Welcome to theUniversity of rizona Dept. of Geosciences
Reflection Seismology Program
Geometries and Fault Scarps in SE Arizona
Although relatively quiescent on human time scales, southeastern Arizona and northern Mexico show ample signs of punctuated seismicity, evidence for which is provided by fault scarps (with displacements up to 4 meters) that cut late-Quaternary alluvial fans along the margins of many of the region's mountain ranges. For instance, the northern end of the Santa Rita Mountains south of Tucson is flanked by fault scarps caused by large-magnitude (M ~6.7-7.6) earthquakes, and the strongest historical earthquake in the region, which occurred in 1887 with an estimated magnitude of about 7.2, rejuvenated the Pitaycachi fault scarp south of Agua Prieta, Mexico.
Industry and University of Arizona seismic reflection data that cross the trend of the Santa Rita fault clearly image a low-angle (~15°) fault-plane reflection that can be traced from a depth of at least 6 km to the late-Quaternary fault scarps at the surface. However, focal mechanisms for large (M >6) earthquakes in extensional terrains suggest that seismogenic normal faults rarely have dips less than ~30°. Here, fossil evidence for seismogenic low-angle normal faulting provides an intriguing clue to the true nature of crustal deformation. Furthermore, the Santa Rita fault, because of its geological significance and proximity to Tucson, has served as an excellent topic to involve students in active and ongoing research. Many of our class seismic data-acquisition field trips have focused on this fault system.
Trey Wagner, a Ph.D. student in Reflection Seismology, currently is analyzing additional industry seismic profiles that bridge the gap between the imaged Santa Rita normal fault and what is interpreted to be a fault-zone reflection from the Catalina detachment fault to the north. The subsurface connection between these structures is key to better understanding the extensional evolution of this region. The Santa Rita fault is in an upper-plate position with respect to the mylonitic rocks of the Catalina-Rincon metamorphic core complex. Consequently, the Santa Rita fault either 1) soles into the Catalina detachment fault and reactivates part of it at depth; 2) cuts the Catalina detachment fault and represents essentially unrelated later extension; or 3) may be at least partly coeval with the Catalina detachment fault and, thus, could be an upper-plate excisement or incisement fault.
Extending our fault-zone investigations ~200 km southeast, we conducted a high-resolution seismic reflection/refraction pilot study on the Pitaycachi fault zone in northern Mexico in November 2001. This study was coordinated with a SESS field trip to the scarp that gave faculty, graduate students, and undergraduates an opportunity to participate in seismic data acquisition activities and to get a close-up look at the very recent (and very dramatic) rupture zone. The goal of the trip was to obtain data that can serve as the basis for independent-study projects and to provide preliminary data for a planned longer-term research project on the fault system. Although the seismic profile was necessarily very short, it is giving us a first look at near-surface disruption across the fault scarp that may provide clues about the fault's reactivation history.