Download Normal faulting in the upper plate of a metamorphic core complex

1
ABSTRACT
The northern Snake Range metamorphic core complex has been studied in depth
and numerous competing models have been proposed for its formation and evolution
(e.g., Wernicke, 1981; Miller and Gans, 1983; Lee, 1995). Most of these models have
utilized relations in lower plate rocks beneath the northern Snake Range decollement
rather than an in depth analysis of upper plate stratigraphy and faulting, which provides
important additional insights into how low-angle detachment faults form. This study
utilizes previously unpublished geologic mapping data from the Sacramento Pass and
Sixmile Canyon Quadrangles, an area of continuous exposure of upper plate geology,
preserved in part due to a younger down-to-the-west normal fault. Map relations indicate
at least two earlier generations of originally down-to-the-southeast normal faults that
repeat section and together tilt bedding 30 to 70 degrees to the northwest. Younger
second generation faults are spaced at least 1 km apart, dip between 20 and 30 degrees to
the E to SE, and display between 1 and 4 km of offset. These faults cut a closely spaced
and complex set of earlier first generation faults that currently dip 10 to 40 degrees to the
W to NW, frequently splay off each other, and display maximum total offsets between
3.5 and 6 km. A third class of previously unidentified faults with apparent strike-slip
offset, strike NW - SE and serve as accommodation faults between the highly extended
northern Snake Range and the less extended southern Snake Range.
These new maps and additional data highlight several problems and
inconsistencies with earlier simplified models for upper plate faulting in the northern
Snake Range that described two generations of rotating domino-style faults. Most
notably, the amount of rotation required to rotate faults to their present positions is not
consistently observed in the dips of the rock units involved in faulting, and the relative
amount of Cambrian Pole Canyon Limestone at the base of the upper plate increases from
west to east. The additional mapping and data suggest a new model for the evolution of
the northern Snake Range decollement that follows the rolling hinge model for the
development of metamorphic core complexes described by Buck (1988) and Wernicke
and Axen (1988). In this new model, first generation faults represent a complex series of
faults and fault splays related to an originally high angle master fault that soled listrically
into a brittle ductile transition at depth. Increased offset and the isostatically induced
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back-rotation of upper portions of the master fault lead to the formation of a series of
second generation faults that soled into steeper portions of the master fault and allowed
continued uplift of lower plate rocks along the master fault. This model explains the
increase in abundance of Cambrian Pole Canyon Limestone from west to east, and is
compatible with the tilts observed on fault planes and in stratigraphic units.
INTRODUCTION
Over the past twenty years, the term metamorphic core complex has been used to
describe uplifted and often domed exposures of metamorphic rocks that occur in highly
extended terranes. In the North American Cordillera, metamorphic core complexes occur
in regions that have experienced previous crustal thickening, such as behind the Sevier
thrust belt, and form a series of isolated metamorphic uplifts that extend in a belt from
Mexico to Canada (Fig. 1) (Coney and Harms, 1984). Metamorphic core complexes are
characterized by a subhorizontal to gently dipping or domed detachment fault that
separates unmetamorphosed and complexly faulted upper plate rocks that have undergone
brittle deformation from metamorphosed lower plate rocks that have deformed ductilely
during extension. Lower plate rocks, which range from slightly metamorphosed breccias
and protomylonites through mylonites and metamorphic tectonites, form by grain size
reduction and plastic flow of minerals such as quartz which begins to deform ductilely at
temperatures above about 300°C. The structural history and evolution of metamorphic
core complexes present many geologic and kinematic problems. To preserve mylonitic
fabrics in the lower plate, the rocks must cool quickly as deformation continues,
indicating that uplift and deformation are closely linked processes. However, the
mechanisms by which metamorphic rocks from the ductile crust are uplifted, brought to
the surface and placed in direct contact with brittlely fauled rocks are not clear. In
addition, rock fracture theory and modern earthquakes in extensional tectonic settings
indicate that motion along low-angle faults like the detachment faults of metamorphic
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Snake
Range
0
300
kilometers
Figure 1: Distribution of the major metamorphic core complexes in the Western
Cordillera with respect to the frontal trace of the Sevier Thrust Belt (black barbs) and the
Laramide uplifts (open barbs) (from Coney, 1979).
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core complexes is not only mechanically difficult and unlikely, but also rarely observed
in seismically active regions of contemporary extension (Jackson and White, 1989). As
more metamorphic core complexes are being described in the literature, these problems
have been addressed with several different kinematic models (Wernicke, 1981; Miller
and Gans, 1983; Buck, 1988; Wernicke and Axen, 1988; Brun, 1994; Miller et al.,
1999c).
The northern Snake Range, located in east-central Nevada in the Basin and Range
province and in the hinterland of the Sevier thrust belt (Fig. 2), is classic example of a
metamorphic core complex. Here, upper and lower plate rocks are separated by a gently
domed detachment, referred to as the northern Snake Range decollement (NSRD)
(Misch, 1960) (Fig. 3-4). Through the years, different researchers have studied the
northern Snake Range and used its geologic relationships to support various models for
the formation of metamorphic core complexes. Wernike (1981) originally proposed that
the NSRD was an example of a low-angle normal fault with up to 60 km of displacement.
Miller et al. (1983) noted the striking lack of stratigraphic omission across the
detachment, and suggested that the SRD was an exhumed brittle-ductile transition zone.
Buck (1988) and Wernike and Axen (1988) developed the concept of the rolling hinge
model for normal faults, in which detachment faults originate as listric high-angle normal
faults that rotate to horizontal through time as they are uplifted to the surface of the earth.
Lee (1995) suggested that the lower plate cooling history of the NSRD was compatible
with the rolling hinge model. Similarities between the northern Snake Range and sand
and silicone analogue models suggest metamorphic core complexes could be underlain
by regions of locally hot and possibly rising material as noted by noted by Brun (1994).
Miller et al. (1999c) suggested that the NSRD represents the top of a rising and extending
mass or welt of heated crustal rocks. Most of these ideas build on the deformational
history and strain geometry of lower plate rocks, placing less emphasis on relationships in
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114 00'
114 15'
39 45'
30
Deep
Creek
Range
Kern
Mtns
113 45'
Range
Snake Valley
n
usio
Con
f
39 00'
?
