Glacial and periglacial geomorphology and chronology around the Nevado de Chañi (Cordillera Oriental of Jujuy): implication for past climate in NW Argentina

MARTINI, MATEO ANTONIO; JORGE A. STRELIN; KAPLAN, MICHAEL; SCHAEFER, JOERG

San Francisco

Congreso; American Geophysical Union 2012 Fall Meeting; 2012

American Geophysical Union

The Nevado de Chañi (24°3? 45?? S; 65° 44? 43?? W) is one of the highest massifs in northwestern Argentina (5882 masl). It is the summit of a N-S trending mountain range located just north of the Arid Diagonal, which separates the southern Pacific westerlies from the South American Summer Monsoon domains. This range consists of Precambrian and Paleozoic sedimentary rocks intruded by Ordovician granites. It has been broadly eroded by glaciers during the Late Pleistocene but today no glaciers exist in the area due to aridity, and periglacial activity takes a main role in the development of the landscape above 4600m. The orographically-driven climate conditions, with seasonal summer wet-air masses moving from the Atlantic, determine that the west side of the massif is clearly drier and probably warmer than the east-facing side. Three main groups of moraines are recognized on both sides of Chañi. We collected moraine boulders from the different glacial landforms for 10Be surface exposure dating, which is in progress. Seven peat bogs were cored to obtain minimum-limiting radiocarbon ages of the deglaciation and limnogeologic change during the post glacial period. Ten preliminary 10Be dates reveal pre-LGM (Last Glacial Maximum) and post LGM ages for the glacier moraines on the west side of the massif. The periglacial geomorphology around Chañi is mainly represented by scree activity, giving way to solifluction and formation of protalus lobes. We focused especially on rock glacier formation. Active, inactive, and fossil rock glaciers were distinguished taking into account geomorphological characteristics such as the frontal slope and ridge and furrows preservation. The lower limit for active and inactive/fossil rock glaciers is located above ~4700m and ~4500m, respectively. Cross-cutting relations with the dated moraines will provide the maximum age of these geomorphic features. Active rock glaciers at present have predominately S and E slopes and are located bellow steep active eroding valley walls. The minimum altitude and the local topographic context suggest that changes in the activity of rock glaciers correspond with local conditions (e.g., debris or talus supply) rather than climatic changes (e.g., temperature) around Chañi. Our geomorphological-based findings allow us to propose a preliminary correlation with other glaciated areas in the arid Andes and the northern Cordillera Oriental and with lake transgression phases recorded on the southern Bolivian Altiplano, during the late Pleistocene. The paleo-equilibrium line altitudes rise from east to west across the region indicating that the source of moisture came from the east and northeast during the past glaciations, which is similar to the present-day circulation scenarios along the north side of the Arid Diagonal.