Current end-member models for the geodynamic evolution of orogenic plateaus predict (a) slow and steady rise during crustal shortening and ablative subduction (i.e., continuous removal) of the lower lithosphere or (b) rapid surface uplift following shortening, which is associated with punctuated removal of dense lower lithosphere and/or lower crustal flow. This review integrates results from recent studies of the modern lithospheric structure, geologic evolution, and surface uplift history of the Central Andean Plateau to evaluate the geodynamic processes involved in forming it. Comparison of the timing, magnitude, and distribution of shortening and surface uplift, in combination with other geologic evidence, highlights the pulsed nature of plateau growth. We discuss specific regions and time periods that show evidence for end-member geodynamic processes, including middle–late Miocene surface uplift of the southern Eastern Cordillera and Altiplano associated with shortening and ablative subduction, latest Oligocene–early Miocene and late Miocene–early Pliocene punctuated removal of dense lower lithosphere in the Eastern Cordillera and Altiplano, and late Miocene–early Pliocene crustal flow in the central and northern Altiplano.

[1] Constructing an accurate kinematic model for crustal thickening in the central Andes is hampered by the inability of 2-D balanced cross sections to resolve the 3-D displacement field within the poorly studied orocline core. This study presents new structural and thermochronometer data from the orocline axis in the central Andes that constrains the magnitude and timing of deformation through (1) a balanced cross section quantifying shortening perpendicular to strike, (2) field observations that constrain translational displacements parallel to strike, and (3) thermochronology that brackets the age of exhumation and deformation. Mapping results show that north directed, right-lateral motion has been accommodated on at least one major strike-slip fault in the Eastern Cordillera (EC). Shortening perpendicular to fault strike is~179 km (41%) for the EC and inter-Andean zone (IA) and 86 km (39%) for the sub-Andes (SA). Apatite fission track cooling ages indicate rapid EC/IA exhumation from 46 to 18 Ma and 15 to 0 Ma in the SA. Zircon and apatite (U-Th)/He ages show that rapid exhumation began 10-5 Ma in the SA. These results are integrated with existing paleomagnetic and GPS rotation data in map view reconstructions to quantify the magnitude of translational faulting related to oroclinal bending. The reconstructions suggest that at least 85 km of north directed translational displacement has been accommodated within the orocline core. Orogen-parallel displacement and vertical axis rotations have focused crustal thickening at the orocline core and facilitated development of a high-elevation plateau, suggesting a genetic link between large-scale oroclines and plateaus.

Orogenic curvature is a common feature in many mountain belts and is strongly linked to the magnitude, direction, and mechanics of crustal shortening. Determining how formation of the Bolivian orocline infl uenced crustal deformation in the central Andes has direct implications for geodynamics of the high-elevation Altiplano plateau. This study presents new reconstructions of the Bolivian orocline constrained by shortening estimates, thermochronology, regional paleomagnetic data, and strain data from lat 12°S to 22°S. The reconstructions investigate paleomagnetically permissible orocline limb rotations of 0°, 6°, and 13° on the kinematic compatibility of shortening constraints. Deformation was restored in 5 m.y. steps from 50 to 0 Ma, and kinematic compatibility was quantifi ed based on the area of map-view overlap at each step. No limb rotation resulted in 14,000 km 2 of overlap, while 13° limb rotations and 50 km of orogen-parallel displacement on known strike-slip faults reduce overlap to 3000 km 2. The preferred model builds on these results by imposing additional rotations at the orocline core and displacement on the Cochabamba fault. This model reduces overlap to 1600 km 2 but predicts map-view shortening estimates 70-90 km greater in the northern limb and 20-30 km greater in the southern limb than determined from cross sections. Of the modeled increase, ~20 km is due to limb rotation, while the remaining 50-70 km is due to transpressional shortening on the Cochabamba and Rio Novillero faults. Total shortening in the preferred model is 370 km in the northern limb, 380 km at the orocline core, and 300-350 in the southern limb.

