Oligocene Laramide Deformation in Southern New Mexico and Its Implications for Farallon Plate Geodynamics

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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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Notes

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 due to interactions between the North Ameri- erates, sandstones, and minor mudstones of the can and Farallon plates (e.g., Dickinson and Gila Conglomerate (Copeland et al., 2010). Cre- The Silver City Range in southwest New Snyder , 1979; Severinghaus and Atwater, 1990), taceous strata lie unconformably on basement to Mexico contains Proterozoic basement rocks which are consistent with geodynamic models the southwest in the Little Burro Mountains due that are overlain by a sequence of Paleozoic, (e.g., Bird, 1988). Spatial and temporal patterns to the removal of the Paleozoic rocks from the Mesozoic, and Paleogene strata. These rocks of magmatism (Coney and Reynolds, 1977) area centered on the Burro Mountains. are folded in a broad, NW-SE–trending, east- have led to several competing ideas, such as All of these rocks are exposed in an east- facing monocline that lies structurally above slab rollback, delamination, and buckling, to facing monocline with moderately dipping an east-directed thrust fault. The youngest explain removal of the Farallon slab beneath (10°–20°) beds in the southwest (backlimb) and rocks folded in the Silver City monocline are North America (Humphreys, 2009). When and steeper (45°–85°) dipping beds in the northeast similar to other late Eocene and early Oligo- where these processes may have occurred hinge (forelimb) (Plate 1). The dip of the forelimb cene volcanic rocks of the Mogollon-Datil vol- on information on the timing of cessation of increases from northeast to southwest along canic fi eld; an ash-fl ow tuff near the bottom Laramide shortening and its temporal relation- strike of the axial trace. A top-to-east thrust of the volcanic sequence gives an 40Ar/39Ar age ship to magmatism. In some work (e.g., Rupert (southwest-dipping) fault is exposed at the base on sanidine of 34.9 ± 0.4 Ma (2σ), and another and Clemons, 1990), Tertiary volcanic rocks are of the forelimb in the southern portion of the tuff near the top of the section contains zircons used as a horizontal reference to retrodeform Silver City Range (Plate 1; Hildebrand et al., that yield a weighted 206Pb/238U age of 34.6 ± the effects of Basin and Range faulting; in this 2008; Copeland et al., 2010). 0.6 Ma (2σ). We interpret similar structures in view, volcanism everywhere postdates shorten- Two orientations of normal faults are present the Little Burro Mountains, Lone Mountain, ing. Results from our study show that this view throughout the range: (1) NE-trending faults and and Bayard area, immediately east and west is invalid for the region around Silver City, New (2) NW-trending faults (Plate 1). The NE-trend- of the Silver City monocline, to all be geneti- Mexico, where the Basin and Range, Colorado ing set of normal faults strike perpendicular to cally related to a system of basement-involved plateau, and Mogollon-Datil volcanic fi eld the trend of the monocline and dip toward the thrust faults. Modeling of these structures come together. NW and SE. Individual faults can be traced for from the Mangas Valley in the southwest to 2–3 km and have displacements ranging from a the Mimbres Valley in the northeast, sug- GEOLOGY few meters to a few hundred meters. This fault gests ~17% total shortening. We conclude that set is located near the hinge of the monocline Laramide shortening was active in southwest The Silver City Range, a NW-SE–trend- between the steeply dipping forelimb and shal- New Mexico generally, and the Silver City ing topographic high ~30 × 15 km, is imme- lowly dipping backlimb. Faults commonly merge region in particular, from the Cretaceous until diately west of the town of Silver City, New with each other along strike and in the downdip the earliest phase of Mogollon-Datil volcanism Mexico (Fig. 1); we have mapped this range at direction. They do not extend across the Silver beginning at ~37 Ma, during which time the 1:24,000 (Plate 1; Copeland et al., 2010). The City Range, but instead tip out on the western earliest extension in the southern Rio Grande rocks there include Proterozoic granite and side of the crest of the range. Range-parallel rift was initiated. The fi nal stage of Laramide metamorphic basement overlain by ~800 m of cross sections show that the NE-SW–trending shortening, recorded in the Silver City mono- Cambrian through Pennsylvanian sandstones normal faults are restricted to shallow structural cline, took place during a lull of volcanism and limestones (Mack, 2004a; Pope, 2004; depths within the monocline and are somewhat (and extension) from ~31.5 to ~29.3 Ma. We Kues, 2004; Armstrong et al., 2004). These are, evenly spaced, between 300 and 600 m. explain the contemporaneity of shortening, in turn, unconformably overlain by the Creta- The NW-trending set of normal faults is signifi cant ignimbrite eruptions, and crustal ceous Beartooth Sandstone and Colorado Shale located along the western fl ank of the Silver extension as the consequence of intermittent (~200–400 m thick; Nummedal, 2004). The City Range. The longest of these faults (the slab breakoff and renewed underthrusting of Cretaceous rocks are overlain with very slight westernmost fault along cross section E–E′) the downgoing Farallon plate. angular unconformity by an Eocene sequence dips steeply to the west toward the Mangas Val- (up to 900 m?) of undifferentiated felsic crystal- ley, a basin interpreted to have developed as a INTRODUCTION rich ash-fl ow and air-fall tuffs and interbedded result of Neogene extension (Mack, 2004b). volcaniclastic sandstones and conglomerates. This fault juxtaposes the Cretaceous Colorado The timing and location of Cenozoic crustal These are capped with angular unconformity by Shale in its hanging wall against Proterozoic shortening, magmatism, and extension in the Neogene basalt and basaltic andesite lava fl ows basement rocks in its footwall. The map pattern western U.S. has long been interpreted to be and up to 300 m of semiconsolidated conglom- in the southern part of Treasure Mountain (near

