DEFORMATION AND METASOMATISM OF THE GRENVILLE-AGED FORELAND FOLD AND THRUST BELT OF WEST TEXAS

Mesoproterozoic rocks exposed in West Texas provide a unique exposure of the Grenville-aged foreland fold and thrust belt along the southern margin of Laurentia that has undergone a complex history of deformation and metasomatism, and has not been significantly modified by later events. Low-grade metasedimentary and metaigneous rocks of the Allamoore Formation record early polyphase deformation and regional metamorphism, which resulted in formation of low-grade talc deposits throughout the region. The major out-of-sequence Streeruwitz thrust juxtaposes high-grade metamorphic rocks of the ~1.35 Ga Carrizo Mountain Group against the ~1.25 Ga Allamoore Formation. Channelization of fluids resulted in high-grade talc deposits along the Streeruwitz thrust.

Detailed mapping and sample analysis in mines along 11 km of the Streeruwitz thrust trace, and up to 2 km away, reveal a consistent regional deformation, whereas increased metasomatic alteration is limited to rocks adjacent to the Streeruwitz thrust. At least four phases of pre-Streeruwitz deformation are observed within the foreland rocks, recording changes in the style of deformation with time. Regional metamorphism formed low-grade talc deposits through interaction of a silica-rich fluid with Mg-carbonate sediments. F₁/S₁ is preserved in dolostones within the talc bodies of the Allamoore Formation. The dominant foliation in the region (S₂) is well defined by alignment of talc axial planar to F₂, and is folded by two later phases. The Streeruwitz thrust and associated imbricate thrusts truncate S₂ as well as post-S₂ folds. Early fluid migration led to localization of high-grade talc along the footwall of the thrusts and albitization of rocks in the hanging-wall. Late stage thrusting resulted in significant brecciation along the Streeruwitz fault with associated cementation and alteration in the hanging-wall rocks. Together, this indicates that the thrusts were a dominant fluid migration pathway during orogenesis. Final Grenville-aged deformation of the area is recorded as domes and basins that fold the Streeruwitz thrust sheet.