The Empalme graben, Sonora, México: magmatism and extensional tectonics associated with the initial opening of the Gulf of California

Abstract

The Empalme graben on the Sonoran coast is interpreted as a structure associated with the opening of the Gulf of California. The graben is approximately 25 km wide and 50 km long, and is limited by two faults striking N–S; one located along the east flank of the Sierra de Santa Úrsula and the other one located along the west side of the Sierra del Bacatete. Both ranges consist of felsic to intermediate volcanic flows and tuffs of Miocene age (11–23 Ma), which unconformably overly late Cretaceous intrusive rocks (63 Ma), and are in turn overlain by 8.5 Ma-old basalts flows. Dikes and small stocks of rhyolitic composition (14 Ma) displaying a granophyric texture intrude the Miocene volcanic section along the edges of the graben. Isolated exposures of granophyre were also identified within the Empalme graben, which are generally overlain by basaltic flows. The younger rocks correspond to calcalkaline and minor tholeiitic basalts, which range in age from 8.5 to 10.5 Ma. The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotope ratios suggest rift-related tholeiitic basalt signatures. These basalts are mostly tilted to the east, and were displaced by the normal faults associated with the younger extensional event recorded in the graben. Because of the widespread cover of recent sediments along the Sonoran coast, the limits of the Extensional Province of the Gulf of California are not well known. However, on the basis of our observations, in this work we propose to reduce the size of this extensional province in the Sonoran coast, to an approximately 100 km-wide belt, parallel to the coast. This is proposed because out of this belt there are no evidences of structures related to the opening of the Gulf of California.