Geosphere; October 2011; v. 7; no. 5; p. 1209–1219; doi:10.1130/GES00672.1; 8 ﬁ gures; 2 tables; 1 plate.

Copeland et al.

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 took place in the Big Bend region of Texas at lion years (Chapin et al., 2004b). Despite these of throw. ~32 Ma. The cessation of Laramide-style defor- diffi culties, some estimates for the age of the The Silver City monocline, which has been mation generally gets younger from Wyoming earliest extension in southwestern New Mex- recognized for almost a century, has character- to New Mexico (Dickinson et al., 1988), so a ico exist that bear on the present discussion. istics of classic Laramide shortening structures, transition from shortening to extension in the Mack (2004b) noted the linearity of 35.4 Ma including basement involvement. The mono- Silver City Range after 36 but before 32 Ma fl ow-banded rhyolite domes and dikes from cline was subdivided by Paige (1916) into three would fi t within this trend. However there is no the Goodsight–Cedar Hills area around Hatch, distinct segments: the Greenwood and Silver particular reason to expect such a gross trend to ~100 km to the east of Silver City, and sug- City segments, to the NE and SW, respectively, hold at every scale, nor should a trend in Wyo- gested that the magmatism was “concentrated of the ~NE-SW normal fault with the largest ming and Colorado necessarily apply in New along incipient faults or fractures.” Mack et al. displacement in the center of our map area, Mexico and Texas. (1994) interpreted the basin in which these and the Lone Mountain segment, ~10 km SE As pointed out by Chapin et al. (2004b), esti- rhyolites formed to be a half graben and not a of Silver City. The fold is exposed over an area mating the initiation of extension could be based volcanomagmatic depression (cf. Seager, 1973). ~5 km wide and ~40 km long, when extended on disparate timing data such as the change in Thus, following the model of Mack et al. (1994), to Lone Mountain. Paige (1916) also described the composition of volcanic rocks, the fi rst the fi rst extension in southwestern New Mexico a monocline in the Little Burro Mountains, normal faulting, the age of oldest sedimentary began ca. 35.5 Ma. Chapin et al. (2004b) also ~10 km southwest of Silver City. deposits preserved in a rift, or the time of the suggested signifi cant subsidence in the Good- Cather (1990, 2004) suggested that ~36 Ma onset of rapid subsidence of rift basins. Obtain- sight–Cedar Hills half graben was tectonic and marked the transition from regional shortening ing estimates for the time of these events is not not magmatic. Outfl ow sheets of tuffs erupted to extension in central New Mexico. Price and always easy, and in central New Mexico these from the nearby Organ cauldron (Fig. 1), which Henry (1984) suggested a similar transition several estimates would span a range of 20 mil- fi ll the space Chapin et al. (2004b) interpreted to

