A COMPREHENSIVE TECTONIC/MAGMATIC MODEL FOR CENOZOIC TRANS-PECOS TEXAS AND ADJACENT MEXICO

Southwestern Texas and adjacent Mexico were profoundly affected by subduction, flattening and subsequent floundering of Farallon oceanic lithosphere. Flattening of subduction led to northeastward expansion of convergent tectonics involving folding of Cretaceous sedimentary units, mostly in Mexico, but also in the Big Bend country extending into early Cenozoic time. Tectonic activity was accompanied by distal Laramide magmatism involving copper porphyry emplacement.

Cenozoic magmatism, beginning about 47 Ma in the Trans Pecos, involved foundering of the subducted slab, which allowed ascent of hot, asthenospheric mantle under the Trans Pecos, causing partial melting of overlying lithospheric mantle. Lithospheric melts interacted with a profound tectonic grain of the Trans Pecos inherited from its Precambrian through Phanerozoic history. In the Big Bend south of the Ouachita suture, mafic magmatism was expressed as basaltic lava units and associated differentiated intrusions. Farther north, undisturbed Grenvillian lithosphere prevented early ascent of asthenospheric magmas; instead, volcanism was delayed about 10 Ma as basaltic magmas were trapped within the lithosphere and eventually manifested in widespread, voluminous eruption at 37 Ma of flood rhyolite of the Davis Mountains volcanic field. North of the Grenville suture, magmatism was delayed further and expressed as minor alkaline intrusions, which formed a linear belt some 400 km long, extending south from the New Mexico border into Big Bend. The concluding phase of Cenozoic activity involved a 10-Ma-long southwestward retreat of silicic magmatism into Mexico as foundering continued, uplift with concurrent basaltic magmatism, which ended ~17 Ma, and continued normal faulting to the present day.