The Late Miocene to Holocene Erosion Pattern of the Alpine Foreland Basin Reﬂ Ects Eurasian Slab Unloading Beneath the Western Alps Rather Than Global Climate Change

Home , Sedimentary basin, Sedimentary budget

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

Ramona Baran1,2, Anke M. Friedrich1,*, and Fritz Schlunegger3 1DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, LUDWIG-MAXIMILIANS-UNIVERSITÄT MÜNCHEN, LUISENSTRASSE 37, 80333 MUNICH, GERMANY 2AIRBORNEHYDROMAPPING GMBH, TECHNIKERSTRASSE 21A, A-6020 INNSBRUCK, AUSTRIA 3INSTITUTE OF GEOLOGICAL SCIENCES, UNIVERSITY OF BERN, BALTZERSTRASSE 1+3, CH-3012 BERN, SWITZERLAND

ABSTRACT

We synthesized published data on the erosion of the Alpine foreland basin and apatite ﬁ ssion-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 (~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 ﬂ ux in response to Quaternary glaciations.

LITHOSPHERE; v. 6; no. 2; p. 124–131; GSA Data Repository Item 2014127 doi: 10.1130/L307.1

INTRODUCTION also explain the increase in erosion rates through surface uplift. However, these studies are based on data from the Alps or the foreland basin without It has been proposed that mountainous erosion increased globally considering either the contribution of erosional recycling of foreland basin around 5 Ma in response to global climate change (e.g., Hay et al., 1988; material to the erosional budget or the shape and size of the region under- Zhang et al., 2001; Herman et al., 2013), mainly because this increase going erosion and surface uplift. coincides with a cooling trend indicated by global isotopic data (e.g., To provide an additional perspective on this debate, we combined sedi- Zachos et al., 2001). Records of erosion rates are typically provided ment budgets from the foreland plus AFT ages from the orogen to infer by low-temperature thermochronological analyses of crystalline rocks a spatial gradient map of erosion rates for the Alps and the Alpine fore- exposed in mountainous regions or by sedimentary budget studies of the land basin. Our database1 consists of published (1) AFT cooling ages for sedimentary rocks of the adjacent sedimentary basin. However, Willen- the Alps (Fig. 2; e.g., Vernon et al., 2008; Luth and Willingshofer, 2008; bring and von Blanckenburg (2010) have challenged the validity of such Wölﬂ er et al., 2012), (2) AFT ages from wells from the Swiss foreland records, based on aliasing effects. basin (e.g., Cederbom et al., 2011), and (3) stratigraphic data from indus- The Alps have played a prominent role in this debate. Kuhlemann try wells in the German and Austrian foreland basin (e.g., Lemcke, 1974; (2000) and Kuhlemann et al. (2002) constructed sediment budgets for Genser et al., 2007). We focus our analysis on the shape and scale of the the western and eastern Alps for the past 35 m.y. Herman et al. (2013) area undergoing erosion since 5 Ma within the foreland and the orogen. inverted apatite ﬁ ssion-track (AFT) ages across the Alps to derive an erosional history of the orogen. Both data sets disclosed a substantial increase THE ALPS AND THEIR FORELAND BASIN in the erosion of the Alps at 5 Ma (dashed line in Figs. 1B and 1C). This temporal coincidence was used to call for a climate driver (Cederbom The western and eastern Alps formed as a result of convergence et al., 2004; Vernon et al., 2008; Herman et al., 2013), mainly because between the Eurasian and Adriatic plates. The most recent collisional this increase was not accompanied by tectonic convergence across the episode occurred before 35 Ma, when the Adriatic plate collided with Alps during this time period (e.g., Schmid et al., 1996). However, several the already deformed southern margin of Eurasia, resulting in frontal authors have emphasized the importance of lithospheric-scale processes collision in the eastern Alps but oblique collision in the western Alps beneath the Alps, (e.g., Lyon-Caen and Molnar, 1989; Andeweg and Clo- (Schmid and Kissling, 2000). The postcollisional deformation history etingh, 1998; Kissling, 1993; Lippitsch et al., 2003; Genser et al., 2007; Sue et al., 2007; Kuhlemann, 2007; Wagner et al., 2010), which could 1GSA Data Repository Item 2014127, Appendix 1, is available at www.geosociety .org/pubs/ft2014.htm, or on request from [email protected], Documents Sec- *Corresponding author: [email protected]. retary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.