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Oligocene Laramide deformation in southern New Mexico le of the plate, please visit http://dx.doi.org/10.1130/GES00672.S1 or the full-text or le of the plate, please visit http://dx.doi.org/10.1130/GES00672.S1 Plate 1. Geologic map and cross sections of the Silver City Range, SW New Mexico (after Copeland et al., 2010). This fi gure is intended to gure This fi Copeland et al., 2010). New Mexico (after City Range, SW sections of the Silver Plate 1. Geologic map and cross fi the full-sized PDF be viewed at a size of 30 × 22 in. For to view Plate 1. article on www.gsapubs.org

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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 irradi- ated in position L-67 for 8 h. Interfering reactions were monitored by including in the irra- diation package samples of CaF and a high-K 2 Figure 2. Outcrop from the Greenwood Canyon section of the Silver City monocline (view to glass. Neutron fl uence was monitored using the west) with welded ash-fl ow tuff (left), unwelded tuff (center), and lahar (right) dipping Fish Canyon Tuff (FCT) sanidine, assuming a 40° to the north. 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 TABLE 1. RESULTS OF 40AR/39AR DATING OF SINGLE SANIDINE CRYSTALS FROM SAMPLE TR98 much as 0.5% from the value we are using; the 39Ar 40Ar* Age 2σ Run ID 36Ar/39Ar 37Ar/39Ar 40Ar*/39Ar (moles) (%) (Ma) ± (Ma) uncertainty of our age assessment is ~1% and K 29/1/12-03 0.00035 0.00566 17.358 1.989E-15 99.2 34.80 ± 1.28 assuming any other published age for FCT will 29/1/12-04 0.00175 0.00916 17.702 1.017E-15 97.0 35.48 ± 1.38 not have any effect on our tectonic conclusions 29/1/12-05 0.00180 0.00632 16.791 1.894E-15 96.8 33.62 ± 1.34 for the Silver City Range. 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 Unknowns and monitors were heated using a 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 CO2 laser to achieve fusion. Gas evolved from this step was cleaned over a GP50 getter for 5 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 min before introduction into the MAP 215–50 29/1/12-12 0.00155 0.02643 17.332 4.531E-16 97.3 34.75 ± 2.70 mass spectrometer in static mode. Masses 40, 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 39, 38, 37, and 36 were analyzed over fi ve 36 37 39 37 Note: Weighted average: 34.9 ± 0.4 Ma. ( Ar/ Ar)Ca = 0.000109 ± 0.000065. ( Ar/ Ar)Ca = 0.000687 ± 0.000092. cycles, and t-zero intercepts were interpolated 40 39 ( Ar/ Ar)K = 0.020 ± 0.003. 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 (LA-ICPMS) using a Varian 810 ICPMS and K. Uncertainty in the ages in Table 1 include a Cetac LSX-213 laser ablation system. Dur- uncertainty in J. Other details of 40Ar/39Ar ing the course of analysis, we used the 1096 analysis can be found in Herman et al. (2010). ± 1 Ma “FC5z” zircon standard to correct for The weighted average of 12 total fusions of instrumental element and mass fractionation. individual sanidine crystals from sample TR98 We ran the 564 ± 4 Ma Peixe standard as an is 34.9 ± 0.4 Ma (2σ) (Fig. 3 and Table 1). unknown relative to FC5z and obtained an aver- Sample FC52, a quartz-latitic ash-fl ow tuff age age of 562 ± 4.2 Ma (n = 5). The laser was (collected at 32° 52.001′ N, 108° 24.853′ W), operated with a 50 μm diameter beam size at was mapped by Finnell (1987) in the Cliff a power of 2 mJ/4 ns pulse at a 20 Hz fi ring quadrangle, just to the west of our fi eld area, as rate. No common Pb corrections were applied the ash-fl ow tuff of Greenwood Canyon (map to these data because there was no measure- unit Tgwu). Zircon was extracted by standard able difference between intensities at mass 204 mineral separation techniques. Zircon grains in the background and during ablation of the Figure 3. Relative probability diagram for were analyzed by laser ablation–inductively sample. Full analytical procedures, including 40Ar/39Ar ages on sanidine single crystals coupled plasma quadrupole mass spectrometry data reduction, propagation of random and from TR98.

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Oligocene Laramide deformation in southern New Mexico

systematic errors, and mass spectrometry are A 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 ﬁ ve different zircons is 34.6 ± 0.6 Ma (2σ) (Fig. 4B and Table 2).

