CONTROL BY PRE-EXISTING STRUCTURES on THE SOUTHEAST DIRECTED PROPAGATION AND TERMINATION OF THE CHALK DRAW FAULT, TRANS-PECOS TEXAS

The Cenozoic Chalk Draw Fault in Trans-Pecos Texas accommodated Basin and Range extension and consists of two main segments; (1) an east-west striking, north dipping portion along its northern half, and (2) a ~36 km northwest striking, northeast dipping normal fault in the south. Numerous studies have focused on the influence of pre-existing features (i.e. earlier structural features and Tertiary plutons) on the geometry and kinematics of faults in the region. In this study we address the role of pre-existing structures on the evolution of the northwest striking portion of the Chalk Draw Fault.

Field observations show Mode I fractures in Tertiary igneous bodies do not continue into surrounding Cretaceous sedimentary rocks, and Mode I fractures are not confined to fault zones but are pervasive throughout the igneous bodies. The pervasive Mode I fractures in the Rosillos Laccolith and Nine Point Mesa Sill are interpreted to be cooling fractures which formed under a Basin and Range stress field prior to the initiation of faulting. Additionally geological cross sections indicate an along-strike asymmetry in throw, with an abrupt decrease in throw across the Chalk Draw Fault to the southeast, within the Rosillos Laccolith.

We interpret that the evolution of the northwest striking portion of the Chalk Draw fault has been heavily influenced by pre-existing features in two ways: (1) The older fractures are interpreted to have aided in the propagation of the Chalk Draw Fault southeast towards the Rosillos Laccolith during extension. (2) After the Chalk Draw Fault propagated into the Rosillos Laccolith, the pre-existing fracture network then acted as a barrier to continued fault growth towards the southeast. Results of this study show the importance of pre-existing fracture networks in the development of Basin and Range normal faults within the Trans-Pecos Texas region.