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Geosphere
Oligocene Laramide deformation in southern New Mexico and its implications
for Farallon plate geodynamics
Peter Copeland, Michael A. Murphy, William R. Dupré and Thomas J. Lapen
Geosphere 2011;7;1209-1219
doi: 10.1130/GES00672.1
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Oligocene Laramide deformation in southern New Mexico
and its implications for Farallon plate geodynamics
Peter Copeland, Michael A. Murphy, William R. Dupré, and Thomas J. Lapen
Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204, USA
ABSTRACT
The Silver City Range in southwest New
Mexico contains Proterozoic basement rocks
that are overlain by a sequence of Paleozoic,
Mesozoic, and Paleogene strata. These rocks
are folded in a broad, NW-SE–trending, eastfacing monocline that lies structurally above
an east-directed thrust fault. The youngest
rocks folded in the Silver City monocline are
similar to other late Eocene and early Oligocene volcanic rocks of the Mogollon-Datil volcanic field; an ash-flow tuff near the bottom
of the volcanic sequence gives an 40Ar/39Ar age
on sanidine of 34.9 ± 0.4 Ma (2σ), and another
tuff near the top of the section contains zircons
that yield a weighted 206Pb/238U age of 34.6 ±
0.6 Ma (2σ). We interpret similar structures in
the Little Burro Mountains, Lone Mountain,
and Bayard area, immediately east and west
of the Silver City monocline, to all be genetically related to a system of basement-involved
thrust faults. Modeling of these structures
from the Mangas Valley in the southwest to
the Mimbres Valley in the northeast, suggests ~17% total shortening. We conclude that
Laramide shortening was active in southwest
New Mexico generally, and the Silver City
region in particular, from the Cretaceous until
the earliest phase of Mogollon-Datil volcanism
beginning at ~37 Ma, during which time the
earliest extension in the southern Rio Grande
rift was initiated. The final stage of Laramide
shortening, recorded in the Silver City monocline, took place during a lull of volcanism
(and extension) from ~31.5 to ~29.3 Ma. We
explain the contemporaneity of shortening,
significant ignimbrite eruptions, and crustal
extension as the consequence of intermittent
slab breakoff and renewed underthrusting of
the downgoing Farallon plate.
INTRODUCTION
The timing and location of Cenozoic crustal
shortening, magmatism, and extension in the
western U.S. has long been interpreted to be
due to interactions between the North American and Farallon plates (e.g., Dickinson and
Snyder, 1979; Severinghaus and Atwater, 1990),
which are consistent with geodynamic models
(e.g., Bird, 1988). Spatial and temporal patterns
of magmatism (Coney and Reynolds, 1977)
have led to several competing ideas, such as
slab rollback, delamination, and buckling, to
explain removal of the Farallon slab beneath
North America (Humphreys, 2009). When and
where these processes may have occurred hinge
on information on the timing of cessation of
Laramide shortening and its temporal relationship to magmatism. In some work (e.g., Rupert
and Clemons, 1990), Tertiary volcanic rocks are
used as a horizontal reference to retrodeform
the effects of Basin and Range faulting; in this
view, volcanism everywhere postdates shortening. Results from our study show that this view
is invalid for the region around Silver City, New
Mexico, where the Basin and Range, Colorado
plateau, and Mogollon-Datil volcanic field
come together.
GEOLOGY
The Silver City Range, a NW-SE–trending topographic high ~30 × 15 km, is immediately west of the town of Silver City, New
Mexico (Fig. 1); we have mapped this range at
1:24,000 (Plate 1; Copeland et al., 2010). The
rocks there include Proterozoic granite and
metamorphic basement overlain by ~800 m of
Cambrian through Pennsylvanian sandstones
and limestones (Mack, 2004a; Pope, 2004;
Kues, 2004; Armstrong et al., 2004). These are,
in turn, unconformably overlain by the Cretaceous Beartooth Sandstone and Colorado Shale
(~200–400 m thick; Nummedal, 2004). The
Cretaceous rocks are overlain with very slight
angular unconformity by an Eocene sequence
(up to 900 m?) of undifferentiated felsic crystalrich ash-flow and air-fall tuffs and interbedded
volcaniclastic sandstones and conglomerates.
These are capped with angular unconformity by
Neogene basalt and basaltic andesite lava flows
and up to 300 m of semiconsolidated conglom-
erates, sandstones, and minor mudstones of the
Gila Conglomerate (Copeland et al., 2010). Cretaceous strata lie unconformably on basement to
the southwest in the Little Burro Mountains due
to the removal of the Paleozoic rocks from the
area centered on the Burro Mountains.
All of these rocks are exposed in an eastfacing monocline with moderately dipping
(10°–20°) beds in the southwest (backlimb) and
steeper (45°–85°) dipping beds in the northeast
(forelimb) (Plate 1). The dip of the forelimb
increases from northeast to southwest along
strike of the axial trace. A top-to-east thrust
(southwest-dipping) fault is exposed at the base
of the forelimb in the southern portion of the
Silver City Range (Plate 1; Hildebrand et al.,
2008; Copeland et al., 2010).
