Sean D. Willett

Tectonics from fluvial topography using formal linear inversion: Theory and applications to the Inyo Mountains, California

Tectonic activity generates topography, and the variability of tectonic forcing is responsible for topographic patterns and variability of relief in fluvial landscapes. Despite this basic relation, the inverse problem, by which features of the topography are used for inferring tectonic uplift rates, has proven challenging. Here we develop formal linear inversion schemes to infer a record of the rate of relative uplift as a function of space and time from the long profiles of rivers. The relative uplift rate is the difference between the rates of rock uplift and of the base level change. The inversion schemes are based on a closed-form analytic solution to the transient linear stream power model, and to increase model resolution they make use of the multiplicity of information made available by multiple rivers and their tributaries. The distribution of the fluvial response time to tectonic perturbations is a key component of the inversion scheme, as this determines which tectonic events are preserved in the topography. We develop two inversion parameterizations that differ in their assumptions about the tectonic forcing: space-invariant and time-space variability with an assumed spatial distribution. The inversion schemes are applied to the Inyo Mountains, an uplifted block along the western boundary of the Basin and Range Province in California. Inversion results indicate that the range has been experiencing an acceleration of the relative uplift in the past ∼2-3 Ma. We use the inversion results to constrain the paleotopography and paleo-erosion rate along the range and to recover the throw rate history along the fault that bounds the Inyo range.

Rapid exhumation in the Western Alps driven by slab detachment and glacial erosion

Identifying topographic and erosion rate response to tectonic and climatic forcing remainsnchallenging. This is in part because of the difficulty in isolating the respective roles of climatenand tectonics. Here we exploit 2500 thermochronometric data points collected over severalndecades of research, using a new inverse technique, to image the space-time evolution of erosionnrate across the European Alps over the past 35 m.y. The most striking feature of ournresults is a two- to three-fold increase in erosion rate over the past 2 m.y. exclusively within thenWestern and Central Alps. This increase appears to be controlled by the inferred high rocknuplift rate due to the progressive detachment of the European slab under the Western Alps.nThe similarity in mean elevation between the Western and Eastern Alps indicates a surprisinglynlow topographic response to this differential tectonic forcing, and points to the role ofnenhanced glacial erosion in response to surface uplift.

The Exhumation history of the European Alps inferred from linear inversion of thermochronometric data

Thermochronometric data collected across the Alps over the last three decades allows for investigation of the evolution of this orogen, which is subject to changes in climate and geodynamics. Exhumation rates are inferred from the thermochronometric ages using a statistical inversion method based on the fact that the distance a sample traveled since closure is equal to the integral of the exhumation rate from the present day to the age of the sample. Exhumation rates are assumed to be spatially correlated but are free to vary through time. This results in the quantification of exhumation rates across the Alps, since 32 Ma, along with assessments of the quality of these inferences. We find that exhumation rates are initially fast in the internal arc of the Western Alps at rates up to 0.8 km/Myr at 30 Ma, decreasing at 20 Ma to 0.3 km/Myr to remain slow to the present. At the same time, around 20 Ma, rates across the External Crystalline Massifs of Western Alps increase to 0.6 km/Myr. We also find that the onset of high exhumation rates in the Tauern Window and the Lepontine Dome occurs at around 20 Ma, a time characterized by major reorganizations in the Alpine chain. A general increase in exhumation rates at around 5 Ma over the entire Alps is not confirmed. Instead we find that the Western Alps exhibit a 2 to 3 fold increase in exhumation rate over the last 2 Ma, during a recent event not seen further east, in spite of very similar topographic characteristics. We attribute this strong signal to detachment of the European slab in the Western Alps, combined with efficient glacial erosion.

Impact of sedimentation on evolution of accretionary wedges: Insights from high-resolution thermomechanical modeling

Syntectonic sedimentation history is a potential cause of differentiated accretionary wedge structures along the subduction margin. Recent efforts to model the role of sedimentation on wedge evolution have highlighted the importance of spatio-temporal history of sedimentation on the evolution of the wedge. Moreover, reconstruction of deformation history of the accretionary wedges using reflection seismic and borehole data have further substantiated the impact of sedimentation on wedge evolution. We conduct several numerical experiments using a high-resolution dynamic 2D thermo-mechanical plate subduction model to systematically investigate and quantify different effects of sedimentation on accretionary wedge evolution. Models with sedimentation suggest migration of deformation to parts of the wedge lying outside the sedimentation zone leading to emergence/reactivation of out-of-sequence thrusts (OOSTs). Frequency and length of new thrust sheets are correlated with sedimentation in the trench. Models undergo a transition period of ~1.5 Myr following the onset of sedimentation, after which they continue to grow under a new steady state. Stabilization of the wedge and increased load on the oceanic plate due to sedimentation create conditions in which smaller wedge-top basins combine to form a large and flat forearc basin. Last but not the least, emergence of OOST in models of accretionary wedges undergoing sedimentation provide important insights in to evolution of potentially tsunamigenic OOSTs like the Megasplay Fault seaward of the Kumano forearc basin.

