RETROARC CRUSTAL SHORTENING AND STRUCTURAL ARCHITECTURE OF THE ANDEAN FOLD-THRUST BELT OF SOUTHERN BOLIVIA (21°S): IMPLICATIONS FOR KINEMATIC DEVELOPMENT AND CRUSTAL THICKENING OF THE CENTRAL ANDES

Reliable estimates of shortening magnitude, deformation kinematics, and thrust belt geometry provide a critical framework for testing the mechanisms influencing the growth of convergent orogenic systems. We calculate total shortening, present a kinematic model for thrust belt development, and assess the crustal thickening budget of the central Andean retroarc region of southern Bolivia (21°S) by incorporating new geologic mapping and existing geophysical data to construct a balanced cross section that encompasses the Subandean Zone (SAZ), Interandean Zone (IAZ), and Eastern Cordillera (EC). The SAZ, defined by 10-20 km wavelength, 4-6 km amplitude fault-bend folds above a 10-12 km-deep regional décollement in Silurian strata, accommodated 82 km (36%) of east-west shortening. The IAZ, a bivergent zone of 2-4 km-thick thrust sheets of mainly Silurian-Devonian rocks that are structurally elevated ~10 km relative to adjacent SAZ levels, accommodated 70 km (70%) of shortening. The EC is a bivergent zone of 2-10 km-thick thrust sheets of Cambrian-Ordovician rocks structurally elevated ~6 km relative to the IAZ and Altiplano. However, we now account for a previously unbalanced segment of the central EC, increasing the previous shortening estimate (95 km) to 120 km (37%). When combined with a published estimate of Altiplano shortening (65 km), total retroarc shortening is estimated at 337±40 km (36 ±3%). We interpret Cenozoic development of the retroarc thrust belt by eastward propagation and stacking of two ~12 km-thick basement megathrust sheets that fed slip from a mid-crustal décollement up regional-scale footwall ramps into an upper décollement at the basement-cover interface. This kinematic model is consistent with the pattern of published cooling ages across the retroarc region and the long-term eastward migration of the central Andean foreland basin system, and also accounts for the large-scale structural steps observed across individual morphotectonic zones. Calculating a crustal thickness budget using the new shortening estimate accounts for 93-101% of the geophysically observed crustal area (assuming 35 km thick initial crust), and produces an excess of ~6-15% crustal area (assuming 40 km thick initial crust), raising the possibility of lower crustal flow or loss due to tectonic thickening.