Southern
39 15'
Sacramento Pass
Transverse Zone
Snake
Range
39 30'
?
Snake
Range
Kern Mountains
Transverse
Zone
Northern
N. Spring Valley
?
Pleasant
Valley
Transverse
Zone
Figure 3: Snake Range Decollement Fault System
0
Footwall
Precambrian-Paleozoic strata and Mesozoic plutons
25
kilometers
Extent of Tertiary ductile deformation (dashes
show stretching lineations)
Hanging Wall
Tilted Tertiary fanglomerates
Upper Paleozoic + Tertiary strata
Paleozoic miogeoclinal units
Interpreted older portions of Snake Range decollement
Younger normal fault systems and interperted younger portions of
Snake Range decooement
Mesozoic thrust faults
Folds
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7
Northern
Snake Range
Schell
Creek Range
pC-C
0
Schell Creek
Fault
Spring
Valley
Confusion Range
Synclinorium
Snake Range
Decollement
Snake Valley
M-TR
C-D
pC-C
5
pC
pC-C
10
5
pC-C
Km
0
10
20
0
No vertical exaggeration
Figure 4: Cross-section through Snake Valley, the northern Snake Range, Spring Valley, and the Schell
Creek Range showing the northern Snake Range decollement cut by the Schell Creek Fault and extending
beneath the Snake Valley and the Confusion Range Synclinorium (from Miller and Dumitru, 1999).
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the upper plate that might provide significant insight into the development of the NSRD
and metamorphic core complexes. The northern Snake Range has perhaps the best
exposures of upper plate rocks of any core complex, and these rocks are characterized by
a straightforward and easily mappable stratigraphy. Thus the northern Snake Range is an
ideal location for a more in-depth analysis of upper plate faulting and its relation to the
evolutionary history of metamorphic core complex detachment faults.
The geology of most of the northern Snake Range has been mapped and published
as a series of seven 1:24,000 quadrangle maps (Miller et al., 1999a; 1999b; Gans et al.,
1999a; 1999b; Lee et al., 1999a; 1999b; 1999c) (see Plate I, Index map). This paper
builds on this previous mapping and includes new geologic data and maps covering the
Sacramento Pass Quadrangle and the southern part of the Sixmile Canyon Quadrangle
(Plate I). New geologic mapping by Elizabeth Miller, Cynthia Martinez and myself was
carried out on 1:24,000 scale topographic maps and airphotos, and this work in progress
will be published as USGS quadrangle geologic maps. The project was partially funded
by the EDMAP program within the USGS. The region covered by the new mapping
spans the geologically poorly understood transition zone between the highly extended
northern Snake Range and the less extended southern Snake Range. Good preservation
of upper plate units and their fault relations in the Sacramento Pass region are related to a
young west-dipping normal fault that cuts the NSRD and downdrops this region with
respect to the rest of the range (Fig. 5). In an attempt to better understand the geometry
of normal faults, two small areas were mapped in greater detail at 1:12,000 scale (Plate I).
In addition, this study identifies a series of NW - SE striking faults and fault zones with
apparent strike-slip offset that help accommodate differential strain between the northern
and southern Snake Ranges. The new data underscore several major problems and
inconsistencies with regard to earlier models for upper plate faulting, and have lead to the
development of a new model. This model is compatible with the rolling hinge model for
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Tertiary and Quarternary
Basin Fill
Cambrian to Permian
Upper Plate Rocks
Precambrian to Cambrian
Lower Plate Rocks
Young down-to-the-west fault
Other major fault sets
Northern Snake Range Decollement
0
1
Miles
Figure 5: Relative location and outcrop pattern of the young down-to-the-west fault.
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the formation of metamorphic core complexes, and suggests that similar processes may
have played a major role in the development of the NSRD.
GEOLOGIC SETTING
The stratigraphy of the northern Snake Range and adjacent ranges is characterized
by a relatively uniform15-km-thick miogeoclinal sequence of sediments that ranges in
age and lithology from upper Precambrian siliciclastic rocks to a Cambrian through
Triassic section that is dominated by carbonate rocks with lesser shale and quartzite.
Detailed descriptions of the lithologies found in the study area are taken from reports
completed by the Stanford Geological Survey and from descriptions in the published
geologic maps of adjacent quadrangles (Miller et al., 1999a; 1999b; Lee et al., 1999c;
Gans and Miller, 1983). These descriptions along with a stratigraphic column are
provided in the Appendix to this thesis.
During the late Mesozoic, several episodes of compression and crustal thickening
affected the region and were coeval with the emplacement of plutons in the Jurassic and
Late Cretaceous (Miller et al., 1989). Peak metamorphism and deformation of lower
plate rocks occurred in the Late Cretaceous and was accompanied by the intrusion of
crustally derived, two-mica granite plutons, metamorphic assemblages up through the
staurolite zone and the regional development of penetrative west-dipping cleavages
(Miller et al., 1989). The orientation of the cleavage and the presence of the plutons
suggest that the crust in this region was ductilely thickened during east-vergent simple
shear, a deep-seated equivalent of supracrustal deformation related to the Sevier thrust
belt (Miller et al., 1989). Unlike the Sevier thrust belt, however, the lack of significant
local or regional angular unconformities between Paleozoic and lower Tertiary rocks, as
well as the ability to reconstruct the entire stratigraphic section within late Cenozoic
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normal fault blocks, indicates that the upper crustal rocks in this region exhibit only
broad folds and small displacement along late Mesozoic thrust faults (Miller et al., 1989).
