LARGE SCALE VERTICAL AXIS ROTATIONS IN COASTAL SONORA: EVIDENCE FOR TRANSTENSIONAL PROTO-GULF DEFORMATION

The Sierra el Aguaje is located in southwestern coastal Sonora, Mexico, a few tens of kilometers north of the town of Guaymas. The range represents the eastern rifted margin of the Gulf of California and is one of the few parts of that margin that is entirely above water, with new ocean crust of the Guaymas basin lying just offshore the western edge of the range.

The Sierra el Aguaje is composed of volcanic units and their corresponding volcaniclastic units that are the result of persistent magmatic activity between 20 and 8.8 Ma. Based on cross cutting relations and geochronologic data for pre-, syn-, and post-tectonic volcanic units, most of the faulting and tilting in the Sierra El Aguaje is bracketed between 11.9 and 9.0 Ma, thus falling entirely within Proto-Gulf time. Existing field relations suggested the possibility of large (>45°) vertical axis rotations in this region. This evidence includes: a) abrupt changes in the strike of tilted strata in different parts of the range, including large domains characterized by E-W strikes b) ubiquitous NE-SW striking faults with left lateral-normal oblique slip, that terminate against major NW-trending right lateral faults, and c) obliquity between the general strike of tilted strata and the strike of faults.

A preliminary investigation into these possible vertical axis rotations in the Sierra el Aguaje has uncovered paleomagnetic evidence of large clockwise rotations of ~107° ±~26°, and no discernable translation. The rotations occurred after 12 Ma and largely prior to 9 Ma, thus falling into the Proto-Gulf period. These results are consistent with field-based structural and kinematic observations for the study area. Such large-scale rotations lend credence to the theory that the area inboard of Baja California was experiencing transtension during the Proto-Gulf period, rather than the pure extension that would be the result of strain partitioning between Sonora and the Tosco-Abreojos fault offshore Baja California.