HOST ROCK FRACTURE ANALYSIS, MCKINNEY HILLS LACCOLITH, BIG BEND NATIONAL PARK, USA

Structural analysis of joints sets and brittle faults adjacent to a laccolithic intrusion was undertaken to evaluate the nature and geometry of the stress field attending magma emplacement and regional deformation during the Cenozoic in Big Bend National Park. Based on fractures and joint set orientations alone, is it possible to differentiate between prior/subsequent tectonic episodes of strain versus deformation overprinted onto the host rocks by flexure from the intrusion? To what magnitude and extent are emplacement-related fractures distributed along the flanks of the deflected host rocks? Research was focused within the Cretaceous and Tertiary siliciclastic host rocks around the NW margin of the Oligocene McKinney Hills laccolith. More than 1700 fractures/joint sets, associated bedding plane orientations and major NW-trending faults have been recorded and placed into 5 structural domains spanning 225 degrees around the intrusion. Balanced cross-sections reveal a shallow intrusion roughly 762 m thick from the present exposed roof to the inferred floor. Bedding-plane dips consistently increase toward the laccolith from sub-horizontal orientations (800 m from the contact) to approx. 50 degrees at the intrusion contact. Contact relations indicate that the laccolith cuts across different units within the sedimentary sequence and so did not preferentially intrude a rheologically weaker stratigraphic horizon. A minimum estimate of the overlying sedimentary thickness during laccolith inflation is 976 m, corresponding to a lithostatic load of 20.2 MPa. The poles to fracture planes reveal a reoccurring NW-trend in all domains. Restoring the fracture orientations by un-tilting the deflected host rocks yields a similar strong preferred NW orientation. Sub-joints, as related cross-joints or an overprint episode, maintain similar attitudes. The NW-trending fracture sets likely reflect regional tectonic stresses associated with basin/range related extension and/or Laramide contraction. However, N-trending radial fractures in host rocks at the northern margin of the laccolith may reflect emplacement related strains.