In the northern Snake Range, most of the Mesozoic metamorphic fabrics in lower
plate rocks are overprinted by Cenozoic metamorphism and deformation related to
extension and deformation along the NSRD. The NSRD forms part of a more extensive
fault system that extends about 150 km along strike from the Deep Creek Range in the
north through the northern and southern Snake Ranges, and has played a major role in the
extensional history of the area (Fig. 3) (Miller et al., 1999c). Of the three ranges, the
northern Snake Range is the most highly deformed with estimates of extension up to 330
- 500% (Gans and Miller, 1983) and is the only range that exposes vast tracts of highly
mylonitized rocks in a lower plate or footwall position with respect to the NSRD. The
NSRD is also inferred to extend underneath the Snake Valley to the east and die out
underneath the Confusion Range, and it is seismically imaged beneath Spring Valley to
the west where it is cut by the Schell Creek Fault (Fig. 4, Plate I: Index map) (Gans et al.,
1985). Two major extensional events in this region are recorded in the uplift and cooling
history of footwall rocks in the northern and southern Snake Ranges, and can be
separated from each other based on geologic relations, using 40Ar/39Ar dating techniques
and fission track analyses of apatites and zircons (Miller et al., 1999c). The first event
began in the late Eocene and continued through the early Oligocene, and is characterized
by a gradual migration of fault activity from west to east (Lee, 1995). The second event
occurred much more quickly at about 17 Ma, and is responsible for at least 12 –15 km of
rapid slip along the NSRD on the eastern front of the northern Snake Range (Miller et al.,
1999c). This second event is coeval with continued arching and doming of the western
portion of the NSRD as a result of reverse drag along the Schell Creek Fault, as well as
by down-to-the-west faulting and folding that uplifted the western front of the northern
Snake Range with respect to the adjacent valleys (Miller et al., 1999).
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In the southern half of the northern Snake Range, stratigraphic units in the lower
plate of the NSRD are mostly Precambrian through Cambrian siliciclastic rocks and the
overlying Cambrian Pioche Shale with occasional scraps of marble belonging to the
Cambrian Pole Canyon Limestone which overlies the Cambrian Pioche Shale. In the Old
Mans Canyon Quadrangle, large portions of the lower plate are comprised of mylonitized
Jurassic Silver Creek Granite and the Jurassic Old Mans Canyon Granite (Plate I) (Miller
et al., 1999a). All of these units exhibit layering or foliation that is parallel to the NSRD,
but they have been severely attenuated, and are increasingly deformed and thinned from
west to east across the range (Plate I, II A) (Lee et al., 1999c; Miller et al., 1999a; Lee et
al., 1987). Lineations associated with the metamorphic foliation in the mylonites of the
lower plate indicate that the principle stretching direction in the northern Snake Range
was N60W with evidence for a top to the east sense of shear increasing and becoming
more conspicuous toward the east (Miller and Gans, 1983). On average, stratigraphic
rock units in the lower plate have been reduced to one third of their original thickness,
which yields an average total extension of 330% upon restoration of these lower plate
rocks to their original thicknesses (Gans and Miller, 1983).
Extensional deformation in the upper plate of the NSRD is characterized by
multiple generations of highly complex NE-SW to N-S trending normal faults (Plate I).
These faults cut and deform Paleozoic rocks down through the Cambrian Pole Canyon
Limestone, but do not cut the Cambrian Pioche Shale or any of the Precambrian to
Cambrian siliciclastics below it. Although all stratigraphic units can be found throughout
the northern Snake Range, exposures of lower Paleozoic strata and especially the
Cambrian Pole Canyon Limestone become distinctly more abundant moving from west to
east across the range. As the different sets of faults evolved, they rotated and tilted the
upper plate Paleozoic strata to their current orientations dipping between 0° and 90°to the
W to NW (Gans and Miller, 1983). The oldest set of faults are closely spaced and
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presumably represent originally down-to-the-east normal faults that have since been
rotated into their current orientations dipping 10° to 30° to the W to NW (Gans and
Miller, 1983). More widely spaced younger faults, dipping 20° to 30° to the east and
abruptly soling into the NSRD, displace earlier generations of faults and upper plate rock
units down to the east (Gans and Miller, 1983). Various methods for calculating total
extension due to faulting in the upper plate of the NSRD have yielded estimates of
between 450 and 500 % extension of upper plate rocks in the northern Snake Range
(Gans and Miller, 1983).
GEOLOGY OF THE SACRAMENTO PASS AND SIXMILE CANYON
QUADRANGLES
The Sacramento Pass and Sixmile Canyon Quadrangles are located on the
southwestern flank of the northern Snake Range (Plate I). Highway 50 passes through
the southern part of the Sacramento Pass Quadrangle, and the area is easily accessible by
way of various Forest Service roads leading from Highway 50. The westernmost
exposures of the Tertiary Sacramento Pass Basin succession are exposed in the
southeastern corner of the Sacramento Pass Quadrangle. Here, Middle Miocene and
younger conglomerate, lacustrine deposits and rare volcanic rocks lie disconformably
upon upper Paleozoic rocks. This relationship is best exposed along Weaver Creek in the
southern part of the Old Mans Canyon Quadrangle (Plate I). The Tertiary section also
includes large breccia sheets composed of various Paleozoic lithologies that have been
interpreted as landslide deposits shed into the basin from the surrounding ranges
(Martinez, 1998; Grier, 1984). The Tertiary section has been tilted 30° to 60° to the
northwest and repeated along a series of rotated low-angle normal faults that sole into the
NSRD and can account for at least 200% extension of the basin (Miller, 1999a).
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As topography rises to the north from the low-lying region underlain by sediments
of the Sacramento Pass Basin, the resistant upper plate Paleozoic carbonates and shales
form a series of rugged ridges that characterize most of the study area. In part, the
spectacular preservation of upper plate rocks and their fault relations in this area are due
to a west-dipping, high-angle normal fault that extends along the east side of the
Sacramento Pass Quadrangle and into the Old Mans Canyon Quadrangle (Fig. 5, Plate I).
This fault could have been localized along the western boundary of the large Jurassic
granitic pluton that might have deformed more rigidly than the surrounding Precambrian
to Cambrian quartzite country rocks. Offset along this fault gradually dies out along
strike to the northeast. This fault cuts and offsets the NSRD, displacing upper plate rocks
at least 300 meters down to the west (Plate II A, B, C2, D), and as such, is a fault related
to the final doming and arching of the NSRD and the uplift and formation of the present
mountain range. Locally, the NSRD underwent normal drag associated with this fault
(Plate II A), and the rock units in the upper and lower plates may have experienced
significant amounts of normal drag near the fault.
