Peter van der Beek

Variable late Neogene exhumation of the central European Alps: Low‐temperature thermochronology from the Aar Massif, Switzerland, and the Lepontine Dome, Italy

[1]xa0Several recent studies proposed an important increase in exhumation rate in the western European Alps since circa 5–4 Ma. In order to assess potential spatial differences in exhumation histories, we present new apatite fission track (AFT) and apatite (U-Th)/He (AHe) ages from the central Aar Massif (Guttannen area, Switzerland) and the western Lepontine Dome (Formazza area, Italy). Internal U/Th zoning in apatites explains alpha-ejection-corrected AHe ages that are older than the corresponding AFT ages in this study. A qualitative interpretation of AFT and AHe age-elevation relationships suggests a two-phase (9–7 and 5–3 Ma) exhumation scenario affecting the central Alps, with a stronger expression of the Pliocene signal in the Formazza area. However, a quantitative evaluation of exhumation scenarios using the 3-D heat equation solver Pecube highlights the existence of several other likely scenarios, casting doubt on the validity of a qualitative interpretation of the age-elevation relationships. In Formazza, scenarios suggested by quantitative modeling include continuous denudation at a rate of ∼750 m/Ma and a one-step exhumation rate change from 300 to 1000 m/Ma at 5 Ma. In Guttannen, they include continuous denudation at a rate of ∼400 m/Ma with valley deepening and two periods of higher exhumation rate (increasing from 300 to 700 m/Ma repeatedly at 9–7 and at 5–3 Ma). Contingent upon further flexural isostatic modeling, the magnitude of exhumation recorded in the axial region of the Alps since circa 5 Ma does not appear sufficient to solely explain the denudation recorded in the North Alpine Foreland Basin.

Growth and lateral propagation of fault-related folds in the Siwaliks of western Nepal: Rates, mechanisms, and geomorphic signature

[1]xa0We study the controls on drainage development in tectonically active regions using a numerical tectonic surface processes model combining tectonic uplift caused by fault-related folding with erosion by fluvial incision, hillslope diffusion, and landsliding. Our model shows the fundamental control exerted by the dip of the detachment underlying the folds on drainage evolution. When the detachment is horizontal, the relative rates of tectonic uplift and fluvial incision control the evolution. For a nonzero dip, in contrast, the lateral displacement gradient associated with fold propagation sets up a lateral slope behind the active structure, which deflects the stream network. We also demonstrate the importance of landsliding for the attainment of realistic and steady state topography. In contrast, discontinuous tectonic movements do not appear to influence the system. We apply our model to the Dundwa fault-related fold ridge (Siwalik, Nepal). We estimate a remarkably low mean propagation rate for different segments of the structure. This finding, together with the structure and morphology of the ridge, leads us to propose that the ridge developed by linkage of several component segments. The drainage evolution predicted when modeling this scenario compares favorably with field observations. Our models provide insights into the dynamics of fault-related fold propagation: In particular, as a result of fault segment linkage, estimates of propagation rate of these structures may be strongly scale-dependent, and the observed morphology and drainage patterns on fault-related fold ridges may be controlled by fault geometry rather than by the relative rates of tectonics and surface processes.

Spatial and temporal patterns of exhumation across the Venezuelan Andes: Implications for Cenozoic Caribbean geodynamics

[1]xa0The Venezuelan Andes formed by complex geodynamic interaction between the Caribbean Plate, the Panama Arc, the South American Plate and the continental Maracaibo block. We study the spatial and temporal patterns of exhumation across the Venezuelan Andes using 47 new apatite fission track (AFT) ages as well as topographic analyses. This approach permits the identification of at least seven tectonic blocks (Escalante, Cerro Azul, Trujillo, Caparo, Sierra Nevada, Sierra La Culata and El Carmen blocks) with contrasting exhumation and cooling histories. The Sierra Nevada, Sierra La Culata and El Carmen blocks, located in the central part of the Venezuelan Andes and separated by the Bocono fault system, cooled rapidly but diachronously during the late Miocene–Pliocene. Major surface uplift and exhumation occurred in the Sierra Nevada block since before 8 Ma. A second phase of uplift and exhumation affected the El Carmen and Sierra La Culata blocks to the north of the Bocono fault during the late Miocene-Pliocene. The highest topography and steepest relief of the belt coincides with these blocks. The Caparo and Trujillo blocks, located at the northeastern and southwestern ends of the orogen, cooled more slowly from the Oligocene to the late Miocene. These blocks are characterized by significantly lower mean elevations and slightly lower mean slopes than the central blocks. Unraveling the cooling history of the individual blocks is important to better understand the control of preexisting faults and regional Caribbean geodynamics on the evolution of the Venezuelan Andes. Our data indicate a strong control of major preexisting fault zones on exhumation patterns and temporal correlation between phases of rapid exhumation in different blocks with major tectonic events (e.g., collision of the Panama arc; rotation of the Maracaibo block).

Spatial correlation between long-term exhumation rates and present-day forcing parameters in the western European Alps

The relative intensity of tectonic and climatic forcing in the western European Alps has been a matter of debate since the recognition of a significant increase in denudation rates over the past few million years. We address this question by quantitatively correlating the spatial pattern of long-term exhumation rates with those of potential short-term tectonic, climatic, and morphologic variables. We find that present-day rock-uplift rates (as measured by geodesy relative to a specific reference point) and mean elevation are correlated with long-term exhumation rates, whereas relief, present-day precipitation, discharge, stream power, and released seismic energy are not, or are only weakly, correlated. We attribute the lack of correlation between long-term exhumation and precipitation to a strong temporal variability in climate and erosional processes during Pliocene–Pleistocene time. The correlations among present-day rock-uplift rates, present-day elevations, and long-term exhumation rates suggest that rock-uplift rates have been sustained for millions of years, consistent with rock-uplift rates being the isostatic response to crustal unloading. The lack of a correlation of the released seismic energy with either rock uplift or long-term exhumation denies active tectonics supporting evidence.

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.