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 206Pb/238U upper crustal structure of the region between the Mangas Valley to the SW and the Mimbres weighted average Valley to the NE (Fig. 5). Figure 5A shows the 34.6 ± 0.6 Ma (2σ) present-day geologic conﬁ guration, 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 fault- bounding blocks parallel to the fault surfaces until hanging wall and footwall cutoffs coin- B cided. Figure 5C shows a reconstruction of the Neogene extension allowing rotation of the fault blocks. The Little Burro block in Figure 5A was Figure 4. (A) Photomicrographs of zircons rotated 8° in order to make the contact between analyzed from sample FC52 showing laser the Eocene and Cretaceous rocks as horizontal pits and associated 206Pb/238U ages. (B) Rela- as possible. The western portion of the rest of tive probability diagram for 206Pb/238U ages the section in Figure 5A was rotated 6°. This on zircons from FC52. 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 tion. If such faulting occurred in the Eocene, parts—see in particular cross sections E–E′ and these dips are solely due to normal fault-related, during the deposition of the volcanic section F–F′ in Plate 1) is even more difﬁ cult to recon- fault-block tilting results in 4.6–2.1 km of dip best exposed in the NW part of the range, this cile with an extensional model for the Silver slip, an amount broadly consistent with separa- could have produced a series of intraformational City monocline. We therefore reject the hypoth- tions shown on cross section E–E′. The rotations angular unconformities within the Eocene vol- esis that this structure was formed as a result of employed in constructing Figure 5C lessen the canic section. However, that would mean the dip Neogene or slightly older extension. regional structural relief as well as the dip of of this section of rocks would get shallower with We wish to bring to the readers’ attention the west limbs of the Silver City and Little Burro decreasing age (it would also require a lot of the several salient points regarding Figures 5B and monoclines when compared to Figure 5B, but Basin and Range structures here to be older than 5C. First, the section is dominated by three no block rotations will unfold the monoclines 34 Ma). However, as we move progressively far- monoclines: from SW to NE, they are the Little seen along the west sides of the Little Burros and ther to the east from the main range-front fault, Burro Mountains and Silver City Range mono- Silver City Range. we see that the dip in the Eocene rocks goes clines (both NE facing) and the Bayard mono- Normal fault-related range tilting could have from ~10°N at the base to ~40°N in the middle cline (SW facing). A broad synclinorium in the occurred after or synchronous with deposition and back to ~15°N at the top of the section. The Arenas Valley separates the Bayard and Silver of the Eocene strata. If such faulting occurred variation in the attitudes of the Paleozoic rocks City monoclines. The structural relief between after the deposition of these rocks, this section in the SE portion of the range (with vertical dips the Little Burro monocline and the Arenas Valley would have uniform dips, contrary to observa- in some parts and dips as shallow as 15° in other 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 U Corrected Uncertainty Corrected Uncertainty 206Pb/238U age 2σ 207Pb/235U 2σ FC52 (ppm) 207Pb/235U (%) (2σ) 206Pb/238U (%) (2σ) (Ma) (Ma) age (Ma) (Ma) c10.1 211 0.03550 4.68 0.00518 3.1 33.31 1.69 35.42 2.34 c10.2 205 0.03379 4.71 0.00519 3.0 33.37 1.67 33.74 2.24 c10.3 66 0.04301 7.14 0.00542 2.8 34.83 1.66 42.75 3.85 c26.1 336 0.04006 2.80 0.00541 1.6 34.80 1.26 39.89 1.89 c26.2 273 0.03842 4.13 0.00546 2.0 35.11 1.40 38.28 2.32 c26.3 188 0.04048 6.24 0.00561 2.3 36.09 1.55 40.29 3.27 C12.1 485 0.03620 2.85 0.00522 2.4 33.56 1.49 36.11 1.73 c12.2 293 0.03682 2.78 0.00556 1.6 35.77 1.29 36.72 1.74 c14.1 60 0.05263 8.52 0.00561 2.9 36.09 1.78 52.08 5.36 c14.2 81 0.04820 8.31 0.00553 2.5 35.55 1.61 47.80 4.83 c14.3 283 0.03726 5.34 0.00521 2.6 33.53 1.53 37.15 2.69 c14.4 349 0.03860 5.55 0.00514 2.8 33.04 1.58 38.46 2.87 C2.1 160 0.04853 7.07 0.00536 2.6 34.44 1.59 48.12 4.29 c2.2 126 0.03830 5.77 0.00541 2.1 34.80 1.42 38.16 2.93 Note: Weighted average of the 206Pb/238U age data is 34.6 ± 0.6 Ma.

