Jason B. Barnes

Linking orography, climate, and exhumation across the central Andes

Quantifying interactions between uplift, climate, deformation, and exhumation remains difficult, in many cases due to a paucity of data relevant to all processes. We synthesize new and existing data to understand the orogen-scale orographic changes across the central Andes, Bolivia. We use a regional climate model and geo-thermochronologic data to identify the correlations between changes in precipitation due to surface uplift and spatiotemporal patterns of deformation and erosional exhumation. Mean orographic rainfall patterns do not reach near present-day gradients and values until the topography grows to >75% modern elevations. New fission-track data near the orocline apex indicate that rapid exhumation moved eastward, beginning in the Eastern Cordillera ca. 50–15 Ma, the Interandean zone ca. 18–6 Ma, and in the Subandes ca. 7–3 Ma. Throughout Bolivia, exhumation is consistent with deformation until ca. 15–11 Ma, after which the pattern corresponds better with the increased rainfall toward modern values. These linked observations suggest that ca. 15–11 Ma, regional elevations reached threshold values (>75% modern) necessary to generate near present-day, enhanced rainfall gradients. These gradients have resulted in variable exhumation implied by the structural level of rocks exposed across the thrust belt and confirmed by fission tracks in apatite. The main insight is that the climate-induced Middle Miocene–recent exhumation varies over scales of a few hundred kilometers across Bolivia and implies that high mean rainfall (>∼3 m/yr) and long time scales (∼10 m.y.) may be necessary for climate to induce orographically driven exhumation patterns recorded by fission tracks.

Temporal Variation in Climate and Tectonic Coupling in the Central Andes

Analog and numerical models predict a coupling between climate and tectonics whereby erosion infl uences the deformation of orogens. A testable prediction from modeling studies is the decrease in width of mountain ranges as a result of increased precipitation. Here we evaluate the effect of climate on a critically tapered orogen, the central Andes, using sequentially restored, balanced cross sections through wet (15°‐16°S) and dry (21°S) regions of the orogen. In these regions, tectonics, basin geometry, and style of deformation are similar, allowing us to use variations in propagation (or changes in percent shortening) to evaluate whether alongstrike changes in width and morphology are climate driven in the north. Results indicate similar total percent shortening along the northern (40%) and southern (37%) sections, suggesting that a wetter climate has not limited the width (propagation) in the north. However, comparison of early (45‐25 Ma) and recent (ca. 20‐0 Ma) shortening indicates that early deformation produced 45% ± 2% shortening of both sections, while recent deformation produced 41% ± 2% (north) versus 32% ± 2% (south) in the actively deforming Subandes. The latter suggests a coupling between climate and tectonics that began between ca. 19 and 8 Ma, and continues to 0 Ma, potentially limiting the width of the northern Subandes by ~40 km.

Spatiotemporal variability of modern precipitation δ18O in the central Andes and implications for paleoclimate and paleoaltimetry estimates

Understanding the patterns of rainfall isotopic composition in the central Andes is hindered by sparse observations. Despite limited observational data, stable isotope tracers have been commonly used to constrain modern-to-ancient Andean atmospheric processes, as well as to reconstruct paleoclimate and paleoaltimetry histories. Here, we present isotopic compositions of precipitation (δ18Op and δDp) from 11 micrometeorological stations located throughout the Bolivian Altiplano and along its eastern flank at ~21.5°S. We collected and isotopically analyzed 293 monthly bulk precipitation samples (August 2008 to April 2013). δ18Op values ranged from −28.0‰ to 9.6‰, with prominent seasonal cycles expressed at all stations. We observed a strong relationship between the δ18Op and elevation, though it varies widely in time and space. Constraints on air sourcing estimated from atmospheric back trajectory calculations indicate that continental-scale climate dynamics control the interannual variability in δ18Op, with upwind precipitation anomalies having the largest effect. The impact of precipitation anomalies in distant air source regions to the central Andes is in turn modulated by the Bolivian High. The importance of the Bolivian High is most clearly observed on the southern Bolivian Altiplano. However, monthly variability among Altiplano stations can exceed 10‰ in δ18Op on the plateau and cannot be explained by elevation or source variability, indicating a nontrivial role for local scale effects on short timescales. The strong influence of atmospheric circulation on central Andean δ18Op requires that paleoclimate and paleoaltimetry studies consider the role of South American atmospheric paleocirculation in their interpretation of stable isotopic values as proxies.

