Complex Structural and Fluid Flow Evolution Along the Grenville Front, West Texas

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 southern 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 Grenville 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

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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).

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 and associated brittle faulting between ca. 1000 and 980 Ma Furthermore, we present a structural and kinematic model for the west Texas (Grimes and Copeland, 2004). Deformation and metasomatism decrease mark- exposures and incorporate it into a tectonic model for Grenville orogenesis edly away from the thrust, and deformation is almost absent ~7 km to the north. along the southern margin of Laurentia. Deformation in the CMG shows a complex and protracted history dominated by zones of dextral transpression and overall northwestward tectonic transport (Grimes, 1999; Grimes and Mosher, 2003). Metamorphic grade and deforma- GEOLOGIC SETTING tional complexity decrease from the southeast to the northwest approaching the Streeruwitz thrust (King and Flawn, 1953; DuBois, 1998; Mosher, 1993, 1998; Precambrian exposures in Texas provide a unique transect through the Grimes, 1999; Grimes and Mosher, 2003; Grimes and Copeland, 2004). The Grenville orogenic belt, from a metamorphic core in the Llano uplift of central highest metamorphic grade CMG rocks (peak metamorphism: 640 ± 50 °C to

Texas to upper crustal and undeformed sections in west Texas. The Grenville 510–530 °C) record at least five phases of deformation (D1–D5) and a medium Front trends southwestward in the subsurface from northeastern Texas to the pressure-temperature metamorphic event dated as 1057 ± 6 Ma (Bristol and

Van Horn region in west Texas, where it is exposed in a Phanerozoic horst block Mosher, 1989; Grimes and Mosher, 2003; Grimes and Copeland, 2004). D1 and D2 (Ewing, 1990; Mosher, 1993). The Llano uplift of central Texas is 300 km inboard in the central and southeastern Carrizo Mountains were characterized by crustal of the Grenville Front and exposes high-grade, polydeformed, metamorphic thickening, formation of northwest-verging folds, and northwest-directed dex-

rocks and syntectonic to posttectonic granites (Mosher, 1998). Deformation tral transpression, whereas D3 and D4 refolded earlier folds under conditions of across the Llano uplift is preserved in as many as six phases of folding (Nelis dextral shearing and retrograde metamorphism (Grimes and Mosher, 2003). et al., 1989; Mosher, 1993, 1998; Reese and Mosher, 2004; Mosher et al., 2004; Precambrian sedimentary units within the foreland include the Allamoore, Levine and Mosher, 2010) and two metamorphic events (see Carlson, 1998, for Tumbledown, and Hazel Formations. The ca. 1260 Ma Allamoore Formation is review). Initial amphibolite transitional to granulite and eclogite facies meta- composed of shallow intrusive and extrusive mafic volcanic rocks interlayered morphism related to collisional tectonics occurred between 1150 and 1119 Ma, with Mg-rich carbonates, thin layers of phyllite and talc, large massive talc waning by 1115 Ma (Mosher, 1998; Carlson, 1998; Carlson et al., 2007; Mosher bodies, minor felsic tuff, and cherty and stromatolitic limestone and dolostone et al., 2008b); a second largely static, low-pressure, high-temperature meta- (Gore, 1985). The sediments were deposited in an intertidal to supratidal set- morphic event associated with emplacement of syntectonic to posttectonic ting within a restricted lagoonal environment with some component of ephem- granitic plutons occurred between 1119 and 1070 Ma (Bebout and Carlson, eral hypersaline lakes or sabkha (Bourbon, 1981; Nyberg and Schopf, 1981; 1986; Reed, 1999; Mosher, 1998). A model was proposed (Mosher, 1998) that Edwards, 1985); this accounts for the high magnesium content of Allamoore included an arc-continent collision of an exotic terrane (Roback, 1996) with an rocks, aiding the mineralization of talc. Massive talc deposits are restricted to unidentified southern continent between 1275 and 1256 Ma, and subsequent the Allamoore Formation. accretion of this arc-continent block to southern Laurentia between 1150 and The ca. 1246 Ma Tumbledown Formation uncomformably overlies the Alla- 1120 Ma, with lower limit of deformation at 1098 Ma (Nelis el al., 1989; redated moore Formation and is a 168-m-thick succession of volcanic lithic sandstone, by Walker, 1992). Structural and kinematic analyses indicate that overall tec- basaltic agglomerate, mafic volcanic flows, rhyolitic felsite, and large gravi- tonic transport within the uplift was to the northeast (Reese and Mosher, 2004; ty-slide blocks of Allamoore carbonate exposed mostly in the Tumbledown Mosher et al., 2004). Evidence to support interpretations involving transcurrent Mountain area 12 km west-northwest of the study area (McLelland, 1996; motion in the uplift has not been observed (Mosher et al., 2004). Glahn, 1997; Bickford et al., 2000). Thrusting involving these two formations Mesoproterozoic exposures in west Texas include polydeformed metamor- resulted in the nonconformably overlying Hazel Formation, an arid alluvial fan phic rocks of the 1.38–1.29 Ga Carrizo Mountain Group (CMG) thrust over ca. 1.26 complex that accumulated in a tectonically active transpressional basin by ero-

sion of advancing thrust sheets to the south (King and Flawn, 1953; Reynolds, the foreland show as much as 1400 m of strike-slip motion (Kwon, 1990). 1985; Soegaard and Callahan, 1994). The basin is of unknown geometry and ex- These faults show sinistral and dextral motion (Kwon, 1990; Glahn, 1997) that tent and was bounded on its southern margin by active uplift of the Allamoore predates (Davis and Mosher, 2006; this study) and postdates major thrusting and Tumbledown Formations, the source terrain for most of the Hazel clasts (King and Flawn, 1953). The protracted histories of strike-slip motion on these (Soegaard and Callahan, 1994). Granite and rhyolite clasts found within the high-angle faults (i.e., Grapevine, Dallas, Yates Spring, and the Carrizo Springs Hazel conglomerate are similar to and coeval with the Red Bluff Granite Com- fault zone) are partly responsible for the west-northwest foreland grain (King plex and rhyolite from the Thunderbird Group (see Thomann, 1980; Shannon and Flawn, 1953). West-northwest–trending faults have been reactivated mul- et al., 1997), both exposed in the Franklin Mountains 160 km west (Roths,1993; tiple times throughout the Phanerozoic in west Texas (King and Flawn, 1953; Bickford et al., 2000). CMG clasts are notably absent in the Hazel Formation Muehlberger, 1980; Soegaard et al., 1993). (King and Flawn, 1953; Soegaard and Callahan, 1994). Red granite clasts in Hazel conglomerate have been dated as 1123 ± 29 Ma and rhyolitic clasts as 1126 +100/–27 Ma (Roths, 1993), giving an older constraint on the age of depo- STRUCTURAL ANALYSIS sition. Overlying the Mesoproterozoic units, the middle Cambrian Van Horn Sandstone (Spencer et al., 2014) is a postorogenic alluvial fan complex derived Although the CMG and the Allamoore Formation are moderately to well from a northern highland and deposited on a highly dissected Precambrian exposed, the Streeruwitz thrust contact between these two disparate groups is surface (McGowen and Groat, 1971). Only tilting and warping, with local offset poorly exposed, and generally inferred (e.g., King and Flawn, 1953). This study on high-angle faults, affects this unit (King, 1965). presents an analysis of new exposures of this northwest- to north-northwest– Grenville-age foreland deformation in west Texas had a long and protracted striking thrust in three open-pit talc mines (Fig. 2). Footwall structures in the Al- history of transpression. High-angle west-northwest–trending faults that cut lamoore Formation predate, postdate, and are coeval with the thrust. The Rosa

–105° 3’ 4.3” –105° 1’ 43” 57”

31° 7’ 31° 80 60 10 80 DEES PIT

STREERUWITZ YAT 45 ROSA BLANCA MINE ES FAUL T Figure 2. Simplified geologic map of the study area showing the location THRUST of the mapped talc mines and major 55 geologic structures (geology modified GLEN RAY MINE from King, 1953; Glahn, 1997; Davis, 2007).

Alluvium Road Allamoore Fm Attitude of Beds CARRIZO SPRING Talc 60 ? 44” Volcanic rocks Attitude of foliation N TEXOLA MINE S Limestone 31° 6’ 31° Mine tailings Carrizo Mtn Group Amphibolite 500 m 45 Metarhyolite

Blanca mine is the farthest northwest with the Streeruwitz thrust exposed along the south wall. The Texola mine is <~3 km southeast of the Rosa Blanca mine A along the trace of the thrust, and the Streeruwitz thrust is exposed in three dimensions (Fig. 3). The thrust is folded into type 1 interference patterns, or dome and basin folds (Ramsey, 1967). The Glen Ray mine is located farthest inboard, e between the two other mines (~2 km southeast of the Rosa Blanca mine, ~800 m northwest of the Texola mine; Fig. 2), and exposes an imbricate thrust within CMG Metarhyolit Amphibolite the Allamoore Formation that truncates early transpressional folds and faults. CMG The structural analysis presented here indicates that four distinct over- Talc Altered Allamoore printing deformation phases affected the foreland (discussed in the following). Limestone

Road STREERUWITZ Phase 1 is characterized by early folding (F1–F3), resulting in S1 and the primary

S2 foliations, and regional metamorphism and metasomatism resulting in the large talc deposits, compatible with early thrusting under ductile conditions. Phase 2 is characterized by high-angle transpressional faults and related folds

(F4) that refolded earlier structures. During phase 3, the Streeruwitz thrust formed, along with subsidiary imbricate thrusts and associated folds (F5). The N thrust faults truncate earlier structures at high angles, indicating a change in Talc the overall kinematics of deformation. Phase 4 resulted in dome and basin type Allamoore Fm. 1 folds of the thrust sheets, and reoriented all previous structures (this causes B Grey Talc the scatter in data observed on stereonets). This final phase is interpreted to White Talc have resulted from renewed transpression. All the structures discussed here Pink Talc Layered Limestone are observed within the Allamoore Formation, unless otherwise noted. Talc Phyllite Breccia Altered Zone

Phase 1. Early Thrusting 62 CMG Amphibolite Metarhyolite Phase 1 is characterized by three generations of folds (F –F ) distinguished 1 3 78 27 on the basis of fold morphology and overprinting relationships between foliations and structures. F folds are rarely observed in the field, typically as 1 56 rootless folds of silicified dolostone enveloped in talc (Fig. 4A). In thin section,

rootless F1 fold hinges are observed in the talc. F1 are tight to isoclinal folds,

46 41 have both arcuate- and chevron-shaped fold hinges, and are complexly re- STREERUWITZ FLOAT folded. F1 folds typically have a wavelength that ranges to 20–30 cm across; no

larger scale F1 folds were identified. 33 In the Rosa Blanca mine, F1 axial planes are east striking and steep dipping, and fold axes are roughly on a steep east-trending girdle, which also parallels the later Streeruwitz thrust in this mine (Davis, 2007). Most of the fold axes

ROAD were measured along a northeast-trending wall composed of intensely folded ? talc with numerous shear planes and shear duplexes. This later intense deformation may account for the spread in fold axis orientations, or the fold hinge N lines may have originally been curved (i.e., sheath folds). F1 folds observed in ROAD the Glen Ray mine are part of a series of the tectonically dismembered seg- 50 m ments of a silicified dolostone layer enveloped within the talc directly below the imbricate thrust on the west side of the mine. Figure 3. A geologic overview of the Texola mine. (A) Southwest panoramic view of the Streeruwitz thrust (black lines) contact between the Carrizo Mountain Group (CMG) F folds developed an axial planar foliation (Fig. 4A), which is rarely ob- 1 and metasedimentary Allamoore Formation within the Texola mine. Thrust sheet is

served in the field. In one less altered dolostone, the 1S foliation in thin sec- folded into complex type 1 folds; foliation is outlined by orange lines. Road and berm tion is defined by very fine grained elongate quartz with a crystallographic on lower picture for scale. (B) Simplified geologic map of the Texola mine.

preferred orientation. S1 is more commonly recognized in thin section in sec- F2 folds can be found in all mines but are relatively uncommon. The best

ond-generation (F2) fold hinges where it has been refolded. S1 is defined by exposures are on the easternmost wall in the Rosa Blanca mine, where altered

alignment of metamorphic or deformed minerals specific to the protolith, with mafic dikes that preserve the S1 foliation are folded into upright, closed to iso-

both talc and tremolite forming S1. Because of intense metasomatism and later clinal folds with the dominant S2 talc foliation parallel to the axial planes. These

foliation development, the S1 foliation is difficult to distinguish, or nonexistent. folds are rootless and have been detached in later shear zones (Figs. 5A, 5B).