SW NE Mangas Little Burro Silver City Arenas Bayard Mimbres Valley Mountains Range Valley Valley A Today

10 km B Pre-Basin and Range, no rotation

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 Creta- ceous 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).

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Oligocene Laramide deformation in southern New Mexico and 3 km in Figure 5C (with rotation). Second, and Bayard monoclines. No attempt was made interpret the 20 or so small- to medium-scale, the thickness of the Cretaceous rocks thins sig- to model the formation of smaller-scale folds. NE-trending normal faults running across the nifi cantly from the center of the section to the The results of our modeling are shown in Fig- crest of the Silver City Range (Plate 1) to be the SW. This refl ects nondeposition and erosion ure 6. Our initial state (stage 1) corresponds to result of fl exure of the monocline into a doubly- of the Colorado Shale in the Burro Mountains, time of deposition of the Colorado Shale; an plunging structure; this fl exure is also not pres- ~50 km to the SW and a general (eastward?) angular unconformity separates the Cretaceous ent in our model. deepening of the western interior seaway during rocks from the Paleozoic in the Silver City area the Cenomanian (Nummedal, 2004; DeCelles, and points east. Jones et al. (1967) interpret the DISCUSSION 2004). Third, we note that the section of Oligo- Colorado Shale to be chronostratigraphically cene volcanic rocks is approximately fl at lying equivalent to the Graneros Shale, which Eicher Structure in the Bayard region (Skotnicki and Ferguson, (1965) interpreted to be entirely Cenomanian 2007) but dips 15° to 40° to the NE on the SW (99.6–93.6 Ma, according to Ogg et al., 2008). The ages reported here are similar to ages of portion of the section in Figure 5B (see cross In stage 2 of our model, slip along a shal- other tuffs seen in the Bayard area to the east sections A–A′ and B–B′ of Plate 1). This places low east-directed thrust fault beneath the Little of the Silver City Range (e.g., Sugarlump Tuff, signifi cant constraints for our estimates of the Burro Mountains results in the formation of 35.17 ± 0.17 Ma; Kneeling Nun Tuff, 34.89 ± timing of some of the shortening in the region. the Little Burro monocline sometime after 0.05 Ma; McIntosh et al., 1992). It is not clear Folding in the Bayard and Arenas Valley areas deposition of the Colorado Shale. This results nor important to our following discussion if must have been largely complete by ~35.2 Ma in ~0.8 km of horizontal shortening across the any of the tuffs seen in the western part of the based on the age of the essentially undeformed section. In stage 3, slip along a deeper east- Silver City Range are the same as these or any Sugarlump Tuff (McIntosh et al., 1992). How- directed thrust fault to the NE of the fault that other well-described unit in southwestern New ever, the folding in the Silver City Range, moved in stage 1 causes the fi rst phase of fold- Mexico (additional candidates include the including the Little Burro monocline, must at ing of the rocks now exposed in the Silver City Datil Well Tuff, the Doña Ana Tuff, the Bluff least in part be younger than ~34.6 Ma (the age monocline. Approximately 0.5 km of shortening Creek Tuff, and the tuff of Woodhaul Canyon, we here report for our deformed sample FC52 at occurs during this stage of the model. In stage which all issued from calderas in SW New the top of the volcanic section in the Silver City 4, slip along a NE-directed thrust fault and SW- Mexico; Chapin et al., 2004a, 2004b; Fig. 1). monocline). directed thrust fault results in formation of the The 40Ar/39Ar and U/Pb data presented here We used the software 2D Move™ to con- Bayard monocline in the NE, a slightly smaller establish that rocks deformed in the Silver City struct a 2-D forward model for the formation of monocline immediately to the SW, and the inter- monocline are as young as 34.6 ± 0.6 Ma. Other the structures shown in Figure 5C. We assumed vening Arenas Valley synclinorium. The timing undated but younger rocks that lie stratigraphi- folding occurred due to slip along blind thrust of the formation of these thrusts and associated cally above our sample FC52 are also folded in faults, and we used trishear fault-propagation monoclines (sequential, simultaneous, or alter- the Silver City monocline (Plate 1). Approxi- fold kinematics. We chose a contractional fault- nating) is not specifi ed, nor are the temporal mately 6 km to the west of the FC52 