Paleomagnetic study of the eastern sector of chapala lake and implications for the tectonics of west-central mexico

Abstract

Western Mexico is characterized by several large-scale tectonic depressions which have been interpreted in terms of active continental rifting, tectonic transpression and coastal sinistral lateral transport of terranes. In this paper we report results of a paleomagnetic study of 148 samples from 22 sites in the Neogene volcanics from the eastern sector of the Chapala graben (western end of the E-W Chapala-Tula fault zone). Characteristic remanent magnetization directions have been isolated after detailed thermal demagnetization for sixteen sites. Six sites present reverse polarities and ten sites present normal polarities. Two sites show a low-latitude VGP and are considered as transitional. The normal and reverse polarity directions are almost antipodal. The overall characteristic pole position for the normal and reverse polarity sites, N = 16, p LAT = 74° N, P long = 160° E, K = 25 and A 95 = 7.6°, lies to the left of the Neogene segment of the North American or northern Mexico apparent polar wander path, which suggest the occurrence of a counterclockwise rotation of −15.5 ± 7.4 to −16.5 ± 6.9°. Results are interpreted in terms of counterclockwise vertical-axis rotation associated with regional transtension and left-lateral shear as a result of oblique subduction of the Cocos plate along the Middle American trench. We suggest that the Chapala graben developed early in the Miocene, in a left-lateral strike-slip environment within the regional E-W Chapala-Tula fault zone. Results support recent studies that propose relative motion for southern Mexico along the volcanic arc and trench-parallel strike-slip faulting.