POLYPHASE FOLDS AND FAULTS WITHIN A WRENCH FAULT ZONE, NORTHERN BIG BEND NATIONAL PARK, TEXAS

Geologic mapping at 1:12,000-scale of a 62 km² area that includes Dog Canyon, N Dagger Mountain, and the S Persimmon Gap laccolith has revealed details of three deformation phases in Cretaceous and younger rocks. The map area is in the E margin of the Texas Lineament, a Precambrian – Recent transpressional and transtensional fault zone that spans all of Big Bend National Park. First-phase (D1) structures are outcrop-scale penecontemporaneous folds found only within the Boquillas Formation. Second-phase (D2) map- and outcrop-scale folds contain NNW-striking, steeply dipping axial planes and subhorizontal fold axes. D2 folds display 90° average interlimb angles and constant limb thicknesses. Two distant localities in Sierra del Carmen contain folds similar in orientation and style. D2 thrust faults dip 2 – 27° NE and cross-cut and are folded by D2 folds. Third-phase (D3) map- and outcrop-scale folds contain NW-striking, steeply dipping axial planes and subhorizontal fold axes. D3 folds display 120° average interlimb angles and constant limb thicknesses. D3 high-angle faults strike N-S to NW, cross-cut D2 thrusts, and contain abundant slickenlines indicating normal offset and rare strike-slip slickenlines. Tertiary dikes and sills mapped to date intruded between D2 and D3: one sill cross-cuts a D2 fault and D3 faults cross-cut the South Persimmon Gap laccolith. Plutons did not reorient existing structures. D2 structures formed by Laramide compression while D3 structures formed during Basin and Range extension. Dominant normal offset on high-angle faults in this area suggests overall displacements were partitioned into separate zones of normal and strike-slip offset. If all intrusions are related to nearby dated Dagger Flat sills (Morgan and others, 2006) and the Rosillos laccolith (Miggins, 2006), then in this area Laramide deformation is pre-31-32 Ma and Basin and Range folding and faulting is post-31-32 Ma and locally Quaternary in age.