Two orientations of normal faults are present
throughout the range: (1) NE-trending faults and
(2) NW-trending faults (Plate 1). The NE-trending set of normal faults strike perpendicular to
the trend of the monocline and dip toward the
NW and SE. Individual faults can be traced for
2–3 km and have displacements ranging from a
few meters to a few hundred meters. This fault
set is located near the hinge of the monocline
between the steeply dipping forelimb and shallowly dipping backlimb. Faults commonly merge
with each other along strike and in the downdip
direction. They do not extend across the Silver
City Range, but instead tip out on the western
side of the crest of the range. Range-parallel
cross sections show that the NE-SW–trending
normal faults are restricted to shallow structural
depths within the monocline and are somewhat
evenly spaced, between 300 and 600 m.
The NW-trending set of normal faults is
located along the western flank of the Silver
City Range. The longest of these faults (the
westernmost fault along cross section E–E′)
dips steeply to the west toward the Mangas Valley, a basin interpreted to have developed as a
result of Neogene extension (Mack, 2004b).
This fault juxtaposes the Cretaceous Colorado
Shale in its hanging wall against Proterozoic
basement rocks in its footwall. The map pattern
in the southern part of Treasure Mountain (near
Geosphere; October 2011; v. 7; no. 5; p. 1209–1219; doi:10.1130/GES00672.1; 8 figures; 2 tables; 1 plate.
For permission to copy, contact [email protected]
© 2011 Geological Society of America
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Copeland et al.
N
Figure 1. Study location and surrounding areas. Blue labels are mountain ranges: AH—Alamo Hueco Mountains; BH—Big Hatchet
Mountains; CH—Coyote Hills; CR—Cooke’s Range; GCH—Goodsite–Cedar Hills; GM—Grandmother Mountain; LB—Little Burro
Mountains; LM—Lone Mountain; OM—Organ Mountains; PM—Pyramid Mountains; SCR—Silver City Range; TH—Tres Hermanos
Mountains; VM—Victorio Mountains. Red labels are basins: Mim—Mimbres Valley; Mag—Mangas Valley. White labels are cities: B—
Bayard; D—Deming; H—Hatch; LC—Las Cruces; L—Lordsburg; P—Playas; SC—Silver City. Green labels show approximate location
of calderas erupted from 36 to 33 Ma (after Chapin et al., 2004a): A—Animas Peak caldera (33.5 Ma); E—Emory caldera (34.9 Ma); J—
Juniper caldera (33.5 Ma); M—Muir caldera (35.2 Ma); O—Organ caldera (35.8 Ma); S—Steins caldera (34.4 Ma); SM—Schoolhouse
Mountain caldera (33.5 Ma); T—Tullous caldera (35.1 Ma). Red lines show approximate location of Laramide-style shortening structures
(after Drewes, 1981; Seager, 2004; Copeland et al., 2010). Dashed purple lines show proposed boundaries of Seager (2004) of Laramide
basins and uplifts: LHTB—Little Hat Top basin; HU—Hidalgo uplift; SRB—Skunk Ranch basin; LU—Luna uplift; KB—Klondike basin;
PU—Potrillo uplift; PB—Potrillo basin. Yellow line shows location of Plate 1.
cross section E–E′) shows this fault has ~2.5 km
of throw.
The Silver City monocline, which has been
recognized for almost a century, has characteristics of classic Laramide shortening structures,
including basement involvement. The monocline was subdivided by Paige (1916) into three
distinct segments: the Greenwood and Silver
City segments, to the NE and SW, respectively,
of the ~NE-SW normal fault with the largest
displacement in the center of our map area,
and the Lone Mountain segment, ~10 km SE
of Silver City. The fold is exposed over an area
~5 km wide and ~40 km long, when extended
to Lone Mountain. Paige (1916) also described
a monocline in the Little Burro Mountains,
~10 km southwest of Silver City.
Cather (1990, 2004) suggested that ~36 Ma
marked the transition from regional shortening
to extension in central New Mexico. Price and
Henry (1984) suggested a similar transition
1210
took place in the Big Bend region of Texas at
~32 Ma. The cessation of Laramide-style deformation generally gets younger from Wyoming
to New Mexico (Dickinson et al., 1988), so a
transition from shortening to extension in the
Silver City Range after 36 but before 32 Ma
would fit within this trend. However there is no
particular reason to expect such a gross trend to
hold at every scale, nor should a trend in Wyoming and Colorado necessarily apply in New
Mexico and Texas.
As pointed out by Chapin et al. (2004b), estimating the initiation of extension could be based
on disparate timing data such as the change in
the composition of volcanic rocks, the first
normal faulting, the age of oldest sedimentary
deposits preserved in a rift, or the time of the
onset of rapid subsidence of rift basins. Obtaining estimates for the time of these events is not
always easy, and in central New Mexico these
several estimates would span a range of 20 mil-
Geosphere, October 2011
lion years (Chapin et al., 2004b). Despite these
difficulties, some estimates for the age of the
earliest extension in southwestern New Mexico exist that bear on the present discussion.
Mack (2004b) noted the linearity of 35.4 Ma
flow-banded rhyolite domes and dikes from
the Goodsight–Cedar Hills area around Hatch,
~100 km to the east of Silver City, and suggested that the magmatism was “concentrated
along incipient faults or fractures.” Mack et al.
(1994) interpreted the basin in which these
rhyolites formed to be a half graben and not a
volcanomagmatic depression (cf. Seager, 1973).
