Much of northern and central Mexico underwent east-northeast extension in the mid- to late Cenozoic; this area constitutes a major but little-recognized part of the Basin and Range province. The extended region is bounded on the east by the Laramide thrust front of the Sierra Madre Oriental. On the west, the relatively unfaulted Sierra Madre Occidental separates the extended area in central Mexico from that around the Gulf of California. Extension occurred as far south as what is now the Trans-Mexican volcanic belt, and in Oaxaca, south of the belt. The Basin and Range province in Mexico constitutes approximately half of the 19 x 105 km2 of western North America that underwent mid- to late Cenozoic extension. North-northwest orientations of numerous epithermal vein systems indicate that east-northeast extension began as early as 30 Ma in areas north of the volcanic belt. Major episodes of faulting began at about 23 to 24 Ma and 12 to 13 Ma, both in Mexico and in the southwestern United States. Faulting was commonly accompanied by eruption of alkali basalts typical of intraplate rifting. Widespread Quaternary fault scarps and alkali basalts indicate that extension continues to the present in the region north of the Trans-Mexican volcanic belt. In contrast, the tectonics in and south of the belt are now probably related to subduction of the Rivera plate. Contemporaneity of the 12 Ma episode with early extension around the Gulf of California attributed to Pacific-North American plate boundary reorganization suggests that extension is related predominantly to plate boundary effects. The beginning of extension at ∼30 Ma may be related to initial encroachment of the East Pacific Rise upon the trench that lay off western North America.

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