Seismic Structure of the Jemez Lineament, New Mexico: Evidence for Heterogenous Accretion and Extension in Proterozoic Time, and Modern Volcanism

Southwestern North America is the result of a long and complex geologic history that spans from the Proterozoic time, when assembly of the southwestern part of the continent began, to the present. Geological and geophysical observations suggest that the lithospheric structures produced during the assembly of the continent profoundly influenced subsequent modifications to the southwest. The primary objective of the Continental Dynamics of Rocky Mountains (CDROM) project is to investigate the processes that have produced the present structure of the Rocky Mountains lithosphere and to understand the legacy of the Archean and Proterozoic accretionary boundaries. One of the enigmatic features investigated in CDROM is the Jemez Lineament, an 800 km long alignment of Tertiary volcanic centers that extends across Arizona and northern New Mexico following the southern margin of the Yavapai-Mazatzal Proterozoic terrane boundary. The Jemez lineament was the target of deep seismic reflection and crustal refraction profiling. The reflection profile extends about 150 km parallel to the front of the southern Rocky Mountains and crosses the southern edge of the lineament at high angle near Las Vegas NM. The seismic reflection profile exhibits a striking difference in reflectivity and crustal structure north and south of Las Vegas. To the north the reflection profile images a broad, south dipping, strongly reflecting, ramp structure, traceable to depths of 30-32km. The ramp is overprinted in places by a complex set of bright layered reflections. We interpret the south-dipping ramp as a suture formed during Proterozoic island arc accretion and the bright reflections as Jemez lineament recent intrusives that have ponded at several crustal depths, and are present locally in outcrop. We speculate that the intrusives used the Proterozoic suture as a pathway through the crust to the surface. To the south, the entire middle crust is characterized by a 35 km wide antiform that may have formed during island arc accretion. Asymmetry of the north side of the antiform may be indicative of extensional shearing at ca. 1.4 Ga, as is observed in outcrop in the Sandia Mountains to the west. The reflection profile also shows distinct deep crustal reflectivity north and south of the southern edge of the Jemez lineament. To the north the lower crust is not defined by any single reflection or reflection zone. Rather the reflectivity dies out at about 30-35km where weak reflectors are observed. The southern part of the profile shows a more abrupt change in reflectivity at around 35-38 km depth. The nearly coincident seismic refraction profile has defined a high velocity lower crustal layer below depths of 35 km, and total crustal thickness of 45-50km.