S1 and S2 commonly form a composite foliation and are described in more F2 folds are also within the main body of talc as centimeter-scale, open to tight,

detail in the following. vertical folds with northeast-striking axial planes. F2 folds are seen, although not so spectacularly, in the other mines, usually as rootless folds of a more competent lithology incorporated in sheared talc. Some F folds show a different morphology. On the easternmost wall of the A 2 Rosa Blanca mine, several meters north of the F2 mafic dike folds, are “flame-

like” F2 folds. These are upright, isoclinal folds with millimeter-scale wave- B lengths that also fold altered mafic dikes and present a stark contrast in geometry and morphology to other F2 folds of the same lithology. The talc foliation is axial planar, indicating that these folds are coeval with the upright closed folds a few meters to the south. The contrast in fold morphology appears to result from a localized zone of intense shearing.

F2 folds show no consistent orientation between the mines (Figs. 6A–6C).

Within the Rosa Blanca mine, F2 axial planes are east striking and steeply dip-

ping, similar to the F1 fold axial planes. Poles to the axial planes roughly define a steep north-trending girdle, indicating that they may have been refolded by a shallow east-plunging fold. Fold axes plunge moderately east. In the Texola

mine, F2 axial planes dip shallowly to moderately southeast to southwest. The poles to the axial planes fall along a steep northeast-trending girdle, indicating that they may have been refolded by a shallow southeast-plunging fold. Fold 5 mm axes have a shallow to moderate south-southeast plunge. In the Glen Ray mine, F axial planes are east striking and shallowly to steeply south dipping. The poles C 2 to the axial planes are along a vertical north-trending girdle, indicating that they

may be folded by a subhorizontal east-plunging fold. The F2 fold axes have a wide range of orientations; most plunge moderately southeast to southwest.

The S2 foliation is the dominant foliation in the mines and is a very closely Qtz spaced, penetrative foliation. It is typically defined by aligned metamorphic minerals such as talc or tremolite where it is best expressed in outcrop; in other lithologies, it is defined by elongate grains of calcite and/or quartz. The Tlc Tr S2 foliation is most consistent in the Rosa Blanca mine, where it generally

is west-northwest striking and steeply dipping (Fig. 6D). The S2 orientation in the Texola and Glen Ray mines shows a wide dispersion with an overall prevalence of northwest strikes and moderate southwest dips; however, the foliation is clearly folded. Three girdles are defined by the foliation poles: the Tr dominant girdle has a p pole that plunges moderately (~40°) southwest; the other two girdles have p poles that plunge ~60° northwest and east (Figs. 6E, 2 mm 6F). These indicate that the foliation has been refolded by multiple sets of

post-F2 folds. The S2 is foliation is also truncated by later faulting. Figure 4. Photographs showing the alteration of a dolostone enveloped by talc from In thin section, S1 and S2 form a composite foliation that varies by lithology. the east wall of the Rosa Blanca mine. (A) Photograph of F1 folds in outcrop. (B) Photo- Within talc units, S2 is the dominant foliation and increases in structural com- micrograph from the limb of the F1 fold showing the S1-S2 foliation and layering of the plexity and purity with proximity to the Streeruwitz thrust. S2 is primarily defined altered dolostone. (C) Close up of S1-S2 foliation defined by tremolite (Tr), talc (Tlc), and quartz (Qtz). Note tremolite alteration to talc (in cross-polarized light, sample RBEW‑10). by talc and other minerals such as tremolite, quartz, and carbonate; the percent-

A B

Figure 5. (A) Overview of folded dike. (B) Folded (F2) altered mafic dike enveloped in talc from the east wall of the Rosa Blanca mine (remnant of dike preserved on right

limb of syncline). (C) Photomicrograph of less altered mafic dike with the S1 foliation preserved. Note the larger albite, the larger percent of biotite, and the presence of tour-

maline. (D) Altered version of mafic dike in B. The foliation S1 has been replaced and the groundmass is mainly quartz and albite with some relict biotite.

C D

1 mm 1 mm

age of other minerals defines the grade of talc (from lowest to highest: talc-rich defining S2 is rare except in the talc-rich phyllites and may reflect either the rock’s phyllite, ceramic-grade talc, and paint-grade talc; see Kyle and Clark, 1990). In bulk chemistry or later nearly complete replacement by talc.

talc-rich phyllite units, located structurally furthest from the thrust front, S2 is In the most abundant gray to black, ceramic-grade talc unit, S2 is defined

a closely spaced, crenulation foliation (Fig. 7A). Here, S2 is dominantly defined by acicular talc grains (to 60%; as much as 2 mm wide, 8 mm long), calcite by tremolite (to 80%), talc, and an increasing percentage of carbonate material and/or dolomite, tremolite, and minor quartz. Carbonate forms thin layers par- away from the thrust. Most of the tremolite grains are aligned laths (1–3 mm allel to the foliation or millimeter-scale nodules; tremolite forms euhedral to

long, 0.2 mm wide) forming a well-developed S2 foliation. The S2 tremolite and subhedral equant or lath-like grains (to 5 mm long) with margins replaced by

talc foliation contains microscale interfolial F2 folds of recrystallized S1 tremo- talc. Interfolial F2 folds of ultrafine-grained (<0.5 mm) talc that is dynamically

lite within F2 fold hinges (Fig. 7A). These recrystallized crenulation foliations of recrystallized in the hinges indicates that talc also defined S1 (Fig. 7B). This

tremolite suggest two periods of tremolite mineralization; however, tremolite relationship, coupled with tremolite forming both S1 and S2 in the talc phyl-

A. Rosa Blanca mine B. Glen Ray mine C. Texola mine

Poles to F2 Axial Planes (n = 9) Poles to F2 Axial Planes (n = 7) Poles to F2 Axial Planes (n = 14)

F2 Fold Axes (n = 8) F2 Fold Axes (n = 8) F2 Fold Axes (n = 14) Calculated Fold Axis (21, 096) Calculated Fold Axis (03, 087) Calculated Fold Axis (19, 149)

D. Rosa Blanca mine E. Glen Ray mine F. Texola mine Poles to foliation (n = 74) Poles to foliation (n = 165) Poles to foliation (n = 190) Poles to girdles Poles to CMG foliation (n = 13) (42, 209; 57, 334; and 61, 083) Pole to girdle (37, 212)

Figure 6. Stereonets (equal area, lower hemisphere projections) for D2 structures measured throughout the field area. (A, B, C) Data from 2F folds identified in three separate mines.

Dashed line is the best fit girdle from the poles to axial plane, star is associated fold axis. (D, E, F) Poles to the 2S surface and best fit girdle. CMG—Carrizo Mountain Group.

A B

S1 Tr S2

50um 50um

Figure 7. Photomicrographs of S2 crenulation cleavage with recrystallization of S1 foliation in F2 fold hinges. Images in cross-polarized light. (A) Talc phyllite where S1

and S2 are defined by both tremolite (Tr) and talc. (B) Ceramic-grade talc where talc defines both1 S and S2.

lites, indicates that early fold generations (F1 and F2) occurred under conditions Elongate recrystallized quartz defines the composite S1-S2 foliation observed in within the stability field of both talc and tremolite, with the second stage of talc most of the dolostone units. and/or tremolite growth favoring talc. Talc shows undulose extinction and kink Carbonate units include carbonate phyllite, banded limestone, and non-

bands, particularly where folded by late (F5) folds. banded limestone; only one foliation is observed, which on the basis of the The pure, white or pink, paint-grade talc is 90%–100% talc and found as com- interlayered relationships with talc and the presence of aligned talc with only

plexly folded slivers beneath thrust sheets. S2 is a closely spaced crenulation minor tremolite, is interpreted to be S2. In the carbonate phyllites, S2 is defined

foliation. Talc parallel to S2 forms centimeter-scale layers or single laths (to 1 cm by elongate calcite and/or dolomite, talc in thin seams between carbonate lay- long). At high magnification, the talc fibers have serrated boundaries, indicating ers, and dynamically recrystallized quartz (Fig. 8C). Dolomite grains are typi- grain boundary migration, undulose extinction, and incipient subgrains. cally larger, as much as 10 mm in diameter, and show undulose extinction and

In silicified dolostones, the composite 1S -S2 anastomosing foliation is de- minor grain boundary migration. In banded limestones, S2 is defined by elon- fined by aligned tremolite, elongate quartz, talc, and elongate dolomite (Figs. gate calcite and/or dolomite, elongate microcrystalline quartz (in chert), quartz

4B, 4C). Talc defines S2 in layers dominated by 0.1–0.5 mm, thin fibrous mats or ribbons (1 mm thick, to 1.5 cm long) that were dynamically recrystallized, and as individual grains in quartz-rich layers. Larger tremolite grains are subhedral, small acicular talc grains. In some units, tremolite appears as millimeter-sized

0.5–1 mm long, acicular to equant grains; some of the larger tremolite porphy- euhedral grains aligned parallel to the foliation. S2 locally is defined by milli- roblasts are rotated and wrapped by smaller tremolite and/or talc that further meter-thick straight or semi-stylolitic seams wrapping dolomite grains, indicat-

defines the composite foliation, indicating two periods of tremolite growth, ing pressure solution. The seams are folded by post-F2 isoclinal folds, and in

the earlier phase defining 1S . Larger S1 tremolites have face-centered pressure the hinges another set of pressure solution seams forms an axial planar cleav-

shadows of quartz, talc, and/or tremolite that are parallel to the composite foli- age to the post-F2 folds, locally enhancing S2 foliation. S2 is poorly expressed in

ation. S1 tremolite grains show undulatory extinction; mimetic replacement by the non-banded limestone by slightly elongate calcite, discontinuous layers of

talc parallel to S2 is observed along the margins of S1 tremolite grains. Quartz- elongate and recrystallized quartz, and some pods of aligned talc. rich layers are either microcrystalline quartz or lenticular clusters (~0.5 mm in Altered Allamoore Formation igneous rocks observed in the mines include length) of moderately large recrystallized quartz with some optical continuity, altered mafic dikes, chloritized olivine basalt, and chloritized basalt interlayered giving the rock a pseudomylonitic texture (Fig. 4C). Quartz recrystallization was with carbonates. Outcrops of altered mafic dikes, best seen in the Rosa Blanca dominated by rotational recrystallization with minor grain boundary migra- mine, are pale green or pinkish crystalline rocks with only one well-developed

tion evident by somewhat serrated grain boundaries on subgrain-sized quartz. foliation (Figs. 5A, 5B). In outcrop, this foliation is parallel to the S2 talc foliation.

