COMPLEX STRUCTURAL AND FLUID FLOW EVOLUTION ALONG THE GRENVILLE FRONT, WEST TEXAS

A narrow (~7 km wide) fold and thrust belt in west Texas that represents the northern most 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 domes and basins. Fluids with an evolving chemistry were channelized along the thrusts metasomatically altering the adjacent rocks.

A minimum of four phases of folding and metasomatic reactions and veining within foreland rocks records changing kinematics and evolving fluids 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 aligned axial planar to F₂ forming the dominant S₂. The Streeruwitz thrust and subsidiary imbricate thrusts truncate vertical S₂ striking slightly oblique to the thrust transport direction and post-S₂ vertically plunging folds, 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 produced mylonites in both footwall (syn-S₂) and hanging wall, which were later brecciated in the final stage of thrusting. Further evolution of fluids along the thrusts is recorded in the breccias starting with albite- or quartz-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 imbricate thrust sheets into complex domes and basins.

Although the deformation history may be compatible with the previously proposed transpressional model, the degree of complexity and interplay of structurally controlled fluids requires a more complex kinematic model.