Structural Style in an En Echelon Fault System, Balcones Fault Zone, Central Texas: Geomorphologic and Hydrologic Implications


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Detailed geologic mapping in the Balcones fault zone in the San Marcos area has revealed a structural style that may have had a profound effect on the geomorphologic and hydrologic evolution of the area. Two major en echelon step-fault zones are present in the area, and a highly faulted ramp structure has formed in the zone between the en echelon fault zones. Differential erosion of rock units in the ramp structure may have determined the course of a stream that captured the Blanco River from an easterly flow direction into the Onion Creek basin to its current southeasterly flow direction. Subsequently, the Blanco may have "tapped" the Edwards Aquifer by down-cutting or side-cutting action at or near the San Marcos Springs location. Thus, both the capture of the Blanco and the current location of San Marcos Springs may have been indirectly caused by the local structural setting between the two major en echelon fault zones of the Balcones system. Similar major ramp structures are apparent by map inspection in at least three other locations in the Balcones fault zone, one near Austin and two west of San Antonio. A fourth structure may also be present near New Braunfels.

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