Lithospheric thinning and the late Cenozoic thermal and tectonic regime of the northern Rio Grande rift
— R. Joe Bridwell

The Rio Grande Rift is a system of narrow, elongate, en echelon tectonic rift valleys extending 1000 km from Colorado to Mexico and separating the Colorado Plateau-Basin and Range provinces from the Southern Rocky Mountains-Great Plains provinces (Fig. 1). These rift valleys are bounded by normal faults and are located between vast, elevated regional uplifts (Kelly, 1952; Chapin, 1971). Regional extension at the north and south ends of the rift began 23-29 m.y. B.P. (Lipman and Mehnert, 1975; Chapin and Seager, 1975) followed by basaltic andesite volcanism 26-20 m.y. B.P. and accelerated basaltic volcanism at 5 m.y. B.P. (Chapin and Seager, 1975), with olivine tholeiites on the Taos Plateau (Lipman, 1969). Hence the Rio Grande Rift is a Cenozoic continental feature involving rift tectonics similar in many respects to other components of the world rift system such as the East African Rift, the Baikal Rift and the Rhinegraben.

 

Current geophysical data across the northern Rio Grande Rift provide a picture of a crust smoothly varying in thickness from 40 km in western New Mexico to 50 km in eastern New Mexico, overlying an upper mantle whose velocity varies from 7.9 to 8.1 km/s (Roller, 1965; Stewart and Pakiser, 1962). Seismicity generally occurs along the western boundary of the rift (Olsen and others, 1976) indicating tectonism west of the Rio Grande. Interpretations of local gravity anomalies along the rift indicate basins, ranging in depth from 2 to 4 km, are filled with continental sediments and volcanics and bounded by normal faults (Andreasen and Kane, 1961; Joesting and others, 1961; Cordell, 1976). Within the rift, microearthquake studies (Sanford and others, 1973) and seismic refraction pro-files (Toppozada and Sanford, 1976) indicate an upper crustal velocity of approximately 5.8 km/s relative to 6.1 km/s external to the rift.

 

The general picture is not consistent with surface structure, seismicity and heat flow. A decrease in crustal velocities within the rift (Sanford and others, 1973) indicates extension and possible thinning of this zone. Microearthquake reflections show an interface at 18 km between Socorro and Albuquerque, with reduced capabilities to propagate S waves (Sanford and others, 1973). Surface heat flow data from Reiter and others (1975) indicate a narrow geothermal anomaly of high heat flow extending from Colorado to southern New Mexico along the western boundary of the rift. Broader anomalies occur over the Zuni Mountains, southern Rocky Mountains and northeastern New Mexico.

 

In this paper, a crust-mantle structure beneath the Colorado Plateau, northern Rio Grande Rift and southern Rocky Mountains is derived from long wavelength negative Bouguer gravity data. Two-dimensional fits of calculated and measured surface heat flow using finite element modeling and gravity models support thinning of the lithosphere and intrusion of material from the low velocity zone into the upper mantle and lower crust beneath the rift. The time term inherent in the requirement of an anomalous heat source to fit the rift thermal anomaly will be used to indicate the need for initiation of convection of material into the lower crust in late Cenozoic time (15 m.y. B.P.) to be consistent with the magnitude of the present thermal anomaly