In addition to this young, relatively minor, down-to-the-west fault, upper plate
rocks in the Sacramento Pass and Sixmile Canyon Quadrangles are cut by at least two
older generations of top to the southeast normal faults that repeat section and together tilt
bedding 30° to 70° to the northwest (Plate I, Figs. 6, 7). Younger second generation
faults are spaced at least 1 km apart, dip between 20° and 30° to the E to SE, and display
between 1 and 4 km of offset (Fig. 6). These faults cut a closely spaced and complex set
of earlier first generation faults that currently dip 10° to 40° to the W to NW, frequently
splay off each other, and display maximum total offsets between 3.5 and 6 km (Fig. 7).
A third class of faults within the study area strikes E–W to SE–NW and display apparent
strike slip-offset (Fig. 8). In the following paragraphs, I will discuss the nature and style
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Tertiary and Quarternary
Basin Fill
Cambrian to Permian
Upper Plate Rocks
Precambrian to Cambrian
Lower Plate Rocks
Second generation faults
Other major fault sets
Northern Snake Range Decollement
0
1
Miles
Figure 6: Distibution and map pattern of second generation faults.
2
16
Tertiary and Quarternary
Basin Fill
Cambrian to Permian
Upper Plate Rocks
Precambrian to Cambrian
Lower Plate Rocks
First generatinon Faults
Other major fault sets
Northern Snake Range Decollement
0
1
Miles
Figure 7
boxes.
2
17
Tertiary and Quarternary
Basin Fill
Cambrian to Permian
Upper Plate Rocks
Precambrian to Cambrian
Lower Plate Rocks
Accommodation faults
Other major fault sets
Northern Snake Range Decollement
0
1
Miles
Figure
sets in the upper plate of the northern Snake Range.
2
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of these three different sets of faults within the upper plate of the NSRD, beginning with
the second generation faults because they are the best exposed and most well understood.
The geometry of second generation faults are well constrained by geologic
mapping and are the most immediately obvious faults in cross-sections (Plate II A, B, C1,
C2, D). These faults displace upper plate rocks down to the east, and repeat section as
well as cut and offset first generation faults. The fault traces of the second generation
faults show that they are relatively planar faults, and vary in length from 2 km to faults
that span the entire study area, a distance of at least 11 km (Fig. 6, Plate I). Second
generation faults are relatively widely spaced with at least 1 km between faults, and
branches and splays are not common and are only observed in several locations (Fig. 6,
Plate I). Exposures of second generation faults along Sixmile Canyon (Plate I) indicate
that at depth, second generation faults sole abruptly into the NSRD, and truncated first
generation faults are often placed in direct contact with the NSRD (Plate II A). This
relationship suggests that slip along second generation faults was coeval with and related
to an additional component of slip along the NSRD. Because these faults flatten into the
subhorizontal NSRD, previous workers have interpreted these faults as originally high
angle normal faults that rotated through time to their present shallowly dipping
orientations as offsets along the faults grew (Gans and Miller, 1983). This interpretation
is consistent with the orientation of earlier sets of normal faults that have been rotated
through horizontal and the maximum dips of bedding, both of which can be achieved
through extensive rotation along second generation faults. However, the relatively
regular spacing of second generation faults, the abrupt intersections between the second
generation faults and the NSRD, as well as evidence for simultaneous slip along second
generation faults and the NSRD suggest that second generation faults within the study
area may have been a series of faults that formed sequentially from west to east in a
similar fashion to the fault slices predicted in the upper plate of the rolling hinge model.
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In this preferred interpretation, originally high-angle second generation faults soled into a
temporarily inclined NSRD, and rotated to their current shallow dips as a result of a
negative isostatic load applied with increased offset and uplift along the NSRD (Plate II
Inset: Fig. 16B, C). This interpretation is also consistent with the observed orientations
of upper plate strata and earlier fault generations, and is described in more detail in
subsequent discussions.
The map pattern of first generation faults is very complex and is hard to
generalize, but many important observations can still be made. Although first generation
faults dip shallowly to the northwest, because bedding dips more steeply to the northwest
than the faults, it is inferred after Gans (1983) that first generation faults were originally
down-to-the-southeast normal faults that were subsequently cut and rotated through
horizontal by younger faults. The first generation faults are very closely spaced,
sometimes only tens of meters apart, and along the westernmost ridge in the study area,
they intersect and splay off each other along strike (Fig. 7, Plate I, II A, B, C1, C2, D).
Although individual faults cannot always be followed along strike, groups of first
generation faults can be followed across the study area along strike and can be roughly
correlated with each other across second generation faults. The fact that first generation
faults do not match perfectly across the second generation faults suggests that their
geometry with abundant splays is complex along strike, and also that they may have
remained active even after they were cut by the younger set of faults (Gans and Miller,
1983). The first generation faults are approximately planar and display fault to bedding
angles between 10° and 30° (Plate II). These low fault to bedding angles are at least
partially the result of intense normal drag that rotated bedding between the closely spaced
faults into near parallelism with the faults (Gans and Miller, 1983), but these
relationships might also indicate that the first generation faults could have cut through
upper plate strata at angles significantly less than 60°, especially in lower parts of the
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stratigraphic section. The interaction between the first generation faults, as well as the
ambiguity with respect to original fault to bedding angles, makes it difficult to estimate
offset along the first generation faults. However, along cross section B-B’ (Plate II B)
several first generation faults intersect placing Permian Ely Limestone on Cambrian
Notch Peak Limestone, and bracket the maximum total offset along first generation faults
in this part of the study area between 6 km and 3.5 km assuming 30° and 60° fault to
bedding angles, respectively. First generation faults are best exposed in the southern part
of the Sixmile Canyon Quadrangle along cross-section A - A’ (Plate I, II A). Here, first
generation faults can be followed from the western side of the Sixmile Canyon
Quadrangle and into the northwest corner of the Old Mans Canyon Quadrangle where
they lie beneath an intact section of upper plate rocks at least 3 km thick that spans the
Upper Ordovician to the Permian (Plate I, II A). This wide stretch of unfaulted section
suggests that the closely spaced first generation faults may not have been penetrative
throughout the section in the northern Snake Range, and may represent one, or a series of
widely spaced fault zones or fault splays. These faults might have operated as distributed
zones of shearing in the lower part of the stratigraphic section, above and along the
evolving NSRD.