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also differs across the orogen, refl ecting counterclockwise rotation of We plotted published estimates of erosion and erosion rates, where the Adriatic plate (Fig. 3). Shortening continued to ca. 5 Ma in the west- available, on Figure 2A. In cases where no erosion rates were provided, ern Alps, but at slow rates (e.g., Schmid et al., 1996), while at present, we estimated erosion or exhumation rates using basic assumptions about orogen-parallel transtension and surface uplift dominate the deforma- (1) the geothermal gradient (between 25 and 30 °C/km), (2) partial tional style (e.g., Sue et al., 2007). Convergence, thrust-faulting, and annealing temperatures for AFT (between 120 °C and 90 °C) studies, lateral escape continue to present day in the east (e.g., Robl and Stüwe, (3) the signifi cance of the AFT cooling ages (exhumation = erosion), 2005; Rosenberg and Berger, 2009). and (4) the timing of the onset of erosion in the foreland basin (5 Ma; The Alpine foreland basin formed on the Eurasian lithosphere in Appendix 1 [see footnote 1]). We then drew contour lines of equal ero- response to loading and fl exural bending starting in late Eocene time sion rate by hand on Figure 2B, using geological, geomorphological, (e.g., Lyon-Caen and Molnar, 1989; Schmid et al., 1996; Andeweg and and geochronological constraints on the position of the contour lines. Cloetingh, 1998; Genser et al., 2007). The basin subsided rapidly dur- We note that erosion rate estimates from the basin often lack precise age ing the Oligocene, but slower during the Miocene, reaching maximum control for the onset of erosion, and that exhumation data from mountain depths of >4 km (e.g., Lemcke, 1974). The end of deposition of sediments belts may be older than the time period under consideration and hence in the basin is diffi cult to reconstruct, because regional-scale erosion has provide a longer-term average with no resolving power over the period removed the youngest sediments from the basin. The age of the youngest considered here (ca. 5 Ma). preserved sediments ranges from 6 Ma (east) to 16 Ma (west) (Pfi ffner et al., 2002; Genser et al., 2007). Regional-scale erosion appears to have RESULTS been active by 5 Ma at the latest, based on high-resolution AFT analyses from boreholes (Cederbom et al., 2004). Sediment Budgets, Recycling, and Glacial Carving Erosion of the Po Basin on the south side of the Alps was limited to a brief episode of deep incision between 5.6 and 5.5 Ma, related to the Mes- Thicknesses of eroded sections (between 0.6 and 2 km; Fig. 2A) of the sinian salinity crisis. This was followed by renewed sedimentation that Swiss foreland (12,000 km2) yield ~7000–24,000 km3 of eroded material. continues to present day (Bertotti et al., 2001). On the eastern margin of The relatively large errors in the erosion rate estimates are mainly due the Alps, the Styrian Basin and part of the Pannonian Basin experienced to uncertainties regarding the amount of basin inversion (Data Reposi- uplift at ca. 5–6 Ma due to lithospheric-scale processes, perhaps due to tory [see footnote 1]). The recycling of the Swiss foreland yields between counterclockwise rotation of the Adriatic plate (e.g., Wagner et al., 2010). ~1400 and <5000 km3/m.y. of the sediment fl ux for the western sector Between 15 and 5 Ma, Kuhlemann et al. (2002) inferred a constant during the past 5 m.y. This is one third of the increase in Kuhlemann’s material fl ux of ~20,000 km3/m.y. for the western Alps, and ~10,000 original curve (Fig. 1B). The remaining two thirds of the remaining mate- km3/m.y. for the eastern Alps, respectively (Fig. 1), based on sediment rial were derived from the western Alps during the past 5 m.y. This implies budgets. At ca. 5 Ma, the sediment discharge gently increased in the east- that erosion rates in the western Alps must have increased at ca. 5 Ma, ern Alps by ~5000 km3/m.y., while the sediment discharge in the western consistent with Kuhlemann (2000). Alps increased signifi cantly to ~30,000 km3/m.y. These values represent For the German and Austrian foreland basin (~35,000 km2), thick- the bulk material fl ux with sources in the Alps and the adjacent foreland nesses of eroded sections (~600–700 m; Lemcke, 1974) amount to basin. As outlined herein, we estimate the contribution of foreland basin 21,000–24,500 km3 of recycled material (Figs. 1 and 2A). This is <5000 recycling and glacial erosion (yellow and blue bars in Fig. 1, respectively), km3/m.y. of the sediment fl ux with sources in the eastern foreland during which we use to show that the size of the eroding system (i.e., the Alps the past 5 m.y., implying a relatively constant erosional fl ux in the east- and the foreland) has increased. We use this observation to invoke a driver ern Alps since the Oligocene. A potential increase appears nonmeasurable situated in the Eurasian lithosphere underlying the western Alps. with the sediment budget approach (e.g., Wagner et al., 2011). The contribution of <6000 km3 of material removed by glacial erosion METHODS in the Alps is signifi cant, but its contribution to the late Miocene–Pliocene sediment budget is relatively small. Accordingly, while a cooling climate Sediment Budgets, Recycling, and Glacial Carving does contribute to a higher erosional fl ux (Herman et al., 2013), glacial erosion alone appears to be insuffi cient to explain the increase in erosion To determine the robustness of the sediment budget curve of Kuhlemann in the western Alps. Likewise, as already pointed out by Champagnac (2000), we fi rst ruled out aliasing effects by recalculating the sediment bud- et al. (2007), erosional rebound induced by glacial erosion alone cannot get in equally spaced 5 m.y. steps (Figs. 1B and 1C). We then considered explain the modern pattern of rock uplift in this part of the Alps. As out- that the younger Alpine-derived foreland basin sediments were eroded lined next, we invoke here an additional driving force rooted in deeper from the foreland basin within a few million years after their deposition, lithospheric levels beneath the western Alps. and we subtracted their contribution (calculated from borehole informa- tion; Appendix 1 [see footnote 1]) from the sediment fl ux curves in Figures Pattern of Late Miocene–Pliocene Erosion Rates 1B and 1C accordingly (see yellow boxes). The difference yields the fl ux from the orogen. We fi nally estimated the contribution of glacial erosion The synthesis shows a coherent, regional-scale pattern of erosion using a mean reduction in the Alpine elevation of ~65 m due to Pleistocene for the western Alps and the foreland basin, and a smaller-scale, fault- glacial carving, as reported by Sternai et al. (2012), and extrapolated this bounded pattern centered on the Tauern Window of the eastern Alps. The value across the Alps (87,000 km2; blue boxes in Figs. 1B and 1C). highest exhumation rates occur in the western Alps (>0.6 km/m.y. over 5 m.y.; cf. area of deeply exhumed crystalline massifs in Fig. 2B), while the Synthesis of Erosion Rates for the Alps and the Foreland Basin lowest detectable erosion rates occur in the easternmost foreland basin, and in the eastern Alps (<0.06 km/m.y.) around the Tauern window. Ero- We compiled published data of erosion for the foreland basin and sion and erosion rates of the Alpine foreland basin are highest in front of exhumation for the Alps, as summarized in Appendix 1 (see footnote 1). the western Alps (>1.5 km, >0.3 km/m.y.; Figs. 2A and 2B) and decrease