sample propagation folding kinematic model because a details in this portion of our model essential location, Finnell (1987) mapped the Bloodgood west-dipping thrust fault is exposed at the base to our conclusions. Approximately 0.8 km of Canyon Tuff (28.05 ± 0.04 Ma, McIntosh et al., of the east-facing limb within the Silver City shortening occurs during stage 4. Stage 5 is the 1992) sitting on top of a sandstone unit dipping monocline (Plate 1; Hildebrand et al., 2008; only part of our model for which we have pre- 30° to the north. Copeland et al., 2010). Although the surface cise temporal information. During this stage, no Basin and Range faulting likely resulted in trace of the thrust fault terminates northward, deformation takes place, but ~2 km of volcanic minor to moderate eastward tilting of rocks in the the monocline continues, implying that if the and volcaniclastic material is deposited between Silver City Range and Little Burros. However, monocline and thrust are related, the fault ~35 and 34 Ma. During stage 6, which we argue the cross sections in Plate 1 strongly suggest that should be present at depth to the northwest of its below occurred at perhaps 30 Ma, renewed slip the moderate to steep NE dips of the Cambrian surface trace. We recognize that monoclines can along the blind thrust underlying the Silver through Eocene strata are a result of pre–Basin form by extensional fault-propagation folding, City monocline results in 2000 feet of horizon- and Range shortening. We rule out the possibil- but this explanation seems inappropriate for the tal crustal shortening, which includes the late ity that the Silver City monocline formed as a Silver City monocline because no normal faults Eocene–early Oligocene volcanic and vol cani- result of slip along a normal fault because no have been observed in the fi eld where concep- clastic section (Tv). This modeling produces east-dipping normal faults within the forelimb tual models (e.g., Mitra, 1993; Schlische, 1995; ~17% shortening from stage 1 to stage 6 (from were recognized (Plate 1). Thus, based on the Hardy and McClay, 1999) predict them to be. 15.4 km to 12.7 km). failure of the extensional model and the pres- For an extensional origin for the Silver City Although our modeling shows that the gross ence of the southwest-dipping thrust fault in a monocline, NE-dipping normal faults would structure of the section shown in Figure 5B can position consistent with the shortening hypoth- be expected proximal to the steep forelimb. be produced by movement along a sequence of esis (Plate 1), we interpret that the Silver City The only signifi cant normal faults observed three NE-directed thrust faults in the SW and on monocline is a consequence of shortening. in the region are SW-dipping faults associated SW-directed thrust in the NE, some differences Laramide-style deformation has long been with the Mangas Valley between the Silver City exist between Figure 5B and stage 6 in Figure 6. recognized in other mountain ranges in south- Range and the Little Burro Mountains, far from We don’t think these differences are signifi cant, ern New Mexico (e.g., Seager and Mack, 1986; the steepest limb of the monocline. primarily because our model does not include Seager et al., 1986; Chapin and Nelson, 1986; The geometry and location of the inferred erosion or the effects of the several Late Creta- Nelson and Hunter, 1986), but the young- blind thrust faults are largely dictated by the ceous to Paleogene intrusions seen throughout est rocks involved in most of these structures spatial extent and geometry of the forelimbs the region (in particular in the Silver City Range are Cenomanian or older, and therefore do not and backlimbs of the Little Burro, Silver City, and Santa Rita area, Jones et al., 1967). We provide signifi cant limits on estimates of the

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SW Little Burro monocline Bayard monocline NE Feet meters 7700′ 0 1000’ 0

–1000 –4000 Silver city monocline –2000 –8000 Stage 6 Renewed shortening

5700′ 0 1000’ 0

–1000 –4000 –2000 –8000 Stage 5 Deposition of Tv Figure 6. Forward structural 5700′ model showing the development 0 1000’ 0 of structures shown in Figure –1000 5B. Dotted lines are future faults. –4000 Black lines are active faults. –2000 Gray lines are inactive faults. –8000 Stage 4 Development of Bayard monocline Color scheme is the same as in Plate 1 and Figure 5: Blue— 4000′ Paleozoic; green—Cretaceous; 0 0 tan—Eocene and Oligo cene. –1000 –4000 –2000 –8000 Stage 3 Development of Silver City monocline

