USING OFFSET MORAINES TO DETERMINE APPARENT SLIP RATES ON THE CORDILLERA BLANCA NORMAL FAULT, PERUVIAN ANDES

Numerical dating of faulted moraines provides a means of determining apparent slip rates on a fault. We are using this approach in the glacierized Cordillera Blanca, Peru, where the active, west-dipping Cordillera Blanca Normal Fault (CBNF) offsets the crests of numerous moraines formed by glaciers flowing west from the high peaks. The Nevado Jeulla Rajo (NJR) massif (10°00'S, 77°16'W, ca. 5600 masl) lies at the southern end of the Cordillera Blanca. The massif hosts a number of small glaciers within a cirque on its southwest face, and large lateral moraines extend onto the Conococha Plain (ca. 4050 masl) from west-facing valleys. Fault scarps on moraines and valley floors on the western side of the massif show the trace of the north-south-trending CBNF. We focused on two southwest-facing valleys, Jeullesh Valley (JV), which originates in the cirque, and Quenua Ragra Valley (QRV) to the south. Surface-exposure ages (¹⁰Be) indicate that the biggest lateral moraines from JV are compound features deposited during both the local last glacial maximum (LLGM; ca. 27-32 ka) and a late-glacial readvance (ca. 15 ka), whereas the large compound lateral moraines from QRV are predominantly late-glacial (ca. 15-18 ka). Using two methods (laser rangefinder, transit and staff), we profiled the fault scarps that offset the crests of the largest moraines in QRV. Preliminary results suggest an apparent slip rate of 2.2-2.8 mm/yr for the CBNF in QRV, based on moraine ages of 15-18 ka. Our estimates can be compared to an apparent slip rate of 0.86-1.36 mm/yr (Schwartz, 1988) for the CBNF at Quebrada Querococha, 30 km to the north. The slip rate calculated for QRV depends mainly on the moraine age: the older the moraine, the lower the slip rate for a given scarp height. If the largest QRV moraines, like those in JV, were initially constructed during a LLGM advance ca. 30 ka, then reoccupied during a late-glacial advance 15-18 ka, the scarps would have had longer to grow. The apparent slip rate would then be consistent with the Querococha rate. Highly weathered boulders observed near the scarps at the undated uppermost ends of QRV and JV moraines support the hypothesis that late-glacial advances did not override sections of LLGM moraines already offset by the CBNF. We plan to use surface exposure dating of boulders adjacent to the scarps to test the hypothesis.