Complex structural and fluid flow evolution along the Grenville Front, west Texas

Abstract

A narrow (∼7 km wide) fold and thrust belt in west Texas that represents the northernmost extent of a Grenville-age collisional belt along the southern margin of Laurentia (Grenville Front), records a complex history of deformation and associated fluid flow. The Streeruwitz thrust that emplaced ca. 1.35 Ga high-grade metamorphic rocks over ca. 1.25 Ga foreland sedimentary and volcanic rocks postdates polyphase deformation in the footwall and is complexly folded into domes and basins. Four phases of tectonism, recording a changing kinematic setting, affected the area and formed: (1) pre-Streeruwitz ductile polyphase folds (F 1 –F 3 ) and associated foliations (S 1 –S 2 ) consistent with northward tectonic transport; (2) dextral oblique-slip, high-angle, west-northwest–trending faults associated with upright vertical sheath folds (F 4 ); (3) Streeruwitz and related subsidiary imbricate thrusts that truncate F 1 –F 4 folds at a high angle and cause localized folding (F 5 ) consistent with north-northeast to northeastward tectonic transport; and (4) complex southeast- and northwest-trending domes and basins (F 6 ) of the thrusts, resulting from continual Grenville-age transpression. Fluids with an evolving chemistry over time were channelized along the thrusts, metasomatically altering the adjacent rocks. Early siliceous fluids caused replacement of mafic dikes and dolostones that preserve F 1 and S 1 and formation of extensive talc bodies with talc aligned axial planar to F 2 , forming the dominant S 2 fabric. Initial thrusting at depth produced mylonites in both footwall (syn-S 2 ) and hanging-wall rocks that were later brecciated in the final stage of thrusting along the Streeruwitz thrust. Further evolution of fluids along the thrusts is recorded in altered rocks adjacent to thrusts, breccias, and veins, starting with silica- and alkali-rich fluids. Lastly, carbonate-rich fluids replaced footwall rocks and cemented breccias in both the hanging wall and footwall. This study documents a previously unrecognized complex structural, metamorphic, and metasomatic history, and fluid evolution in the foreland. This history, coupled with differences from that in the overriding older metamorphic rocks, requires a new kinematic model for the southern margin of Laurentia. In addition, the disparity in deformation timing and kinematic evolution between west Texas and the central Texas Llano uplift requires active subduction in west Texas after collision in central Texas. We propose that collision of a north-verging continental indenter with southern Laurentia initially occurred in the Llano area (ca. 1150–1120 Ma) and that continued subduction along strike caused clockwise rotation of the indenting continent and collision in west Texas (ca. 1060–980 Ma).