At lower structural levels in certain locations, first generation faults are more
difficult to map in the thick Cambrian section. Previous mapping showed some of these
faults to bound units whose dips indicated anomalously thick stratigraphic sections. In an
attempt to get a better understanding of this deformation, two different locations were
mapped in detail. The first location, just to the south of Miller Basin (Fig. 7-9), has
apparently been affected by a left-lateral, dip-slip accommodation fault that extends along
the northeast corner of the area (see discussion on accommodation faults below), and as a
consequence, stratigraphic contacts and strikes and dips of bedding have been bent and
folded from their more regional orientations. Because similar faulting patterns are
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Op
Op
OS
Dg
6
24
34
34
Opd
34
G'
Qol
38
26
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Oe
Qal
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OS
Op
42
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Opk
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Opb
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Oe
Cn
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Opc
40
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Opd
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26
34
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Opd
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Opc
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Opl
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Cn
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Opk
Opa
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29
45
25
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Cl
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OS42
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H
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Opl
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Cn
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G
Contour Interval 20 meters
0
Meters
H'
Cl
Opa
Opb10
34
48
34
19
20
Cpc
45
33
25
Oe
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Cn
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Opa
Opl
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500
1000
Figure 9: Detailed mapping at 1:12,000
scale near Miller Basin in the Sacramento
Pass Quadrangle. Here, stratigraphic
contacts are preserved and repeated
perpendicular to, and within a series of gently
west dipping first generation faults. An
accommodation fault with left-lateral downto-the-southwest sense of displacement is
mapped in the northeast corner of this
study area. For location, map explanation
and rock descriptions, refer to Plate I and
the Appendix.
22
observed in other locations in both quadrangles, it can be assumed that the original
structural relationships have been preserved despite this younger rotation. Here, a series
of rotated, top-to-the-east normal faults dip 10° to 40° to the west, and repeat intact
section that strikes roughly east-west within the fault slivers. As these faults rotated, a
series of faults with considerably less offset were also developed in the shaley Ordovician
Pogonip Group and Cambrian Lincoln Peak Formation lithologies stratigraphically above
and below the more resistant Cambrian Notch Peak Limestone (Fig. 9-10). These smaller
faults helped to alleviate space problems that were created on either side of the more rigid
Cambrian Notch Peak Limestone as offset along the larger faults grew.
Similar styles of faulting are observed along the southern side of Sixmile Creek
(Fig. 7, 11). In this area, the structurally lowest first generation fault dips 25° to the NW,
and exposes an intact section of SW-dipping upper Cambrian Notch Peak Limestone and
Ordovician Pogonip Group that has been downdropped onto stratigraphically lower
sections of the Cambrian Notch Peak Limestone (Fig. 11-12). Unlike the area near Miller
Basin, smaller displacement faults are not observed within the Op or Cl. However, a
series of faults with offsets on the scale of meters are well developed within more
resistant beds in the Cambrian Lincoln Peak Formation and suggest that space problems
in this area may have been accommodated through a suite of unmappable microfaults on
the meter scale or smaller. Immediately above this fault there are least three more closely
spaced first generation faults as documented by klippe of Ordovician Eureka Quartzite
and Ordovician Silurian Dolomite above the Ordovician Pogonip Group (Fig. 11-12).
Two tight folds in the Ordovician Pogonip Group display fold axes that strike 199° and
232° (Fig. 13A, B), roughly parallel to the strike of the first generation faults. This
deformation suggests that original bedding relationships may not be well preserved as a
result of shear and normal drag operating between the closely spaced first generation
23
G
G'
Opl
Oe
Oe
Opl
Opl
9000
Opk Opd
Opb
Opd
Opk
Opc
Opb
Opa
Cn
Opk
Opl
Opd
Opc
Opb
Opl
Opc
Opa
9000
Opk
Opb
Opa
Opd
Cn
Cn
Cn
Qol
Dg
8000
8000
Cl
?
Cl
Opk
Opd
9000
Oe
OS
Dse
Opl
Opk
Opd
8000
H'
Opl
H
Opc
9000
Opl
Opk
Opd
Opa
Opb
Cn
Opc
8000
Cl
Cl
No vertical exaggeration.
0
1000
Feet
Figure 10: Vertical cross-sections through the Miller Basin detailed study area in the Sacramento Pass Quadrangle.
2000
24
Cn
Cpi
Cpc
7
12
Cpc
10
28
7
6
28
22
Op
28
Qal
22
8
10
18
16
15
Cn
Oe
Os
Cpi
30
25
Cpi
21
Opa
Cl
32
51
26
Oe
Dsi
13
27
46
28
21
21
17
25
Cpc
Cpc
13
16
57
17
30
Opb
40
J
29
20
40
54
Os
Mc
Dg
24
18
54
56
22
54
26
Oe
17
14
69
76
10
36
39
35
Os
17
16
40
64
Dse
42
28
33
51
Os
Opk
34
31
43
Opl
Opk
12
36
25
70
59
15
39
16
30
49
Os
Cl
26
49
25
31
19
61
35
Dse
41
29
Oe
24
26
35
45
Opk
Opd
68
64
24
19
55
35
Figure 11: Detailed mapping at 1:12,000
scale near Sixmile Canyon in the Sixmile
Canyon Quadrangle. Here, stratigraphic and
fault contacts between the Cambrian Notch Peak
and the Ordovician Pogonip Group are observed
beneath a major set of first generation faults.
Mylonitized lower plate rocks outcrop in Sixmile
Canyon in the northern section of this study
area. For location, map explanation and rock
descriptions, refer to Plate I and the Appendix.
43
Opc
46
27
Qol
27
19
24
Opl
15
Os
Cn
Oe
44
30
Oe
20
Pe
42
39
27
46
28
Cn
Dse
Mc
44
Op
25
Contour Interval 20 meters
0
Meters
500
Mj
1000
J'
25
J
J'
Oe
9000
Mj
Pe
Mc
Opl
MDp
Cn
Cl
Opd
Cpc
Oe
Mc
Dg
9000
Os
NSRD
Opk
Opl
7000
Cpi
7000
Cpi
Cpm
Cpm
No vertical exaggeration.
