Fritz Schlunegger

Climate-induced rebound and exhumation of the European Alps

Foreland basins record the regional isostatic compensation of mountain belts; during periods of crustal thickening, they subside, and when erosion unloads the mass of the mountains, the basins rebound and are eroded. In order to evaluate this mechanism for rebound, it is critical that the timing and magnitude of erosion are documented. We present data estimating the timing and magnitude of late orogenic or postorogenic erosion in the North Alpine Foreland Basin of Switzerland. Mineral cooling ages demonstrate that the basin underwent 1-3 km of erosion soon after 5 Ma. This erosion coincided with a decline in structural deformation in the Swiss Alps, and a doubling of sediment accu- mulation rates in surrounding depocenters. We propose that accelerated erosional un- roofing of the Swiss Alps triggered isostatic rebound and erosion of the foreland basin after 5 Ma. A projection of the isostatic rebound of the basin into the mountains suggests that at least 6.5 km of erosion should have occurred in the high topography of the Aar Massif. Accelerated erosion in the Swiss Alps at that time is explained by an increase in atmospheric moisture driven by an intensification of the Atlantic Gulf Stream at 4.6 Ma. Consequently, we propose that the changing erosional capacity of the climate triggered late orogenic to postorogenic mass reduction and isostatic rebound of the Swiss Alps and their neighboring foreland basin.

Messinian climate change and erosional destruction of the central European Alps

At the end of the Miocene, the European Alps ceased outward expansion, and tectonic uplift and exhumation shifted into the orogen interior. This shift is consistent with a change from orogenic construction to orogenic destruction, reflecting an increase in the ratio of erosional flux to accretionary flux. The coincidence of this change with an increase in sediment yield from the Alps suggests a climate-driven increase in erosional flux. The timing of deformation and sediment release from the southern Alps indicates that the tectonic change occurred synchronous with the last phase of the Messinian salinity crisis. We attribute the increase in erosional flux to a climatic shift to wetter conditions throughout Europe, likely augmented by the base-level fall that occurred during the Mediterranean dessication. This climate change is represented in the stratigraphic record by the Lago Mare deposits of the Mediterranean salinity crisis.

Glacial conditioning as an erosional driving force in the Central Alps

Unparalleled data availability in the European Alps has led to an ongoing debate about the driving mechanism behind the concurrent patterns of surface denudation and modern rock uplift. Analysis of stream channels reveals that oversteepened stream segments are primarily located in landscapes with strong glacial inheritance. This leads to a transient signal in the landscape, with the result that erosion is spatially focused by a combination of glacial conditioning and lithologic controls. We postulate that the effect of glacial forcing is a positive feedback cycle between erosion and rock uplift, driving rapid rates of both in the Alpine landscape. This mechanism may explain the observed increases in sediment flux since the late Pliocene.

Spatial and temporal variations in exhumation of the central Swiss Alps and implications for exhumation mechanisms

Abstract Information about the structural and thermal evolution of the Alps, interpreted through thermal and mechanical models, provides an improved understanding of the processes that led to the exhumation of the Alps. We synthesize published thermochronometric data, analyse these data in terms of cooling rates and interpret the spatial and temporal patterns of cooling. Cooling rates are interpreted in terms of exhumation rates, aided by the use of a one-dimensional thermal model. Our study reveals that rapid exhumation of the Lepontine core occurred during the interval of 35-20 Ma. Existing data cannot determine whether this was by rapid erosion at rates exceeding 1 km Ma−1 or by a relatively brief period of tectonic exhumation, although the correlation between extensional fault motion and high cooling rates supports the tectonic exhaumation hypothesis. Peripheral regions of the Alps cooled at rates consistent with low to moderate exhumation rates of 400–500 m Ma−1, initiating later than cooling of the Lepontine core, consistent with outward growth of the orogen. Outward growth of the orogen is potentially the result of either lower exhumation rates or higher rates of crustal accretion as demonstrated by a two-dimensional, coupled erosion-deformation model. In particular, the growth of the Southern Alps after c. 20 Ma is evidence for a decrease in the exhumation rate relative to the crustal accretion rate. This could represent a decrease in exhumation rate after cessation of normal faulting, or it could reflect initiation of accretion of a larger fraction of the European crust.

