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The potential structural controls on exhumation across the southern Peruvian Andes are not well understood, in part due to limited structural studies that co-locate with thermochronometric datasets. We integrate these two datasets and evaluate the relative contribution that fault geometry, magnitude, and shortening rate have on predicted cooling ages. Here we present a balanced cross-section constructed using new structural observations. This section, combined with existing thermochronometer data and a thermokinematic model, investigates the drivers of high exhumation and young canyon thermochronometric ages along the deeply incised Marcapata canyon in southern Peru. Together, these approaches constrain the timing and magnitude of exhumation in this portion of the southern Peruvian Andes and provide a mechanism for documenting how the internal architecture changes along strike. The balanced cross-section (oriented N30E) covers the Subandean Zone to the northeast, the Marcapata canyon on the eastern flank of the southern Peruvian Andes, and the Altiplano-Eastern Cordillera boundary to the southwest (13–18◦ S). Exhumation is constrained by four low-temperature thermochronometer systems, including apatite and zircon (U-Th)/He (AHe and ZHe, respectively) and fission-track (AFT and ZFT, respectively). The youngest AHe (∼1–3 Ma), AFT (∼3–7 Ma), ZHe (∼4–7 Ma), and ZFT (∼14–17 Ma) ages are located in the center and valley bottom of the Marcapata canyon. The thermokinematically modeled cross-section produces cooling ages determined by fault geometry and kinematics. Reset ZFT ages require burial of Ordovician rocks in excess of 5.5 km above the original 6.5 km depositional depth. We find that the ZFT and ZHe ages in the Eastern Cordillera are sensitive to the history and magnitude of burial, age and location of uplift, and canyon incision. Canyon incision is required to reproduce the youngest canyon thermochronometric ages while slow shortening rates from ∼10 Ma to Present are required to reproduce interfluve thermochronometric ages. Shortening is accommodated by basement faults that feed slip up through three different décollement levels before reaching the surface. The proposed stacked basement geometry sets the first-order cooling signal seen in modeled ages. We determined that the total shortening in this section from the Subandean Zone to the Altiplano is 147.5 km, similar to shortening estimates in an adjacent thermo-kinematically modeled section in the San Gabán canyon 50 km to the southeast. Both the ZHe and ZFT ages in the Marcapata section (4–5 and 14 Ma) are noticeably younger than cooling ages from the San Gabán section (16 and 29 Ma). The Marcapata section’s higher magnitude of exhumation is due to a repetition of basement thrusts that continues to elevate the Eastern Cordillera while active deformation occurs in the Subandean Zone. The youngest thermochronometric ages in all four systems are co-located with the overlapping basement thrust geometry. This basement geometry, kinematic sequence of deformation, and canyon incision co-conspire to produce the young cooling ages observed in the Eastern Cordillera. 

This study assesses the impact of fold-thrust belt driven deformation on the topographic evolution, bedrock exhumation and basin formation in the southeastern Peruvian Andes. We do this through a flexural and thermokinematically modelled balanced cross-section. In addition, published thermochronology samples from low-elevation (river canyons) and high-elevation (interfluves) and Cenozoic sedimentary basin datasets along the balanced cross-section were used to evaluate the age, location, and geometry of fault-driven uplift, as well as potential relationships to the timing of ∼2 km of canyon incision. The integrated structural, thermochronologic, and basin data were used to test the sensitivity of model results to various shortening rates and durations, a range of thermophysical parameters, and different magnitudes and timing of canyon incision. Results indicate that young apatite (U-Th)/He (AHe) canyon samples from ∼2 km in elevation or lower are consistent with river incision occurring between ∼8–2 Ma and are independent of the timing of ramp-driven uplift and accompanying erosion. In contrast, replicating the young AHe canyon samples located at >2.7 km elevation requires ongoing ramp-driven uplift. Replicating older interfluve cooling ages concurrent with young canyon ages necessitates slow shortening rates (0.25–0.6 mm/y) from ∼10 Ma to Present, potentially reflecting a decrease in upper plate compression during slab steepening. The best-fit model that reproduces basin ages and depositional contacts requires a background shortening rate of 3–4 mm/y with a marked decrease in rates to ≤0.5 mm/y at ∼10 Ma. Canyon incision occurred during this period of slow shortening, potentially enhanced by Pliocene climate change. 