F F3 S A N B 3 S2/So

S2/So F3

F3 7 mm

F C 5

F4 Figure 8. (A) F4 rollover fold refolding an F3 1st isoclinal fold exposed on the east side of F3 the Glen Ray mine. F4 folds postdate the

F3 isoclinal folds. Inset diagram is a step- wise diagram showing the formation of the superposed structures. (B) Photomi-

crograph (cross-polarized light) of F3 fold

(red dashed) of S1-S2 foliation (black line). Layering is original bedding; albite-quartz

vein offsets F3 fold limb. (C) F5 fold (black dashed line) refolding F folds of the S -S 2nd 3 1 2 foliation (red line) defined by talc, tremolite, and elongate calcite and/or dolomite (cross-polarized light).

0.5 mm

In thin section, the S2 foliation is defined by elongate mineral grains in alter- observed in the albite are deformation twins and undulose extinction. Some nating quartz-dominated or chlorite- and/or talc-dominated layers. Quartz-rich grains have inclusions of biotite and overprint the foliation, whereas others layers have a smooth foliation defined by a groundmass of fine-grained (~0.2 are wrapped by the foliation, suggesting that original and metasomatic albite

mm) elongated quartz and minute (<0.05 mm) laths of chlorite. Subparallel is present. An adjacent sample of the same rock shows that the S1 foliation to the quartz foliation are 5-mm-thick layers of quartz ribbons. Deformational has been completely destroyed by silica-rich metasomatism (Fig. 5D). Only

features in the quartz ribbons include continuous to discontinuous undulose relict thin seams of the S1 biotite foliation and some associated albite, which

extinction with boundaries that are subparallel to the foliation, numerous sub- shows evidence of grain boundary migration, are preserved. The S2 foliation is grains 1 mm in length, and some blurry or serrated grain boundaries. The dom- defined by the elongate quartz and minute (~0.5 mm) laths of chlorite. Quartz inant recrystallization mechanism in these layers is rotational recrystallization elongation was accomplished by lattice reorientation, rotational recrystalliza- with a minor degree of grain boundary migration. In the quartz-poor layers, the tion, and grain boundary migration. Chlorite is roughly aligned with the elon-

S2 foliation is defined by aligned grains of talc, chlorite, and tremolite. gate quartz, but in some parts of the sample it has a more random orientation.

In the hinge of one of the F2 folds of these mafic dikes, the 1S foliation is The timing of foliation development in the chloritized basalts is not as

observed and is defined primarily by aligned biotite with some albite grains evident. The foliation is similar to the S1 foliation described here for the less oriented parallel to the foliation (Fig. 5C). The only deformational features altered part of the mafic dike. The foliation is defined exclusively by aligned

biotite and chlorite. It wraps relict olivine phenocrysts, which are now altered Glen Ray mine to epidote and albite. Some of the albite has altered to sericite. The foliation

contains a mineralogy similar to the S1 foliation of the mafic dike, and therefore it is reasonable to assume that this foliation is probably the same foliation;

however, it could also be a temporal equivalent to S2.

F3 folds are found in all the mines but are best exposed and exemplified in the limestone units in the Glen Ray mine (Fig. 8A). Third-generation folds are

tight to isoclinal and refold previous folds and the S2 foliation (Fig. 8). Two morphologies for the isoclinal folds have been observed, parallel folds (class 1B or 1C; Ramsey, 1967) and similar folds (class 2). The orientations of the two sets of folds are similar; both fold the previous foliations and lack a penetrative axial planar cleavage, and both appear to have formed under less ductile conditions than previous fold generations and more ductile conditions than later fold gen- A B erations. Therefore, they are treated together as a single fold generation. The

morphological contrast is thought to be a reflection of rheological differences Poles to F3 Axial Planes (n = 32) Axes (n = 35) between heterogeneous limestone layers. F folds are observed from thin sec- F3 Fold 3 Poles to girdle (39, 147 and 69, 232) tion (millimeter scale) to mesoscopic (wavelengths to 2 m) scale (Fig. 8); some larger folds display parasitic folds with wavelengths of ~1 m.

Orientations for F3 folds are similar between the Glen Ray and Texola mines. Poles to F axial planes for both mines plot along a northeast-trending girdle 3 Texola mine where the associated p pole plunges moderately southeast (Fig. 9). Another

girdle, from the poles to axial planes of F3 fold in the Glen Ray mine, can be defined that trends northwest with a steep southwest-plunging fold axis. Both

girdle orientations can be explained by later refolding. F3 fold axes show a wide range of orientations but are mostly constrained to either the southeast quad- rant or show a cluster plunging west. Despite the scattered stereonet pattern, fold axes for the Glen Ray mine in map view are within the overall plane of the north wall of the large west pit, which strikes west-northwest and dips steeply south (Fig. 10). Meter-scale upright isoclinal folds oriented parallel to the strike of the north wall of the Glen Ray mine were observed along the eastern portion of the wall. The orientation of this wall appears to be a result of these large meter-scale F isoclinal folds, which are oblique to the strike of the imbricate CD 3 of the Streeruwitz thrust that cuts the wall in the northwest corner of the mine. Poles to F3 Axial Planes (n = 11) No penetrative and pervasive foliation is associated with F3 folds; the S3 sur- Pole to girdle (29, 151) face is a weakly defined axial planar cleavage observed only in thin section and F3 Fold Axes (n = 10)

localized in F3 fold cores. The lack of an axial planar foliation and the folding of Figure 9. (A, B) Stereonets for F3 folds from Glen Ray mine. (C, D) Stereonets for F3 folds from S1–S2 surfaces are characteristics of F3 and all the subsequent fold generations. Texola mine. Note that poles to axial planes from both mines show folding about a southeast-plunging fold axis. Equal area, lower hemisphere projections.

Phase 1. Interpretation

These earliest structures consist of fold generations F1–F3, and the asso- these early structures and require a different kinematic setting. F1 and F2 folds

ciated S1 and S2 foliations. These folds are grouped together because they are usually rootless folds and both have axial planar foliations, S1 and S2, re-

formed early under ductile conditions, have similar mineralogies defining 1S spectively. F1–F3 folds are tighter and more ductile in nature than later fold

and S2, and could have progressively formed in a noncoaxial shear environ- generations. S1 is defined by biotite in mafic dikes, talc in ceramic-grade talc

ment associated with thrust motion. Alternatively the fold generations could units, and tremolite in dolostone. The tremolite is replaced by S2 talc and is

be genetically unrelated. Subsequent structures, however, clearly truncate only found parallel to S2 in talc-rich phyllites, most likely indicating higher tem-

–105° 1’ 58.72” Allamoore Fm. N Limestone Banded Limestone Talc Chloritized Basalt 63

52 49

34 33 36 F 38 F5 3 Roads ? Fault truncatedF by 6 float Figure 10. Simplified geologic map of later thrust F4 63 U the main pit of the Glen Ray mine (for

5.79” geology of the north wall, see Fig. 12). 33 D

U—upthrown; D—downthrown. 31° 7’ 31°

Water float

Roads FE 54 ?

10 m

peratures during S1, although changing fluid composition could also cause this fied in the competent units of the Glen Ray mine. F4 folds are tight to isoclinal,

change. F2 folds have the dominant talc foliation, axial planar, indicating that fold previous folds and foliations, and did not develop an axial planar foliation.

the main foliation in the mines is S2, and most talc formed during formation Some folds could not be uniquely assigned to either F3 or F4 fold generations,

of the F2, S2 phase. The abundance of talc and a decrease in tremolite in talc- but most F4 folds have unique morphologies that aid in their recognition, de-

rich units approaching thrusts suggests later fluid influx altering the mineral scribed in the following, and many fold F3 folds (Fig. 8A).

assemblages. F3 are isoclinal folds that lack a pervasive axial planar foliation. F4 folds, termed rollover folds, are typically asymmetrical, moderately to

Upright isoclinal F3 folds make up the north wall of the Glen Ray mine, which steeply plunging folds with a consistent vergence. F4 folds have curved or is oriented obliquely to the imbricate thrust in the mine, requiring a change anastomosing fold axes; some are incipient to moderately developed sheath in the kinematic setting (see Phase 3 discussion). The mineralogy (talc and/or folds (Figs. 11 and 12). These folds typically die out along their length and

tremolite) defining S1 and S2 and deformational and recrystallization features change morphology from tight, overturned folds in the middle of the fold trace

exhibited in quartz and calcite indicate that greenschist facies metamorphic to more open, upright folds where they die out. Unlike F3 folds, which are

conditions were achieved during F1 and F2. Thus, these folds are best explained mainly constrained to limestone units, F4 folds are found in other lithologies.

by pre-Streeruwitz, noncoaxial shear deformation associated with thrusting at F4 folds in talc show the characteristic fold geometry of rollover folds; i.e., they depth; tectonic transport was most likely northward. have curved fold axes that appear to have been rolled differentially, and are

usually steeply plunging (Fig. 11). The largest F4 folds have a wavelength of only ~3 m, due to their isoclinal geometries. The morphology of the F folds gives an indication of the type of kinematic Phase 2. F4 Folds and Transpressional Faulting 4 setting in which they were formed. The steep north wall of the Glen Ray mine

The second group of structures encompasses one recognized fold gener- roughly parallels bedding (Fig. 10) and displays abundant F4 folds at multi-

ation, F4, and associated transpressional motion on high-angle faults or shear ple scales (Figs. 11A and 12). The best example is the largest F4 fold, which is

zones. F4 folds are seen in each mine, but like F3 folds, they are best exempli- steeply plunging, asymmetrical, and westward verging with a wavelength of

~3 m. Its morphology shows that it underwent differential motion, as the top A part of the fold is tighter and offset from the lower portion of the fold by a tear fault (Fig. 12). Numerous parallel en echelon tear faults that accommodated

strain during F4 folding are present on the limbs of this and adjacent rollover folds, resulting in offset hinge lines. These characteristics indicate that the fold formed by oblique dextral motion under brittle and/or ductile conditions, which is consistent with the anastomosing, sheath-like geometries. Thrust Timing for the F4 folding is well constrained. Field observations show F3

folds being refolded by F4 (Fig. 8A). Furthermore, many of the F4 folds are truncated by shear zones in the talc and the Streeruwitz and associated imbricate Imbricate thrusts (Figs. 11 and 12). The orientations of these folds, the crosscutting rela- F4 tionships with faults, folding of the S2 surface, and fold superposition give a

well-constrained timing for the formation of F4 folds. F4 The orientation of F4 folds, although different between mines (Fig. 13), is internally consistent, controlled by the orientation of the layers on which they

formed. In the Texola mine, poles to F4 axial planes are along an east-trending, north-dipping girdle, suggesting that they have been folded by a mod-

erately south-plunging fold (Fig. 13A). The majority of F4 fold axes plunge

CMG moderately south; some plunge steeply west. A few of the F4 folds were rotated as they were tectonically incorporated into a talc décollement; how-

ever, the majority appears to be unrotated. Only a few F4 folds in the Rosa Streeruwitz Thrust Blanca mine were observed; these are nearly vertical, south-plunging folds with east-northeast–striking axial planes (Fig. 13B). This orientation is com-

patible with the F4 folds of the other mines, and some are truncated by the Streeruwitz thrust (Fig. 11B).

In the Glen Ray mine, poles to the axial planes of F4 folds are along a northeast-trending, northwest-dipping girdle, suggesting that they were

folded by a shallow southeast-plunging fold (Fig. 13C). F4 fold axes generally plunge moderately southeast. Most were measured from the north wall of the main pit in the Glen Ray mine, along the east-striking, moderately to steeply southward dipping layers. Although the fold axes appear consistently oriented in the field, they show a spread when plotted on a stereonet. The spread results from anastomosing, curved hinges of sheath folds, reorienta-

tion by later broad folds (F6), and the folding by F4 of a previously folded (i.e., nonplanar) surface.

