EOCENE TO RECENT EXHUMATION PATTERNS CORRELATED WITH DEFORMATION ACROSS 200 KM OF THE CENTRAL ANDEAN FOLD-THRUST BELT, NORTHERN BOLIVIA

Quantifying the evolution orogenic plateaus has been hampered by limited age constraints on their deformation and exhumation histories. Palinspatic restorations of the central Andean fold-thrust belt in Bolivia propose a model of Andean plateau development controlled by basement shortening. To test this model, we combine a synthesis of apatite and zircon fission track (AFT, ZFT) and zircon (U-Th)/He (ZHe) cooling ages and erosion magnitudes with structure from a balanced cross section along a 200 km transect of the thrust belt at 15-16ºS. New AFT data were obtained from meta-sedimentary units and interpreted by grain-age decomposition and inverse thermal modeling to constrain each samples most recent exhumation history.

We group results regionally from west to east. The Altiplano (AL) records AFT cooling from ~2-18 Ma with an erosion magnitude of ~3.5 km. The Eastern Cordillera (EC) records AFT cooling from ~5-20 Ma, ZHe cooling from 16-33 Ma, and ZFT cooling from ~25-50 Ma with erosion magnitudes from ~4.4-10.4 km. The Interandean zone (IAZ) records cooling from ~5-40 Ma with an erosion magnitude of ~4.4 km. The Subandes (SA) record AFT cooling from ~4-19 Ma and ZHe cooling from 220-660 Ma with Andean-related exhumation magnitudes bracketed between ~3.9-7.0 km.

Results suggest (1) initial erosion migrated eastward from the plateau from ~50 Ma, (2) the entire region experienced distributed erosion from ~20 Ma, and (3) the magnitude of erosion decreases eastward. In the EC and IAZ, exhumation trends indicate regional erosion events with little age variation across tightly folded individual structures with minor offsets (1-5 km). This suggests the need for a more deep-seated and broad exhumation mechanism. Individual structures and exhumation are more directly associated in the SA where multiple detachment levels allow for larger thrust sheet offsets (6-15 km). The exhumation patterns are consistent with the basement shortening model. Deformation-driven exhumation migrated eastward from ~50 Ma coincident with the emplacement of an upper basement thrust sheet generating relief and driving erosion. The widespread exhumation since ~20 Ma records distributed deformation in the AL, IAZ, and SA combined with erosion-driven exhumation in the EC associated with emplacement of a lower basement thrust sheet.