Zircon (U-Th)/He Thermochronologic Investigation of the Long-term Thermal History of Proterozoic Rocks in West Texas and Southern New Mexico

The tectonic history of west Texas and southern New Mexico is well documented, however existing geologic data only provide limited snapshots of the region’s prolonged and complex tectonic evolution. Zircon (U-Th)/He (ZHe) thermochronology is used to examine the long-term thermal history of Proterozoic rocks exposed in the southern Rio Grande rift. ZHe data were collected from the Carrizo Mountains (10 dates), Cookes Range (16 dates), and Franklin Mountains (30 dates) in order to constrain the long-term (\u3e1 Ga) thermal history of rocks in this region and relate this history to known tectonic or climatic events. Individual ZHe dates range from 6-731 Ma in the Carrizo Mountains, 44-446 Ma in the Cookes Range, and 19-649 Ma in the Franklin Mountains. Each of these datasets show negative correlations with effective uranium concentrations. Forward and inverse modeling of samples from each of these ranges successfully constrains the \u3e1 Ga thermal history of each of these ranges and documents periods of exhumation and/or burial associated with Precambrian assembly of the North America, the Paleozoic Ancestral Rocky Mountain event, Paleozoic-Mesozoic sedimentary burial, the Laramide orogenic event, and formation of the Rio Grande rift. The final inverse model for the Carrizo Mountains suggest a large pulse of cooling from ~1100-1000 Ma, likely associated with the collision of a microcontinent with the southern margin of Laurentia at that time. Farther to the north, thermal history modeling of the Franklin Mountains documents a previously unrecognized period of Precambrian burial and subsequent uplift. This may have been associated with the formation of an orthogonal rift during final assembly of Rodinia, or alternatively with the breakup of Rodinia. In the Cookes Range, the Precambrian thermal history is somewhat unconstrained. However, exhumation likely occurred during either final assembly or breakup of Rodinia. These times of exhumation in all three ranges are therefore likely a record of the assembly and breakup of supercontinents. The data do not support models where Precambrian erosion was a result of Snowball Earth glaciations