Geomorphic evidence for enhanced Pliocene-Quaternary faulting in the northwestern Basin and Range

Mountains in the U.S. Basin and Range Province are similar in form, yet they have different histories of deformation and uplift. Unfortunately, chronicling fault slip with techniques like thermochronology and geodetics can still leave sizable, yet potentially important gaps at Pliocene–Quaternary (∼105–106 yr) time scales. Here, we combine existing geochronology with new geomorphic observations and approaches to investigate the Miocene to Quaternary slip history of active normal faults that are exhuming three footwall ranges in northwestern Nevada: the Pine Forest Range, the Jackson Mountains, and the Santa Rosa Range. We use the National Elevation Dataset (10 m) digital elevation model (DEM) to measure bedrock river profiles and hillslope gradients from these ranges. We observe a prominent suite of channel convexities (knickpoints) that segment the channels into upper reaches with low steepness (mean k sn = ∼182; θref = 0.51) and lower, fault-proximal reaches with high steepness (mean k sn = ∼361), with a concomitant increase in hillslope angles of ∼6°–9°. Geologic maps and field-based proxies for rock strength allow us to rule out static causes for the knickpoints and interpret them as transient features triggered by a drop in base level that created ∼20% of the existing relief (∼220 m of ∼1050 m total). We then constrain the timing of base-level change using paleochannel profile reconstructions, catchment-scale volumetric erosion fluxes, and a stream-power–based knickpoint celerity (migration) model. Low-temperature thermochronology data show that faulting began at ca. 11–12 Ma, yet our results estimate knickpoint initiation began in the last 5 Ma and possibly as recently as 0.1 Ma with reasonable migration rates of 0.5–2 mm/yr. We interpret the collective results to be evidence for enhanced Pliocene–Quaternary fault slip that may be related to tectonic reorganization in the American West, although we cannot rule out climate as a contributing mechanism. We propose that similar studies, which remain remarkably rare across the region, be used to further test how robust this Plio–Quaternary landscape signal may be throughout the Great Basin.

Testing fault growth models with low‐temperature thermochronology in the northwest Basin and Range, USA

Common fault growth models diverge in predicting how faults accumulate displacement and lengthen through time. A paucity of field-based data documenting the lateral component of fault growth hinders our ability to test these models and fully understand how natural fault systems evolve. Here we outline a framework for using apatite (U-Th)/He thermochronology (AHe) to quantify the along-strike growth of faults. To test our framework, we first use a transect in the normal fault-bounded Jackson Mountains in the Nevada Basin and Range Province, then apply the new framework to the adjacent Pine Forest Range. We combine new and existing cross sections with 18 new and 16 existing AHe cooling ages to determine the spatiotemporal variability in footwall exhumation and evaluate models for fault growth. Three age-elevation transects in the Pine Forest Range show that rapid exhumation began along the range-front fault between approximately 15 and 11Ma at rates of 0.2–0.4 km/Myr, ultimately exhuming approximately 1.5–5 km. The ages of rapid exhumation identified at each transect lie within data uncertainty, indicating concomitant onset of faulting along strike. We show that even in the case of growth by fault-segment linkage, the fault would achieve its modern length within 3–4Myr of onset. Comparison with the Jackson Mountains highlights the inadequacies of spatially limited sampling. A constant fault-length growth model is the best explanation for our thermochronology results. We advocate that low-temperature thermochronology can be further utilized to better understand and quantify fault growth with broader implications for seismic hazard assessments and the coevolution of faulting and topography.

A global perspective on the topographic response to fault growth

Precise factors controlling the coevolution of deformation and topography in tectonically active landscapes remain poorly understood due to complex feedbacks between numerous possible variables. Here we examine the links between fault kinematics, emergent topography, and environmental factors on a global data set of active fault-driven mountain ranges (n = 41). Using simple regressions between tectonic, climatic, and topographic variables, we explore the controls on fault-driven landscape development at the range scale. For each fault in our Google Earth accessible database, we compiled (1) topographic metrics from a 30-m digital elevation model including along-strike changes in elevation and relief, fault length, and tip zone length (the along-strike distance from fault tip to where the associated relief stops increasing) and gradient; (2) long-term (10 4–6 yr) tectonic variables including fault slip rate, displacement rate, displacement, and age; (3) climatic variables including annual precipitation; and (4) rock type from geologic maps. Our results show that all mountain ranges reach a uniform value of relief within some distance from their tips and the length scale of this relief growth correlates with long-term vertical displacement rate (R = 0.55) and slip rate (R = 0.51). We apply a well-established framework for fault growth as the tectonic boundary condition to estimate the time required to achieve this uniform relief (∼10 4–6 yr) and suggest that this threshold time indicates regional tectonomorphic equilibrium. Strong correlations between annual precipitation and deformation rates (R > 0.60), and between lithologic strength and mountain relief (R > 0.70), allude to other principal forces affecting emergent landscape form that are often ignored. Our findings demonstrate that fault-driven topography always saturates in relief, suggest there are quantifiable fault-kinematic controls on landscape form, and hint that landscape relief patterns may, in turn, be used to estimate rates of faulting.