Thus, following the model of Mack et al. (1994),
the first extension in southwestern New Mexico
began ca. 35.5 Ma. Chapin et al. (2004b) also
suggested significant subsidence in the Goodsight–Cedar Hills half graben was tectonic and
not magmatic. Outflow sheets of tuffs erupted
from the nearby Organ cauldron (Fig. 1), which
fill the space Chapin et al. (2004b) interpreted to
Plate 1. Geologic map and cross sections of the Silver City Range, SW New Mexico (after Copeland et al., 2010). This figure is intended to
be viewed at a size of 30 × 22 in. For the full-sized PDF file of the plate, please visit http://dx.doi.org/10.1130/GES00672.S1 or the full-text
article on www.gsapubs.org to view Plate 1.
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Oligocene Laramide deformation in southern New Mexico
Geosphere, October 2011
1211
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Copeland et al.
be the result of tectonic subsidence, are as old as
36.2 Ma (McIntosh et al., 1991) and suggest that
extension must be older still.
In the following, we present data that bear on
the timing of the convergence in southwestern
New Mexico.
GEOCHRONOLOGY
The youngest rocks folded in the Silver City
monocline are a sequence of volcanic (mostly
rhyolitic) and volcaniclastic rocks exposed
mostly in the NW part of the map area (Plate 1
and Fig. 2). Two of these units were analyzed
for geochronology at the University of Houston.
Sanidine was extracted from sample TR98,
a white, welded tuff with quartz, sanidine,
and minor biotite phenocrysts (collected at
32° 47.658′ N, 108° 21.900′ W) by standard
mineral separation techniques. This material
was sent to the Ford Nuclear Reactor at the
University of Michigan, where it was irradiated in position L-67 for 8 h. Interfering reactions were monitored by including in the irradiation package samples of CaF2 and a high-K
glass. Neutron fluence was monitored using
Fish Canyon Tuff (FCT) sanidine, assuming a
monitor age of 27.90 Ma (Cebula et al., 1986).
We are aware of other published estimates of
the age of FCT sanidine, which vary by as
much as 0.5% from the value we are using; the
uncertainty of our age assessment is ~1% and
assuming any other published age for FCT will
not have any effect on our tectonic conclusions
for the Silver City Range.
Unknowns and monitors were heated using a
CO2 laser to achieve fusion. Gas evolved from
this step was cleaned over a GP50 getter for 5
min before introduction into the MAP 215–50
mass spectrometer in static mode. Masses 40,
39, 38, 37, and 36 were analyzed over five
cycles, and t-zero intercepts were interpolated
based on the obtained data. All data in Table 1
are corrected for the decay of 37Ar and 39Ar
as well as the interfering reactions on Ca and
K. Uncertainty in the ages in Table 1 include
uncertainty in J. Other details of 40Ar/39Ar
analysis can be found in Herman et al. (2010).
The weighted average of 12 total fusions of
individual sanidine crystals from sample TR98
is 34.9 ± 0.4 Ma (2σ) (Fig. 3 and Table 1).
Sample FC52, a quartz-latitic ash-flow tuff
(collected at 32° 52.001′ N, 108° 24.853′ W),
was mapped by Finnell (1987) in the Cliff
quadrangle, just to the west of our field area, as
the ash-flow tuff of Greenwood Canyon (map
unit Tgwu). Zircon was extracted by standard
mineral separation techniques. Zircon grains
were analyzed by laser ablation–inductively
coupled plasma quadrupole mass spectrometry
1212
Figure 2. Outcrop from the Greenwood Canyon section of the Silver City monocline (view to
the west) with welded ash-flow tuff (left), unwelded tuff (center), and lahar (right) dipping
40° to the north.
TABLE 1. RESULTS OF 40AR/ 39AR DATING OF SINGLE SANIDINE CRYSTALS FROM SAMPLE TR98
40
39
Ar
Ar*
Age
2σ
36
37
40
Run ID
Ar/ 39Ar
Ar/ 39Ar
Ar*/ 39ArK
(moles)
(%)
(Ma)
±
(Ma)
29/1/12-03
0.00035
0.00566
17.358
1.989E-15
99.2
34.80
±
1.28
29/1/12-04
0.00175
0.00916
17.702
1.017E-15
97.0
35.48
±
1.38
29/1/12-05
0.00180
0.00632
16.791
1.894E-15
96.8
33.62
±
1.34
29/1/12-06
0.00366
0.00394
18.141
1.076E-15
94.3
36.35
±
1.44
29/1/12-07
0.00094
0.02320
17.143
7.107E-16
98.3
34.37
±
1.84
29/1/12-08
0.00022
0.02428
17.425
2.364E-15
99.5
34.93
±
1.32
29/1/12-09
0.00005
0.01846
17.584
2.116E-15
99.8
35.25
±
1.34
29/1/12-10
0.00009
0.01692
17.399
1.552E-15
98.4
34.88
±
1.34
29/1/12-11
0.00127
0.02501
17.217
1.917E-15
97.7
34.52
±
1.32
29/1/12-12
0.00155
0.02643
17.332
4.531E-16
97.3
34.75
±
2.70
29/1/12-13
0.00158
0.03248
17.400
1.640E-15
97.3
34.88
±
1.44
29/1/12-14
0.00062
0.01982
17.201
1.998E-15
98.8
34.48
±
1.36
Note: Weighted average: 34.9 ± 0.4 Ma. (36Ar/ 37Ar)Ca = 0.000109 ± 0.000065. (39Ar/ 37Ar)Ca = 0.000687 ± 0.000092.