Controls of erosional denudation in the orogen on foreland basin evolution: The Oligocene central Swiss Molasse Basin as an example

A high-resolution three-dimensional reconstruction of the 25-m.y.-old central Swiss Molasse Basin reveals two sedimentary domains separated by a ∼5-km-wide flood-plain. The proximal domain of the basin attained a width of 20 km, and its basement is steeply flexed (6°-7° dip). Petrographic data indicate that it was filled by sediment from the Rigi dispersal system derived from the central Alps of eastern Switzerland and by locally sourced bajadas. In contrast, the distal sedimentary domain, located farther north, was gently dipping (<2°) and was filled by the meandering Lac Leman and Honegg dispersal systems. Chronological data reveal that sedimentation in the northern proximal part of the basin started at ∼27 Ma, when sediment supply to the basin started to increase. Deflection of the foreland plate at ∼25 Ma is successfully simulated by flexural modeling of the thrust load and the sediment load. The model reveals that the Lac Leman and Honegg dispersal systems are located on a buried flexural bulge. Furthermore, it shows that burial and suppression of the flexural bulge at ∼27 Ma as well as an increase of the basin wavelength were controlled by the contemporaneous increase in the sediment supply rate of the Rigi system. The model presented suggests that the tectonic subsidence of the Molasse Basin was mainly controlled by tectonic events in the northern part of the orogen, within ∼70 km distance from the tip of the orogenic wedge. Crustal thickening in this part of the orogen is reflected in the proximal Molasse by sedimentary cycles characterized by an increase in the sediment accumulation rates up section and by the presence of locally sourced bajada fans at the top of each cycle. Although south vergent back thrusting along the Insubric Line ∼150 km south of the foreland basin contributed little to flexure, it resulted in an increase of the sediment supply to the foreland basin. This is reflected in the Molasse by coarsening and thickening upward trends, an increase of the basin wavelength, basinward shifts of the depocenters of the dispersal systems, and uplift and erosion of the proximal basin border.

Magnetostratigraphic constraints on relationships between evolution of the central Swiss Molasse basin and Alpine orogenic events

Magnetostratigraphic chronologies, together with lithostratigraphic, sedimentological, and petrological data enable detailed reconstruction of the Oligocene to Miocene history of the North Alpine foreland basin in relation to specific orogenic events and exhumation of the Alps. The Molasse of the study area was deposited by three major dispersal systems (Rigi, Hohronen, Napf). Distinguished by characteristic heavy mineral suites, conglomerate clast populations, and the presence of key clasts, these systems record three major phases of denudation of the Alpine edifice. The Rigi system eroded the Austroalpine and Penninic nappes of eastern Switzerland from 30 to 25.5 Ma as a result of backthrusting and uplift of these units along the Insubric Line. Subsequent uplift of the Aar massif some 40 km to the north appears to have controlled the duration of the Hohronen and Napf dispersal systems, spanning 24–22 Ma and 21.5–15 Ma, respectively. They record downcutting into the crystalline cores of the Penninic and Austroalpine nappes of eastern (Hohronen) and western (Napf) Switzerland. High-resolution reconstruction of the structural and geometrical evolution of the proximal Molasse reveals in-sequence and out-of-sequence thrusting events at the Alpine front and incorporation of the Molasse into the orogenic wedge by in-sequence thrusting and underplating. Furthermore, it reveals close relationships between periods of rapid denudation in the central Alps and phases of increased sediment accumulation rates at the proximal basin border. An initial increase in Molasse accumulation rates to >1 km/m.y. occurred between 30 and 25.5 Ma and coincides with the Insubric phase of backthrusting along the eastern Insubric Line, where >10 km of vertical displacement is interpreted. During the same time span, the Alpine wedge propagated forward along the basal Alpine thrust, as indicated by the coarsening- and thickening-upward megasequence and by occurrence of bajada fans derived from the Alpine border. The end of this tectonic event is marked by a basinwide unconformity, interpreted to have resulted from crustal rebound after initial loading. A subsequent increase in accumulation rates to >1 km/m.y. between 23 and 21.5 Ma coincides with initial uplift of the eastern Aar massif by at least 4 km. This phase of high accumulation rates is associated with incorporation of early Chattian conglomerates into the orogenic wedge. The third advance of the Alpine wedge between 21 and 15.5 Ma caused underplating of Molasse deposits, resulting in synsedimentary backthrusting of previously deposited Molasse sequences and in the development of a progressive unconformity. A rapid increase in accumulation rates from 0.35 to >1 km/m.y. between 15.5 and 15 Ma marks the final loading event in the wedge, which may be caused by further major displacement and loading of the Aar massif. This deformation is coeval with out-of-sequence thrusting of the Helvetic border chain and of the piggyback stack of North Penninic and Ultrahelvetic Flysch nappes along the basal Alpine thrust.