The abundant F4 folds in a plane showing the same sense of shear indicates that they are part of a dextral oblique-slip shear (or fault) zone that paralleled the length of the Glen Ray pit; the adjacent talc was removed by mining. Ex- F4 amination of the nearly inaccessible main pit east wall, where the fault zone is projected to intersect, shows layers of highly contorted paint-grade talc and B a zone of intense deformation and localized metasomatism. Numerous contorted beds and duplication and folding of basaltic rocks suggest that the fault Figure 11. F4 folds being truncated by thrusts. (A) Well-developed had a large component of strike-slip motion, and some possible later reactiva- F4 sheath folds truncated by an imbricate thrust; location is in the northwest corner of the Glen Ray mine (see Fig. 10). (B) Steeply tion as a thrust. On the west end of the pit, the imbricate thrust would truncate plunging F folds on vertical foliation cut by the Streeruwitz thrust. 4 the fault within the excavated pit; this map interpretation is supported by the Transport direction is to the left, subparallel to the strike of the foliation; location is the east side of the Texola mine. CMG—Carrizo truncation of associated F4 folds in the northwest corner by the lower segment Mountain Group. of the imbricate thrust (Figs. 10 and 11A).

E W

F4 F6 Qal F6 F4 Chloritized Talc F Basalt Allamoore Limestone 4 F4 F4

NE F3 F4 F6

F5 F6

Imbricate Thrust

2nd 1st

Figure 12. The north wall of the Glen Ray mine showing the folds formed along the walls. The inset is a schematic diagram showing the oblique dextral motion that formed the F4 folds and the relationship with imbricate thrust (arrows show thrust transport direction).

Phase 2. Interpretation of these F4 folds suggest the presence of an east-striking, dextral oblique-slip fault parallel to the northern wall within the excavated pit (Figs. 10 and 12). The The steeply to moderately dipping layers that form the north wall of the proposed fault is substantiated by the presence of sheared and deformed Alla-

west pit in the Glen Ray mine best expose the F4 folds and display the timing moore Formation rocks, especially talc, along the eastern wall of the main pit relationships with other structural generations. The orientation of the layers is in the Glen Ray mine, where the fault would intersect the exposed outcrop. Al-

a result of isoclinal F3 folding, and the F4 folds exposed along this wall formed though the trace of this fault cannot be followed out of the pit, a regional view

on the limbs of these larger isoclinal F3 folds. The consistent vergence of the F4 of the fault trend suggests that it is related to other high-angle west-northwest– folds coupled with their morphology (anastomosing, curved hinge lines that trending faults mapped in the foreland. die out along their length with associated changes in tightness and degree of King and Flawn (1953), Kwon (1990), and Glahn (1997) mapped a series overturning and sheath fold geometry) indicate that they formed as a result of of high-angle faults throughout the foreland (Fig. 1), all with at least some dextral oblique shear parallel to the layering. This shear is supported by the indication of strike-slip motion. Glahn’s (1997) map shows a segment of the

differential motion along associated tear faults exhibited by the largest F4 fold Carrizo Springs fault trending toward the Glen Ray mine (Fig. 2). Although only located in the center of this wall and adjacent smaller folds. The characteristics mapped as far as the hills bordering the Texola mine to the east, extrapolating

A BC

Texola mine Rosa Blanca mine Glen Ray mine

Poles to F4 Axial Planes (n = 11) Poles to F4 AP (n = 3) Poles to F4 Axial Planes (n = 18)

Pole to girdle (41, 174) F4 Fold Axes (n = 3) Pole to girdle (23, 151)

F4 Fold Axes (n = 11) F4 Fold Axes (n = 23)

Figure 13. Stereonets (equal area, lower hemisphere projections) for F4, or rollover folds. (A) Texola mine. (B) Rosa Blanca mine. (C) Glen Ray mine. Poles to axial planes in the Texola and Glen Ray mines are folded about a southeast- and south-plunging fold axis. Fold axes from the Texola, Rosa Blanca, and Glen Ray mines are mostly moderate to steeply plunging to the south-southeast. Note the clockwise rotation of fold measurements between the Glen Ray and the Texola mines.

the fault along its trend projects it into the Glen Ray mine. Thus, it seems rea- by rotated porphyroclasts of albite and quartz. Matrix quartz was elongated by sonable that one of the regionally mapped high-angle, oblique- and/or strike- lattice reorientation, shows undulose extinction, and 120° grain boundaries.

slip faults was responsible for the formation of the F4 folds. The presence of Muscovite shows undulose extinction and microscale kinks. Foliation wraps

oblique- and/or strike-slip faults, the nature of associated F4 folds, and relative the porphyroclasts; albite porphyroclasts are rotated with pressure shadows of timing indicate that this phase was dominated by transpressional deforma- quartz parallel to foliation and shear sense is either sinistral or indeterminate. tion. Clear crosscutting relationships demonstrate that it postdates the earlier Quartz pressure shadows show continuous to discontinuous undulose extinc-

phases (F1–F3, S1–S2) and predates the Streeruwitz thrust and associated imbri- tion, elongation by lattice reorientation, and rotational recrystallization with cate thrusts. This interpretation of transpression-dominated foreland deforma- minor grain boundary migration. tion is also supported by the narrow width of foreland deformation. Unaltered CMG amphibolites have a coarse, anastomosing, spaced foliation defined by aligned elongate quartz and biotite altered to clinochlore, forming a groundmass that wraps albite and blue-green hornblende porphy- Phase 3. Streeruwitz Thrust and Related Structures roblasts. The porphyroblasts show a rough alignment parallel to the foliation. A crude metamorphic layering parallels the foliation and is defined by alternat- The Streeruwitz thrust is the master structure emplacing high-grade met- ing felsic- and mafic-rich layers. Isoclinal millimeter-scale folds of the foliation amorphic rocks of the CMG over low-grade metasedimentary rocks of the Al- were observed that augment the wavy cleavage. Opaque minerals, principally lamoore Formation. The thrust is exposed in three dimensions in the Texola pyrite, are aligned with the foliation. Fe-oxide-rich pressure solution seams (Figs. 3 and 14) and Rosa Blanca mines, with an associated imbricate thrust parallel the foliation and wrap porphyroblasts and postdate the foliation. exposed in the Glen Ray mine (Fig. 10) that places volcanic rocks over metased- Open to isoclinal, centimeter-scale folds of the foliation were observed in imentary rocks. both the metarhyolite and amphibolite. These folds have no axial planar cleav- The CMG in the hanging wall contains metarhyolite composed of fine- age and do not appear to be refolded. The only timing constraint is that they grained, well-foliated mylonite that is interleaved with amphibolite. A mylo- postdate the CMG mylonitic foliation. Large-scale folds of the CMG are ob- nitic foliation is defined by aligned elongate quartz, albite, and muscovite and served and related to the folding of the Streeruwitz thrust. The timing of all

A N S 74 33 46 CMG Talc 34 28 27 42 43 38 71 CMG 41 12 44 Streeruwitz Streeruwitz 21 52 Allamoore Talc 16 25 13 56 Carrizo Mountain Group

45 Road Thrust Streeruwitz 44 Alz B C. 63 39 41 44 80 Talc 46

CMG Thrust Float tz Berms wi eru re St

Water Level Nov 2005 ? 54 p 77

34 Strike and dip of thrust 7 plane measurement 65 39 72 34 Strike and dip of S ShearedShheheararereddA AAllamoorllalaamomoooorere 72 2

54 11 Carrizo Mountain Grou foliation Road 36 68 58 N 74 B. ? Talc 10 meters C

Figure 14. Smaller east pit of Texola Mine. (A) Simplified geological map showing Carrizo Mountain Group (CMG) klippes folded by a southwest-plunging anticline-syncline pair of 6F folds (blue axial traces) oriented at high-angle to a south-plunging anticline (red axial trace). See Figure 3 for unit key. (B) Photograph view to the northeast, showing the CMG klippes folded

by a synform-antiform pair of F6 folds (blue axial traces in A). (C) Photograph view to the south showing F6 anticline of the Streeruwitz thrust zone (red axial trace in A).

CMG structures not related to or postdating the Streeruwitz thrust is unknown tourmaline. King and Flawn (1953) noted slickensides of tourmaline associated relative to those in the Allamoore Formation rocks in the footwall. with the Streeruwitz thrust elsewhere in the foreland. Typically, the Streeru- In the Texola mine, CMG foliation and compositional layering strike south- witz thrust has CMG in thrust contact with talc, using talc as a décollement, east and dip moderately southwest and are truncated by the thrust (Fig. 3). Lo- although slivers of limestone and small imbricate thrusts are noted directly calized zones of metasomatism, where CMG rocks were replaced by a mixture below the main thrust (Figs. 3 and 14). of albite, vein tourmaline, iron carbonate, and quartz, obscure the thrust and In the Texola mine, the orientation of the thrust plane varies dramatically, lithological contacts within the CMG (Fig. 15). Breccias composed of clasts of indicating folding after thrusting. Poles to shear planes define two rough gir- CMG mylonites and Allamoore limestones are observed along the thrust; they dles indicative of folding on southwest- and northeast-plunging fold axes are cemented with ankerite, dolomite, albite, and quartz, with local veins of (Figs. 16A, 16B). Talc bodies directly below the thrust behave as a décollement,

Metarhyolite Amphibolite Amphibolite

Altered CMG Altered CMG

A B

Figure 15. Metasomatically altered Carrizo Mountain Group (CMG) along the southern wall of the Rosa Blanca mine. (A) Less altered CMG metarhyolite and amphibolites are shown in the top half of photograph above highly altered CMG amphibolite. Altered CMG, termed jasperoid, is a typically a nondescript, fine-grained, pinkish rock with large amount of albite. (B) Close- up showing the highly altered CMG amphibolite jasperoid adjacent to unaltered amphibolites. Diameter of coin in center left of picture is ~21 mm.

and some thrust-related strain is partitioned onto shear planes within the talc. 3–5 m mixed zone of sheared, intensely metasomatized rock (Fig. 17) in which

The preexisting foliation, S2, is deflected into the shear planes and folded by many various lithologic clasts were incorporated (southern boundary com-

shear-related F5 folds. The orientations of these shear planes vary because of plex). CMG rhyolites and amphibolites are altered to a nearly indistinguish- the anastomosing morphology and later folding (Figs. 16D, 16E). In general, able nondescript pink rock (jasperoid; Fig. 15) with abundant albite forming the most dip moderately to steeply southwest or to a lesser extent northeast. groundmass. Below the CMG rocks is an ~3-m-thick zone of a hard, dark brown Overall the apparent sense of shear appears to be to the northeast, on the carbonate-rich rock. Also associated with this complex are off white to pale basis of shear related folds, deflection of the foliation, and the regional orien- yellow sections of a quartz-ankerite(?) rock in which the protolith texture was tations of the shear planes and the thrust. This direction of tectonic transport is destroyed by shearing and metasomatism. Within the plane of this complex essentially parallel to the strike of the truncated bedding and foliation. shear zone is series of nested boudins; the largest boudin spans 2.5 m. Struc- Along the southeastern wall in the Texola mine, a meter-thick shear zone turally below this zone and pinching out to the east is a sliver of chloritized adjacent to the Streeruwitz thrust is composed of strongly foliated talc with olivine basalt. This entire Streeruwitz thrust package is riding on 25–30 m of a inclusions of folded carbonate clasts (Fig. 14C). Along the upper portion of this talc décollement. zone, a thin (~0.5 m) zone of mylonitized carbonate is in direct contact with Within the talc adjacent to the thrust are numerous shear planes, duplexes, altered CMG. The mylonitized carbonate is exclusively sheared iron carbonate and shear-related folds (Fig. 18). The shear planes generally strike northwest concretions or elongate folded dolomite. This thrust zone cuts vertical beds of and are steeply dipping (Fig. 16E). Like those in the Texola mine, there is a

the S2 talc foliation and vertical F4 (rollover) folds (Figs. 11B and 14C). F3 folds range of orientations; however, in this mine the variation appears to be pri- were also truncated by the thrust in other areas of the mine. marily related to the anastomosing nature of the shear planes rather than later In the Rosa Blanca mine the Streeruwitz thrust strikes east and dips ~70º folding. Folds within the talc with curved axes are common and record the south along the southern portion of the mine. About ~350 m north, in the north-northeast direction of thrusting. Dees pit, the thrust strikes north and dips ~60 º west, indicating that the thrust In the Glen Ray mine, the westernmost wall is chloritized basalt and sheet is complexly folded on a larger scale (Fig. 2). Instead of a discrete fault sheared talc of the Allamoore Formation thrust over previously deformed car- or zone, juxtaposed CMG and Allamoore Formation rocks are separated by a bonate units of the Allamoore Formation. The thrust strikes southeast and dips