(40Ar/ 39Ar)K = 0.020 ± 0.003.
(LA-ICPMS) using a Varian 810 ICPMS and
a Cetac LSX-213 laser ablation system. During the course of analysis, we used the 1096
± 1 Ma “FC5z” zircon standard to correct for
instrumental element and mass fractionation.
We ran the 564 ± 4 Ma Peixe standard as an
unknown relative to FC5z and obtained an average age of 562 ± 4.2 Ma (n = 5). The laser was
operated with a 50 μm diameter beam size at
a power of 2 mJ/4 ns pulse at a 20 Hz firing
rate. No common Pb corrections were applied
to these data because there was no measureable difference between intensities at mass 204
in the background and during ablation of the
sample. Full analytical procedures, including
data reduction, propagation of random and
Geosphere, October 2011
Figure 3. Relative probability diagram for
40
Ar/ 39Ar ages on sanidine single crystals
from TR98.
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Oligocene Laramide deformation in southern New Mexico
systematic errors, and mass spectrometry are
described in Shaulis et al. (2010).
Figure 4A shows photomicrographs of the
zircons after being shot with the laser; Figure
4B shows the relative probability plot for the
ages obtained from this analysis. The weighted
average 206Pb/238U age from 14 laser spots on
five different zircons is 34.6 ± 0.6 Ma (2σ) (Fig.
4B and Table 2).
A
STRUCTURAL MODELING
In order to offer a wider view of the structure
of the region, we combined the cross sections in
Plate 1 with data taken from Paige (1916) and
Hedlund (1978a, 1978b) from the Little Burro
Mountains, Skotnicki and Ferguson (2007) from
the Bayard quadrangle to the east, and Jones
et al. (1967) from the Santa Rita area to the east,
to compile a regional cross section showing the
upper crustal structure of the region between
the Mangas Valley to the SW and the Mimbres
Valley to the NE (Fig. 5). Figure 5A shows the
present-day geologic configuration, and Figure 5B
shows the same region palinspastically restored
to pre–major Basin and Range faults. Slip along
normal faults was restored by translating the faultbounding blocks parallel to the fault surfaces
until hanging wall and footwall cutoffs coincided. Figure 5C shows a reconstruction of the
Neogene extension allowing rotation of the fault
blocks. The Little Burro block in Figure 5A was
rotated 8° in order to make the contact between
the Eocene and Cretaceous rocks as horizontal
as possible. The western portion of the rest of
the section in Figure 5A was rotated 6°. This
magnitude of rotation allows for ~1 km of dip
slip along the normal fault bounding the Silver
City Range. The magnitude of fault-block rotation may be higher as shown by the attitudes of
the Eocene Tr in the northern part of the mapped
area in the Silver City Range (Plate 1). Bedding
in the Tr adjacent to the W-dipping normal faults
ranges from 35° to 15° to the NE. Assuming that
these dips are solely due to normal fault-related,
fault-block tilting results in 4.6–2.1 km of dip
slip, an amount broadly consistent with separations shown on cross section E–E′. The rotations
employed in constructing Figure 5C lessen the
regional structural relief as well as the dip of
the west limbs of the Silver City and Little Burro
monoclines when compared to Figure 5B, but
no block rotations will unfold the monoclines
seen along the west sides of the Little Burros and
Silver City Range.
Normal fault-related range tilting could have
occurred after or synchronous with deposition
of the Eocene strata. If such faulting occurred
after the deposition of these rocks, this section
would have uniform dips, contrary to observa-
206 Pb/ 238 U
weighted average
34.6 ± 0.6 Ma (2σ)
B
Figure 4. (A) Photomicrographs of zircons
analyzed from sample FC52 showing laser
pits and associated 206Pb/ 238U ages. (B) Relative probability diagram for 206Pb/ 238U ages
on zircons from FC52.
tion. If such faulting occurred in the Eocene,
during the deposition of the volcanic section
best exposed in the NW part of the range, this
could have produced a series of intraformational
angular unconformities within the Eocene volcanic section. However, that would mean the dip
of this section of rocks would get shallower with
decreasing age (it would also require a lot of the
Basin and Range structures here to be older than
34 Ma). However, as we move progressively farther to the east from the main range-front fault,
we see that the dip in the Eocene rocks goes
from ~10°N at the base to ~40°N in the middle
and back to ~15°N at the top of the section. The
variation in the attitudes of the Paleozoic rocks
in the SE portion of the range (with vertical dips
in some parts and dips as shallow as 15° in other
Geosphere, October 2011
parts—see in particular cross sections E–E′ and
F–F′ in Plate 1) is even more difficult to reconcile with an extensional model for the Silver
City monocline. We therefore reject the hypothesis that this structure was formed as a result of
Neogene or slightly older extension.
We wish to bring to the readers’ attention
several salient points regarding Figures 5B and
5C. First, the section is dominated by three
monoclines: from SW to NE, they are the Little
Burro Mountains and Silver City Range monoclines (both NE facing) and the Bayard monocline (SW facing). A broad synclinorium in the
Arenas Valley separates the Bayard and Silver
City monoclines. The structural relief between
the Little Burro monocline and the Arenas Valley
synclinorium is ~4 km in Figure 5B (no rotation)
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Copeland et al.
