Research Paper GEOSPHERE Complex structural and fluid flow evolution along the Grenville Front, west Texas GEOSPHERE; v. 11; no. 3 Ben R. Davis* and Sharon Mosher Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA doi:10.1130/GES01098.1 ABSTRACT occurred in the Llano area (ca. 1150–1120 Ma) and that continued subduction 24 figures; 1 table along strike caused clockwise rotation of the indenting continent and collision A narrow (~7 km wide) fold and thrust belt in west Texas that represents in west Texas (ca. 1060–980 Ma). CORRESPONDENCE: [email protected] the northernmost extent of a Grenville-age collisional belt along the south- ern margin of Laurentia (Grenville Front), records a complex history of defor- CITATION: Davis, B.R., and Mosher, S., 2015, Com- mation and associated fluid flow. The Streeruwitz thrust that emplaced ca. INTRODUCTION plex structural and fluid flow evolution along the Gren ville Front, west Texas: Geosphere, v. 11, no. 3, 1.35 Ga high-grade metamorphic rocks over ca. 1.25 Ga foreland sedimentary p. 868–898, doi:10.1130/GES01098.1. and volcanic rocks postdates polyphase deformation in the footwall and is Mesoproterozoic Grenville orogenesis (ca. 1.3–0.9 Ga) resulted in the complexly folded into domes and basins. Four phases of tectonism, recording formation of the supercontinent Rodinia (e.g., Dalziel et al., 2000) with the Received 1 July 2014 a changing kinematic setting, affected the area and formed: (1) pre-Streeru- Laurentian continental block in a central position. Grenville-age rocks crop Revision received 20 January 2015 witz ductile polyphase folds (F –F ) and associated foliations (S –S ) consis- out in a south-southwest trend from Canada’s Maritime Provinces through Accepted 26 March 2015 1 3 1 2 Published online 13 May 2015 tent with northward tectonic transport; (2) dextral oblique-slip, high-angle, the Appalachian Mountains into the subsurface and are also exposed in cen- west-northwest–trending faults associated with upright vertical sheath folds tral and west Texas. Numerous studies indicate that these Grenville-age rocks (F4); (3) Streeruwitz and related subsidiary imbricate thrusts that truncate underwent polyphase deformation associated with arc-continent and conti- F1–F4 folds at a high angle and cause localized folding (F5) consistent with nent-continent collision (e.g., see Tollo et al., 2004). One of the key debates north-northeast to northeastward tectonic transport; and (4) complex south- for Grenville collisional orogenesis hinges on which continent or continents east- and northwest-trending domes and basins (F6) of the thrusts, resulting collided with Laurentia during the Proterozoic, resulting in the assembly of from continual Grenville-age transpression. Rodinia (Hoffman, 1991; Dalziel, 1991; Karlstrom et al., 1999; Dalziel et al., Fluids with an evolving chemistry over time were channelized along the 2000; Tohver et al., 2002; Torsvik, 2003; Meert and Torsvik, 2003; Whitmeyer thrusts, metasomatically altering the adjacent rocks. Early siliceous fluids and Karlstrom, 2007; Li et al., 2008; Ibanez-Mejia et al., 2011). Integral parts of caused replacement of mafic dikes and dolostones that preserve 1F and S1 and testing Rodinia plate reconstructions are structural, kinematic, and geochro- formation of extensive talc bodies with talc aligned axial planar to F2, forming nologic analyses from key areas. the dominant S2 fabric. Initial thrusting at depth produced mylonites in both Two tectonic models have been proposed for the Grenville orogeny along footwall (syn-S2) and hanging-wall rocks that were later brecciated in the final the southern margin of Laurentia. One of us (Mosher, 1998) proposed that a stage of thrusting along the Streeruwitz thrust. Further evolution of fluids along southern continent acted as an indenter that collided with Laurentia between the thrusts is recorded in altered rocks adjacent to thrusts, breccias, and veins, the Llano uplift and the Van Horn region. This collisional model predicts north- starting with silica- and alkali-rich fluids. Lastly, carbonate-rich fluids replaced east tectonic transport for the Llano uplift and northwest tectonic transport for footwall rocks and cemented breccias in both the hanging wall and footwall. the Van Horn region of west Texas. In this model, a different continental block This study documents a previously unrecognized complex structural, collided with the eastern margin of Laurentia. Bickford et al. (2000) proposed metamorphic, and metasomatic history, and fluid evolution in the foreland. dextral transcurrent motion along the southern margin of Laurentia resulting This history, coupled with differences from that in the overriding older