Thrust measurements from the Thrust measurements from the east pit of the Texola mine. entire Texola mine. Poles to thrust plane measurements (n = 11) Poles to thrust plane measurements (n = 70) Poles to thrust plane measurements (n = 21) Pole to girdle (40,182; 27, 238; 35, 094; 49, 332) Pole to girdle (25, 238; 43, 191; 32, 080)

C DE Glen Ray mine Texola mine Rosa Blanca mine Poles to shear planes (n = 31) Poles to shear zones (n = 30) Poles to shear zones (n = 26) Pole to girdle (44, 215) Pole to girdles (41, 299, and 44, 055)

Figure 16. Stereonets (equal area, lower hemisphere projections) of thrust plane measurements and defined girdles from the Streeruwitz thrust. (A) Thrust plane measurements from the east pit in Texola mine; red circles are from the south side of the east pit, white circles are from the klippes north of east pit. (B) All thrust plane measurements from around the entire Texola mine. (C, D, E) Orientation of shear planes in talc décollement throughout the mines, and girdles defined by poles to the planes are shown for Glen Ray, Texola, and Rosa Blanca mines.

A NE SW

Shear melange

thrust contact

F5 Talc

S2 Shear Planes

So A

1 mm B

Figure 17. Shear mélange complex of the Streeruwitz thrust from the south wall of the Rosa Figure 18. (A) Eastward or strike-parallel view of anastomosing shear duplexes in the Blanca. (A) Southeast corner of the mine showing the thrust contact between the south east wall of the Rosa Blanca mine. Within the shear duplexes (outlined in red) are

boundary complex along the south wall and the talc along the east wall. Note the mafic box or chevron-shaped F5 (yellow) folds that refold earlier folds, in this case F3 folds dikes in the east wall talc on the left. Height of wall is ~3 m. (B) Close-up of a carbonate(?) (green). The change in color of the talc probably related to original bedding. Height of

clast incorporated into the shear zone (compass for scale). wall is ~2.5 m. (B) Photomicrograph of paint-grade talc S2 foliation folded by F5 box folds (image in cross-polarized light).

moderately southwest, but is gently folded by a late southwest-plunging fold. adjacent to shear duplexes within talc bodies; thus all crenulations of S2 are

The thrust is oriented at a high angle to the prominent east-striking, steeply grouped with the F5 folds.

dipping layers containing the F4 (rollover) folds on the northern wall of the F5 fold axes directly below various thrust exposures are curvilinear and mine, truncates these folds (Fig. 11A), and would also truncate the postulated convex in the direction of thrusting, compatible with north-northeast–directed

dextral oblique-slip fault (Figs. 10, 11A, and 12). Thick chloritized basalt of the thrusting. F5 fold axes are almost exclusively in the southern half of all stereon- overriding plate is exposed on the far eastern edge of the mine as well, indi- ets (Fig. 19), reflecting the generally southeast to southwest dip of the Streeru-

cating that the imbricate thrust sheet extends across the mine and is folded witz and associated imbricate thrusts. Axial planes for all F5 folds in each mine like the Streeruwitz thrust. Associated with the thrust zone is intensely sheared strike roughly east and dip south (Fig. 19). talc with numerous anastomosing shear duplexes. In general, most shear duplexes dip moderately to the southwest or west; however, the poles to the planes define a rough girdle indicative of folding on a southwest-plunging fold Phase 3. Interpretation axis (Fig. 16C). Folds within the talc have curved axes that record the northeastward direction of thrusting. The Streeruwitz thrust is exposed in the Texola and Rosa Blanca mines,

In all the mines, folds (F5 folds) are spatially related to thrusts or are in where it cuts previously formed structures at high angles in both the hanging shear duplexes, and although they appear in other lithologies, they are best walls (CMG) and footwalls (Allamoore Formation). Mylonite breccias observed

expressed within the talc décollement (Figs. 18A, 18B). F5 folds that fold the along the fault indicate that the Streeruwitz exhumation history traversed from

S2 foliation range from open to isoclinal with arcuate or chevron-shaped ductile to brittle conditions. The rocks adjacent to the Streeruwitz thrust were hinges (Fig. 8C). Some box-shaped folds are also observed away from thrust also intensely metasomatized by silica, alkali, and carbonate-rich fluids that faults. Fold wavelengths range from millimeter-scale crenulations in talc to acted as cementing agents for the breccias. The Streeruwitz thrust utilized talc meter-scale folds (Fig. 18B). A poorly to moderately well defined crenulation as a décollement horizon, and some of the motion was partitioned into shear cleavage (and kinking) in the talc appears to be related to these folds. In addi- planes within the talc. Thrusting and development of shear duplexes in the talc

tion, the talc is extremely crenulated by multiple sets that are most abundant and adjoining rocks formed curvilinear F5 folds.

A B C Glen Ray mine Texola mine Rosa Blanca mine

Poles to axial planes (n = 15) Poles to axial planes (n = 27) Poles to axial planes (n = 38) Fold axes (n = 24) Fold axes (n = 31) Fold axes (n = 41)

Thrust planes Thrust planes

Figure 19. Stereonets (equal area, lower hemisphere projections) for F5 folds from three mines in the study area with thrust plane plotted. (A) Glen Ray mine. (B) Texola mine (thrust omitted due to complex folding). (C) Rosa Blanca mine.

Klippes of the Streeruwitz thrust north and northeast of the mines (Fig. 2) sin morphology (Fig. 20A). This wall is directly below the Streeruwitz thrust confirm that the Streeruwitz thrust was an out of sequence thrust that truncated and can be considered an example of the geometry of the thrust plane. Dome all previous faults as it moved over the foreland. The imbricate thrust exposed structures along this wall have a wavelength of ~5 m. The axial planes and fold

in the Glen Ray mine, as well as those exposed in the Texola mine, also indi- axes from these F6 folds have a “shotgun” pattern as would be expected from cates that the foreland fold-thrust belt was the result of thin-skinned thrusting. measurements of domed shaped structures (Fig. 20B). Many of the fold axes Kinematic indicators below the thrust sheets, such as deflected foliation measurements are either part of one fold with a curvilinear fold axis curving and curvilinear fold axes, and the orientation of associated shears and the over a domal structure, or were calculated from foliation measurements taken thrusts, indicate that motion on the Streeruwitz and related imbricate thrusts around domal structures. Calculated and measured fold axes from the Texola is toward the north-northeast to northeast. This orientation matches the struc- mine shows a preponderance of plunges to the northwest, southeast, and tural grain of the foreland, and differs from the northeast structural grain of the southwest. The large-scale dome and basin structures in the Texola mine cre- Carrizo Mountains. ated room problems where the already present talc décollement was pinched

Earlier folds (F1–F4) are truncated by the Streeruwitz and associated imbri- and squeezed into the core of one of the domes. F6 folds observed in the Texola cate thrusts, and are typically oriented at a high angle to these thrusts. In the can be traced northwest along strike into the Glen Ray mine, where they are Glen Ray and part of the Texola mines, the apparent thrust transport direction is also exposed.

nearly parallel (Texola) or at a low angle (Glen Ray) to the strike of the underly- F6 folds in the Glen Ray mine are broad to open, 5–10-m-wide folds, some ing beds and foliation. Thus, a change in the direction of shearing is required to of which have a dome-shaped morphology. They fold the foliation and bedding

explain these observations. If early F1–F3 folds resulted from noncoaxial shear, and have no axial planar cleavage. One of the best and largest examples of an

as proposed, then a change from apparent northward transport to north-north- F6 fold in the Glen Ray mine is along the north wall (Fig. 12); it plunges mod-

east– to northeast-directed transport occurred between the formation of F1–F3 erately to the southeast and F4 folds are found on both limbs. This dominant folds and the Streeruwitz thrusts. The proposed dextral oblique motion on ap- structure reorients all previous folds and foliations (i.e., all are along stereonet

proximately west-northwest–striking faults that produced the F4 folds occurred girdles that have p poles that plunge moderately to the southeast; Fig. 20C).

between these two phases. Other, nearly perpendicular, large F6 folds are observed along and nearly parallel to the wall (inaccessible to measurement) and fold the generally north-

west-trending imbricate thrust. The axial planes of F6 folds have a wide range Phase 4. Transpression: F6 folds of orientations but show a somewhat consistent northwest strike. Fold axes mostly plunge southeast or southwest (Fig. 20C). The final deformational phase to affect the foreland is evidenced by the The effects of these post-Streeruwitz folds can be seen in the Rosa Blanca refolding of all previous folds and folding of the thrust sheets, including the and adjacent Dees mine. The southern wall of the Rosa Blanca mine is paral- Streeruwitz thrust. At least two sets of nearly orthogonal folds are observed. lel to the roughly east-west trace of the Streeruwitz thrust. Mining company These folds are not differentiated as separate phases because no conclusive maps, based on drill hole data from the Rosa Blanca mine, show that the thrust timing relationships between them could be determined. They form type 1 fold trace bends to the north just west of the Rosa Blanca (Davis, 2007), indicating interference patterns (Ramsey, 1967) or dome and basin structures. (Anoma- that the thrust sheet is folded. The Dees pit, which is ~150 m north of the Rosa

lous folds, termed FE folds, associated with eastward-directed thrust faulting, Blanca mine (Fig. 2), exposes a generally north-trending Streeruwitz thrust that are described in the following.) has been folded by several northwest-plunging folds and some tight, nearly

A three-dimensional view of the F6 folds of the Streeruwitz thrust can be vertical folds with east-striking axial planes (Taylor and Mosher, 2011). Along seen in the Texola mine (Fig. 3). Two sets of folds oriented at high angles to the south wall of the Rosa Blanca mine, many boudinaged layers are observed each other are evident in the folded trace of the thrust throughout the mine. with shallow east-southeast–plunging necks. These boudins could be related Figure 14A is a simplified map of the smaller east pit of the mine showing the either to extension during thrusting or to extension localized on the limbs of complex folding of the thrust sheet. A syncline-anticline pair with a moderate the late folds. plunge to the southwest forms an interference pattern with a south-south- An anomalous set of north-northeast–striking, steeply west dipping thrusts

west–plunging anticline (Figs. 14A–14C). The stereonets in Figure 16A show with associated brittle fault propagation folds, FE, are observed in the Glen Ray that one of the two sets of folds plunges either south-southeast or north-north- mine. Fold axes of these folds are along the adjacent fault planes, and the axial west, and the other set plunges east or southwest. Changes in plunge along planes are similar to or somewhat steeper than related fault planes. The data the trend of the fold axes suggest curvilinear fold axes, consistent with these are internally consistent, and these folds do not appear to be refolded. The

folds defining domes and basins. relative timing of the FE folds has not been determined, other than to note that The main northeast wall of the Texola mine does not expose the thrust, they are the result of a late event. These structures are classed with the domes but the foliation and bedding surfaces form a well-expressed dome and ba- and basins because of their more brittle nature and lack of refolding. As only

one outcrop of the folds and faults was observed, they are interpreted as being

related to a space problem associated with the F6 folding event.