TABLE 2. RESULTS OF U-PB DATING OF ZIRCON CRYSTALS FROM SAMPLE FC52
206
U
Corrected
Uncertainty
Corrected
Uncertainty
Pb/ 238U age
2σ
207
206
FC52
(ppm)
(Ma)
Pb/ 235U
(%) (2σ)
Pb/ 238U
(%) (2σ)
(Ma)
c10.1
211
0.03550
4.68
0.00518
3.1
33.31
1.69
c10.2
205
0.03379
4.71
0.00519
3.0
33.37
1.67
c10.3
66
0.04301
7.14
0.00542
2.8
34.83
1.66
c26.1
336
0.04006
2.80
0.00541
1.6
34.80
1.26
c26.2
273
0.03842
4.13
0.00546
2.0
35.11
1.40
c26.3
188
0.04048
6.24
0.00561
2.3
36.09
1.55
C12.1
485
0.03620
2.85
0.00522
2.4
33.56
1.49
c12.2
293
0.03682
2.78
0.00556
1.6
35.77
1.29
c14.1
60
0.05263
8.52
0.00561
2.9
36.09
1.78
c14.2
81
0.04820
8.31
0.00553
2.5
35.55
1.61
c14.3
283
0.03726
5.34
0.00521
2.6
33.53
1.53
c14.4
349
0.03860
5.55
0.00514
2.8
33.04
1.58
C2.1
160
0.04853
7.07
0.00536
2.6
34.44
1.59
c2.2
126
0.03830
5.77
0.00541
2.1
34.80
1.42
Note: Weighted average of the 206Pb/ 238U age data is 34.6 ± 0.6 Ma.
207
Pb/ 235U
age (Ma)
35.42
33.74
42.75
39.89
38.28
40.29
36.11
36.72
52.08
47.80
37.15
38.46
48.12
38.16
2σ
(Ma)
2.34
2.24
3.85
1.89
2.32
3.27
1.73
1.74
5.36
4.83
2.69
2.87
4.29
2.93
SW
NE
Mangas Little Burro
Mountains
Valley
Silver City
Range
Arenas
Valley
Mimbres
Valley
Bayard
A Today
B Pre-Basin and Range, no rotation
10 km
C Pre-Basin and Range, with rotation
Figure 5. Regional cross section from the Mangas Valley to the Mimbres Valley compiled from data in Plate 1 as well as cross sections
reported by Paige (1916), Jones et al. (1967), and Skotnicki and Ferguson (2007). Symbols as in Plate 1 with the addition of Kab—a Cretaceous andesitic breccia (Skotnicki and Ferguson, 2007). (A) Present day; (B) restored to pre–Basin and Range conditions with translation
only along Neogene normal faults; (C) pre–Basin and Range restoration using rotation and translation (see text for details).
1214
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Oligocene Laramide deformation in southern New Mexico
and 3 km in Figure 5C (with rotation). Second,
the thickness of the Cretaceous rocks thins significantly from the center of the section to the
SW. This reflects nondeposition and erosion
of the Colorado Shale in the Burro Mountains,
~50 km to the SW and a general (eastward?)
deepening of the western interior seaway during
the Cenomanian (Nummedal, 2004; DeCelles,
2004). Third, we note that the section of Oligocene volcanic rocks is approximately flat lying
in the Bayard region (Skotnicki and Ferguson,
2007) but dips 15° to 40° to the NE on the SW
portion of the section in Figure 5B (see cross
sections A–A′ and B–B′ of Plate 1). This places
significant constraints for our estimates of the
timing of some of the shortening in the region.
Folding in the Bayard and Arenas Valley areas
must have been largely complete by ~35.2 Ma
based on the age of the essentially undeformed
Sugarlump Tuff (McIntosh et al., 1992). However, the folding in the Silver City Range,
including the Little Burro monocline, must at
least in part be younger than ~34.6 Ma (the age
we here report for our deformed sample FC52 at
the top of the volcanic section in the Silver City
monocline).
We used the software 2D Move™ to construct a 2-D forward model for the formation of
the structures shown in Figure 5C. We assumed
folding occurred due to slip along blind thrust
faults, and we used trishear fault-propagation
fold kinematics. We chose a contractional faultpropagation folding kinematic model because a
west-dipping thrust fault is exposed at the base
of the east-facing limb within the Silver City
monocline (Plate 1; Hildebrand et al., 2008;
Copeland et al., 2010). Although the surface
trace of the thrust fault terminates northward,
the monocline continues, implying that if the
monocline and thrust are related, the fault
should be present at depth to the northwest of its
surface trace. We recognize that monoclines can
form by extensional fault-propagation folding,
but this explanation seems inappropriate for the
Silver City monocline because no normal faults
have been observed in the field where conceptual models (e.g., Mitra, 1993; Schlische, 1995;
Hardy and McClay, 1999) predict them to be.
For an extensional origin for the Silver City
monocline, NE-dipping normal faults would
be expected proximal to the steep forelimb.
The only significant normal faults observed
in the region are SW-dipping faults associated
with the Mangas Valley between the Silver City
Range and the Little Burro Mountains, far from
the steepest limb of the monocline.