0 2500′ 0

–1000 –4000 –2000 –8000 Stage 2 Development of Little Burro monocline

0 0

–1000 –4000 –2000 –8000 Initial condition Post-Cenomanian 0 8000 16000 24000 32000 40000 48000 56000 Feet

timing of shortening, other than “post–90 Ma,” signifi cant clastic sequence of this age has been uplift-basin pair in Luna and Doña Ana counties which doesn’t exclude any of the canonical identifi ed as having been sourced by the rocks to the southeast. range of the Laramide orogeny (80–40 Ma). now exposed in the Silver City area. Such mate- Our interpretation of the geology of the Sil- Seager (2004) concluded that “Laramide con- rial may have been deposited to the north and ver City area is consistent with Seager’s (2004) tractile deformation culminated in southern now lie beneath the substantial deposits of the conclusion “that the southwestern New Mexico New Mexico during the Paleocene and early to Mogollon-Datil volcanic fi eld. Conversely, it crust failed under Laramide stresses primar- middle Eocene, as determined by the ages of may be that the Silver City area deformation lies ily by breaking into a series of basement-cored syn- to post-orogenic [sedimentary] deposits.” at the NW edge of the Potrillo uplift of Seager block uplifts and intermontane basins,” but the Although our modeling suggests 17% short- (2004) and development of the basin may not Potrillo basin (as shown in fi gure 6 of Seager, ening in the Paleocene through Oligocene, no have been as pronounced in the center of the 2004) cannot be extended along strike into our

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ﬁ eld area, given our interpretation of the struc- Tectonic Implications ture of the Silver City area in Figure 5B. Recog- 20 nition of Laramide shortening in the Silver City The intermittent switching between shorten- region not present in Seager’s (2004) analysis ing, arc magmatism, and extension suggests a requires a truncation of the Potrillo basin and tectonic model for the interaction of the North the merging of the Potrillo and Rio Grande American and Farallon plates in the south- Oligocene 30 Oligocene uplifts (mostly beneath Oligocene and Miocene ern Rockies and southern Basin and Range volcanic cover). (Fig. 8) in which ﬂ at subduction of the Faral- lon plate causes crustal shortening (plus per-

