Apparent Slip Rates on the Cordillera Blanca Normal Fault, Peruvian Andes, Determined From Offset Moraine Crests

In the glacierized Cordillera Blanca, Peru, where the active, west-dipping Cordillera Blanca Normal Fault (CBNF) vertically offsets the crests of numerous moraines, we are dating faulted moraines as a means of determining apparent slip rates on the CBNF. We profiled CBNF scarps on moraines in six valleys: 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; and Llaca, Cojup, and Querococha valleys in the central Cordillera Blanca (9°28'-45'S, 77°28'-21'W). The NJR massif hosts a number of small glaciers within a cirque on its southwest face, and large lateral moraines extend onto the Conococha Plain from west-facing valleys. Fault scarps on moraines and valley floors on the western side of NJR show the trace of the north-south-trending CBNF. Surface-exposure ages (¹⁰Be) indicate that the largest lateral moraines from Jeullesh Valley are compound features deposited during both the local last glacial maximum (ca. 27-32 ka) and a late-glacial readvance (ca. 15 ka), whereas the large compound lateral moraines from Quenua Ragra Valley are predominantly late-glacial (ca. 15-18 ka). To the north of NJR in the central Cordillera Blanca, the faulted Rurec moraine in Cojup Valley has a ¹⁰Be age of ca. 30 ka (Farber et al., 2005). These surface-exposure ages suggest an apparent slip rate of 1.2-1.4 mm/yr for the CBNF in Quenua Ragra Valley (based on moraine ages of 15-18 ka), and 0.5-0.6 mm/yr for Jeullesh Valley and 2.0-2.7 mm/yr for Cojup Valley (based on moraine ages of 30 ka). Samples for ¹⁰Be dating have been collected in the Tuco, Llaca, and Querococha valleys, where moraine crests are displaced vertically ca. 44 m , 46-50 m, and 17-26 m, respectively. Field observations suggest an age of 30 ka for faulted moraines in Tuco, Llaca, and Querococha Valleys, which would yield slip rates of ca. 1.5, 1.5-1.7 mm/yr, and 0.6-0.9 mm/yr, respectively. Our estimates are largely consistent with an apparent slip rate of 0.86-1.36 mm/yr calculated for the CBNF on the valley floor at Querococha Valley (Schwartz, 1988). 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.