Drainage basin response to climate change in the Pisco valley, Peru

The Quaternary development of the Pisco valley in central Peru has been characterized by multiple phases of sediment accumulation and erosion that formed distinct levels of cut-and-fill terraces and alluvial fans. Luminescence dating shows that they formed in response to at least two different stages of sediment accumulation and erosion during the past 60 ka, the main phase of sediment aggradation occurring between ca. 54 and 38 ka ago. The ages show that sediment accumulation was contemporaneous with the time intervals of the Minchin (47.8–36 ka ago, with enhanced precipitation beginning ca. 54.8 ka ago) and Tauca (26–14.9 ka ago) paleolakes on the Altiplano, where the headwaters of the Pisco River are located. We conclude that sediment accumulation was triggered by shifts toward a more humid climate, whereas erosion is the response of the fluvial system to the depletion of the hillslope sediment reservoirs.

The late Miocene to Holocene erosion pattern of the Alpine foreland basin reflects Eurasian slab unloading beneath the western Alps rather than global climate change

We synthesized published data on the erosion of the Alpine foreland basin and apatite fission-track ages from the Alps to infer the erosional sediment budget history for the past 5 m.y. The data reveal that erosion of the Alpine foreland basin is highest in front of the western Alps (between 2 and 0.6 km) and decreases eastward over a distance of 700 km to the Austrian foreland basin (similar to 200 m). For the western Alps, erosion rates are >0.6 km/m.y., while erosion rates for the eastern foreland basin and the adjacent eastern Alps are <0.1 km/m.y., except for a small-scale signal in the Tauern Window. The results yield a large ellipsoidal, orogen-crossing pattern of erosion, centered along the western Alps. We suggest that accelerated erosion of the western Alps and their foreland basin occurred in response to regional-scale surface uplift, related to lithospheric unloading of the Eurasian slab along the Eurasian-Adriatic plate boundary. While we cannot rule out recent views that global climate change led to substantial erosion of the European Alps since 5 Ma, we postulate that regional-scale tectonic processes have driven erosion during this time, modulated by an increased erosional flux in response to Quaternary glaciations.

Possible erosional control on lateral growth of the European Central Alps

Between the middle and late Miocene, the Central Alps of Switzerland and northern Italy underwent a major phase of lateral crustal growth as recorded by the formation of the southern Alps and the Jura fold-and-thrust belt. This period of dominantly outward-directed deformation differs from the late Oligocene and early Miocene deformation style, which was characterized mainly by rapid vertical exhumation. Sediment budgets from circum-Alpine sedimentary basins indicate a decrease in the erosional efficiency in the Alpine hinterland several million years before lateral orogen growth was initiated, and decreasing magnitudes of sediment discharge thereafter. This decrease in erosional efficiency of the hinterland coincides approximately with widespread exposure of the crystalline core in the Alpine hinterland as indicated by clast types in conglomerates of synorogenic deposits, and with a contemporaneous increase in the continental influence in the paleoclimate, as suggested by the fossiliferous plant record. We suggest that the observed decrease in erosional efficiency caused gravitational forces to increase relative to tectonic forces driving the orogenesis, which led to a transition from dominantly vertical to horizontally directed extrusion. The data thus can be interpreted to indicate an active link between surface erosion and orogenic evolution.

Slab rollback orogeny in the Alps and evolution of the Swiss Molasse basin

The stratigraphies of foreland basins have been related to orogeny, where continent–continent collision causes the construction of topography and the downwarping of the foreland plate. These mechanisms have been inferred for the Molasse basin, stretching along the northern margin of the European Alps. Continuous flexural bending of the subducting European lithosphere as a consequence of topographic loads alone would imply that the Alpine topography would have increased at least between 30 Ma and ca. 5–10 Ma when the basin accumulated the erosional detritus. This, however, is neither consistent with observations nor with isostatic mass balancing models because paleoaltimetry estimates suggest that the topography has not increased since 20 Ma. Here we show that a rollback mechanism for the European plate is capable of explaining the construction of thick sedimentary successions in the Molasse foreland basin where the extra slab load has maintained the Alpine surface at low, but constant, elevations.