Paleoelevation histories from the central Andes in Bolivia have suggested that the geodynamic evolution of the region has been punctuated by periods of large-scale lithospheric removal that drive rapid increases in elevation at the surface. Here, we evaluate viable times and locations of material loss using a map-view reconstruction of the Bolivian orocline displacement field to forward-model predicted crustal thicknesses. Two volumetric models are presented that test assumed predeformation crustal thicknesses of 35 km and 40 km. Both models predict that modern crustal thicknesses were achieved first in the northern Eastern Cordillera (EC) by 30-20 Ma but remained below modern in the southern EC until ≤ 10 Ma. The Altiplano is predicted to have achieved modern crustal thickness after 10 Ma but only with a predeformation thickness of 50 km, including 10 km of sediment. At the final stage, the models predict 8-25% regional excess crustal volume compared to modern thickness, largely concentrated in the northern EC. The excess predicted volume from 20-0 Ma can be accounted for by: 1) crustal flow to the WC and/or Peru, 2) localized removal of the lower crust, or 3) a combination of the two. Only models with initial crustal thicknesses > 35 km predict excess volumes sufficient to account for potential crustal thickness deficits in Peru and allow for lower crustal loss. However, both initial thickness models predict that modern crustal thicknesses were achieved over the same time periods that paleoelevation histories indicate the development of modern elevations. Localized removal of lower crust is only necessary in the northern EC where crustal thickness exceed modern by 20 Ma, prior to paleoelevation estimates of modern elevations by 15 Ma. In the Altiplano, crustal thicknesses match modern values at 10 Ma and can only

No abstract

Current end-member models for the geodynamic evolution of orogenic plateaus predict (1) slow-and-steady rise during crustal shortening and ablative subduction (i.e., continuous removal) of the lower lithosphere, or (2) rapid surface uplift following shortening, associated with punctuated removal of dense lower lithosphere and/or lower crustal flow. We will review results from a recent multidisciplinary study of the modern lithospheric structure, geologic evolution, and surface uplift history of the Central Andean Plateau to evaluate the geodynamic processes that have formed the Plateau. Comparison of the timing, magnitude, and distribution of shortening and surface uplift, in combination with other geologic evidence, highlights the pulsed nature of plateau growth. We will discuss specific regions and time periods that show evidence for end-member geodynamic processes, including middle-late Miocene surface uplift of the southern Eastern Cordillera and Altiplano associated with shortening and ablative subduction, latest Oligocene-early Miocene and late Miocene-Pliocene punctuated removal of dense lower lithosphere in the Eastern Cordillera and Altiplano, and late Miocene-Pliocene crustal flow in the central and northern Altiplano.

Carefully selected 2D transects contain an abundance of structural information that can constrain 3D analyses of petroleum systems. Realizing the full value of the information in a 2D transect requires combining multiple, independent structural analysis techniques using fully interactive tools. Our approach uses quantitative structural geologic software that instantaneously displays structural computations and analyses, eliminating time-intensive manual measurements and calculations. By quickly testing multiple hypotheses, we converged on an optimal solution that is consistent with available data. We have combined area-depth-strain (ADS) analysis, structural restoration, and forward modeling of a structural interpretation of a fault-propagation fold in the Niger Delta. These methods confirmed the original interpretation and furthermore quantified displacement, strain, detachment depth, and kinematic history. ADS analysis validated the interpreted detachment depth and revealed significant layer-parallel strain (LPS) that varied systematically with stratigraphic depth. The stratigraphic distribution of the LPS was diagnostic of structural style and, in this example, discriminated against fixed-axis and constant-thickness fault-propagation folding. A quantitative forward model incorporating backlimb shear and trishear fault-propagation folding accurately reproduced folding and faulting in the pregrowth section and folding in the growth section. The model-predicted strain distributions were consistent with those from ADS analysis. The highest local strains on the back limb of the structure were spatially coincident with two backthrusts, which accommodated these strains. Animations of a more complete model including the backthrusts revealed that the backthrusts formed sequentially as rock passed through the main fault bend.

No abstract