PREFERRED ORIENTATION OF GRAINS IN FAULTED POORLY LITHIFIED SEDIMENTS FROM THE RIO GRANDE RIFT STUDIED BY AMS, SEM, AND MICROCT

Permanent strain in poorly lithified sediments within shallow fault zones can be difficult to characterize. Anisotropy of magnetic susceptibility (AMS) data were obtained from oriented samples along 4 faults cutting Mio-Pliocene basin-fill sediments of the Rio Grande rift of north-central New Mexico. The San Ysidro, Sand Hill, and West Paradise faults within the northern Albuquerque basin have normal slip whereas an unnamed fault near Buckman in the western Española basin has oblique normal-strike slip. Previous studies have shown that detrital magnetite controls magnetic susceptibility in the rift sandstones. In a 50-m-long hanging wall traverse of the San Ysidro fault, non-gouge samples have typical sedimentary AMS fabrics with Kmax and Kint axes (defining magnetic foliation) scattered within bedding. For the normal-fault sites, samples from fault core gauges or adjacent mixed zones that lie within 1 m of the principal slip surface developed deformation fabrics with (1) magnetic foliation inclined in the same azimuth but more shallowly dipping than the fault, and (2) magnetic lineation (Kmax axis) plunging down foliation dip with similar trend as fault striae. For a 2-m-long traverse at the Buckman site, horizontal sedimentary AMS foliation persists to < 15 cm from the fault slip surface, whereas foliation in sand and clay gouge rotates toward the steeply dipping fault plane in a sense consistent with sinistral-normal slip.

Scanning electron microscope (SEM) images of grains separated using a magnet from both fault-core and host sediment samples show elongate, subrounded, detrital magnetite grains whose long axes aligned parallel to an externally imposed magnetic field. High-resolution x-ray tomographic (MicroCT) imaging of two oriented samples demonstrated that high-density grains of similar size and elongate shape to magnetite grains in SEM images are dispersed throughout the samples. Together these relations indicate that the poorly lithified fault sediments deformed by particulate flow leading to alignment of magnetite grains within < 1 m of faults. Preferred orientation of grains in fault samples as detected from AMS fabrics can provide kinematic information for faults cutting sediments that may be difficult to determine otherwise.