The Aljibes half-graben; active extension at the boundary between the trans-Mexican volcanic belt and the Basin and Range Province, Mexico

Abstract

The Aljibes half-graben is located 140 km west-northwest of Mexico City and is characterized by several parallel, > or =10-km-long, south-dipping, and east-striking normal faults. Late Miocene basalt flows with super 40 Ar/ super 39 Ar ages of 7.7 + or - 0.1 Ma and 7.1 + or - 0.5 Ma are displaced along these faults. With reference to this basalt, the composite throw and heave of the half-graben are 480 m and 270 m, respectively, and the relative extension is 5%, whereas the vertical slip rate along the faults of the Aljibes half-graben after the deposition of the basalt is > or =0.07 mm/yr. The Aljibes half-graben is located at the northern margin of the trans-Mexican volcanic belt and perpendicularly intersects north-striking normal faults of the Basin and Range Province. The relative sequence of activity of the two stress fields and fault systems cannot be resolved in the study area, and the two systems may have been active simultaneously. We explain this structural configuration by horizontal migration of the boundary between the two stress provinces with time, which requires intermittent permutations between the intermediate and least principal stresses sigma sub 2 and sigma sub 3 . This is supported by the low-stress ratio phi = sigma sub 2 -sigma sub 3 /sigma sub 1 -sigma sub 3 obtained from modeling the stress tensor associated with the slip along the faults of the Aljibes half-graben. The orientations of the fault planes and slip vectors of the Aljibes half-graben are practically identical to those of the nearby Acambay graben and the Venta de Bravo fault, which caused the Acambay earthquake of November 19, 1912 M sub S = 6.9, and the Maravatio earthquake of February 22, 1979 m sub b = 5.3. Furthermore, the Aljibes half-graben forms the extrapolated western continuation of the northern master fault of the Mezquital graben, which ruptured partly in the Cardonal earthquake of March 26, 1976 m sub b = 5.3. This suggests the Aljibes half-graben may also be seismically active, although no historical earthquake is known to have occurred along this structure. Further evidence for the likely seismic activity of the Aljibes half-graben are two east- to east-northeast-trending epicenter alignments between the Aljibes half-graben and the Mezquital graben, and structural indication for stick-slip behavior. Based on the structure of the Aljibes half-graben, we believe the maximum credible earthquake that may occur along the faults of the Aljibes half-graben is on the order of 2 X 10 super 27 dyne.cm > or = M sub 0 > or = 6 X 10 super 25 dyne.cm and 7.5 > or = M sub w > or = 6.5.

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