Abstract

CARCIUMARU, Dana  and  ORTEGA, Roberto. Paleostress and Geometric Analysis of the Southern Rio Grande Rift in the Northern Chihuahua Trough: Franklin and Potrillos Mountains as examples. Bol. Soc. Geol. Mex [online]. 2017, vol.69, n.3, pp.669-689. ISSN 1405-3322.  https://doi.org/10.18268/bsgm2017v69n3a9.

We present a deformation history of Potrillo and Franklin Mountains in El Paso, TX, using paleostress and geometric analysis. Application of paleostress and geometric analysis shows that low angle normal faults exposed at Anthony Gap and regions to the north of the Franklin Mountains record differential displacement that increase from North to South along the strike. As displacement on the faults increases, the hanging walls of the faults rotated counterclockwise and transverse faults which accommodate flexure developed as a consequence of that movement. The hanging wall of the faults rotates in a counter-clockwise direction and the transverse faults accommodate the developing flexion. The transverse faults are thought as the result of an isostatic rebound of the footwalls due to the tectonic denudation of the hanging wall. The average elastic thickness of the lithosphere is 23 km. This flexure and isostatic rebound take place in a late stage which was concurrently developed to the mountain range genesis. However, transverse faults do not cut the low angle faults suggesting that the two sets of faults are part of the same system. Increase in displacement along the faults is mirrored by an overall increase in elevation in the Franklin Mountains, which suggests that fault activity is critical to the uplift of the mountain range with respect to the surrounding basins. Estimates of the stresses from small scale faults in the Franklin Mountains indicate that the greatest extension direction was ENE-WSW and the greatest compression direction was subvertical. Low angle normal faults in the Potrillo Mountains belong to two different deformation episodes. The first family of faults occurs along the eastern side of the mountain range. This is cut by younger NNW trending normal faults which bound the range at the East. Fault slip analysis, calcite vein arrays and the geometry of the faults indicate that the first set of low angle normal faults record NNE directed extension and vertical shortening rotated approximately sixty degrees in a counter-clockwise sense as inferred from the trend of the east Potrillo fault. The second generation of faults have strike parallel slickenlines which trend to the ESE. Fault slip inversion indicates that these faults were formed in response to left oblique slip, with a moderately Southeast plunging shortening direction and a shallowly plunging WNW trending extension direction. These observations are most consistent with the first low angle faults in the Potrillo Mountains tracking an early phase of extension distinct from low angle faults in the Franklin Mountains. The second set of faults may have formed in a similar stress field to that recorded in the Franklin Mountains. The combined dataset is most consistent with two phases of extension across low angle normal fault in the southern Rio Grande rift, the first was caused by NNE extension, and the second by ESE directed extension.

Keywords : geometric analysis; low angle normal faults; paleostress; Rio Grande rift.

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