0
1000
Feet
Figrue12: Vertical cross-section through the Sixmile Canyon detailed study area in the Sixmile Canyon Quadrangle.
2000
26
A.
B.
F.A. = 232, 38 SW
F.A. = 200, 16 SW
C.
D.
Figure 13: Structural data from the study area plotted on equal area stereonets.
A - Broad fold in Ordocician Pogonip Group, Unit D. B - Tight fold in Ordovician
Poganip Group, Lehman Formation. C - Poles to to fault plane surfaces preserved in
jasperoid along an accommodation fault zone. D - Trend and plunge of lineations on
fault surfaces.
27
faults. Although the exact nature of the first generation faults is still not fully understood,
this more detailed mapping emphasizes the complexity of their geometry.
First and second generation faults are both affected by a third set of faults that
strike NW – SE and show apparent strike-slip sense of displacement. These faults are
most prominent in map view (Fig. 8, Plate I), and are described here for the first time.
Apparently serving as accommodation faults that separate areas of the upper plate that
have experienced somewhat different amounts of extension, these accommodation faults
are observed on a variety of scales and occur where first generation normal faults end
abruptly or splay in slightly different directions (Fig. 8, Plate I). Stratigraphic units and
structures observed near these accommodation faults are often bent and dragged into
parallelism with the accommodation fault zones (Plate I). Because first generation faults
cannot be easily restored or traced across them (Plate II E), these accommodation faults
could have been synkinematic with the first generation faults, and some of them appear to
have remained active and also provided boundaries for the termination of second
generation faults. Two of the most conspicuous accommodation faults are observed in
the middle of the Sacramento Pass Quadrangle between cross-section lines B-B’ and D –
D’. Both faults juxtapose rock units in such a way as to suggest apparent left-lateral
offset and down-to-the-SW sense of slip (Plates I, II E). This type of offset is best
observed along the northernmost of these two faults. Here, first generation faults along
B-B’ place Permian rocks against the middle Cambrian section, while more widely
spaced fault splays along C-C’ to the south place Permian strata against Devonian strata
indicating at least 2 km less offset. As this fault system evolved, second generation faults
also splayed or branched off the accommodation fault to the south, and the entire system
was gradually rotated and reoriented to show the apparent left-lateral sense of
displacement that is presently observed (Plate I). The southernmost of these two
accommodation faults is characterized by massive exposures of red jasperoid that occur
28
along the length of the fault (Plate I). Lineations on small fault surfaces in the resistant
jasperoid are varied, but generally trend to the NE and SW with relatively high plunge
values, and suggest that the sense of shear in this accommodation fault zone was normal
and down-to-the-SW (Fig 13C, D).
Another major accommodation zone striking NW – SE that indicates left-lateral
and down-to-the-south sense of offset is exposed on the east side of the Sacramento Pass
Quadrangle (Plate I). To the east, this fault extends across the southern edge of the Old
Mans Canyon quadrangle where it serves as a major strike-slip fault juxtaposing upper
plate and lower plate rocks. Here, this accommodation fault acts as the northern
boundary of the Sacramento Pass Basin and as the terminus of the major faults that cut
the Sacramento Pass Basin and gradually merge into this strike-slip zone (Plate I). The
fact that the faults that cut the Sacramento Pass Basin merge with this accommodation
fault suggests that the two fault systems were synkinematic or operated together,
indicating that at depth, the NSRD may have remained active to the south of the
accommodation fault while extension along the NSRD to the north of the accommodation
fault had slowed or ceased. As this fault extends into the Sacramento Pass Quadrangle, it
juxtaposes mylonitized lower plate rocks to the north against unmylonitized Cambrian
Prospect Mountain Quartzite to the south (Plate I). The fact that Cambrian Prospect
Mountain Quartzite normally resides in a lower plate position, suggests that this fault
may have cut the NSRD and exhibits significant strike-slip offset. Further to the west,
offset along this fault decreases and is presumably transferred to the series of
accommodation faults discussed above, which suggests that they might have also cut the
NSRD. As the region to the west of the younger normal fault is covered by upper plate
rocks, the nature and location of the boundary between mylonitized and unmylonitized
rocks is not well understood. The southernmost fault belonging to this set of
accommodation faults lies concealed beneath Tertiary alluvium along Highway 50 (Plate
29
I). The existence of this fault is inferred by the fact that Cambrian carbonates to the north
of Sacramento Pass strike into sections of similarly striking Precambrian siliciclastic
rocks to the south of Sacramento Pass. Map scale drag of units as evidenced by rotation
of stratigraphic boundaries as well as by rotation of bedding indicate that this fault
displays a right-lateral sense of offset (Plate I). Although the kinematic history of these
accommodation faults that cut through the study area are not fully understood, the
geologic relationships as well as the fact that they are developed parallel to the direction
of maximum stretching indicate that they are related to differential amounts of extension
associated with the transition zone between the more extended northern Snake Range and
the less extended southern Snake Range.
KINEMATIC INTERPRETATIONS
Previous models for upper plate faulting in the northern Snake Range have
simplified the complex arrays of upper plate faults by describing them as two generations
of planar, rotating, domino-style faults that soled into a brittle-ductile boundary at depth
(Fig. 14, 15) (Miller and Gans, 1983; Gans and Miller, 1983). A closely spaced set of
first generation faults originated at high angles (60°) and rotated 30° to 40° before they
were cut by a second, more widely spaced set of second generation faults which, after
moving, accomplished an additional 30° to 40° of rotation. Together, the two sets of
faults produced the dramatic thinning observed in the upper plate, and resulted in total
extension values estimated between 450% and 500% (Fig. 14). Because both sets of
faults cut stratigraphic section down through, but no deeper than the Cambrian Pole
Canyon Limestone and there was no omission of stratigraphic units across the NSRD,
researchers supported an in situ stretching or pure shear model for the creation of the
NSRD (Miller and Gans, 1983; Gans and Miller, 1983).