The geometry and location of the inferred
blind thrust faults are largely dictated by the
spatial extent and geometry of the forelimbs
and backlimbs of the Little Burro, Silver City,
and Bayard monoclines. No attempt was made
to model the formation of smaller-scale folds.
The results of our modeling are shown in Figure 6. Our initial state (stage 1) corresponds to
time of deposition of the Colorado Shale; an
angular unconformity separates the Cretaceous
rocks from the Paleozoic in the Silver City area
and points east. Jones et al. (1967) interpret the
Colorado Shale to be chronostratigraphically
equivalent to the Graneros Shale, which Eicher
(1965) interpreted to be entirely Cenomanian
(99.6–93.6 Ma, according to Ogg et al., 2008).
In stage 2 of our model, slip along a shallow east-directed thrust fault beneath the Little
Burro Mountains results in the formation of
the Little Burro monocline sometime after
deposition of the Colorado Shale. This results
in ~0.8 km of horizontal shortening across the
section. In stage 3, slip along a deeper eastdirected thrust fault to the NE of the fault that
moved in stage 1 causes the first phase of folding of the rocks now exposed in the Silver City
monocline. Approximately 0.5 km of shortening
occurs during this stage of the model. In stage
4, slip along a NE-directed thrust fault and SWdirected thrust fault results in formation of the
Bayard monocline in the NE, a slightly smaller
monocline immediately to the SW, and the intervening Arenas Valley synclinorium. The timing
of the formation of these thrusts and associated
monoclines (sequential, simultaneous, or alternating) is not specified, nor are the temporal
details in this portion of our model essential
to our conclusions. Approximately 0.8 km of
shortening occurs during stage 4. Stage 5 is the
only part of our model for which we have precise temporal information. During this stage, no
deformation takes place, but ~2 km of volcanic
and volcaniclastic material is deposited between
~35 and 34 Ma. During stage 6, which we argue
below occurred at perhaps 30 Ma, renewed slip
along the blind thrust underlying the Silver
City monocline results in 2000 feet of horizontal crustal shortening, which includes the late
Eocene–early Oligocene volcanic and volcaniclastic section (Tv). This modeling produces
~17% shortening from stage 1 to stage 6 (from
15.4 km to 12.7 km).
Although our modeling shows that the gross
structure of the section shown in Figure 5B can
be produced by movement along a sequence of
three NE-directed thrust faults in the SW and on
SW-directed thrust in the NE, some differences
exist between Figure 5B and stage 6 in Figure 6.
We don’t think these differences are significant,
primarily because our model does not include
erosion or the effects of the several Late Cretaceous to Paleogene intrusions seen throughout
the region (in particular in the Silver City Range
and Santa Rita area, Jones et al., 1967). We
Geosphere, October 2011
interpret the 20 or so small- to medium-scale,
NE-trending normal faults running across the
crest of the Silver City Range (Plate 1) to be the
result of flexure of the monocline into a doublyplunging structure; this flexure is also not present in our model.
DISCUSSION
Structure
The ages reported here are similar to ages of
other tuffs seen in the Bayard area to the east
of the Silver City Range (e.g., Sugarlump Tuff,
35.17 ± 0.17 Ma; Kneeling Nun Tuff, 34.89 ±
0.05 Ma; McIntosh et al., 1992). It is not clear
nor important to our following discussion if
any of the tuffs seen in the western part of the
Silver City Range are the same as these or any
other well-described unit in southwestern New
Mexico (additional candidates include the
Datil Well Tuff, the Doña Ana Tuff, the Bluff
Creek Tuff, and the tuff of Woodhaul Canyon,
which all issued from calderas in SW New
Mexico; Chapin et al., 2004a, 2004b; Fig. 1).
The 40Ar/39Ar and U/Pb data presented here
establish that rocks deformed in the Silver City
monocline are as young as 34.6 ± 0.6 Ma. Other
undated but younger rocks that lie stratigraphically above our sample FC52 are also folded in
the Silver City monocline (Plate 1). Approximately 6 km to the west of the FC52 sample
location, Finnell (1987) mapped the Bloodgood
Canyon Tuff (28.05 ± 0.04 Ma, McIntosh et al.,
1992) sitting on top of a sandstone unit dipping
30° to the north.
Basin and Range faulting likely resulted in
minor to moderate eastward tilting of rocks in the
Silver City Range and Little Burros. However,
the cross sections in Plate 1 strongly suggest that
the moderate to steep NE dips of the Cambrian
through Eocene strata are a result of pre–Basin
and Range shortening. We rule out the possibility that the Silver City monocline formed as a
result of slip along a normal fault because no
east-dipping normal faults within the forelimb
were recognized (Plate 1). Thus, based on the
failure of the extensional model and the presence of the southwest-dipping thrust fault in a
position consistent with the shortening hypothesis (Plate 1), we interpret that the Silver City
monocline is a consequence of shortening.
Laramide-style deformation has long been
recognized in other mountain ranges in southern New Mexico (e.g., Seager and Mack, 1986;
Seager et al., 1986; Chapin and Nelson, 1986;
Nelson and Hunter, 1986), but the youngest rocks involved in most of these structures
are Cenomanian or older, and therefore do not
provide significant limits on estimates of the
1215
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Copeland et al.
