Matthias Bernet

Short-lived, fast erosional exhumation of the internal western Alps during the late early Oligocene: Constraints from geothermochronology of pro- and retro-side foreland basin sediments

Apatite and zircon fission-track analysis and single zircon fission-track–U/Pb double dating of Oligocene to Miocene pro- and retro-side foreland basin sedimentary rocks provide evidence for short-lived but fast erosional exhumation of at least 1.5–2 km/m.y. in the internal western Alps between ca. 30 and 28 Ma. This period of fast erosion is seen as a result of rapid surface uplift coupled with increasing orographic precipitation during this phase of orogenesis. Surface uplift may have been caused and sustained by different plate-tectonic processes such as a change in convergence direction, intermediate-depth slab breakoff, and emplacement of the Ivrea body during continental collision. The occurrence of contemporaneous volcanic activity on the pro-side of the western Alps on the subducting European plate between ca. 36 Ma and 30 Ma is seen in connection with slab rollback of the Apennine slab and upwelling of hot mantle material beneath the western Alps. Single zircon double dating shows that the exhumational signal in the detrital thermochronologic data is not compromised by volcanically derived zircons, as volcanic grains can be identified and removed from the zircon fission-track data set to obtain a pure exhumational signal. The signal of fast exhumation is observed in the zircon fission-track data of the pro-side foreland basin and in the apatite fission-track data and published 40 Ar- 39 Ar data in the retro-side foreland basin. During late Oligocene times, erosion rates slowed down to rates similar to present-day erosion rates in the western Alps.

Dynamic ups and downs of the Himalaya

Fast uplift and exhumation of the Himalaya and Tibet and fast subsidence in the foreland basin portray the primary Neogene evolution of the Indian-Eurasian collision zone. We relate these events to the relative northward drift of India over its own slab. Our mantle-flow model derived from seismic tomography shows that dynamic topography over the southward-folded Indian slab explains the modern location of the foreland depocenter. Back in time, our model suggests that the stretched Indian slab detached from the Indian plate during the indentation of the Eurasian plate, and remained stationary underneath the northward-drifting Indian continent. We model the associated southward migration of the dynamic deflection of the topography and show that subsidence has amounted to ∼6000 m in the foreland basin since 15 Ma, while the dynamic surface uplift of the Himalaya amounted to ∼1000 m during the early Miocene. While competing with other processes, transient dynamic topography may thus explain, to a large extent, both the uplift history of the Himalaya and subsidence of its foreland basin, and should not be ignored.

Strong tectonic and weak climatic control on exhumation rates in the Venezuelan Andes

We studied the relationships among present-day relief, precipitation, stream power, seismic energy, seismic strain rate, and long-term exhumation rates for the Venezuelan Andes. Average long-term exhumation rates were determined for seven large catchments in the Venezuelan Andes from fission-track analysis of detrital apatite. A quantitative comparison between eight new detrital apatite fission-track (AFT) age distributions presented here and previously published bedrock AFT age patterns shows that detrital AFT ages can be used for predicting exhumation patterns across the mountain belt. Catchment-averaged exhumation rates estimated from the raw data range from 0.48 ± 0.02 km m.y. –1 to 0.80 ± 0.26 km m.y. –1 Accounting for variable sediment yield and assuming that short-term sediment production rates scale with long-term exhumation rates, these rates vary from 0.33 ± 0.07 km m.y. –1 to 0.48 ± 0.08 km m.y. –1 No variation in rates is observed between the northwestern and southeastern flanks of the mountain belt, despite a threefold increase in precipitation from the northwest to the southeast. Long-term exhumation rates are strongly correlated with relief in the different catchments, but no or negative correlations exist with precipitation data or present-day erosion indexes, while the correlation with seismic energy released by earthquakes is weak to moderate. This lack of correlation may be caused by the insufficient temporal range of the available precipitation and seismicity data, and the different time scales involved in the comparison. Long-term exhumation rates are, however, strongly correlated with seismic strain rates (which take the temporal earthquake magnitude-frequency scaling into account), suggesting that the moderate correlation with seismic energy is indeed related to the different time scales and that tectonic control on exhumation is significant. In contrast, given that precipitation patterns in the Venezuelan Andes should have been installed during Miocene times, we suggest that decoupling of relief and exhumation from present-day climate explains the lack of correlation between exhumation and precipitation.

Crustal-scale block tilting during Andean trench-parallel extension: Structural and geo-thermochronological insights

Despite a long history of plate convergence at the western margin of the South American plate that has been ongoing since at least the Early Paleozoic, the southern Peruvian fore-arc displays little to no evidence of shortening. In the light of this observation, we assess the deformation history of the southern Peruvian fore-arc and its geodynamic implications. To accomplish this, we present a new structural and geo-thermochronological data set (zircon U-Pb, mica 40Ar/39Ar, apatite and zircon fission-track and zircon (U-Th)/He analyses) for samples collected along a 400 km long transect parallel to the trench. Our results show that the Mesoproterozoic gneissic basement was mainly at temperatures ≤350°C since the Neoproterozoic and was later intruded by Jurassic volcanic arc plutons. Along the coast, a peculiar apatite fission-track age pattern, coupled with field observations and a synthesis of available geological maps, allows us to identify crustal-scale tilted blocks that span the coastal Peruvian fore-arc. These blocks, bounded by normal faults that are orthogonal to the trench, suggest post-60 Ma trench-parallel extension that potentially accommodated oroclinal bending in this region. Block tilting is consistent with the observed and previously described switch in the location of sedimentary sources in the fore-arc basin. Our data set allows us to estimate the cumulative slip on these faults to be less than 2 km and questions the large amount of trench-parallel extension suggested to have accommodated this bending.