30
Figure 14: Schematic models of extension in the northern Snake Range showing bedding
to fault relationships as a result of two generations of normal faults above a horizontal
NSRD. This model also illustrates the space problems that are created at the bases of these
faults as they rotate from high to low angles along a horizontal detachment that must be
compensated by splays or smaller scale faults (from Miller and Gans, 1983).
31
Figure 15: Interpretive cross-sections of the evolution of the northern Snake Range utilizing the model for two
generations of normal faults in the upper plate, and suggesting an overall in situ stretching or pure shear model for the
development of the NSRD (from Gans, 1985).
32
However, this model for faulting in the upper plate presents several major
problems. First, although 30° to 40° of rotation between normal fault generations is
mechanically possible (Nur et al., 1986), in the Singatse, Wassuck and Egan Ranges of
Nevada where this amount of rotation is observed between normal fault sets, the second
generation faults are typically younger, steeper, and more widely spaced Basin and Range
style faults, while first generation faults have not been rotated through horizontal (Proffet,
1977; Gans and Miller, 1983; Surpless, 1999). Thus, two sets of such closely spaced
normal faults as proposed by the model for the northern Snake Range are not commonly
observed. Second, because first generation faults within the study area dip between 10°
and 40° to the west, the existing model requires that first generation faults must have
rotated a total of 70° to 100°, but significantly lesser dips in the rock units involved in
faulting suggest that the units may not have been rotated by a comparable amount. This
discrepancy was previously explained by high amounts of normal drag along the first
generation faults that strained units into parallelism with the closely spaced first
generation faults, creating the low angles between faults and bedding that are currently
observed. However, even in the best preserved, least faulted and most resistant units of
the upper plate as observed in the northwest corner of the Old Mans Canyon Quadrangle,
stratigraphic units have only been rotated 60° to 70°. In contrast, in the Singatse and
Wassuck Ranges, comparable tilts ≥60° on rock units are achieved, even though first
generation faults have not been rotated through horizontal (Proffet, 1977; Surpless,
1999). Third, in order to thin the crustal column and uplift a flat brittle-ductile transition
zone, faults within individual fault sets must be closely spaced and synchronous across
the width of the uplifted zone (Buck, 1988). In the northern Snake Range, however,
thermochronologic and geochronologic data indicate an asymmetric uplift history with
fault activity gradually moving from west to east (Lee, 1995). In addition, although first
generation faults are closely spaced, there is no direct evidence that they were pervasive
33
over a region as wide as the current exposures of the NSRD. In fact, palinspastic
reconstructions of the upper plate of the northern Snake Range indicate that first
generation faults originally spanned only 1.7 km horizontally (Miller and Gans, 1983).
Fourth, because all upper plate faults are assumed to cut down through the Cambrian Pole
Canyon Limestone directly above the brittle-ductile transition zone, this model does not
explain the relative increase of Cambrian Pole Canyon Limestone exposures from west to
east across the range. Last, rigid fault block rotation above a flat detachment fault creates
increasingly significant space problems where the faults contact the detachment (Fig. 14).
In the northern Snake Range, space problems would be most prominent at the base of the
widely spaced second generation faults, and are accounted for by suggesting that less
resistant units were warped and folded, and that first generation faults were reactivated as
fault splays off the second generation faults. Although possible, this seems highly
unlikely as it requires that first generation faults remain active as normal splays even as
they are rotated into thrust position. In addition, neither deformation nor first generation
faults are consistently observed in the immediate hanging wall of second generation
faults.
The additional mapping and data presented in this paper, in conjunction with
previously published maps of the northern Snake Range suggest a new model for upper
plate faulting and the evolution of the NSRD. This model utilizes and builds on the
rolling hinge model of Buck (1988) and Wernicke and Axen (1988) as shown in Fig. 16
(inset in Plate II). This model calls for the origin of the NSRD as a master fault (MF) that
formed at high angles to bedding, and which soled in a listric fashion into a zone of
decoupling along a subhorizontal brittle-ductile transition (BDT) (Fig. 16 A). As
movement along the MF increased, a complex system of related faults and fault splays
developed synthetically to it, and soled in a listric fashion into the basal BDT to help
accommodate space problems created at depth (Fig. 16 B). Between these closely spaced
34
Plate II Inset: Figure 16:
Interpretive cross-sections showing the evolution of the NSRD through time.
Scale - no vertical exaggeration (km)
Scale - no vertical exaggeration (km)
0
0
8
4
Scale - no vertical exaggeration (km)
0
B.
A.
MF
MF
0
1
MF
Pe
Dg
Dg
Cpc
4
Cn
Brittle
Ductile
Cpm
Depth (km)
Pe
Dg
Cn
Cpc
Cn
Cpc
8
Scale - no vertical exaggeration (km)
8
4
Brittle
Ductile
Cpm
8
0
1 234
4
Brittle
Ductile
Cpm
8
8
0
Pe
4
4
C.
0
Depth (km)
Depth (km)
8
4
Scale - no vertical exaggeration (km)
12
0
8
4
12
16
20
E.
D.
2
1
MF
0
3456789
0
Pe
4
5
6
7
8
9
MF
Pe
Dg
Zone of
backrotation
Dg
Depth (km)
Depth (km)
3
2
1
Listric
Normal
Fault
4
Cn
4
Cn
Brittle
Cpc
Zone of ductile
flow and stretching
Cpm
Zone of ductile
flow and stretching
Cpc
Ductile
Cpm
8
Brittle
Ductile
8
Scale - no vertical exaggeration (km)
0
4
8
12
20
16
28
24
36
32
40
44
46
F.