Timing Time (Ma) 40 haps strike-slip deformation) until ~37–36 Ma, when the downgoing slab detached beneath Eocene Eocene Based on the structural style and timing, our the Rocky Mountains, initiating the fi rst stage data indicate that some of the shortening in the of Mogollon-Datil volcanic fi eld ignimbrites Silver City Range is younger than ~34.6 Ma. In 50 (McIntosh et al., 1992). Closely following the the following, we consider the tectonic variabil- initiation of regional volcanism, extension Figure 7. Tectonic history of southwestern ity in southwest and south-central New Mexico begins in the southern Rio Grande rift at ~36 Ma New Mexico from 50 to 20 Ma. See text for in the late Eocene and early Oligocene (Fig. 7) (Mack, 2004b; Chapin et al., 2004b). Following details. to put the deformation in the Silver City Range this initial phase of volcanism, we hypothesize in a regional context. continued underthrusting produced the short- The Mogollon-Datil and Boot Heel volcanic ening seen in the Silver City Range during the fields in New Mexico, the Trans-Pecos vol- correlate to similar late Miocene basalts in the interval 31.5 to 29.3 Ma. By 28 Ma, extension canic fi eld in Texas, and the San Juan and Cen- Mimbres valley (Faulkenberry, 1999), but con- dominated the structural style in southwest tral Colo rado volcanic fi elds in Colorado were sideration of regional tectonics above suggests New Mexico and the ignimbrite fl areup was in sites of signifi cant volcanism between 37.5 and our more restricted estimate in the middle of full bloom in Nevada (Dickinson, 2006), New 23.5 Ma (Chapin et al., 2004a, 2004b). Volcanic the Oligocene. Chapin et al. (2004b) suggested Mexico (McIntosh et al., 1992), and the Sierra activity in these fi elds was more or less continu- “regional extension began rather haltingly”; we Madre Occidental (Ferrari et al., 2007) due to ous save for two ~2 m.y. lulls in magmatic activ- conclude that the end of regional shortening was rollback of the Farallon plate. ity beginning at ca. 31.5 and 26.8 Ma (Chapin similarly halting with a hiatus of ~2 m.y. pre- Our tectonic model (Fig. 8) is consistent with et al., 2004b). For the purposes of our discussion ceding the fi nal stage of shortening beginning the tomographic data of Schmid et al. (2002), below we are assuming that Eocene and Oligo- sometime after 31.5 Ma. who concluded, “part of the [Farallon] plate at cene volcanism occurred in a neutral, or more Hedlund (1978a, 1978b, 1980) has described depth did not move with the same convergence likely, extensional stress fi eld. the Tertiary volcanic section in the Little Burro velocity as the plate fragments at the surface.” During the late Eocene and early Oligocene, Mountains and northernmost Burro Mountains Sigloch et al. (2008) came to a similar conclu- Laramide-style shortening, subduction-related as consisting of a basal andesite overlain by sion, based on different tomographic data; how- magmatism, and extension all played important three signifi cant ash-fl ow tuffs and associated ever, we suggest a slightly different chronology roles in the tectonic evolution of southwestern clastic rocks. The youngest of the tuffs, the tuff than Sigloch et al. (2008) based on our data. New Mexico (Fig. 7), although it seems unlikely of Wind Mountain, is reported to have a K-Ar The recognition of Oligocene Laramide all at the same time. Taking the well-dated sanidine age of 27.1 ± 0.9 Ma (however, this deformation in southwestern New Mexico early phase of volcanism from 37.5 to 31.5Ma was only reported as “written communication” requires modifying the standard model of slab (Chapin et al., 2004a, 2004b) as a starting point, from other workers in Hedlund, 1978a, 1978b). rollback of the Farallon plate (e.g., Coney and and noting the interpretation of early exten- Preliminary structural investigation in the Little Reynolds, 1977; Atwater, 1989; Humphreys, sion in the Hatch area at 36–35 Ma, we suggest Burros is consistent with the interpretation of 1995; Lawton and McMillan, 1999) by adding that the time of the fi rst pulse of magmatism Paige (1916) of a broad monocline that folds more underthrusting of the subducting plate was accompanied by minor extension and no the tuff of Wind Mountain. We know of no more after slab breakoff but before complete roll- shortening. After this, SW New Mexico expe- modern (U-Pb zircon or 40Ar/39Ar) geochronol- back (Fig. 8). Alternatively, sporadic episodes rienced a pause in volcanic activity, from ~31.5 ogy for rocks in the Little Burros that would be of shortening and extension could be consis- to ~29.3 Ma. We suggest this lull in magmatism more trustworthy than this essentially unpub- tent with Humphreys ’ (1995) model of slab coincided with the fi nal stage of shortening lished K-Ar date. This date, if taken at face buckling. However, late-stage buckling would recorded in the Silver City monocline; if fold- value, in combination with the observed map not induce the same orientation of stresses as ing of the type shown in our model was active, pattern, suggests tilting similar to that seen in seen during the previous stage of normal (and it seems reasonable that this would be a time the Silver City Range (and therefore also prob- fl at) subduction, and the orientation of the without signifi cant extension. This then places ably related to development of a Laramide-style post–34 Ma deformation in the Silver City our estimate for the latest Laramide shorten- monocline) continued into the late Oligocene Range is consistent with most of the Laramide ing in southwestern New Mexico to be within (Chattian). Thus, an additional shortening stage structures seen throughout southwest New the range of 31.5 to 29.3 Ma, entirely within the may have occurred in southern New Mexico. Mexico (Seager, 2004). Rupelian age of the Oligocene epoch. The only However, because of the uncertainty in the DeCelles et al. (2009) have noted a broad aspect of the geology of Silver City Range that geochronology of the Little Burros, we did not cyclicity in Cordilleran orogenic systems in restricts the possible range of youngest deforma- extend the bar in Figure 7 corresponding to the which the end of prolonged shortening events tion is the age of nearly fl at-lying basalts above fi nal stage of Laramide shortening past 29.3 Ma, is accompanied by high-fl ux magmatic events. the Silver City monocline (Plate 1), which likely and left Figure 6 with only six stages. Our data suggest that, in the local environment

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