Resolving spatial heterogeneities in exhumation and surface uplift in Timor‐Leste: Constraints on deformation processes in young orogens

Although exhumation and surface uplift are important parameters in understanding orogenesis, the opportunity to measure both in close proximity is rare. In Timor-Leste (East Timor), deeply exhumed metamorphic rocks and piggyback deepwater synorogenic basins are only tens of kilometers apart, permitting direct relation of uplift and exhumation by comparing micropaleontology to thermochronology interpreted through one-dimensional thermal modeling. Foraminifera in two deepwater synorogenic basins suggest basin uplift from depths of 1–2u2009km to depths of 350–1000u2009m between 3.35 and 1.88u2009Ma. Thermochronologic sampling was conducted in the central mountain belt between these basins. Of four muscovite 40Ar/39Ar samples, one provides a reset age of 7.13u2009±u20090.25u2009Ma in the Aileu high-grade belt that suggests ~9–16u2009km of exhumation since that time. Eighteen zircon (U-Th)/He samples contain a group of reset ages in the Aileu Complex ranging from 4.4 to 1.5u2009Ma, which suggest exhumation rates of 1.0–3.1u2009mm/yr with 2.7–7.8u2009km of exhumation since these ages. Thirteen apatite (U-Th)/He ages in the Gondwana Sequence range from 5.5 to 1.4u2009Ma, suggesting 1–2u2009km of exhumation and defining a pattern of exhumation rates (ranging from 0.2 to 1.3u2009mm/yr) that positively correlates with average annual rainfall. Seven apatite fission track samples display varying degrees of partial resetting, with greatest resetting where apatite (U-Th)/He ages are youngest. Together, these data demonstrate extreme variability in surface uplift and exhumation over small spatial scales. We propose ongoing subsurface duplexing driven by subduction and underplating of Australian continental crust as the predominant driver for surface uplift and uplift-induced exhumation.

Linking Deep Earth and Surface Processes

One of the important developments in Earth science over the past decade has been recognition of the significance of linking deep Earth dynamic processes with surface and near-surface geologic processes [e.g., Braun, 2010]. Deep Earth research, encompassing fields such as seismology and mantle geodynamics, has traditionally operated distinctly from fields focusing on dynamics of the Earths surface, such as sedimentology and geomorphology. However, these endeavors have in common the study of Earths topography and the prediction of changes in its surface. Observables from surface studies, such as basin stratigraphy, geomorphology of landscapes, changes in surface elevation, and changes in sea level, provide some of the principal constraints on geodynamic and tectonic models. Conversely, deep geodynamic processes give rise to the topography, erosion, and sediment generation that are the basis of surface geology. Surface manifestations of deep geodynamic processes have significant societal impact by creating natural hazards, such as earthquakes and mass movements, and controlling the distribution of natural resources such as fossil fuels or geothermal energy. The relevance of research conducted in both the deep Earth and surface regimes is thus enhanced through a focus on their interaction.

Evaluating igneous sources of the Taveyannaz formation in the Central Alps by detrital zircon U–Pb age dating and geochemistry

Late Palaeogene syn-tectonic volcanic products have been found in the Northern Alpine foreland basin and in the South Alpine hemipelagic basin. The source of abundant volcanic fragments is still in debate. We analyzed the geochronology and geochemistry of detrital zircons, and evaluated their temporal and genetic relationships with potential volcanic sources. The study shows that the detrital zircon U–Pb age patterns have two major age groups: a dominance (ca. 90%) of pre-Alpine zircons was found, as commonly observed in other Alpine flysch formations. These zircons apparently derived from erosion of the early Alpine nappe stack in South Alpine and Austroalpine units. Furthermore, a few Neo-Alpine zircons (ca. 10%) have ages ranging from Late Eocene to Early Oligocene (~xa041–29xa0Ma). Both source materials were mixed during long riverine transport to the basin margins before being re-deposited by gravity flows. These Palaeogene ages match with the activity of Peri-Adriatic magmatism, including the Biella volcanic suite as well as the Northern Adamello and Bergell intrusions. The values of REE and 176Hf/177Hf(t) ratios of the Alpine detrital zircons are in line with the magmatic signatures. We observe an in time and space variable supply of syn-sedimentary zircons. From late Middle Eocene to Late Eocene, basin influx into the South Alpine and Glarus (A) basins from the Northern Adamello source is documented. At about 34xa0Ma, a complete reorganisation is recorded by (1) input of Bergell sources into the later Glarus (B) basin, and (2) the coeval volcaniclastic supply of the Haute-Savoie basin from the Biella magmatic system. The Adamello source vanished in the foreland basin. The marked modification of the basin sources at ~xa034xa0Ma is interpreted to be initiated by a northwestern shift of the early Alpine drainage divide into the position of the modern Insubric Line.