SW
Little Burro monocline
Bayard monocline
NE
Feet
meters
7700′
0
1000’
–1000
Silver city monocline
–4000
0
–2000
–8000
Renewed shortening
Stage 6
5700′
0
1000’
0
–1000
–4000
–2000
–8000
Figure 6. Forward structural
model showing the development
of structures shown in Figure
5B. Dotted lines are future faults.
Black lines are active faults.
Gray lines are inactive faults.
Color scheme is the same as in
Plate 1 and Figure 5: Blue—
Paleozoic; green—Cretaceous;
tan—Eocene and Oligocene.
Deposition of Tv
Stage 5
5700′
0
1000’
0
–1000
–4000
–2000
–8000
Development of Bayard monocline
Stage 4
4000′
0
0
–1000
–4000
–2000
–8000
Development of Silver City monocline
Stage 3
2500′
0
0
–1000
–4000
–2000
–8000
Development of Little Burro monocline
Stage 2
0
0
–1000
–4000
–2000
–8000
0
timing of shortening, other than “post–90 Ma,”
which doesn’t exclude any of the canonical
range of the Laramide orogeny (80–40 Ma).
Seager (2004) concluded that “Laramide contractile deformation culminated in southern
New Mexico during the Paleocene and early to
middle Eocene, as determined by the ages of
syn- to post-orogenic [sedimentary] deposits.”
Although our modeling suggests 17% shortening in the Paleocene through Oligocene, no
1216
Post-Cenomanian
Initial condition
8000
16000
24000
32000
Feet
significant clastic sequence of this age has been
identified as having been sourced by the rocks
now exposed in the Silver City area. Such material may have been deposited to the north and
now lie beneath the substantial deposits of the
Mogollon-Datil volcanic field. Conversely, it
may be that the Silver City area deformation lies
at the NW edge of the Potrillo uplift of Seager
(2004) and development of the basin may not
have been as pronounced in the center of the
Geosphere, October 2011
40000
48000
56000
uplift-basin pair in Luna and Doña Ana counties
to the southeast.
Our interpretation of the geology of the Silver City area is consistent with Seager’s (2004)
conclusion “that the southwestern New Mexico
crust failed under Laramide stresses primarily by breaking into a series of basement-cored
block uplifts and intermontane basins,” but the
Potrillo basin (as shown in figure 6 of Seager,
2004) cannot be extended along strike into our
Downloaded from geosphere.gsapubs.org on October 10, 2011
Oligocene Laramide deformation in southern New Mexico
Based on the structural style and timing, our
data indicate that some of the shortening in the
Silver City Range is younger than ~34.6 Ma. In
the following, we consider the tectonic variability in southwest and south-central New Mexico
in the late Eocene and early Oligocene (Fig. 7)
to put the deformation in the Silver City Range
in a regional context.
The Mogollon-Datil and Boot Heel volcanic
fields in New Mexico, the Trans-Pecos volcanic field in Texas, and the San Juan and Central Colorado volcanic fields in Colorado were
sites of significant volcanism between 37.5 and
23.5 Ma (Chapin et al., 2004a, 2004b). Volcanic
activity in these fields was more or less continuous save for two ~2 m.y. lulls in magmatic activity beginning at ca. 31.5 and 26.8 Ma (Chapin
et al., 2004b). For the purposes of our discussion
below we are assuming that Eocene and Oligocene volcanism occurred in a neutral, or more
likely, extensional stress field.
During the late Eocene and early Oligocene,
Laramide-style shortening, subduction-related
magmatism, and extension all played important
roles in the tectonic evolution of southwestern
New Mexico (Fig. 7), although it seems unlikely
all at the same time. Taking the well-dated
early phase of volcanism from 37.5 to 31.5Ma
(Chapin et al., 2004a, 2004b) as a starting point,
and noting the interpretation of early extension in the Hatch area at 36–35 Ma, we suggest
that the time of the first pulse of magmatism
was accompanied by minor extension and no
shortening. After this, SW New Mexico experienced a pause in volcanic activity, from ~31.5
to ~29.3 Ma. We suggest this lull in magmatism
coincided with the final stage of shortening
recorded in the Silver City monocline; if folding of the type shown in our model was active,
it seems reasonable that this would be a time
without significant extension. This then places
our estimate for the latest Laramide shortening in southwestern New Mexico to be within
the range of 31.5 to 29.3 Ma, entirely within the
Rupelian age of the Oligocene epoch. The only
aspect of the geology of Silver City Range that
restricts the possible range of youngest deformation is the age of nearly flat-lying basalts above
the Silver City monocline (Plate 1), which likely
Oligocene
40
Eocene
30
Oligocene
20
Eocene
Timing
Tectonic Implications
Time (Ma)
field area, given our interpretation of the structure of the Silver City area in Figure 5B. Recognition of Laramide shortening in the Silver City
region not present in Seager’s (2004) analysis
requires a truncation of the Potrillo basin and
the merging of the Potrillo and Rio Grande
uplifts (mostly beneath Oligocene and Miocene
volcanic cover).
50
Figure 7. Tectonic history of southwestern
New Mexico from 50 to 20 Ma. See text for
details.
correlate to similar late Miocene basalts in the
Mimbres valley (Faulkenberry, 1999), but consideration of regional tectonics above suggests
our more restricted estimate in the middle of
the Oligocene. Chapin et al. (2004b) suggested
“regional extension began rather haltingly”; we
conclude that the end of regional shortening was
similarly halting with a hiatus of ~2 m.y. preceding the final stage of shortening beginning
sometime after 31.5 Ma.
