SEISMIC REFLECTION IMAGE OF THE JEMEZ LINEAMENT OF NEW MEXICO: EVIDENCE FOR A SOUTH-DIPPING PALEOSUBDUCTION BOUNDARY BETWEEN THE YAVAPAI AND MAZATZAL PROVINCES

A 160 km long seismic reflection profile across the Jemez lineament, an inferred Proterozoic province boundary in northern New Mexico, shows a marked change in crustal-scale reflection geometries that coincides with the surface trace of the Lineament. To the north, upper crustal reflections image 2 km of Paleozoic strata that are underlain by a set of north-verging recumbent folds and related thrusts in the Proterozoic Ortega Quartzite that match surface structures. South of the Jemez Lineament, Paleozoic strata are almost entirely absent and the upper crust is largely transparent. Within the middle crust of the northern half of the profile lies a 5-km-wide south-dipping set of reflections that extends from 10 km depth to the north to about 35 km depth under the Jemez Lineament. We interpret this feature to be a paleosubduction zone that marks the thrusting of the Mazatzal province northwards over the Yavapai province at about 1.65 Ga. To the south, north-dipping reflectors at mid crustal depth define an antiformal feature that is interpreted as a south-verging thrust belt imbricated during accretion of Matzatzal crust at ca. 1.65 Ga. High amplitude reflectors that crosscut these features over much of the profile are interpreted to represent basaltic dikes and/or magma of either 1.1 Ga or Cenozoic age. These intrusions may have taken advantage of zones of weakness created by older structures in their ascent towards the Ocate and Raton volcanic fields. This study supports the hypothesis that older assembly features dominate lithospheric structure to great depths and have been important in influencing younger tectonism. Diminished reflectivity in the lower crust along the profile is associated with a 10-km thick high-velocity (> 7.0 km/s) layer found at the base of the crust in a velocity model derived from refraction data.