Lithospheric Structure from the Jemez Lineament to the Cheyenne Belt, Southern Rocky Mountains

We present the modeling and interpretation of a 955km long refraction profile and a 170km long deep reflection profile acquired as part of the Continental Dynamics of the Rocky Mountains (CD-ROM) project. The refraction profile extends across the major features of the Rocky Mountain region, from the Jemez Lineament (JL), in northern New Mexico through the Proterozoic Mazatzal and Yavapai Provinces, to the Cheyenne belt in southern Wyoming. The reflection profile parallels the refraction profile in northern New Mexico and targets the lithospheric structure of the Jemez Lineament. We inverted travel-time data from the 10 refraction records using both layer based travel-time inversion and first-arrival travel-time tomography methods. In the shot records we identified two refracted arrivals, one from the crystalline crust and one from the upper mantle (Pg, and Pn) as well as prominent reflections from the middle crust (PcP) and from the Moho (PmP). We modeled the Bouguer gravity along the profile using the interfaces of the layer based refraction model and densities derived from a standard density-velocity relationship. Calculated gravity fits the observations to +/- 14 mgal. The upper and middle crust shows low velocities (6.0-6.2 km/s) beneath the JL and at the center of the profile, beneath the Colorado Mineral Belt. Upper and mid crustal velocities increase (6.2-6.5 km/s) north of the Cheyenne belt and south of the JL, where the deep reflection profile images a Proterozoic crustal duplex. The lower crust across the profile exhibits a relatively low velocity, with an average velocity of 6.7 km/s, and a variable thickness, with the thinnest section (10 km) beneath the JL and the thickest section beneath the northern Colorado Mineral Belt (22 km). Moho depths vary from near 40 km beneath the JL to more than 50 km in the northern half of the profile. Upper mantle velocities are low (7.85 km/s) with the lowest velocities (7.70-7.76 km/s) found beneath the thinnest crust (40-41 km) across the JL. The average CD-ROM one dimensional velocity function has lower values for the crust and the upper mantle than the Christensen-Mooney global average velocities for continents and for orogens. Low velocities in the upper and middle crust can be explained by widespread Proterozoic and Phanerozoic felsic batholiths. Low velocities throughout the crust are also a result of high heat flow (63 to >105 mWm-2) in the Rocky Mountains south of the Cheyenne Belt. Attenuated velocities of the lower crust and the upper mantle, together with the Quaternary basalt flows, recent uplift and high heat flow, suggest temperatures in the mantle that are above the peridotite solidus, particularly beneath the JL. The reflection profile in the JL images bright linear reflections at mid-crustal depths which we interpret as modern mafic sills, consistent with the hypothesis that the mantle beneath the JL is partially molten and provides basaltic melt to the crust.