Paleogene Extension in the Southern Basin and Range Province of Mexico: Syndepositional Tilting of Eocene Red Beds and Oligocene Volcanic Rocks in the Guanajuato Mining District

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Abstract
The Guanajuato region, located in the southern part of the Basin and Range tectonic province, is characterized by a rhombohedral fault pattern. Extensional faulting-as well as related uplift, erosion, and deposition of continental clastic deposits-was in progress at 49 Ma, as indicated by K-Ar dating of intercalated mafic volcanic lavas. Earliest extension is registered in the lower member of the Guanajuato conglomerate and produced NW-trend-ing, down to the southwest, normal faults and northeast tilting of the hanging-wall clastic sequence. Beds in the upper member of the Guanajuato conglomerate display lateral thickness variations, changes in size and composition of the clasts, and ENE and SE inclination, all of which suggest activity along a second set of NE-trending younger faults. Extension continued also after emplacement of the overlying Bufa ignimbrite (∼37 Ma, K-Ar). Dip differences between the conglomerate and the Calderones Formation (30 Ma), which is part of the volcanic sequence that conformably overlies the Guanajuato conglomerate, indicate at least 13° and probably more than 20° of rotation prior to 37 Ma. Consistent northeast tilting in the Calderones Formation indicates that subsequent extension preferentially reactivated the NW-trending faults. This sequence of deformational events produced the rhombohedral fault pattern observed in the region. Later pulses of extension (late Oligocene and younger) produced simultaneous movement on both sets of faults, as suggested by others. In addition to this strong evidence for Paleogene extension in the Guanajuato region, indication of Paleogene extension also may be present at scattered localities throughout central and southern Mexico, where continental red beds occur stratigraphically between pre-Cenozoic rocks and Tertiary volcanic rocks (Eocene-Oligocene). Detailed structural and stratigraphic data are lacking for these localities, but they may provide an important record of a regional Paleogene extension episode that has been largely overshadowed by the obvious evidence for extension during the Neogene.