Crustal structure and location of the Ouachita orogenic belt in northern Mexico

Abstract

The location of the Paleozoic southern margin of Laurentia in Mexico has been much debated. Determining its location is important for the evaluation of hypotheses that suggest large pieces of southwestern North America were translated toward South America during the breakup of Laurentia (early Cambrian) and Pangea (early Mesozoic) and that there were large lateral movements in northern Mexico in Mesozoic time that displaced the Ouachita orogenic belt. Using limited Paleozoic outcrops, lead isotopic data, regional seismic data and regional gravity anomalies, we show that the Ouachita orogenic belt and the Laurentian margin in Mexico trend either directly south into Coahuila or south-southwest across Chihuahua toward Durango from the Big Bend region of west Texas. In order to evaluate these two possibilities, integrated crustal models were constructed along three profiles in northern Mexico and west Texas. Because constraints on deep structure are sparse, each profile could be modeled with either a southward or a south-southwestward extension of the Ouachita orogenic belt. The southward extension model suggests that two regional gravity minima delineate a Paleozoic foreland basin (Mapimi basin) along the Chihuahua/Coahuila border, and an accreted terrane (Coahuila terrane) in western Coahuila. The south-southwestward extension model suggests that these gravity minima both delineate accreted terranes. We favor a south-southwestward extension because we can trace the gravity maximum associated with the known Ouachita interior zone 300 km into eastern Chihuahua and south toward Durango in a way that is consistent with trends in lead isotopic data, pre-Mesozoic geologic data from drill holes and outcrops, and Ouachita-style lithologies and structures found in western Sonora. Spanning the gap between the postulated Ouachita orogenic belt in southern Chihuahua with possible similar lithologies in Sonora with a northwest-trending margin provides a simple way to complete our picture of the southern portion of Laurentia during the early Paleozoic.