HR: 0800h
AN: T51A-01    [Abstracts]
TI: The Half-Graben Structure of Bahia de Banderas, Mexico
AU: * Alvarez, R
EM: rab@leibniz.iimas.unam.mx
AF: Union Geofisica Mexicana Roman Alvarez, IIMAS-UNAM Circuito Interior, CU, Mexico, DF 04510, Mexico
AB: Bahia de Banderas contains important records of the detachment and initial separation of Baja California from mainland Mexico; however, its relief and structure are poorly known. The structure has been identified with a canyon associated with a fault striking E-W, named Banderas fault; it has also been proposed that the canyon may be a graben but no structural model was presented. To help in the description of its relief, a digital elevation model of the bay is constructed with individual soundings, ships soundings from 1970 to date, and satellite- derived depths, which shows an irregular topography of the canyon along the fault. The deepest portion of the bay reaches 1600 m; the south flank of the canyon dips at angles ranging from 15° to 22°, while the north flank ranges from 5° to 9°. This asymmetry between the flanks first suggested that the structure of the canyon might be that of a half-graben. A model is presented based on previous developments for half-graben structures of the fault growth type with reverse drag geometry; the model is based on the lengthening of the fault through seismically induced slip events; a seismic study of the area reported elsewhere complements the model and shows that the region is active. For Banderas canyon the footwall corresponds to the south flank, and the hanging wall corresponds to the north flank, including the shallow platform to the north of the bay. A fault length of 63 km is inferred and pertinent parameters are derived for the model calculations. Theoretical profiles are superposed and compared to actual topographic profiles of the canyon, concluding that the model describes well the central part of the structure, within 14 km of its mid-point in either direction, reproducing with less accuracy the geometries at the ends of the active portion of the fault, which is attributed to the perturbing effects of additional faults acting on those regions. The model allows for calculations of the age of the half-graben; however the parameters required for such a calculation are not well constrained at present and assumptions are made regarding the frequency of slip events along the fault. The maximum displacement of the fault at its center is determined as 2800 m, indicating an accumulation of sediments and slumped materials of 1200 m thickness, with respect to the oeanic depth of 1600 m reported above; assuming a rate of slip events of one every 5000 years the age of the fault is close to 10.6 M years. If this rate were changed to one slip event every 3000 or 7000 years, the corresponding ages would be of 6.36 and 14.8 M years. These results support the hypothesis that the structure of Banderas canyon is a half-graben, and strengthen the idea that it is the limit between the region to the north that underwent extension in the Miocene, and the region to the south that did not experienced it.
DE: 0905 Continental structures (8109, 8110)
DE: 8109 Continental tectonics: extensional (0905)
DE: 8110 Continental tectonics: general (0905)
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Tectonophysics [T]
MN: 2007 Joint Assembly