High heat flow in the Jornada del Muerto: a region of crustal thinning in the Rio Grande rift without upper crustal extension

Abstract

The Jornada del Muerto is a large basin located in the Rio Grande rift of southern New Mexico. Although syn-rift crustal thinning is likely in the region, geologic data suggest little if any syn-rift tectonism has occurred in the upper crust. This implies crustal thinning occurred by processes affecting the mid to lower crust. There is little syn-rift subsidence or sedimentation and little Cenozoic volcanism in the immediate study area; instead, the Jomada del Muerto seems to be a stable crustal block exhibiting remnant Laramide compression. Interestingly, high heat flows are estimated regionally in the area (~95 mW m −2), with very high heat flows in the southern Jornada del Muerto (~ 125 mW m −2). The N-S heat flow profile across the study area has a small half width (~ 5–11 km) which implies relatively shallow thermal sources. From analysis of possible surface anomalies generated by subsurface isothermal steps, one may suggest that the thermal sources generating the very high heat flows must be within ~ 10–15 km of the surface. A high crustal thorium concentration, measured in Precambrian rocks at one of the heat flow sites, may indicate that the surface heat flow anomaly in the southern Jomada del Muerto is caused by radiogenic anomaly. A variety of other heat sources also seem possible, such as: convective overturn in the ductile mid to lower crust, mid to lower crustal intrusion, and volatile streaming. It is suggested that these crustal heat and mass-transfer mechanisms are in turn generated by deeper thermal sources. Possibilities might include a mantle plume, or local thickening of a massive lower crustal tabular intrusion thought to underlie large regions of the southern Rio Grande rift.