Geophysical modeling of Valle de Banderas graben and its structural relation to Bahía de Banderas, Mexico

Abstract

A gravimetric survey consisting of five lines and 483 stations, as well as a magnetotelluric (MT) survey consisting of 17 observation sites, were made in the Valle de Banderas region for the determination of the structural characteristics of the valley. Additionally, data from a previous aeromagnetic survey were analyzed to correlate them with our geophysical measurements. The gravimetric and MT models confirm that the valley corresponds to a graben structure with slumped blocks that vary from ~2000 m deep close to Bahía de Banderas to shallow burial depths (~100 m) toward the NE end of the valley. Faults flanking the valley, inferred from the gravity and magnetic models, trend toward offshore faults in Bahía de Banderas, indicating a structural connection between land and sea. From the MT measurements we conclude that a 2-D resistivity behavior is observed within the graben whereas outside of the graben the behavior is 1-D, in spite of the rough topography of this region. Our gravimetric models also indicate the occurrence of basin-like structures within the graben, coinciding with similar structures reported offshore in the Bahía de Banderas, suggesting that such structures may be typical erosional features of the graben. The aeromagnetic analysis correlates with the gravimetric and MT models, and suggests that the graben structure is an extensional zone on granitic crust; it also indicates that the extensional zone continues to the NE, beyond the topographic limits of Valle de Banderas on land and towards Bahía de Banderas seawards. We propose that this region belongs to the same tectonic structure in spite of an approximate change in orientation of 30° between the bay and the valley, and strengthen the idea that this graben constitutes part of the NW limit of the Jalisco Block. A tectonic model for the region is presented at various stages, from around 100–75 Ma to the Present. We propose that initiation of the Bahía de Banderas structure started at ~14 Ma along an E-W trend, and that at ~5 Ma the graben axis was reoriented to a SW-NE direction, that induced the formation of Valle de Banderas.