9
Listric
Normal
Fault
0
1
8
2
3
MF
Schell
Creek
Fault
10
11
12
13
14
15
16
17
18
19
20
21
7
6
Pe
6
Depth (km)
Dg
5
4
4
Zone of
backrotation
Cn
Cpc
Cpm
8
Brittle NSRD
Zone of ductile
flow and stretching
Brittle
Ductile
35
faults, normal drag deformed and bent stratigraphic units to form low fault to bedding
angles (Fig. 16 B). Continued extension and thinning of the upper plate to the east of the
MF created a negative isostatic load on footwall rocks which lead to the gradual uplift
and backrotation of the structurally highest and westernmost portions of the MF (Buck,
1988; Wernicke and Axen, 1988). Eventually, as this section of the fault continued to
rotate, it reached a position unfavorable for slip, and the first in a series of younger
second generation faults formed and soled into a deeper section of the MF that remained
steep (Fig. 16 C). As displacement along this younger fault grew, upward truncations of
first generation splays slid down onto the deeper portions of the MF, while the upper
section of original MF and its associated faults and splays were tilted in the footwall of
the second generation fault, and continued to be rotated toward horizontal (Fig. 16 C, D).
It is likely that the lower portions of first generation faults and splays remained active
even after they had been cut by the second generation faults and possible that additional
first generation splays continued to form fanning out further onto the BDT (Fig. 16 D).
Like the original MF, the first second generation fault was gradually back-rotated by
negative isostatic load applied to the footwall, and a younger second generation fault was
formed at high angles soling into a more steeply inclined portion of the original MF zone
(Fig. 16 D). This process repeated itself many times, each time cutting through the first
generation faults with slip occurring along new second generation faults, the lower
portion of the original MF and along the basal BDT (Fig. 16 D, E, F). This model
requires that mylonitized lower plate rocks are brought into contact with upper plate
rocks in the footwall of the MF with increased slip along the fault system, and as the MF
rotates to horizontal, it is eventually exposed as the NSRD, separating upper and lower
plate rocks. In order to conserve rock unit volume in the footwall of the MF or NSRD,
rock units near the BDT are thinned to some extent, and a series of listric normal faults
are developed that tilt brittle rock units and migrate from west to east (Fig. 16 D, E). The
36
Schell Creek Fault represents the youngest of these listric normal faults as it cuts through
the western portion of the NSRD (Fig 16 D, E).
DISCUSSION
This model for the evolution of the NSRD presents several problems related to the
history of upper plate faulting. One major uncertainty lies in the mechanics and timing of
the accommodation faults, the affect they had on first and second generation faults, and
the role they played between the differentially extending northern and southern Snake
Ranges. This is partially the result of poor exposures and a lack of detailed mapping, and
as a result, a variety of interpretations for different offsets can be created depending on
the timing and on the position of rocks involved with respect to the NSRD. However, it
seems likely that major left-lateral down-to-the-south offsets observed throughout the
study area were created as sections of the NSRD to the north of the accommodation faults
became inactive due to rapid uplift and back-rotation, while sections of the NSRD to the
south of the accommodation faults continued to drop upper plate rocks along the MF
(Fig. 17). As lower plate rocks were brought to the surface in the footwall of the MF,
they cooled, became brittle, and were eventually affected and cut by the accommodation
faults as differential extension and uplift continued. This interpretation is consistent with
observations in the considerably less extended southern Snake Range that suggest a
similar style of extensional deformation along an original major down-to-the-west normal
fault and a series of back-rotating second generation faults (McGrew, 1993), and explains
the major left-lateral, down-to-the-south offsets observed at the northern boundary of the
Sacramento Pass Basin.
Another major uncertainty lies in the original coverage of the first generation
faults and fault splays, and the extent to which they interacted with and splayed off the
MF as well as second generation faults. Again, this uncertainty is related to the lack of
37
Slo
w
Ex
ten
sio
n
Ra
pid
Ex
ten
sio
n
North
Figure 17: A cartoon portraying a proposed mechanism for the formation of major accommodation faults in the upper plate of the northern
Snake
generati
the observed accommodation structures (shown in blue).
38
exposure and detailed mapping of upper plate rocks throughout the northern Snake
Range. Mapping at 1:12,000 scale revealed the complexity of upper plate faulting and
also pointed out the necessity for simplification even at this enlarged scale. These
simplifications lead to difficulties when generalizing relations and creating a model for
the evolution of the NSRD. At the same time, the model presented above is compatible
with the data currently available based on geologic mapping of the upper plate rocks, and
successfully addresses some of the problems that arose with earlier models for upper
plate faulting. Because this preferred model describes the NSRD as a master high-angle
normal fault that cut through upper plate stratigraphic units at angles near 60° and
subsequently rotated to subhorizontal orientations, the least deformed upper plate rocks in
contact with this fault would be expected to show dips only on the order of 60° to the
northwest. The interpretation of first generation faults as a series of lower-angle splays
from a master fault is compatible with an originally narrow horizontal zone of first
generation faults. Also, second generation faults would not cut first generation faults at
lower structural levels until several sets of second generation faults had run their course
and significant uplift had occurred, which explains the relative increase in the abundance
of Cambrian Pole Canyon Limestone outcrops from west to east across the range. This
model is also compatible with the migration of uplift along the NSRD from west to east,
and because second generation faults intersect the MF at low angles, they do not rotate
with respect to the NSRD, thus eliminating space problems that are developed at the
bases of rotating domino faults.
CONCLUSION
New mapping of excellently exposed upper plate rocks in the Sacramento Pass
and Sixmile Canyon Quadrangles has revealed several major problems with the in situ
pure shear model for extension that has previously been applied to upper plate rocks in
39
the northern Snake Range. Although more detailed mapping needs to be completed
before the mechanics of some of these faults are fully understood, the geologic relations
observed in the study area suggest that first generation faults represent a major system of
splays related to a MF. A more widely spaced set of second generation faults soled into
temporarily steep portions of the MF and formed sequentially as the MF was isostatically
uplifted and back-rotated to horizontal. In addition, a set of dip-slip accommodation
faults was formed as different sections of the NSRD experienced extension and uplift at
different rates. This model for upper plate faulting is compatible with the rolling hinge
model for the evolution of metamorphic core complexes. The interpretation of upper
plate faults as part of, and the consequence of a rolling hinge system, brings the analysis
of upper plate faulting in the northern Snake Range into agreement with related studies
on lower plate rocks that suggest the rolling hinge model (Lee, 1995), and is compatible
with models for the origin of the southern Snake Range decollement (McGrew, 1993)
that also indicate large amounts of isostatically induced vertical uplift.
40
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