Millennial scale variability of denudation rates for the last 15 kyr inferred from the detrital 10Be record of Lake Stappitz in the Hohe Tauern massif, Austrian Alps

Reconstructing paleo-denudation rates over Holocene timescales in an Alpine catchment provides a unique opportunity to isolate the climatic forcing of denudation from other tectonic or anthropogenic effects. Cosmogenic 10Be on two sediment cores from Lake Stappitz (Austrian Alps) were measured yielding a 15-kyr-long catchment-averaged denudation record of the upstream Seebach Valley. The persistence of a lake at the outlet of the valley fixed the baselevel, and the high mean elevation minimizes anthropogenic impacts. The 10Be record indicates a decrease in the proportion of paraglacial sediments from 15 to 7 kyr cal. BP after which the 10Be concentrations are considered to reflect hillslope erosion and thus can be converted to denudation rates. These ones significantly fluctuated over this time period: lower hillslope erosion rates of ca. 0.4 mm/year dated between 5 and 7 kyr cal. BP correlate with a stable climate, sparse flooding events and elevated temperatures that favoured the widespread growth of stabilizing soils and vegetation. Higher hillslope erosion rates of ca. 0.8 mm/year over the last ~4 kyr correlate with a variable, cooler climate where frequent flooding events enhance denudation of less protected hillslopes. Overall, our results suggest a tight coupling of climate and hillslope erosion in alpine landscapes as it has been observed in other parts of the Alps.

Stratigraphic signatures of forearc basin formation mechanisms

Tectonic deformation of accretionary wedges is often interpreted using stratigraphic patterns observed in forearc basins. However, similar stratigraphic patterns could evolve from different deformational processes making it difficult to uniquely reconcile stratigraphic patterns and the geodynamic history of natural accretionary wedges. Therefore, it is important to test the dynamic consistency of interpreted deformation histories. One approach is to compare synthetic stratigraphy of forearc basins generated in numerical accretionary wedge models, to the stratal patterns observed in natural forearcs. We present a simple method to simulate synthetic stratigraphy in numerical accretionary wedge models, comparable to the stratigraphic patterns observed in seismic reflection data. As calibration, we use reflection seismic and borehole data for the Kumano forearc basin in Nankai. We observe that the stratigraphy in retro-forearc basins remains predominantly undisturbed and unaffected by wedge deformation. Sediment-stabilized wedge-top basins in wedges with trenchward surface slope typically form landward of an active out-of-sequence thrust, leading to a landward tilting of the basin. These stratigraphic characteristics are diagnostic of the forearc basin forming mechanism. We also infer that sedimentation in the trench can potentially influence the generation/reactivation of Megasplay Fault activity in the Nankai accretionary wedge. Activity on the Megasplay Fault helps create accommodation space landward to it that leads to the formation of Kumano forearc basin.

Transience of the North American High Plains landscape and its impact on surface water

Ecosystem diversity and human activity in dry climates depend not just on the magnitude of rainfall, but also on the landscape’s ability to retain water. This is illustrated dramatically in the High Plains of North America, where despite the semi-arid modern and past climate, the hydrologic conditions are diverse. Large rivers sourced in the Rocky Mountains cut through elevated plains that exhibit limited river drainage but widespread surface water in the form of ephemeral (seasonal) playa lakes1, as well as extensive groundwater hosted in the High Plains aquifer of the Ogallala formations2. Here we present a model, with supporting evidence, which shows that the High Plains landscape is currently in a transient state, in which the landscape has bifurcated into an older region with an inefficient river network and a younger, more efficient, river channel network that is progressively cannibalizing the older region. The older landscape represents the remnants of the Ogallala sediments that once covered the entirety of the High Plains, forming depositional fans that buried the pre-existing river network and effectively ‘repaved’ the High Plains3–6. Today we are witnessing the establishment of a new river network that is dissecting the landscape, capturing channels and eroding these sediment fans. Through quantitative analysis of the geometry of the river network, we show how network reorganization has resulted in a distinctive pattern of erosion, whereby the largest rivers have incised the older surface, removed fan heads near the Rocky Mountains and eroded the fan toes, but left portions of the central fan surface and the Ogallala sediments largely intact. These preserved fan surfaces have poor surface water drainage, and thus retain ephemeral water for wetlands and groundwater recharge. Our findings suggest that the surface hydrology and associated ecosystems are transient features on million-year timescales, and reflect the mode of landscape evolution.The High Plains region of North America is in a transient state, with a younger, efficient network of river channels progressively cannibalizing an older, less efficient region, aiding water retention for wetlands and groundwater recharge.