Hedlund (1978a, 1978b, 1980) has described
the Tertiary volcanic section in the Little Burro
Mountains and northernmost Burro Mountains
as consisting of a basal andesite overlain by
three significant ash-flow tuffs and associated
clastic rocks. The youngest of the tuffs, the tuff
of Wind Mountain, is reported to have a K-Ar
sanidine age of 27.1 ± 0.9 Ma (however, this
was only reported as “written communication”
from other workers in Hedlund, 1978a, 1978b).
Preliminary structural investigation in the Little
Burros is consistent with the interpretation of
Paige (1916) of a broad monocline that folds
the tuff of Wind Mountain. We know of no more
modern (U-Pb zircon or 40Ar/39Ar) geochronology for rocks in the Little Burros that would be
more trustworthy than this essentially unpublished K-Ar date. This date, if taken at face
value, in combination with the observed map
pattern, suggests tilting similar to that seen in
the Silver City Range (and therefore also probably related to development of a Laramide-style
monocline) continued into the late Oligocene
(Chattian). Thus, an additional shortening stage
may have occurred in southern New Mexico.
However, because of the uncertainty in the
geochronology of the Little Burros, we did not
extend the bar in Figure 7 corresponding to the
final stage of Laramide shortening past 29.3 Ma,
and left Figure 6 with only six stages.
Geosphere, October 2011
The intermittent switching between shortening, arc magmatism, and extension suggests a
tectonic model for the interaction of the North
American and Farallon plates in the southern Rockies and southern Basin and Range
(Fig. 8) in which flat subduction of the Farallon plate causes crustal shortening (plus perhaps strike-slip deformation) until ~37–36 Ma,
when the downgoing slab detached beneath
the Rocky Mountains, initiating the first stage
of Mogollon-Datil volcanic field ignimbrites
(McIntosh et al., 1992). Closely following the
initiation of regional volcanism, extension
begins in the southern Rio Grande rift at ~36 Ma
(Mack, 2004b; Chapin et al., 2004b). Following
this initial phase of volcanism, we hypothesize
continued underthrusting produced the shortening seen in the Silver City Range during the
interval 31.5 to 29.3 Ma. By 28 Ma, extension
dominated the structural style in southwest
New Mexico and the ignimbrite flareup was in
full bloom in Nevada (Dickinson, 2006), New
Mexico (McIntosh et al., 1992), and the Sierra
Madre Occidental (Ferrari et al., 2007) due to
rollback of the Farallon plate.
Our tectonic model (Fig. 8) is consistent with
the tomographic data of Schmid et al. (2002),
who concluded, “part of the [Farallon] plate at
depth did not move with the same convergence
velocity as the plate fragments at the surface.”
Sigloch et al. (2008) came to a similar conclusion, based on different tomographic data; however, we suggest a slightly different chronology
than Sigloch et al. (2008) based on our data.
The recognition of Oligocene Laramide
deformation in southwestern New Mexico
requires modifying the standard model of slab
rollback of the Farallon plate (e.g., Coney and
Reynolds, 1977; Atwater, 1989; Humphreys,
1995; Lawton and McMillan, 1999) by adding
more underthrusting of the subducting plate
after slab breakoff but before complete rollback (Fig. 8). Alternatively, sporadic episodes
of shortening and extension could be consistent with Humphreys’ (1995) model of slab
buckling. However, late-stage buckling would
not induce the same orientation of stresses as
seen during the previous stage of normal (and
flat) subduction, and the orientation of the
post–34 Ma deformation in the Silver City
Range is consistent with most of the Laramide
structures seen throughout southwest New
Mexico (Seager, 2004).
DeCelles et al. (2009) have noted a broad
cyclicity in Cordilleran orogenic systems in
which the end of prolonged shortening events
is accompanied by high-flux magmatic events.
Our data suggest that, in the local environment
1217
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Copeland et al.
~40 Ma
Sevier
CP
Laramide
Moho
Fa r
a llo n
sla b
~37 Ma
CP
Figure 8. Tectonic model, modified after Humphreys (2009) for
the late Eocene–early Oligocene. Cross sections are roughly
E-W at ~34°N. CP—Colorado
Plateau; RGR—Rio Grande
Rift; B&R—Basin and Range.
Inflowing
asthenosphere
Slab
breakoff
~31 Ma
CP
Laramide
Continuted
underthrusting
~28 Ma
B&R
CP
RGR
Slab
rollback
of southwest New Mexico, there was as much as
8 m.y. between the beginning of the ignimbrite
flareup seen in the Bootheel and Mogollon-Datil
volcanic fields in the Eocene and the end of shortening associated with the Laramide orogeny in
the late Oligocene.
ACKNOWLEDGMENTS
Much of the University of Houston’s field program in the Silver City area was built on a foundation of work by Max Carman and Carl Norman.
Alex Woronow, Kevin Cook, Sylveen Robinson, and
Wayne Ericson provided logistical support in the field.
Tim Lawton, Nadine McQuarrie, Jolante van Wijk,
Dan Miggins, and Terry Pavlis provided helpful comments and suggestions.
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REVISED MANUSCRIPT RECEIVED 7 JUNE 2011
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