NEW DETRITAL ZIRCON DATA FROM THE WESTERNMOST OUACHITA-MARATHON-SONORA SUTURE BELT: IMPLICATIONS FOR WESTERN LAURENTIAN TECTONICS AND CLOSURE OF THE RHEIC OCEAN

The closure of the Rheic Ocean and collision of Gondwana with Laurussia in Late Paleozoic time is considered the final stage in formation of the Pangean supercontinent. Increasing evidence indicates that the Ouachita-Marathon orogenic belt, which marks the suture between continents, extends well into Sonora and from there into Baja California (BC), Mexico, forming the Ouachita-Marathon-Sonora (OMS) belt. We report here on detrital zircon in a sedimentary unit from strata in the Sierra Las Pintas (SLP) in northern BC that likely correlate to strata in Sonora, and propose that the final western collision took place synchronously with earliest manifestations of the Cordilleran arc.

Low-grade metasandstone, metasiltstone and metalimestone with Middle Pennsylvanian-lower Permian crinoids exposed in the northern SLP are lithologically similar to Carboniferous turbidites and calcareous debris-flow deposits of the Rancho Nuevo formation in central Sonora. Zircon grains from a metasandstone sample from near the base of the turbiditic succession in the SLP were analyzed by LA-ICPMS and yielded 158 concordant ages that range from Archaean to Permian. Nearly 50 of the grains have ages from 285 – ca. 265 Ma with a maximum depositional age of 270±3 Ma (MSWD = 1.2).

Biostratigraphic and stratigraphic similarity strongly support correlation between strata in BC and those in Sonora that are assigned to the OMS. We propose that the collision between Gondwana and Laurussia, which younged toward the west, peaked in its westernmost extent in early Permian time, such that lithostratigraphic units are time transgressive between the two areas. Contractional forces on the transcurrent fault present along the western margin of Laurentia caused induced nucleation of subduction, and deposition of the youngest allochthonous strata in BC coincides with some of the earliest arc magmatism.