ESTIMATING APPARENT SLIP RATES ON THE CORDILLERA BLANCA NORMAL FAULT, PERUVIAN ANDES

In the Cordillera Blanca, Peru, the active, west-dipping Cordillera Blanca Normal Fault (CBNF) vertically offsets the crests of numerous moraines, some of which have been dated as a means of determining apparent slip rates on the CBNF. We profiled CBNF scarps on eight moraines in six valleys: Llaca, Cojup, and Querococha valleys in the central Cordillera Blanca (9°28’-45’S, 77°28’-21’W); and Jeullesh, Quenua Ragra, and Tuco valleys in the Nevado Jeulla Rajo (NJR) massif (10°00’S, 77°16’W) at the southern end of the Cordillera Blanca. The faulted moraines in glaciated Llaca and Cojup valleys have vertical offsets (VO) of 46 m and 60 m and ¹⁰Be ages of 15-18 and ca. 30 ka, respectively (Farber et al., 2005). The NJR massif has a number of small glaciers within a cirque on its southwest face, and large lateral moraines with fault scarps extend onto the Conococha Plain from west-facing valleys. Surface-exposure ages (¹⁰Be) indicate that the largest lateral moraines from Jeullesh Valley (VO = 18 m) are compound features deposited during both the local last glacial maximum (ca. 30 ka) and a late-glacial readvance (ca. 15 ka), whereas the large compound lateral moraines from Quenua Ragra Valley (VO = 21 and 30 m) are predominantly late-glacial (ca. 15-18 ka). The surface-exposure ages suggest an apparent slip rate for the CBNF of 2.6-3.1 mm/yr for Llaca Valley, 2.0 mm/yr for Cojup Valley, 1.0-1.2 mm/yr for Jeullesh Valley, and 1.2-2.0 mm/yr in Quenua Ragra Valley. Samples for ¹⁰Be dating have been collected in the Tuco Valley (VO = 44 m) and Querococha Valley (VO = 17 and 27 m), but not yet analyzed. Field observations suggest an age of 15-18 ka for faulted moraines in Tuco and Querococha Valleys, yielding slip rates of 2.4-2.9 mm/yr and 0.9-1.8 mm/yr, respectively. Our estimates are comparable to slip rates calculated for the CBNF of 0.86-1.36 mm/yr on the valley floor at Querococha Valley (Schwartz, 1988) and 3±1 mm/yr for moraines ~70 km to the north of Llaca Valley (Siame et al., 2006). Differences in slip rates along the length of the seismogenic CBNF are known to exist, with the fastest slip occurring to the north of the study area, coincident with the highest topography. Our estimated slip rates appear to be consistent with an increase in slip rates from south to north in the southern section of the Cordillera Blanca.