OFR-578 refines understanding of the geologic architecture within Rio Grande del Norte National Monument

A technical completion report for the village of Questa NM released as OFR-578, by the NM Bureau of Geology, in close collaboration with U.S. Geological Survey, refines our understanding of the geologic architecture of the new Rio Grande del Norte National Monument. The report contains two oversize color plates; one is a preliminary geologic map of the Questa and Guadalupe Mountain quadrangles; the second contains three, parallel, geologic cross sections. These east-west sections are unique, detailed depictions of the architecture of this part of the San Luis Basin of the Rio Grande rift. They combine insights gleaned from: 1) new, detailed (1:24,000) geologic mapping through the STATEMAP and FEDMAP programs; 2) a high-resolution aeromagnetic survey; 3) new ground-based magnetic traverses; 4) a new gravity model of the San Luis Basin; 5) a new volcanologic model of the Taos Plateau volcanic field; and 6) a conceptual model of the stratigraphy and structural geology of the southern San Luis rift basin.

Important Findings

1. The new geologic map of the Questa area has revealed many details of the complex Cenozoic history of this part of the Rio Grande rift, including the development of the Questa sub-basin, the eruptive history of the Taos Plateau volcanic field, the evolution of the Rio Grande and its gorge, the geomorphic history of the Red River valley, the spread of syntectonic alluvial fans, and the connections among tectonism, volcanism, sedimentation, and erosion.
2. The detailed geologic mapping shows that the course of the ancestral Rio Grande has changed through time. The ancient, small, and slow-moving river was temporarily blocked by basalt ledges that were uplifted along the southwestern side of the Red River fault zone. Downstream of the fault zone, the high gravels deposited by the ancestral Red River continue southward within the area now containing the Rio Grande gorge, indicating that the headwaters of the middle-Quaternary ancestral Rio Grande watershed were not the San Luis Basin, but instead the Red River watershed. Eventually, the Red River cut down through the basalt ledge, capturing the northern Rio Grande watershed, and greatly increasing the amount of flow in the ancestral river system.
3. Although the topographic expression of the Rio Grande rift (from the foothills of the Tusas Mountains to the foothills of the Sangre de Cristo Mountains) at the latitude of Questa is about 27 miles wide, the principal structural rift basin (from the Rio Grande gorge to the foothills of the Sangre de Cristo Mountains) is only about 6 miles wide.
4. The central San Luis rift basin does not conform to the classic, asymmetric “trap-door” rift geometry. Rather, the deepest part of the basin (under Guadalupe Mountain) is flanked by nearly symmetrical, highly faulted, pre-rift basement rocks.
5. The deepest basin-fill in the Questa area is about 4000 ft thick.
6. Faults defined by geologic mapping show a good correlation with faults defined by aeromagnetic modeling. In many cases, the ends of mapped faults can be extended laterally with a high degree of confidence by using the aeromagnetic interpretations.
7. According to the gravity model and the conceptual structural model of the area, buried, medium-displacement, normal faults are likely ubiquitous throughout the structural rift basin.
8. The existence of single, large-displacement “master” faults along the topographic east edge of the rift is problematic. Although some large faults do separate basin fill and bedrock along the Sangre de Cristo fault zone, in general, extensional strain seems to be distributed among large numbers of complex normal faults that branch, curve, and terminate.
9. The normal faults located within the basement rocks of the Sangre de Cristo Mountains are important rift structures, and actually represent a large component of the cumulative throw on the Sangre de Cristo fault system. These faults can be difficult to identify and characterize, due to the general lack of marker stratigraphy in the crystalline rocks.
10. Guadalupe Mountain consists of three, distinct eruptive lobes that can be mapped at the surface and in the subsurface by high-resolution aeromagnetic methods.
11. The subsurface shapes of the local volcanoes are complex, and represent an elaborate interaction of volcanism, sedimentation, tectonics, and erosion during Pliocene and Pleistocene time.
12. Modern gravity modeling is a powerful, and essential, tool for drawing deep cross sections through the Rio Grande rift, especially in areas where deep boreholes are scarce.
13. Modern, high-resolution aeromagnetic and ground-based magnetic modeling is a compelling tool for characterizing the buried structural geology and buried volcanic rocks in the Rio Grande rift.