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eastward over a distance of 700 km to the eastern portion of the Alpine foreland basin (~200 m; <0.06 km/m.y.). N A Our synthesis also reveals very low average erosion rates for the east- strian Mola n and Au sse rma 48°N ern Alps, the Northern Calcareous Alps, and the Alps east of the Tauern Ge Folded Jura asse Window. This is consistent with the pre-Miocene paleosurfaces described Mol E a s t e r n A l p s iss by Hejl (1997) and Frisch et al. (2001) (Fig. 2). The regional-scale pat- Sw Periadriatic Line tern of erosion rates is perturbed by an anomaly centered on the Tauern W e s t e r n A l p s Window, which is characterized by the highest local spatial gradients in Milano 46°N erosion rates (>0.3 km/m.y. over <10 km), reﬂ ecting young tectonic activ- Po Basin ity (e.g., Scharf et al., 2013; Frisch et al., 2000). Apennines Adriatic Sea 100 km Tyrrhenian Erosion Rate Pattern and Tectonic Features Sea 44°N 10°E 14°E The center affected by high erosion rates is situated in the western Alps and the adjacent portion of the Alpine foreland basin, all of which 40 Recycling of B overlie the Eurasian lithosphere (Figs. 3 and 4). While the southwestern /Ma) Swiss Molasse sediments km margin of this pattern is poorly known (e.g., Sissingh, 1997), the south- 3