Geological record of flat slab–induced extension in the southern Peruvian forearc

The long-lived Andean subduction zone underwent several flat slab episodes and is therefore ideal to study the consequences of a complete cycle of slab flattening and steepening on the upper plate deformation pattern. In the modern Peruvian forearc (15°–17°S), slab flattening caused a Paleogene (52–30 Ma) landward migration of volcanic activity. Combining structural geology and a source-to-sink thermochronological study, we show that the flat slab period is contemporaneous with uplift accommodated by large-scale crustal extension in the forearc. In this light, we argue that the study area is an ancient analog to the modern Mexican and northern Peruvian forearcs located above modern flat slab segments and currently undergoing widespread extension.

Exhumation Studies of Mountain Belts Based on Detrital Fission-Track Analysis on Sand and Sandstones

Fission-track (FT) analysis of detrital apatite and zircon from modern sediments and ancient sandstone is a commonly used approach for studying and quantifying the long-term exhumation history of convergent mountain belts. Being aware of potential bias in the age spectra because of sampling, sample preparation and statistical data treatment such as peak-fitting, FT ages from sediments and sedimentary rocks of known depositional age can be readily transferred into long-term average exhumation or erosion rates using the lag-time concept. Double dating of single grains with the FT and U–Pb methods provides additional valuable provenance information, for example, for identifying volcanically derived grains, which may obscure the exhumation signal. Applying both apatite and zircon FT dating on the same samples allows combining the study of source area exhumation and the thermal evolution of sedimentary basins.

Differential Exhumation Across the Longriba Fault System: Implications for the Eastern Tibetan Plateau

The deformation processes at work across the eastern margin of the Tibetan Plateau remain controversial. The interpretation of its tectonic history is often polarized between two deformation models: ductile flow in the lower crust and shortening and crustal thickening accommodated by brittle structures in the upper crust. Many geological investigations on this plateau margin focused on the Longmen Shan, at the western edge of the Sichuan Basin. However, the Longriba fault system (LFS) located 200 km northwest and parallel to the Longmen Shan structures provides an opportunity to understand the role of hinterland faults in eastern Tibet geodynamics. For this reason, we investigate the exhumation history of rocks across the LFS using (U-Th)/He and fission track ages from apatite and zircon. Results show a significant contrast in cooling histories across the Maoergai fault, the southernmost fault of the LFS. South of the Maoergai fault, the bedrock records a rapid increase in exhumation rate since ~10-15 Ma. In contrast, the area north of the fault has experienced steady cooling since ~25-35 Ma. We attribute this cooling contrast to ~2 km of differential rock uplift across the Maoergai fault, providing the first evidence of activity of the LFS in the Late Cenozoic. Our results indicate that deformation of the eastern Tibetan margin has been partitioned into the LFS and the Longmen Shan over an ~200 km wide block, which should be incorporated in future studies on the regions deformation, and in both above-mentioned deformation models.

Extensional deformation along the Footwall Fault below the Hyde-Macraes Shear Zone, Otago Schist, New Zealand

ABSTRACT We describe ductile-to-brittle structures across the Footwall Fault directly below the Hyde-Macraes Shear Zone in the Otago Schist. These indicate that pervasive deformation along the Footwall Fault was due to northeast–southwest crustal extension. Structures below the Footwall Fault are ductile top-to-the-northeast extensional shear bands that formed in a penetrative lower greenschist facies foliation. With progressive deformation, shear bands formed at higher angles to the foliation, culminating in the shear bands being cut by low- and high-angle normal faults. In the hanging wall, extensional structures are much less pervasive and comprise a few high-angle normal faults. This sharp contrast in structural evolution of footwall and hanging wall resembles the tectonic evolution of major extensional shear zones worldwide. We propose that mid Cretaceous normal movement on the low-angle Footwall Fault accompanied the formation of the steeper Waihemo Fault further north. On a regional scale, much of the boundary between the Otago Schist core and its northern flank is cut by mid Cretaceous normal faults. Zircon fission-track ages from the footwall range from 79 ± 13.7 to 46.1 ± 6.2 Ma (2σ uncertainties) and young in the direction of hanging wall transport. One sample from the hanging wall yielded a zircon fission-track age of 76 ± 12.9 Ma. The fission-track ages are hard to interpret as zircon across the fault zone was affected by partial or full annealing. In part, the ages confirm mid Cretaceous movement but may also be interpreted to suggest additional reactivation of the Footwall Fault at slow inferred slip rates.