Tertiary Extension in Sonora, Mexico

Collaborative research with Mike Oskin (PhD, Caltech, 2001) focuses on several unmapped Tertiary basins in coastal Sonora that probably formed during late Miocene-Pliocene extension associated with rifting of the Gulf of California.  Steeply tilted basal unconformities are commonly exposed, revealing stratigraphically deep megabreccias and conglomerates overlain by a variety of volcanic flows.  These tilted strata are overlain by basalts of the Quaternary Pinacate volcanic field, which itself shows evidence of deformation.  One proposal we have submitted to NSF is designed to document variations in magnitude and timing of extension along a northeast-southwest transect through northwestern Sonora.

Simplified geologic map showing highly extended regions of coastal Sonora and southern Arizona.  Red boxes indicate locations of current research areas.

Geologic map of the Caborca focus area, showing the detached Tertiary, Cretaceous, and Paleozoic rocks of the Caborca basin relative to Mesozoic footwall rocks.  Modified from Figure 2 of Nourse (2001).

Below are some photographs showing typical landscape and field relationships

in the coast al Sonora extended terrane:

Normal fault displaces Early Miocene conglomerate in the Caborca basin

Lighthouse at Puerto Lobos on the Gulf of California.  View is to the northwest across east-dipping Upper Miocene andesite flows.

Tilted upper Miocene andesite flows near Rancho Golindrina

Rancho Golindrina, view looking  northwest toward unmapped tilt block

Abandoned ranch in center of valley east of Sierra Picu.