Proterozoic Basement Geology of the Northwest Sonora-

Southwest Arizona International Border Region

Since 1997, Cal Poly Pomona students and I have been mapping the lithology and structure of Paleoproterozoic and Mesozoic crystalline rocks exposed in several ranges straddling the northwestern Sonora-Arizona border (see photographs from this remote field area).  These rocks are exciting because they constitute the most southwestern Proterozoic basement known in this part of the Laurentian craton.  Two USGS collaborators (Wayne Premo and Alexander Iriondo) are helping me characterize the geochronology and isotope geochemistry of this belt to create a detailed "fingerprint" useful in reconstructions of the pre-750 Ma Rodinian supercontinent.  The rocks are superbly exposed and only modestly overprinted by Phanerozoic thermal and structural activity.  Some typical outcrops are shown below after the maps.  

This work is finally in press in GSA Special Paper #393!!  (Click here for link to PDF of related article)

Proterozoic rocks in this area have provided  undergraduate senior thesis projects for three students.  New U-Pb SHRIMP  geochronological results from Sierra Hornaday and Cerro El Aguila  were recently presented at GSA by student Miguel Espinoza.  Miguel continues to document relationships in these isolated ranges at the edge of the Pinacate volcanic field.

Index map showing locations of two current research areas in northwestern Sonora.  (Click here to view article on Upper Jurassic stretched pebble conglomerate of the Caborca area)

Generalized geologic map of  the northwest Sonora-southwest Arizona international border region, showing location of focus areas and dated samples.  This map plus more detailed geologic maps of Figures 3 and 6 are presented in GSA Special Paper #393, Chapter 4. 

Below are photographs illustrating representative field relationships:

Panorama of Sierra Los Alacranes, viewed from the southwest.   Augen gneisses from the northwest and southeast ends of the range yield U-Pb SHRIMP ages of 1722+/-12 Ma and ~1725+/-19 Ma, respectively.  The dark colored mafic body that transects the center part range has yet to be dated.

View of Cerro los Ojos showing 1.1 Ga(?) diabase dikes intruding Paleoproterozoic metasedimentary rocks and monzogranite.  View is toward the west.

Xenolith of Paleoproterozoic meta-arkose included in nonfoliated 1432 Ma coarsely porphyritic biotite granite

Pre-1.73Ga mafic gneiss interlayered with quartzofeldspathic gneiss

Folded dike of 1725 Ma granite augen gneiss intruded into ampibolite/mafic gneiss and quartzofeldspathic gneiss country rocks

Click here to view additional photographs from this remote and beautiful desert field area!