eastern extent has a well-deﬁ ned spatial gradient and coincides largely, (10 20 Sediment discharge from southeast to the northeast, with (1) the Periadriatic Line, (2) the Erosional flux from the western Alps western limit of the Austroalpine nappes, and (3) the northern Alpine 0 0 1 5 10 15 20 25 30 thrust front (Fig. 2B). High erosion does not affect any other circum- Time (Ma) Alpine basin (e.g., the Po Basin), nor any portion of the southern Alps, 20 Recycling of German and

which sit above Adriatic lithosphere. /Ma) Austrian Molasse sediments C km 3 DISCUSSION Erosional flux from eastern Alps (10 0 0 1 5 10 15 20 25 30 Sediment discharge Time (Ma) The acceleration of erosion rate, by itself, cannot be used to rule out any

tectonic or climatic processes, because both drivers may have occurred on a 2 short time scale and at high rate, not detectable over a 5 m.y. period. How- O (‰) 18 ever, our new synthesis provides a high spatial resolution of exhumation far δ 5 Time span of rapid glacial D outside the core of the Alps to the foreland, which allows us to eliminate erosion in the Alps models that do not result in the shape of the erosion rate pattern described Erosion of Molasse herein. A viable climate model should result in erosion rates that are higher Sedimentation in Molasse basin in the mountains than in neighboring low-relief basins. However, the erosion rate pattern shows the opposite behavior (Fig. 2B): The erosion con- Erosional flux for the Alps (Kuhlemann et al., 2002) tour lines cross the mountain-foreland border at an oblique angle. On this Our estimate of the erosional flux for the Alps, derived by basis, we rule out global climate change as the main driving force. subtracting the recycled foreland basin sediments from the The onset of accelerated erosion (Fig. 1) also coincided with the end Kuhlemann et al. (2002) curve and averaging over 5 Ma bins. of shortening in the Jura (Schmid et al., 1996; Sue et al., 2007), imply- Our estimate of erosional flux related to foreland basin recycling ing that horizontal convergence rates across this portion of the orogen Contribution of glacial carving to erosional flux were negligent by ca. 5 Ma. Instead, the erosion pattern refl ects vertical, buoyancy-driven lithospheric processes through unbending and unloading Age of youngest preserved sediments in western foreland of the downward-bent Eurasian slab (e.g., Genser et al., 2007), as already Age of youngest preserved sediments in eastern foreland proposed by Lyon-Caen and Molnar (1989) and Sue et al. (2007). The eastward-decreasing gradient in foreland basin erosion may then refl ect Figure 1. (A) Overview fi gure showing the Alps and surrounding basins. the increasing fl exural plate strength (e.g., Stewart and Watts, 1997). Slab (B–C) Sediment discharge curves. We have calculated the contributions related to the erosional recycling of the foreland basins for the western unloading is also consistent with high uplift rates beneath the area of Mont and eastern Alps, respectively (modifi ed after Kuhlemann, 2000). (D) Blanc, where current erosion rates have been highest during the past few Oxygen isotope curve of Zachos et al. (2001) used as a proxy for climate millions of years (Figs. 2B and 3). Lippitsch et al. (2003) explained this change. All x-axes (time) are at the same scale. rebound through positive buoyancy resulting from possible tearing, break- age, and separation of the Eurasian slab (Figs. 3 and 4). Effects from the counterclockwise rotation of Adria were invoked by Wagner et al. (2011) to explain the young uplift of the Styrian Basin in the ern Window (e.g., Frisch et al., 2001), or active buckling of the Aar massif, eastern Alps, which has resulted in crustal shortening and underthrusting particularly at its eastern tip (labeled with Ch on Fig. 3), may be conse- of the Pannonian fragment. The same mechanism can be used to explain quences of deformation along an unevenly shaped plate boundary geom- active uplift and erosion in the vicinity of the Montello thrust (Fig. 2B) etry of the rotating Adriatic indenter. In particular, simultaneous extension on the southern side of the Alps (Rosenberg and Berger, 2009). For the and slab unloading in the western Alps would result if the Euler pole of western Alps, Sue et al. (2007) attributed extensional deformation and dex- rotation between the Eurasian and the Adriatic plate is located along the tral escape as consequences of a rotating Adriatic plate. In this context, it plate boundary, between the two regions (Fig. 3); changes in the motion of seems plausible that other tectonic events, such as reactivation of the Tau- Adria then result in changes in the kinematics of the Alps (Figs. 3 and 4B).

