Late Jurassic Pull-Apart Basins in Northern Sonora, Mexico

Since the mid 1990's, collaborative research with Tom Anderson of the University of Pittsburgh has focused on a group of Mesozoic sedimentary  basins in southern Arizona and northern Sonora that appear to have common pull-apart origins.  According to Anderson's original hypothesis (1994), these basins accumulated in left-stepping releasing bends near faults associated with the Late Jurassic Mojave-Sonora megashear.  The ductilely deformed remnants of one large basin preserved in the lower plate of the Magdalena metamorphic core complex may represent a late Jurassic pull-apart basin, based on its gross stratigraphy and regional outcrop  patterns.  See my 1995 GSA Special Paper article for more details.

In several ranges north of Caborca I have been mapping a deformed conglomerate belt of controversial age and paleogeographic setting.  These rocks are situated immediately northeast of the trace of the hypothetical Mojave-Sonora megashear.  My recent article in Journal of South American Earth Sciences proposes that this conglomerate belt was deposited in a Late Jurassic transtensional basin.  Debate on its origin continues because of structural overprinting and the fact that Mesozoic conglomerates with four different stratigraphic ages exist in northern Sonora.  Future stratigraphic and provenance studies, detailed facies mapping, and ⁴⁰Ar/³⁹Ar dating of intercalated volcanic strata should resolve the issue. 

A recent article by Anderson and Nourse (GSA Special Paper 393, Chapter 3) provides current details of the pull-apart basin hypothesis.     

Below are some maps showing areas of current interest in Arizona and Sonora:

An updated version of the above geologic map, showing proposed southeastward extension of the Mojave-Sonora fault system  to the Gulf of Mexico, constitutes Plate 1 of GSA Special Paper 393, Chapter 3

Portions of the green areas mapped above contain Upper Jurassic stretched-pebble conglomerate and volcanic flows, hypothetically deposited in a pull-apart basin.  See my 2001 Journal of South American Earth Sciences article for detailed maps, cross sections, and discussion.