met- from continental collision of a single block along the eastern margin of Lau- amorphic rocks, requires a new kinematic model for the southern margin of rentia. This model also predicts northwest-directed tectonic transport for the Laurentia. In addition, the disparity in deformation timing and kinematic evo- west Texas exposures, although it does not explain the northeastward tectonic lution between west Texas and the central Texas Llano uplift requires active transport observed in the Llano uplift (e.g., Reese and Mosher, 2004; Mosher et subduction in west Texas after collision in central Texas. We propose that col- al., 2004). Grimes and Copeland (2004) demonstrated that deformation in the lision of a north-verging continental indenter with southern Laurentia initially west Texas exposures occurred ~60 m.y. after orogenesis in the Llano uplift, requiring modification of both models (Mosher et al., 2008A). For permission to copy, contact Copyright *Present address: SM Energy, 6301 Holiday Hill Road, Building #1, Midland, Texas 79707, USA; Mesoproterozoic exposures near Van Horn, where high-grade metamor- Permissions, GSA, or [email protected]. [email protected]. phic rocks were thrust over low-grade rocks in a 5–7-km-wide foreland fold and © 2015 Geological Society of America GEOSPHERE | Volume 11 | Number 3 Davis and Mosher | Complex structural and fluid flow evolution Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/3/868/3338498/868.pdf 868 by guest on 01 October 2021 Research Paper SIERRA DIABLO BEAN HILLS STREERUWITZ GRAPEVIN HILLS A’ E TUMBLEDOWN STREERUWITZ MOUNTAIN 10 THRUST U MILLICAN D HILLS Fig. 2 CARRIZO SPRING S MILES FAULT 0 12345 PHANEROZOIC COVER A PLUTONIC ROCKS 0 2 4 6 8 ALLAMOORE KILOMETERS HAZEL FORMATION D HILLSIDE SANDSTONE U FAUL CONGLOMERATE T TUMBLEDOWN FORMATION FRANKLIN AGGLOMERATE MOUNTAINS FRONT ALLAMOORE FORMATION LLANO CARBONATE/PHYLLITE CARRIZO 10 VAN BASALT VAN HORN LLANO HORN UPLIFT CARRIZO MOUNTAIN GROUP MOUNTAINS METAVOLCANIC ROCKS METASEDIMENTARY ROCKS 31° A –105° A A’ t us thr er v pCHc pCCi O itz pCAl w pCHs u reer pCCs St B Figure 1. (A) Simplified geologic map of the Precambrian exposures in Van Horn area (modified from Soegaard et al., 1993). Inset map of Texas shows other Proterozoic rocks exposed in central and west Texas, and location of the Grenville Front (Llano Front of Mosher, 1993). (B) Schematic cross section across the foreland showing metamorphic rocks of the Carrizo Mountain Group (CMG) thrust over Allamoore and Hazel Formations along the Streeruwitz thrust. Note the overturned folded structure of the Allamoore and Hazel Formations; the interfolding of the CMG with the Allamoore in the hanging wall is inferred, not observed (after King and Flawn, 1953). GEOSPHERE | Volume 11 | Number 3 Davis and Mosher | Complex structural and fluid flow evolution Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/3/868/3338498/868.pdf 869 by guest on 01 October 2021 Research Paper thrust belt (Fig. 1), provide a key area for testing tectonic models for Grenville Ga (Grimes, 1999; Bickford et al., 2000) foreland sedimentary rocks of the Alla- orogenesis. Exposures in west Texas display a transect across a mid-crustal moore and Tumbledown Formations, and younger Hazel Formation, of the Van metamorphic core to upper crustal foreland rocks over a distance of ~20 km. Horn area (this study, Fig. 1A); undeformed sedimentary and igneous equiva- Furthermore, these rocks are north and west of the deformational fronts of lents are found in the Franklin Mountains near El Paso (Roths, 1993; Pittenger et Phanerozoic orogens and show little evidence of overprinting deformation or al., 1994), and in igneous rocks in the Hueco Mountains (Masson, 1956). younger translation. Rocks of equivalent age to the north and west are unde- In the Van Horn region, the Streeruwitz thrust is the primary structural fea- formed, indicating that this area represents the Grenville Front in west Texas ture, which translated high-grade metamorphic rocks of the CMG ~19 km north- (Llano Front of Mosher, 1993). ward over low-grade, highly deformed sedimentary and volcanic rocks of the In this paper we present results of a detailed structural analysis of new ex- Allamoore and Hazel Formations (King and Flawn, 1953; Wiley, 1970; Reynolds, posures in talc mines within the Grenville foreland that constrain its kinematic 1985; Haenggi, 2001; Fig. 1B). The 40Ar/39Ar dating of mylonites near the Streeru- and geologic evolution and demonstrate that both the tectonic transport direc- witz thrust indicates mylonitization ca. 1035 Ma (Bickford et al., 2000), and tion and the chemistry of fluids channelized along faults changed over time. rapid exhumation