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46°N 48°N Figure 2. Shaded relief map of the Alpine region illustrating (A) estimates of depth of erosion since ca. 5 Ma, and representat 5 Ma, since ca. of depth erosion (A) estimates illustrating Alpine region map of the Shaded relief 2. Figure tec and important since 5 Ma, rates of erosion and (B) pattern sources), 1] for footnote Appendix 1 [text Alps (see the eastern based on ﬁ rates erosion where and the major crystalline massifs, in the foreland, dep of eroded thicknesses Appendix 1 for See lines. contour yellow by represented Alps, (2008)the western for et al. Vernon of footnote 1). ( 1). footnote

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Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/2/124/3045009/124.pdf by guest on 02 October 2021 BARAN ET AL. Basin Vienna Pannonian Strh1 16°E Po Basin: Pliocene to recent sedimentation and basin subsidence Estimate of the pattern erosion rates averaged over the period from 5 to 0 Ma Symbols as in Figure 1A - Locations of boreholes (see DR-1) also Table 250 km Stkr1

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Comparison of the 5-Ma-period erosion rate pattern with large-scale tectonic features ? 250 km N Eurasian plate 0.1 0.06 + Vienna Molasse foreland basin Munich

48°N ? 0.3 + SEMP + region of high Fig. 4A 0.6 surface uplift Jura mountains Ch < 0.06 TW > 0.3 StB and AL > 0.6 + G + 0.06 lithospheric PF Pannonian rebound MB HF Bg Ad Basin PAFS Monthello km Thrust Pliocene behavior of 46°N . 80 LF c circum-alpine basins Ec EP > 150 km Adriatic plate subsidence and sedimentation Venice ? (Po Basin) Po Basin basin inversion, uplift + and erosion (Molasse basin, Styrian basin) Marseille Ap Ligurian plate ennines Estimate of the pattern 0.3 of erosion rates averaged over the

44°N 4°E 8°E 12°E km/Ma period from 5 to 0 Ma

Surface features Eurasian plate Eurasian-Adriatic plate boundary Surface trace of southern edge of subducted m major fault systems Fault systems 0 k c. 8 Eurasian lithosphere LF-HF-PF Longitudinal-Houiller-Penninic Frontal Fault Crystalline massifs Approximate position of the top of the detached > 150 km subducted Eurasian lithosphere, based on PAFS Periadriatic fault system tomography Tauern window SEMP Salzachtal-Ennstal-Mariazell-Puchberg fault Numbers indicate top of slab, based on tomography Austroalpine results by Lippitsch et al. (2003) TW Tauern window fault system nappes (Apulia) Position of present-day Euler pole between Adriatic plate EP StB Styrian Basin Adria and Eurasia: counterclockwise motion Trace of the plate boundary at the Moho depth of Adria relative to Eurasia, from Nocquet (c. 50 - 60 km) from Schmid and Kissling (2000) For abbreviations see Figure 2A and Calais (2003)

Figure 3. Schematic map of the Alpine region showing the relationship between the erosion rate pattern and the orogen-scale tectonic features, includ- ing boundaries between the Adriatic and Eurasian plates during the Pliocene. G—Geneva.

It is possible that reactivation along the Periadriatic Line (acceler- streams (Ziegler and Fraefel, 2009) could have occurred in response to the ated exhumation of the Bergell area since 4 Ma; Fig. 2) decoupled most inferred slab tear in the west-central Alps (Mont Blanc area). of this rock uplift from the southern Alps. The same mechanism can be invoked for the Longitudinal-Houiller-Penninic frontal fault (LF-HF-PF; CONCLUSIONS Fig. 3) in the western Alps, which delineates a western block with late Miocene to Pliocene ﬁ ssion-track ages from an eastern block with Oli- We suggest a model that is capable of explaining the following obser- gocene to Miocene ﬁ ssion-track ages (Malusà et al., 2005). In particular, vations: (1) a coherent “banana-shaped” pattern of fast erosion during the the Periadriatic Line has juxtaposed the subsiding Po Basin next to the past ~5 m.y. over an area of ~700 × 150 km from the western Alps to Alpine blocks where exhumation has persisted to the present (Carrapa the eastern portion of the Alpine foreland basin, (2) an increase in ero- and Garcia-Castellanos, 2005). Counterclockwise rotation of the Adriatic sion rates of >0.6 km/m.y. toward the center of the erosional region, (3) a plate about an Euler pole located south of the central Alps (Fig. 3) could switch from depositional to erosional processes over most of the Alpine contribute to unloading of the Eurasian slab below the western Alps (and foreland basin contemporaneous with acceleration of erosion in the Alps, thus to surface uplift through positive buoyancy), and likewise to uplift (4) an erosional signal that is strongest in the western Alps, (5) and con- and compressional escape in the eastern Alps due to indentation of the current sedimentation in the Po Basin south of the Periadriatic Line. We Adriatic plate (Fig. 4B). propose that the erosion rates depicted here largely accommodate surface The notion of lithospheric slab unloading is also consistent with the uplift due to unloading of the Eurasian lithospheric slab beneath the west- recent reorganization of the entire drainage pattern of the major rivers. ern Alps, from where it affected the adjacent mountains and the Alpine By ca. 5 Ma or even later, cannibalization between the Rhine and Danube foreland basin to a distance of over 700 km along strike. The erosional

