RIFT INITIATION AND EVOLUTION RECORDED IN THE OBLIQUE TUSAS-ABIQUIU SEGMENT OF THE RIO GRANDE RIFT, NORTH-CENTRAL NEW MEXICO

Understanding the tectonic processes during the development of a continental rift is critical for both scientific pursuits and social demands such as natural resources exploration and hazard assessment. We study the deformation history and associated magmatic and sedimentary records along the Tusas Mountain - Abiquiu Embayment segment, arguably the longest oblique segment, on the west flank of the Rio Grande rift in north-central New Mexico, USA, in order to deepen our knowledge on the initiation and evolution of continental rifts. Based upon field mapping and structural analysis, we identify six stages of structural events in the study area. (1) Discrete normal faults were generated at the onset of the Rio Grande extension along NE-SW direction, by reactivating pre-existing crustal weaknesses. (2) The normal faults propagated vertically and grew laterally, connected with neighboring faults, and established an oblique boundary of the Rio Grande rift by Late Oligocene. Stress field rotated clockwise, and extensional fault-propagation folds, soft-linkages, and hard-linkages were developed under near E-W extension. (3) Syn-kinematic growth strata of the Late Oligocene – Early Miocene age were deposited in the shallow Abiquiu Embayment, tapping the coeval Latir Volcanic Field to the northeast, while the border fault system continued to accommodate extensional strain. (4) The extension migrated basinward, generating distributed, rift-parallel normal faults with small offsets. Basinward tilting of rift strata also occurred. (5) As the Rio Grande rift evolved into a more mature stage, the extensional strain was accommodated predominantly in the major axial basins, while tectonics and deposition in the Abiquiu embayment waned and faults abandoned. (6) The arrival of the Jemez Mountains Volcanic Field loaded a large pile of igneous rocks to the south of the Abiquiu area, causing elastic flexure and brittle faulting in the outskirt area of the Valles caldera. This study portrays the tectonic sequences at the embryonic and young stages of a continental rift, and demonstrates the geological significance of early-stage, shallow platforms that are preserved on the flanks of many rift systems.