
S41E,61,SW
N87W,86,NE
N38E,25,NW
N59W,60,NE
S10E,43,SW
S50E,39,SW
S29E,55,SW
S27E,29,SW
S61E,40,SW
N27W,29,SW
S59E,90,SW
S09E,48,SW
S49E,70,SW
S15E,46,SW
S29E,79,SW
S23E,34,SW
S49E,88,SW
N51E,51,SE
S55E,54,NE
S49E,65,SW
S07E,61,SW
S70E,73,NE
S70E,66,NE
S50E,39,NE
N74W,81,NE
S76E,62,NE
N73W,79,NE
Appended Structural Data
Augen Gneiss
57
Faults
Biotite Gneiss
S80W,71,SE
S77E,81,NE
S49E,84,SW
N16W,90,SW
N39W,39,NE
N90W,44,NE
N62W,77,NE
N41E,81,SE
N00W,75,SW
S29W,89,NW
S24E,88,SW
S55W,62,NW
S59W,71,NW
N46E,60,NW
S82E,88,SW
N31E,88,NW
S82E,69,NE
S80E,80,SW
S74E,75,SW
S26E,64,NE
N90E,52,SE
N31E,87,SE
S46W,79,NW
S04W,82,NW
S35W,90,NW
N32E,90,NW
N36E,90,NW
S40E,71,NE
N79W,87,NE
N60E,81,SE
N25W,63,NE
S10E,71,SW
S20E,40,SW
N78E,80,NW
N84E,52,NW
S82W,76,SE
S84W,72,SE
N83W,80,NE
N78E,84,NW
S76E,76,SW
S63E,61,NE
S36E,61,NE
N61W,63,NE
S67E,61,NE
N85W,75,NE
S60E,85,SW
S85W,85,NW
N87W,75,NE
S85E,90,NE
S56E,69,NE
N62W,85,NE
N85W,86,NE
N62W,90,NE
N62W,90,NE
N76W,87,SW
N65W,90,NE
N39W,63,NE
N44W,84,SW
N79W,24,NE
S81W,28,NW
S65E,75,NE
S52E,89,SW
Quartzofelspathic Gneiss
N81W,80,NE
N61W,43,NE
N64W,80,SW
S61E,88,NE
N79W,78,NE
N61W,83,NE
N50W,75,NE
S78W,82,SE
S86E,72,NE
N88W,64,NE
S76E,60,NE
N76W,24,NE
S84E,90,SW
N51W,90,SW
N52W,85,NE
N76W,79,NE
Augen Gneiss
S54E,69,SW
S27E,53,SW
S74W,61,NW
S71E,62,NE
N71W,81,SW
N77W,90,NE
Triassic
Intrusive
S61E,64,NE
S59E,65,NE
Appended Structural Data Continued
58