Phase 4. Interpretation

F6 folds resulted in folding of the Streeruwitz-related thrusts and refolding

of previous folds. The superposing of two sets of F6 folds resulted in type 1 fold interference patterns, dome and basin structures (Ramsey, 1967). The effect of

the F6 dome and basin folding on the Streeruwitz thrust is the most obvious manifestation of this set of structures. However, the impact on previous fold generations is widespread, as shown by the wide variety of orientations of all previous fold axial planes and axes. Previous fold and foliation measurements and thrusts are along girdles with p poles that plunge either to the south or

southeast and northwest or to the east and southwest, the orientations of F6

domal fold axes. F6 folds are interpreted as being the result of continual transcurrent motion on oblique- and/or strike-slip faults. A summary of events and their tectonic significance is presented in Table 1.

A TIMING OF METASOMATISM AND METAMORPHISM

Several lines of evidence indicate that widespread metasomatism, with evolving fluid composition, affected foreland rocks during deformation (King and Flawn, 1953; Reid, 1974; this study), i.e., large monomineralic talc deposits, replacement of mafic dikes with silicates, partial to complete replacement of rocks adjacent to fluid pathways (faults), and the presence of veins and cemented breccias. Focused fluid flow along faults was also noted in the hinterland (Grimes, 1999, p. 208–210) and in the CMG in the hanging wall of the Streeruwitz thrust (this study). Fluid evolution starts with an early silica-rich phase, followed by an alkali-rich phase, and then a flux of Fe, Mg carbonate,

and silica-rich fluids. Mineral assemblages associated with S1, however, suggest an earlier isochemical greenschist facies metamorphism. B C

Texola mine Glen Ray mine Early Metamorphism

Poles to F6 axial planes (n = 26) Poles to F6 axial planes (n = 8) Evidence for an earlier isochemical greenschist facies metamorphism de- rives from volcanic rocks and dolostones within the Allamoore Formation. The F6 Fold axes (n = 29) F6 Fold Axes (n = 14) mafic volcanic suite typically has a mineral assemblage of biotite + chlorite + albite ± epidote ± tourmaline, indicating greenschist facies metamorphism. Figure 20. (A) Exposure of a domal structure in carbonate phyllite resulting from the interfer- Edwards (1985) also noted the rare presence of epidote, clinozoisite, and zoisite ence of two sets of F6 folds (in red) along the northeastern section of the Texola mine. Global positioning system unit for scale is 14 cm long. (B) Stereonet (equal area, lower hemisphere in Allamoore volcanic rocks further from the thrust, indicating low greenschist

projection) of F6 fold axes and axial planes from the Texola mine. (C) Stereonet (equal area, lower facies metamorphism. hemisphere projection) of F fold axes and axial planes from the Glen Ray mine. The wide spread 6 The dolostones have a mineral assemblage of talc + tremolite + quartz + of orientations is due to the domal nature of these folds. dolomite ± calcite, indicating greenschist facies metamorphism, and tremolite

is locally observed in limestones. In dolostones, the S1 foliation is defined pri-

TABLE 1. SUMMARY OF FOLD PHASES, ASSOCIATED FOLIATIONS, AND TECTONIC SIGNIFICANCE Fold description Foliation Tectonic signiﬁcance

F1 Isoclinal cm-scale folds only preserved in S1 deﬁned by recrystallized quartz, calcite, tremo- Ductile thrusting at depth altered dolostone lite and minor talc; annite, hornblende, and albite in maﬁc rocks

F2 Isoclinal “ﬂame” to closed meter-scale folds S2 dominant foliation in Allamoore rocks, cren- Peak period for talc mineralization, and thrusting at depth, possibly ulation cleavage in talc, elongate calcite and north directed dolomite, and minor aligned tremolite

F3 Isoclinal mm- to meter-scale folds S3 usually not present, locally present in the core North-directed thrusting oblique to Streeruwitz thrust trace of the fold

F4 Vertical isoclinal folds with consistent vergence No axial planar foliation developedOblique dextral wrench faulting along a WNW-trending high-angle and anastomosing, curved fold axes fault, transpression dominated, truncated by Streeruwitz thrust

F5 Chevron, kind, box-like folds, talc crenulations, No axial planar foliation developedRelated to the Streeruwitz thrust, hanging wall transported to the folds with curvilinear fold axes NNE to NE

F6 Two sets of large, broad folds No axial planar foliation developedF6 folds reorient all previous structures, fold Streeruwitz thrust into complex dome and basin structures; continued transpression

marily by tremolite and dynamically recrystallized quartz, with dolomite and prevalence of talc over tremolite could have been in response to more CO2-rich calcite as accessory phases. Mimetic replacement by talc is observed along the fluids and/or a decline in temperature. A major influx of silica-rich fluids- oc

margins of tremolite grains (Fig. 4C). The mineral assemblage defining S1 is a curred during the formation of S2, and most likely started during S1 formation. possible product of isochemical metamorphism, assuming that the siliceous Timing for silica-bearing fluid influx is supported by the metasomatism -ob

carbonate protolith had the appropriate bulk chemistry. At pressures of 2 kbar, served affecting an F2 folded mafic dike along the east wall of the Rosa Blanca tremolite forms at 450 °C and is stable up to ~650 °C and over a range of X mine (Fig. 5). S in an altered part of a dike is almost completely replaced by CO2 1 values (Winkler, 1979). Talc replacing tremolite occurs as retrograde reactions recrystallized microcrystalline quartz, indicating post-S1 metasomatism. S2 in as the temperature drops below 500 °C at values higher than ~0.2 X (e.g., talc the altered dike (axial planar to the F folding the dike) is defined by deformed CO2 2 is stable at lower temperatures, see Winkler, 1979). Below that X level, fluid quartz; thus the metasomatism of this dike was pre-S and post-S . CO2 2 1 composition becomes the main variable between the two stability fields, with Deformed limestone away from the Streeruwitz thrust contains as much

more water-rich fluid stabilizing tremolite and more CO2-rich fluid favoring talc. as 60% secondary quartz. Thus, silica-bearing fluids were widespread along Thus, the greenschist mineral assemblages displayed in dolostones and pre-Streeruwitz faults and affected rocks distal to the Streeruwitz thrust. Early mafic volcanic rocks may be the result of isochemical metamorphism. Identifi- quartz veins are folded, vein quartz has been dynamically recrystallized, and cation of tremolite in altered dolostone, limestone, and talc-bearing rocks indi- the early veins are cut by veins of albite and less deformed quartz, some of

cates that metamorphic conditions in the Allamoore Formation were a higher which are post F3 (Fig. 8B). grade than previously thought (e.g., Edwards, 1985). The purity of talc is observed to increase with proximity to the Streeruwitz thrust. The pre-Streeruwitz complex folding of talc and the truncation of these structures by the Streeruwitz thrust preclude this being an original composi- Siliceous Fluid Phase tional effect; therefore, it must be fluid controlled. The change from talc-rich phyllites to ceramic-grade talc involves a change from 80% tremolite to 60% The most dramatic effect of silica-rich fluid influx was the formation of eco- talc, along with a decrease in carbonate. The paint-grade talc found along the nomic talc bodies. Throughout the region, talc deposits form lenticular bodies thrust is 90%–100% talc. During fluid channelization along the fault, an increase

that pinch and swell from a few meters to as much as 60 m thick (Chidester et in CO2 content in water-rich fluids could cause tremolite to retrograde to talc.

al., 1964). The main foliation of the talc throughout the study area is S2. Rarely In summary, silica-rich fluid was dominant during the earliest ductile, lo-

preserved S1 foliation is defined by recrystallized tremolite and/or talc in 1F fold cally high-temperature deformational phase, occurring syn-F1 to pre-F3, and

hinges. Talc defining 2S is observed replacing the S1 tremolite, although trem- predominantly syn-F2. These conditions indicate greenschist facies metamor-

olite is also observed parallel to S2 (Fig. 7). Both tremolite and talc were stable phism during talc mineralization and silica-dominated metasomatism. The during the formation of both foliations, but talc clearly postdates tremolite, and increase in purity (paint-grade talc) immediately adjacent to the Streeruwitz

the main phase of talc mineralization occurred during the formation of S2. The thrust suggests additional synthrusting to postthrusting fluid flow.

Albitization tourmaline found throughout the region in both the hanging walls and footwalls of the thrust. Tourmaline has also been reported to form slickensides An alkali-rich fluid phase metasomatically altered rocks along the trace of along faults within the Hazel Formation (King and Flawn, 1953; Reid, 1974). the Streeruwitz and related thrusts in the foreland. Mineralization includes dis- Note, however, that the CMG jasperoid amphibolite (affected by albite- and seminated albite, veins of albite, tourmaline, and sodic-amphiboles richterite silica-rich fluids) is now juxtaposed with chloritized Allamoore basalt lacking and magnesioriebeckite. Albitization adjacent to the thrust appears to have albitization and siliceous alteration. This comparative relationship shows a dif-

had a protracted history. F2 folds of veins with albite as a minor constituent ference in the timing and spatial effects between these evolving fluids, indicat- indicates alkali fluids early in the deformational history; however, these are ing continued movement on the Streeruwitz thrust postdating local albite-rich

not as volumetrically significant as they are post-F2. Albite veins cut the S2 fluid influx and juxtaposition with rocks not affected by those fluids.

foliation, are observed offsetting an F3 fold (Fig. 8B), and crosscut early quartz Near end-member magnesioriebeckite (Davis, 2007) replaces albite and

veins, suggesting that the main alkali-fluid influx was at least post-F3. quartz in a siderite–iron oxide–dolomite carbonate. Magnesioriebeckite occurs

The presence of albite growth or replacement adjacent to the Streeruwitz as spherical clusters or as laths (Fig. 21) folded by F6 folds. Spherical clus- thrust and its general absence further from the thrust indicate that albite-rich flu- ters indicate that the minerals grew in an environment free from differential ids were focused along the Streeruwitz thrust. The presence of albite as an early stresses, and the amphiboles overgrow albite. These relationships suggest that cementing agent for associated breccias also implies that alkali-rich fluids were magnesioriebeckite in a host rock of dolomite and/or albite carbonate grew present during late thrusting. Tapering deformational twins, some bent, kink after the presence of albite-rich fluids, or late in the alkali phase. bands, and a minor degree of grain boundary migration (possibly enhanced by In summary, an alkali-rich fluid phase that was focused along the Streeruwitz fluids) within the albite indicate subsequent low- to medium-grade deformation thrust produced a protracted history of metasomatism. Although present during

conditions (300–500 °C) for some of the albite. Thus, although some albitization F2, the alkali fluids were most abundant post-F3 and pre-F6. Tourmaline seems to

occurred early, it is mainly a post-F3 to synthrusting (and F5) event. have crystallized just before or synchronously with albite, and magnesioriebeck- Further evidence of albitization and silica-rich fluids adjacent to the Streeru- ite crystallized afterward. Carbonate- and quartz-bearing veins cut albite veins, witz thrust is found in the highly metasomatized metarhyolite and amphibolite however, and cement breccias consisting of fragments with disseminated albite. units of the CMG exposed in the Texola and Rosa Blanca mines. These units are the least deformed and lowest metamorphic grade of the CMG. In altered me- tarhyolites, albite that postdates the foliation is commonly confined to veins, Carbonate Phase but also may exist as disseminated grains distinct from the earlier porphyroblasts wrapped by the foliation. CMG amphibolite is locally metasomatized to The final influx of fluids during deformation was dominated by iron-mag- a pink fine-grained rock that is difficult to distinguish from the metarhyolite nesium carbonate- and silica-rich fluids. These fluids mineralized the rocks in (Fig. 15). The altered amphibolite is now composed of quartz, albite, iron ox- veins and cemented breccias that were localized along thrusts, mainly the ides, and carbonate, and biotite is no longer present. Streeruwitz thrust. Carbonate and quartz veins crosscut all other veins and

