New Mexico Geological Society Annual Spring Meeting — Abstracts

Recent geologic mapping and geophysical investigations have elucidated the structure and upper Santa Fe Group stratigraphy of the Velarde graben, which in turn has refined interpretations regarding late Cenozoic rift tectonism. The Velarde graben is a 6-10 km-wide, NE-trending extensional feature in the northern Española Basin of the Rio Grande rift. We restrict the Velarde graben to a 19 km-long gravity low between the town Hernandez to the south and the north tip of Black Mesa. Slip associated with the Embudo fault system to the north is interpreted to have been partitioned amongst the major faults in the Velarde graben. In the southern graben, a significant lateral transfer of slip likely occurs between these faults and the down-to-the-east Hernandez fault (new name) to the south. The area of this transfer coincides with transverse folds and may be considered a fault segment boundary. Throw values along the Velarde graben border faults since 7.7-8.4 Ma range from 65 m to 435 m. On the southern tip of Black Mesa, comparison of vertical slip rates for the Hernandez fault over two time periods yields slightly higher vertical slip rate values for 3-8 Ma (48-56 m/Myr) compared to 0-3 Ma (35-48 m/Myr). Comparison of vertical offset values in the upper Santa Fe Group with inferred throw of the bedrock and lower Santa Fe Group indicate that the Velarde structure was active prior to the late Miocene. This is consistent with a steepening of seismic reflectors with depth along the eastern margin of the Velarde graben.