2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 3
Presentation Time: 2:05 PM


MCQUARRIE, Nadine1, BARNES, Jason B.2 and EHLERS, Todd A.2, (1)Department of Geosciences, Princeton Univ, Guyot Hall, Princeton, NJ 08544, (2)Geological Sciences, University of Michigan, 2534 C.C. Little Building, 1100 N. University, Ann Arbor, MI 48109, nmcq@princeton.edu

Along strike changes in topography, relief, climate and thrust-belt geometry has generated proposals that north-south changes in climate exert a first order control on the deformational history of the Andean fold-thrust belt. In northern Bolivia (15-16°S), increases in the magnitude of precipitation, the deepest exposed portions of the Andean plateau and little to no morphological expression of the Subandean (SAZ) portion of the fold-thrust belt all suggests climatically driven tectonics in this region. We present a sequentially restored, balanced cross section through the northern Bolivian, Andean fold-thrust belt in conjunction with new mineral cooling ages to highlight the relationship between deformation and exhumation.

The cross section was balanced and restored using the sinuous bed method. The restored section was then sequentially forward modeled using 2-dmove. In general, the Eastern Cordillera (EC) records the highest magnitudes of shortening (123 km or 55%), oldest ages of shortening (40-25 Ma), but some of the youngest ages of exhumation (15-2 Ma). The Interandean zone (IAZ) has shortened 48 km or 30%. In both the EC and IAZ individual thrust sheets are tightly folded and have minor offsets of 1-5 km. Cooling ages in the EC and IAZ do not seem affected by individual thrusts but rather cooled broadly as zones with EC cooling at 50-25 Ma and ~20-5 Ma, and IAZ cooling from 40-5 Ma. The fold-thrust belt of the SAZ has multiple levels of detachments allowing for thrust sheets with large 6-15 km of offset. Total shortening in the SAZ is 66 km or 40%. Model constrained cooling ages (19-4 Ma) are more diverse among the individual thrust sheets in the SAZ confirming both 1) that motion along individual thrusts was much larger, and 2) the zone lacks any structural mechanism to produce broad/ uniform uplift.

Total magnitude of shortening across the northern Bolivian portion of the fold-thrust belt is 267 km (40%), 30-60 km less shortening, but same percentage as estimates to the south. Early exhumation supports kinematic models of plateau growth via the eastward emplacement basement thrust sheets at ~40-15 Ma, and ~20-0 Ma. The correlation between predicted basement fault motion and cooling implies relief generated through basement thrust faulting drove initial erosion, late 20-5 Ma exhumation of the EC is due to other processes.