Intense albite metasomatism associated with Streeruwitz thrusting altered foliations, implying that these fluids postdate F2 and most of the albitization. both CMG and Allamoore Formation rocks to a nondescript, pink fine-grained Furthermore, because these fluids acted as cementing agents for breccias lo- siliceous jasperoid, obscuring the normally sharp contact in the Rosa Blanca calized along thrust planes (Fig. 22), they are at least synchronous with if not

mine. Altered units have a groundmass of disseminated albite and quartz, and younger than the Streeruwitz thrust. F6 folding of tectonic slivers of cemented an absence of mafic minerals in the amphibolite units. Biotite inclusions in Allamoore breccia along the thrust implies that they were at least partially ce- metasomatic albite grains define the relict foliation. Metasomatic alteration mented at that time. All the carbonate and quartz grains associated with this decreases markedly away from the thrust. fluid flow phase are somewhat deformed, indicating that they predate the ces-

Tourmaline is in juxtaposed CMG and Allamoore Formation rocks, suggest- sation of motion along the Streeruwitz thrust or were deformed during F6. The ing that the two formations were contiguous during the alkali metasomatic deformational features of calcite, quartz, and the iron-magnesium carbonates phase. In the footwall, tourmaline commonly parallels the foliation but is demonstrate that they were deformed under low-temperature conditions. usually randomly oriented and occurs in veins that cut the foliation. Tourma- Several episodes of carbonate cementation of breccias were roughly co- line is interspersed with disseminated albite, and albite veins cut the tourma- eval with each other. The earliest appears to have been dolomite cementation line-bearing veins, implying that tourmaline may have precipitated early in (Fig. 22B), which was closely followed by siderite and quartz cementation. De- the alkali fluid phase. The best constraint on the tourmaline mineralization is formational features and mineralogical associations indicate that alkali-rich

post-S2 and F2 and coeval with the albitization phase. fluids were still present at the time of brecciation, but predate the cessation of King and Flawn (1953) proposed that Na-rich fluids introduced albite into movement on the Streeruwitz thrust. By the time the thrust sheet was folded, rocks adjacent to the Streeruwitz thrust coeval with fluids responsible for the the voluminous fluids percolating through the foreland had ceased.

A B

mr mr

mr 1 mm 0.5 mm

Figure 21. Photomicrographs, in cross-polarized light, showing the occurrence and morphology of magnesioriebeckite (mr, light blue mineral) from the Glen Ray mine. (A) Photomicrograph of Allamoore carbonate with matrix of dolomite, iron oxides, and albite. Magnesioriebeckite occurs as laths in thin layers, possibly veins, and as

clusters associated with albite or iron oxide minerals. (B) Photomicrograph of F6 (?) (red dashed line traces folded layering) fold of a limestone and basalt section from the Glen Ray mine. Magnesioriebeckite forms as distinct laths dominantly in the limestone section, with individual grains showing kinks and undulose extinction.

A B

CMG

5 mm

Figure 22. (A) Photograph of carbonate and quartz cemented breccia from the Allamoore Formation along the trace of the Streeruwitz thrust in the Rosa Blanca mine (compass for scale). Pink portions are dominated by disseminated albite. (B) Photomicrograph of Carrizo Mountain Group (CMG) breccia along the Streeru- witz thrust in the Texola mine. CMG metarhyolite in upper left is overprinted by two phases of carbonate mineralization. First phase is evident by smaller grains of dolomite (Do) in lower left, and the second phase by larger grains of ankerite (An) and/or siderite.

Deformational features of quartz that formed from the early silica-bearing surface exposure. Thrust tips were eroded into an alluvial fan complex that fluids and the presence of brecciated mylonites along the thrust indicate that became the Hazel Formation (Reynolds, 1985, 1988; Soegaard and Callahan, the rocks adjacent to the Streeruwitz thrust record a ductile to brittle history. 1994). Soegaard and Callahan (1994) interpreted the Hazel Formation as being deposited in a transpressional basin on the basis of temporal variation in clast composition and juxtaposition of source terrane and sedimentary DISCUSSION basin coupled with aggradation of megasequences. This deformation could be related to initial deformation at depth (ductile shearing and polyphase The Streeruwitz thrust is an out of sequence thrust that emplaces the CMG folding, phase 1 of this study; mylonitization of CMG rocks and beginning of metamorphic rocks on greenschist facies polydeformed Allamoore Formation exhumation) with north to northwest transport, compatible with oblique con- rocks, truncating all previous structures. The Streeruwitz thrust also truncates vergence (Fig. 23A). Observed transport in the hinterland (CMG) was to the folded syndepositional thrusts that involve the unmetamorphosed Hazel For- northwest within a dextral zone of transpression (Grimes, 1999; Grimes and

mation, therefore postdating thrusting at the surface. We interpret thrusting Mosher, 2003). The early structures (F1–F3 of this study) are compatible with that resulted in Hazel Formation deposition as an early surface manifestation northward transport and dextral transpression. The Hazel Formation has no of shearing at depth farther south that resulted in the ductile deformation in the clasts of CMG; therefore, the CMG must not have been exposed or in the near CMG and Allamoore Formation and ultimately led to emplacement of the CMG vicinity at this time, but it probably was in the processes of being exhumed along the Streeruwitz thrust. CMG rocks underwent ductile deformation and along oblique convergent shear zones. mylonitization from 1057 to 1035 Ma and were exhumed along the Streeruwitz Silica-bearing fluids of unknown origin metasomatically altered rocks ad- thrust between 1000 and 980 Ma (Grimes and Copeland, 2004). Mylonites near jacent to developing thrusts throughout the region. These fluids provided the the thrust yielded the youngest deformational (1035 Ma; Bickford et al., 2000) necessary silica and mobilized other ions for the formation of talc in the Al- and cooling ages (980 Ma; Grimes and Copeland, 2004). Note that Elston and lamoore Formation. The juxtaposition of silicified (and albitized) mafic CMG Clough (1993), using paleomagnetic data from the Hazel Formation, suggested rocks against Allamoore talc indicates that these fluids were widespread (i.e., that Hazel Formation deposition was between ca. 1100 and 1080 Ma; however, affected rocks later juxtaposed). This first deformational phase was dominated this timing would predate the deformation in the CMG and be unrelated to by ductile deformation, peak metamorphism that reached greenschist facies, emplacement of the Streeruwitz thrust and associated foreland thrust belt, and and an influx of silica-rich fluids. therefore is unlikely (see Mosher, 1998). As convergence and transpression continued, high-angle oblique- and/or strike-slip faults formed and were reactivated in the foreland (Figs. 23A, 23B). This second phase of deformation was dominated by steeply plunging folds Structural and Fluid Flow Evolution along high-angle west-northwest–trending dextral oblique- and strike-slip faults under brittle and/or ductile conditions, and marks a shift from thrust-re- Structural analysis of new exposures in talc mines along the trace of lated noncoaxial shear in phase 1 to transcurrent motion in phase 2. This de- the Streeruwitz thrust indicates that the Grenville-age foreland underwent formation is compatible with the dextral transpression observed in the CMG polyphase deformation in four distinct phases. Deformation was accompanied (e.g., Grimes and Mosher, 2003) and the proposed deposition of the Hazel For- by fluids with an evolving chemistry that were focused along foreland faults, mation in transpressional basins (Soegaard and Callahan, 1994; Glahn, 1997). including the major Streeruwitz thrust, and resulted in three phases of syntec- As shortening progressed, thin-skinned imbricate thrusts associated with

tonic metasomatism. Early polyphase folding (F1–F3, S1–S2) occurred at green- the thrust zone developed and cut upsection through the foreland (Fig. 23B).

schist facies and was accompanied by early silica-rich fluids syn-F1 and pre-F3, Allamoore and Hazel thrusts eventually ceased without the CMG being ex- resulting in formation of the economic talc bodies. Subsequently, oblique- and/ posed, and the thrust package was folded into conical folds, inverting much or strike-slip faults and associated shear zones deformed the earlier structures of the foreland (Fig. 23C) (King and Flawn, 1953; Reynolds, 1985, 1988). Con- producing vertical rollover folds. All these later structures were truncated at tinued exhumation along the Streeruwitz thrust moved rocks through a duc- a high angle by the Streeruwitz and imbricate thrusts. Alkali-rich fluids were tile-brittle transition.

locally present pre-F2, but predominantly caused metasomatism early in the Fluids focused along the developing Streeruwitz thrust evolved to an alka- evolution of the Streeruwitz and imbricate thrusts. Later during thrusting, the li-rich fluid phase (Fig. 23B). Albite was emplaced as veins and disseminated fluids became iron magnesium carbonate and silica rich. Lastly, the region was grains in the groundmass of rocks adjacent to the Streeruwitz thrust. The CMG folded into large-scale domes and basins. The model for structural and fluid adjacent to the Streeruwitz thrust was extensively altered, as were rocks in the flow evolution is discussed in detail in the following. associated shear zone. Tourmaline was precipitated along shear zones and in Initial deformation at shallow crustal levels in the foreland (Fig. 23A) be- faults further north of the Streeruwitz. King and Flawn (1953) noted tourmaline gan with thrusting in the Allamoore and Tumbledown Formations, causing slickensides in faults found in the Hazel Formation and thought that tourmaline

Allamoore/Hazel ACThrust Streeruwitz Thrust A.

Streeruwitz

Thrust 5 km 5 km Silica-rich fluids N N

Silica- & Carbonate-rich fluids

Allamoore/Hazel BDThrusts

StreeruwitThrust

N 5 km

Alkali-rich fluids 5 km N

Figure 23. Model of early evolution of the foreland. The Streeruwitz thrust (sensu stricto) is unlikely to be active at this time, but is shown to represent the relative position of rocks undergoing ductile deformation that are currently adjacent to thrust. (A) Phase 1, thrusting at depth in the Carrizo Mountain Group (CMG) and Allamoore Formation and shallower thrusting of Allamoore

and Tumbledown Formations that deposit the Hazel Formation during north to northwest transport in a transpressional environment. F1–F3 folds are developed during this event. Early silica-rich fluids mineralized talc and replaced mafic dikes. (B) Phase 2, transpressional environment and continued exhumation of the thrust currently beneath Streeruwitz thrust. Alkali fluids focused along faults are most active after this regime. (C) Phase 3, out of sequence Streeruwitz overthrust being exhumed, truncated structures in CMG and foreland rocks, and Allamoore-Hazel thrust sequence

deforming into conical folds, with associated carbonate and quartz cementing the breccias. F5 folds formed during this regime. Streeruwitz thrust was active. Note change in N arrow from A and B.