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surface uplift and erosion subsidence A & sedimentation

Periadriatic fault system NNW Messinian Molasse foreland basin Austroalpine nappes unconformity (5.5 Ma) SSE nappes (sedimentation until c. 5 Ma) Penninic He Ivrea 0 Siviez - Monte Rosa 0 Sesia Mischabel Strona- Aar Ceneri Depth Eurasian plate distal Adriatic (km) lower crust European European lower crust Lithospheric mantle margin Adriatic plate

40 40

Age of most recent pre-Quaternary Major alpine units 50 km sedimentary basin fill Flysch Mesozoic-Cenozoic 10 to 1 Ma Ophiolites sedimentary rocks 30 to 10 Ma Helvetic nappes

B Alps, before c. 5 Ma Western Alps, after c. 5 Ma Eurasian plate Adriatic plate Eurasian plate Adriatic plate flexural bending and unloading & subsidence & erosion MR orogenic loading EP sedimentation ccw rotation

Eu loading rasian lithospheric Eurasian unloading mantle lithospheric mantle ? subsidence detac Euras

hed/bent ian slab lithospheric-scale uplift

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m local-scale

k uplift & lateral extrusion

150 km

Figure 4. (A) Schematic structural geological profi le across the western Alps (after Pfi ffner et al., 2002) showing the spatial rela- tionships among late Miocene to Holocene surface uplift, erosion rate, and the lithospheric structure at depth, and (B) sketch fi gures at the lithospheric scale showing the situation prior to and after the inferred slab unloading in the west and the east, respectively (modifi ed after Lippitsch et al., 2003).

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signal is strongest above the Eurasian lithosphere and weakest above the Luth, S.W., and Willingshofer, E., 2008, Mapping the post-collisional cooling history of the eastern Alps: Swiss Journal of Geosciences, v. 101, supplement 1, p. 207–223, doi:10.1007 Adriatic plate. Local tectonic exhumation of the Tauern Window and /s00015-008-1294-9. active shortening in the southeastern Alps interfere with this pattern. In Lyon-Caen, H., and Molnar, P., 1989, Constraints on the deep structure and dynamic processes contrast, subsidence continues in the Po Basin adjacent to the Periadriatic beneath the Alps and adjacent regions from an analysis of gravity anomalies: Geophysi- cal Journal International, v. 99, p. 19–32, doi:10.1111/j.1365-246X.1989.tb02013.x. Line, implying decoupling between the Eurasian and the Adriatic plates. Malusà, M.G., Riccardo, P., Zattin, M., Bigazzi, G., Martin, S., and Piana, F., 2005, Miocene to Counterclockwise rotation of Adria contributes to east-west gradients in present differential exhumation in the western Alps: Insights from fi ssion track themo- shortening and erosion via lithospheric loading in the east and unloading chronology: Tectonics, v. 24, p. TC3004, doi:10.1029/2004TC001782. Nocquet, J.M., and Calais, E., 2003, Crustal velocity fi eld of western Europe from permanent in the west. Spatial patterns of erosion rate on a regional scale may be GPS array solutions: Geophysical Journal International, v. 154, p. 72–88, doi:10.1046/j.1365 helpful in identifying lithospheric-scale tectonic processes. -246X.2003.01935.x. Pfi ffner, O.A., Schlunegger, F., and Buiter, S., 2002, The Swiss Alps and their peripheral foreland basin: Stratigraphic response to deep crustal processes: Tectonics, v. 21, p. 3-1–3-16, ACKNOWLEDGMENTS doi:10.1029/2000TC900039. R. 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