Complex structural and fluid flow evolution along the Grenville Front, Trans-Pecos Texas

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Davis, Benjamin R., 1977-

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A narrow (~7 km wide) fold and thrust belt in west Texas that represents the northernmost extent of the Grenville collision belt along the southern margin of Laurentia (Grenville Front) records a complex history of deformation and fluid flow. The main Streeruwitz thrust that emplaced ~1.35 Ga high-grade metamorphic rocks over ~1.25 Ga foreland sedimentary and volcanic rocks postdates polyphase deformation in the footwall and is itself complexly folded into local domes and basins. Deformation can be characterized by four stress regimes. A pre-Streeruwitz ductile deformation phase, dextral oblique strike-slip motion, deformation associated with Streeruwitz thrusting, and formation of complex domes and basins resulting from continual transpression. Fluids with an evolving chemistry were channelized along the thrusts, metasomatically altering the adjacent rocks. Deformation consisting of six phases of folding, metasomatic reactions, and veining within foreland rocks record a change in kinematics and evolving fluid chemistry with time. An early phase of siliceous fluids during D2 caused replacement of mafic dikes and dolostones that preserve F1/S1 and formation of extensive talc lenses aligned axial planar to F2, forming the dominant S2. Subsequent upright vertical sheath folds, F4, indicate dextral oblique motion on high-angle west-northwest-trending faults. The Streeruwitz thrust and subsidiary imbricate thrusts truncate F1-F4 folds at a high angle requiring a change in the overall kinematics of the deformation. Late chevron to box folds and kinks of S2 are generally associated with shears locally related to thrusting. Initial thrusting at depth produced mylonites in both footwall (syn-S2) and hanging wall rocks that were later brecciated in the final stage of thrusting along the Streeruwitz. Further evolution of fluids along the thrusts is recorded in the breccias and veins, starting with siliceous- and alkali-rich fluids. Lastly, carbonate-rich fluids replaced footwall rocks and cemented breccias in both the hanging and footwalls. Final deformation of the area folded the Streeruwitz and imbricate thrust sheets into complex domes and basins trending SW and SE. The degree of structural complexity and fluid flow interaction requires a new kinematic model for the southern margin of Laurentia, one where a north-verging indenter collided with southern Laurentia and then under went clockwise rotation. The disparity in timing of deformation, and of kinematics, between west Texas and the Llano uplift of central Texas requires active subduction in west Texas after collision in central Texas. Furthermore, the complex structural histories and differences in kinematics with time between the hanging wall and the footwall of the Streeruwitz thrust can be explained by clockwise rotation of an indenting continent after docking on southern Laurentia.