LACCOLITH HOST ROCK JOINT SET ANALYSIS: A FINITE ELEMENT MODEL AND FIELD OBSERVATIONS

Limited research has focused on the micro- & macro-structural deformation mechanisms that accommodate folding and flexure of the roof rocks above laccolith intrusions. 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 via finite element modeling? We have measured bedding plane attitudes, 1700 fractures/joint sets, and at least 4 major faults in the Cretaceous siliciclastic host rocks of the McKinney Hills laccolith in Big Bend National Park. Mapping was focused into 5 structural domains that span the western margin of the intrusion. Balanced cross-sections reveal a shallow laccolith body roughly 760 m thick from the present exposed roof to the floor with a mean horizontal diameter of 3000 m. Bedding plane dips consistently increase toward the laccolith from sub-horizontal orientations (800 m outward) to approx. 50 degrees near the intrusion contact. The estimated host rock lithostatic load (909 m of vertical section) at the time of emplacement is 20.2 Mpa. The excess vertical magma driving pressure was calculated at 3.11 Gpa. The poles to fracture planes, collected from the structural domains around the intrusion, reveal a reoccurring NW trend. Restoring the fracture orientations within the domains to horizontal reflects the preferred NW trend. Sub-joints, as related cross-joints or an overprint episode, maintain similar attitudes. Using a finite element model with the program FEMLAB, we have constructed and calculated the stress distribution within the host rocks above the intrusion associated with flexure from emplacement. We have modeled the stress concentrations around different geometries of assumed pre-existing fracture sets in a 2 dimensional model, therefore simulating depositional or regional tectonic jointing. By comparing the stress concentration factors or fiber strength of the rock to the modeled stresses, we question if the joints or sub-joints were formed by emplacement of the laccolith or are merely reactivated structures from previous Laramide contraction in the Trans Pecos region.