Downloaded from geosphere.gsapubs.org on October 10, 2011 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 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geosphere Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. 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Notes © 2011 Geological Society of America Downloaded from geosphere.gsapubs.org on October 10, 2011 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. For permission to copy, contact [email protected] 1209 © 2011 Geological Society of America Downloaded from geosphere.gsapubs.org on October 10, 2011 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 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.