Quantifying the impacts of past changes in tectonics or climate on mountain topography has proven challenging. The incision of the eastern Central Andean Plateau has been interpreted as both a result of deformation-related uplift and erosion and climate-driven erosion. Here, we contribute >100 new apatite and zircon (U-Th)/He and fission-track dates from 51 new and eight previous bedrock samples. These samples were combined with previous thermochronometer data from three ∼190-km-long and ∼200-km-apart across-strike transects along the eastern margin of the Andean Plateau in southern Peru. We discuss age-distance, age-elevation, and inverse thermal history model results along these transects to constrain the timing and extent of recent canyon incision compared to the region’s long-term (∼40 Myrs) exhumation history. Results indicate that, along the plateau flank, long-term, deformation-related exhumation is superimposed by a regional, synchronous canyon incision-related signal since ∼4–3 Ma. This incision is traceable from at least the Abancay Deflection in southern Peru to southern Bolivia along the eastern Central Andes. Based on the regional and synchronous character of canyon incision across areas with different deformation histories and exhumation magnitude, we suggest that paleoclimate change was a significant contributor to incision. However, structural processes resulting in surface uplift, erosion, and exhumation continued post-mid Miocene and contributed to the observed exhumation magnitude. 

Constraining the subsurface structural geometry of the central Himalaya continues to prove difficult, even after the 2015 Gorkha earthquake and the resulting insights into the trajectory of the Main Himalayan thrust (MHT). To this end, we apply a thermokinematic model to evaluate four possible balanced cross section geometries based on three estimates of the MHT in central Nepal. We compare the effect of different décollement and duplex geometries on predicted cooling ages and compare these to new and published ages. We find that the best‐fit geometry able to reproduce the cooling ages at the surface is a hinterland‐dipping duplex, which has been translated over a mid‐crustal ramp located ~110 km north of the Main Frontal thrust. We find that the temporal evolution of the duplex and MHT mid‐crustal ramp both play an integral role in producing the observed cooling ages, implying that the common assumption that the active décollement and ramp geometry solely control the distribution of cooling ages is incorrect. Furthermore, results indicate that the Ramgarh‐Munsiari thrust was emplaced between 17 and ~10 Ma, followed by the Trishuli thrust. Duplex growth occurs between 6.5 and 0.75 Ma, with its constituent thrust sheets moving at variable rates between 10 and 42 mm/yr. Young out‐of‐sequence thrusting (5 km of displacement) in the hinterland produces a slightly improved fit to the cooling ages. Finally, the resulting thermal field modeled from our best‐fit geometry suggests a possible basis for the nucleation and rupture characteristics of the Gorkha earthquake.

 

Understanding, and ideally quantifying, the relative roles of climatic and tectonic processes during orogenic exhumation is critical to resolving the dynamics of mountain building. However, vastly differing opinions regarding proposed drivers often complicate how thermochronometric ages are interpreted, particularly from the hinterland portions of thrust belts. Here we integrate three possible cross‐section geometries and kinematics along a transect through the eastern Bhutan Himalaya with a thermal model (Pecube‐D) to calculate the resulting thermal field and predict potential ages. We compare predicted ages to a suite of new and published cooling ages. Our results argue for ramp‐focused exhumation of the Main Central thrust from 16 to 14 Ma at shortening rates of 40–55 mm/year, followed by slower rates (25 mm/year) during the last 50 km of Main Central thrust displacement and growth of the Lesser Himalayan duplex from 14 to 11 Ma. Emplacement of frontal Lesser Himalayan thrust sheets occurred rapidly (55–70 mm/year) between ~11 and 9 Ma, followed by a decrease in shortening rates to ~10 mm/year during motion on the Main Boundary thrust. Modern shortening rates (17 mm/year) and out‐of‐sequence motion on the Main Boundary thrust from 0.5 Ma to present reproduce the young cooling ages near the Main Boundary thrust. We show that the dominant control on exhumation patterns in a fold‐thrust belt results from the evolution of ramps and emphasize that the geometry and kinematics of structures driving hinterland exhumation need to be evaluated with their linked foreland structures to ensure the viability of the proposed geometry, kinematics, and thus cooling history.