(D) Phase 4, continued transpressional environment in which the Streeruwitz thrust is folded into complex domes and basins by two sets of F6 folds.

and albitization were coeval. Sodic amphiboles, including magnesioriebeckite, Ma, and syntectonic to posttectonic plutonism continued until ca. 1070 Ma also were precipitated in veins and as spherical clusters in Allamoore rocks late (Mosher, 1998, and references therein; Mosher et al., 2008b). Deformation in during the alkalic metasomatism. the west Texas Van Horn exposures occurred from ca. 1060 to 980 Ma, thus As the out of sequence Streeruwitz thrust was emplaced over the foreland, postdating deformation in the Llano uplift by at least 60 m.y. (Grimes and Co- it truncated all early structures within the Allamoore Formation and the CMG peland, 2004). In the Llano uplift, kinematic and structural observations are

(Fig. 23C). Folds related to the Streeruwitz thrust (F5, this study), other kine- consistent with a model that associates deformation and metamorphism with matic indicators, and mapping by Reynolds (1988) all show that motion on a southern-colliding continent (Mosher, 1998; Mosher et al., 2008b), and are the Streeruwitz thrust had a north-northeast– to northeast-directed transport. not consistent with large zone of dextral transcurrent motion (Bickford et al.,

F1–F4 structures were truncated at a high-angle by the Streeruwitz and asso- 2000; see Reese and Mosher, 2004). The complex structural evolution with ciated imbricate thrusts, indicating a change in the overall tectonic transport changing directions of tectonic transport, documented in this study for the Van direction. Motion on the Streeruwitz thrust was at a high angle to the north- Horn exposures, is also inconsistent with dextral transcurrent motion as pro- west-directed transport in the hinterland (Grimes and Mosher, 2003). Elston posed by Bickford et al. (2000). and Clough (1993) noted a 30° clockwise rotation after deposition of the Hazel Any tectonic model of this region must account for the difference in tim- Formation, based on paleomagnetic data. In Bristol and Mosher (1989), a pro- ing of deformation between the central Texas Llano uplift (1150–1115 Ma) and gressive clockwise rotation of fold phases in the northwest Van Horn Moun- west Texas Van Horn exposures (1050–980 Ma) and different tectonic transport tains was noted. Both Glahn (1997) and Leaf (1999) noted a change in direction directions between the two areas. Such models must explain the temporal of principal stress in foreland rocks. Thus several lines of evidence indicate variation in tectonic transport from northwest-, to north-northeast–, to north- that there was a clockwise rotation of shortening directions that happened late east-directed motion, followed by continued transpression, as seen in the Van during the emplacement of the Streeruwitz thrust. Horn exposures. The simplest model consistent with these constraints is that a Breccias along the Streeruwitz thrust are cemented dominantly by iron-rich different continental block or island arc collided in what is now the west Texas carbonates and quartz, with some albite. Breccia clasts indicate a ductile to area ~60 m.y. after a southern continent collided in the Llano uplift area. It is brittle transitional history. The minerals cementing the breccias show features difficult, however, to explain the observed changes in tectonic transport ob- characteristic of brittle deformation, indicating continued deformation. The flu- served in the Van Horn exposures with collision of a single, separate block in ids that formed the cements may have been a mixture of meteoric water with that area. Thus we propose a modification to the indenter model as outlined connate alkali fluids already present within the rocks. in the following. The final phase of deformation in the foreland folded the Streeruwitz The Grenville-age orogenic events as recorded in the Llano uplift of cen- thrust sheet and adjacent rocks into complex domes and basins (Fig. 23D). We tral Texas are well explained by collision of a southern continental block and propose that reactivation and movement along preexisting strike-slip faults in an exotic arc terrane with the Laurentian continent between 1150 and 1115 Ma the foreland caused the formation of domes and basins after thrusting ceased, (Fig. 24; see Mosher et al., 2008b). We propose that subduction along the through transpressive inversion, as proposed by Allen et al. (2001) for south- southern margin of Laurentia, that resulted in collision of a southern con- ern Kazakstan. Glahn (1997, p. 76) noted two sets of transcurrent faults (north- tinent in what is now central Texas, continued along the rest of the plate east-striking faults with general sinistral motion, and northwest-striking faults boundary at least as far as present-day west Texas. The southern continental with general dextral motion) both sets postdating “major thrust sheet em- block underwent clockwise rotation and eventual collision along the rest of placement and subsequent folding.” In addition, Kwon (1990) and Soegaard its margin as this narrow basin closed in a “zipper-like” fashion (Fig. 24A). et al. (1993) proposed that many of the high-angle transcurrent faults have This rotation may have been enhanced by the slab breakoff and intrusion been reactivated numerous times and some show both dextral and sinistral of late syntectonic to posttectonic granites (1119–1070 Ma) in the Llano area motion. Cooling of the CMG through 300 °C is determined to have taken place (proposed in Mosher et al., 2008b). The rotation of this large-scale continental between 1000 and 980 Ma (Grimes and Copeland, 2004) as a result of exhu- block resulted in the change in tectonic transport directions seen in the Van mation along the Streeruwitz thrust. Thus, transpression must have occurred Horn exposures. During the initial closure, northwest transport and dextral after 980 Ma. transpression occurred in the CMG (ca. 1060–1035 Ma). As the intervening basin between the irregular margins of the two colliding continents closed, tectonic transport changed to north-northeast and then northeast directed Evolution of the Southern Margin of Laurentia (ca. 1035–980 Ma), forming the structures in the foreland. The interaction between these two plates as they collided reactivated preexisting faults and Our results show that previously proposed models (e.g., Mosher, 1998; caused transpression and formation of domes and basins in the foreland af- Bickford et al., 2000) must be reconsidered. In the Llano uplift of central Texas, ter 980 Ma. An additional possibility is that an island arc (or small continental the main collision-related deformation occurred between ca. 1150 and 1120 block) was present between the two colliding continents and collided, first

A ~1150-1120 Ma t B ~1150-1060 Ma Carrizo Mt. Gp. Allamoore Fm, Tumbledown Fm Laurentia Laurentia Llano Deformation Fron FM AF Arc CBP C C LU CM D CCD c 1 Streeruwitz Thrust 2 ~980 Ma b 1 N a ust Laurentia ContinentalCrust StreeruwitzHazel Fmthrust

Figure 24. (A) Tectonic map showing proposed plate configuration ca. 1150–1120 Ma. Northward movement of a southern continent relative to Laurentia (arrow 1) results in collision from the Llano uplift (LU) area to the Central Basin Platform (CBP), with northeastward tectonic transport in the Llano area and extension perpendicular to the plate margin near the CBP. Subduction continues along the western margin. Clockwise rotation of the southern continent during “zipper-like” closing of the remaining narrow basin (arrow 2), results first in northwest (2a), then north-northeast (2b), and then northeast (2c) tectonic transport from ca. 1035 to 980 Ma. CM—Carrizo Mountain Group; AF—Allamoore Formation; FM—Franklin Mountains; CCD—Coal Creek Domain. (Northern part of modern-day Texas is shown for reference; modified from Mosher, 1998.) (B) Schematic tectonic cross sections of the west Texas exposures ca. 1050–1060 Ma showing subduction under the southern continent and ca. 980 Ma, after collision. Possible thinned continental crust is shown for previously proposed backarc basin (see Mosher, 1998).

causing the northwest transport. Then final collision between the two conti- Rodinia reconstructions must satisfy constraints on the timing of deforma- nents resulted in north-northeast to northeast transport. Such an island arc tion and direction of tectonic transport documented along the southern mar- was proposed in the west Texas area on the basis of the small exposures in gin of Laurentia. Chihuahua, Mexico (Mosher, 1998). Interaction between a small intermediate block and the two larger continents would also help explain the later reactivation of preexisting faults with multiple senses of shear and formation of CONCLUSIONS the domes and basins. Reconstructions of Rodinia have changed over the past few decades The Grenville foreland exposed in west Texas underwent transpressional (summarized in the review by Evans, 2013), and still rely heavily on paleo- deformation that can be separated into four distinct phases; fluids with an magnetic data. Integration of geochronologic, structural, and metamorphic evolving chemistry were focused along faults and/or shear zones through- work in Grenville-age orogenic belts is essential in developing valid recon- out the deformation. Early polyphase ductile folding and foliation develop- structions. Future Rodinia reconstructions must satisfy the timing and kine- ment were concurrent with greenschist facies metamorphism and a major matic constraints identified in this study for Van Horn exposures in the west influx of silica-rich fluids that resulted in extensive metasomatism and the Texas area, as well as for the Llano uplift in the central Texas area. In the Llano formation of economic talc bodies. Observed structures are most compat- uplift, metamorphism (and associated deformation) did not occur until ca. ible with northward tectonic transport. These structures were deformed by 1150–1120 Ma. The earliest medium-T eclogites are indicative of subduction west- to west-northwest–striking, dextral transcurrent shear (fault) zones of continental crust. Subsequent regional amphibolite facies metamorphism forming vertical sheath-like folds, and were cut at high angle by the out of and deformation with northeast tectonic transport were followed by intru- sequence Streeruwitz and associated imbricate thrusts with north-northeast sion of syntectonic to posttectonic plutons (1119–1070 Ma) with a juvenile to northeast transport directions. Early in the thrust history, deformation was signature (Mosher et al., 2008b). In west Texas, metamorphism and deforma- more ductile and alkali-rich fluids caused further metasomatism. Later brittle tion did not begin until ca. 1060 Ma and were initially expressed as dextral deformation along the Streeruwitz thrust was associated with an influx of transpression and northwest-directed tectonic transport. After ca. 1035 Ma, Fe- and Mg-rich carbonate and silica-rich fluids. During the last deformation transport directions changed to north-northeast, then northeast in the fore- phase, the thrusts and all previous structures were deformed by complex land, with final uplift and exhumation occurring until ca. 980 Ma. Successful dome and basin formation, most likely as a result of movement on preexist-

ing transcurrent faults. The kinematic and structural analysis of the foreland, Geological Society of America South-Central Section Annual Meeting: Dallas, University of in comparison with the hinterland (CMG) and the Llano uplift of central Texas, Texas, p. 36–49. Evans, D.A.D., 2013, Reconstructing pre-Pangean supercontinents: Geological Society of America indicates that Grenville orogenesis along the southern margin of Laurentia Bulletin, v. 123, p. 1735–1751, doi:10.1130/B30950.1. involved arc-continent and continent-continent collision with a southern in- Ewing, T.E., 1990, Tectonic map of Texas: Bureau of Economic Geology, University of Texas at Aus- denter that subsequently underwent clockwise rotation, causing orogenesis tin, scale 1:750,000, 11 sheets. Glahn, J.E., 1997, Tumbledown Formation, Van Horn, Texas: Analysis of a Grenville-age transpres- in west Texas ~60 m.y. later. sive basin [M.S. thesis]: Richardson, University of Texas at Dallas, 117 p. Gore, L.D., 1985, The sedimentology, paleontology, and depositional environment of the Precam- brian Allamoore Formation, Culberson County, Texas [M.S. thesis]: College Station, Texas ACKNOWLEDGMENTS A&M University, 139 p. We thank Steve Whitmeyer, Tim Wawrzyniec, and Raymond Russo for their insightful reviews of Grimes, S.W., 1999, The Grenville orogeny in west Texas: Structure, kinematics, metamorphism this paper, and Mark Helper and Rich Kyle for their help with previous versions. We also thank and depositional environment of the Carrizo Mountain Group [Ph.D. thesis]: Austin, University Zemex Industrial Minerals for granting access to the talc mines and Steve Cox and Jonathon of Texas at Austin, 372 p. Gant for help in the field. This project was made possible by funding from a Geological Society of Grimes, S.W., and Copeland, P., 2004, Thermochronology of the Grenville Orogeny in west Texas: America Penrose grant. 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