FAULT-KINEMATIC PATTERN OF EXTENSIONAL STRAIN TRANSFER BETWEEN BASINS OF THE MIDDLE RIO GRANDE RIFT, NORTHERN NEW MEXICO

The right-stepping, en-echelon geometry of late Cenozoic extensional basins of the Rio Grande rift in northern New Mexico has been long recognized, but the structural mechanisms by which rift faults link and transfer strain between basins are not well understood. To improve understanding, we collected kinematic data (fault surface orientations, slickenline rakes, and slip-sense determinations) at 340 fault exposures in and bordering the Santo Domingo Basin (SDB) and adjacent southern Española Basin (EB). Faults with a wide range of displacements (<1 to >500m) were measured in Miocene and younger basin-fill sediments and volcanic rocks and in flanking pre-rift rocks. Faults in the central parts of the basins are typically north-striking with normal slip (rakes >70°). In contrast, faults have a broad range of orientations (northeast strikes common) and exhibit normal-oblique slip (rakes 30-70°) or strike slip (rakes <30°) on either primary or overprinted slickenlines at sites along the northern and eastern margins of SDB, in <3Ma basaltic rocks of the Cerros del Rio volcanic field (CDRVF) between SDB and EB, and in southwest EB. Sinistral components of slip dominate along northeast-striking faults bordering the east side of SDB, including the Tetilla fault zone that projects beneath lavas of the CDRVF to southwest EB. In other domains, dextral and sinistral oblique- and strike-slip faults coexist, either as conjugate pairs (e.g., in CDRVF, with ~270° azimuth of acute bisector) or with identical fault strikes (e.g., in southern EB west of Santa Fe). Paleomagnetic data suggest synkinematic CW and CCW vertical-axis rotations from Eocene rocks within the Tetilla fault zone, whereas Pliocene lavas of the CDRVF express only CCW rotation (~7° mean).

The observed structural patterns suggest that extensional strain was relayed between rift basins by nonuniform oblique- and strike-slip faulting. This strain involved localized and, perhaps, brief rotations of tectonic principal stresses and, in some places, fault-block domains. Such stress and strain rotations may have been caused by lateral shear focused at large extensional strain gradients along and between basin margins.