E. Kissling

Geophysical‐geological transect and tectonic evolution of the Swiss‐Italian Alps

A complete Alpine cross section integrates numerous seismic reflection and refraction profiles, across and along strike, with published and new field data. The deepest parts of the profile are constrained by geophysical data only, while structural features at intermediate levels are largely depicted according to the results of three-dimensional models making use of seismic and field geological data. The geometry of the highest structural levels is constrained by classical along-strike projections of field data parallel to the pronounced easterly axial dip of all tectonic units. Because the transect is placed close to the western erosional margin of the Austroalpine nappes of the Eastern Alps, it contains all the major tectonic units of the Alps. A model for the tectonic evolution along the transect is proposed in the form of scaled and area-balanced profile sketches. Shortening within the Austroalpine nappes is testimony of a separate Cretaceous-age orogenic event. West directed thrusting in these units is related to westward propagation of a thrust wedge resulting from continental collision along the Meliata-Hallstatt Ocean further to the east. Considerable amounts of oceanic and continental crustal material were subducted during Tertiary orogeny, which involved some 500 km of N-S convergence between Europe and Apulia. Consequently, only a very small percentage of this crustal material is preserved within the nappes depicted in the transect. Postcollisional shortening is characterized by the simultaneous activity of gently dipping north directed detachments and steeply inclined south directed detachments, both detachments nucleating at the interface between lower and upper crust. Large scale wedging of the Adriatic (or Apulian) lower crust into a gap opening between the subduced European lower crust and the pile of thin upper crustal flakes (Alpine nappes) indicates a relatively strong lower crust and detachment between upper and lower crust.

The arc of the western Alps in the light of geophysical data on deep crustal structure

Recently, two international deep seismic campaigns in the western and central Alps (Etude Continentale et Oceanique par Reflexion et Refraction Sismique - Progetto Strategico Crosta Profonda, ECORS-CROP [Roure et al. 1996b]; and National Research Program 20, NRP-20 [Pfiffner et al. 1997]) have been completed. Here we present a synoptic interpretation of the wealth of geophysical data about deep crustal structure of the Alps collected during the past 40 years, including the two above mentioned geophysical projects. The tectonic reinterpretation of the ECORS-CROP high-resolution seismic transect is based on an evaluation of the deep crustal structure by integrating new and literature data on surface geology. Combined with previously published interpretations regarding the central Alps [Schmid et al., 1996], this study reveals substantial differences in geometry and kinematics between transects across the western and central Alps, respectively. At depth the transition between the central and western Alps is marked by the western limits of an Adriatic lower crustal wedge-shaped structure and the northeastern limits of a similar structure made of European lower crustal material. At the surface it coincides with a corridor of dextral strike-slip along the Tonale and Simplon lines. In addition, the thickness of the seismogenic zone shows a remarkable variation from over 40 km beneath the Penninic realm of the western Alps to less than 20 km in the central Alps. The formation of the western Alpine arc was initiated during convergence and collision before 35 Myr ago, when the Adriatic micro-plate, moving northward with respect to the European foreland, caused sinistral transpression in the western Alps while the central and eastern Alps underwent head-on convergence and collision. During the post-collisional stage, i.e., after 35 Myr ago, the arcuate shape of the western Alps was accentuated by WNW-directed movement and anticlockwise rotation of the Adriatic microplate, decoupled from the central and eastern Alps along the Tonale-Simplon dextral shear zone. This led to wedging of lower crustal slices both in the western Alps and in the central Alps. The new tectonic interpretation of the ECORS-CROP transect allows a kinematic model to be established for crustal shortening in the western Alps during the past 35 Myr. The Ivrea mantle plays the role of a backstop in our tectonic model. We distinguish three episodes of post-collisional crustal shortening along the ECORS-CROP transect: From 35 to 30 Ma the Brianconnais basement was back-thrusted over the Gran Paradiso units, leading to 30 km of crustal shortening (first episode). In the early Miocene, movements concentrated mainly along the Penninic frontal thrust and resulted in about 60 km shortening (second episode). Post-12 Ma shortening within the external massifs is associated with folding in the Jura mountains when the crust was shortened by an additional 30 km (third episode).

Lithosphere structure and tectonic evolution of the Alpine arc: new evidence from high-resolution teleseismic tomography

Abstract Several continental and oceanic plates and/or terranes amalgamated during the formation of the tectonically complex Alpine arc. Reliable knowledge of the present structure of the lithosphere-asthenosphere system throughout the Alpine arc from the Western through the Central to the Eastern Alps is crucial for understanding the evolution of this orogen and the current interaction of lithospheric blocks, and additionally, for assessing the amount and orientation of lithosphere subducted in the geological past. We have compiled results from earlier geophysical studies and reinterpretations of existing seismic and geological data for the Alpine crust and Moho. High-resolution teleseismic tomography was used to produce a detailed 3D seismic model of the lower lithosphere and asthenosphere. The combination of these techniques provides new images for the entire lithosphere-asthenosphere system, showing significant lateral variations to depths of 400 km. Over the years the crustal structure has been determined extensively by active seismic techniques (deep seismic sounding) with laterally variable coverage and resolution. For a closer view three international seismic campaigns, using mainly near-vertical reflection techniques in the Western, Central and Eastern Alps, were carried out to assess the crustal structure with the highest possible resolution. The synoptic reinterpretation of these data and an evaluation of existing interpretations have allowed us to construct four detailed deep crustal transects across the Alps along the ECORS-CROP, NFP-20/EGT and TRANSALP traverses. In addition, contour maps of the Moho for the wider Alpine region and of the top of the lower crust were compiled from existing seismic refraction, near-vertical and wide-angle reflection data. Substantial structural differences in the structure of the deep crust appear between the Western, Central and Eastern Alps: doubling of European lower crust in the west resulted from collision with the Ivrea body; indentation of lower Adriatic crust between European lower crust and Moho occurred in the Central Alps; and a narrow collision structure exists under the transitional area between the western and eastern subduction regime under the Tauern Window of the Eastern Alps, where the crustal structure resembles a large-scale flower structure. Most recently, high-resolution teleseismic tomography based on the a priori known 3D crustal structure and compilation of a high-quality teleseismic dataset was successfully developed and applied to derive reliable detailed images of the lower lithosphere. Along strike of the Alps a fast slab-like body is revealed which in the western part is subducted beneath the Adriatic microplate. In the Western Alps detachment of parts of the lower continental slab occurred, possibly induced by the Ivrea body, which acted as a buttress in the collision process of the European and Adriatic plates. The generally SE-directed subduction of the European continental lithosphere changes gradually from west to east to almost vertical under the westernmost part of the Eastern Alps (western Tauern Window and Giudicarie lineament). Unexpectedly, some 50 km further east the subducted continental lower lithosphere is now part of the Adriatic lithosphere and dips NE beneath the European plate. Our tomographic image documents clear bipolar slab geometries beneath the Alpine orogen. The depth extent of the subducted continental lithospheric slab agrees rather well with estimates of post-collisional crustal shortening for the Western and Central Alps. This kinematic control on amounts of lateral motion of the collision zone in the west also allows estimates of the subduction and collision process in the Eastern Alps. The new 3D lithospheric picture for the wider Alpine region to 400 km depth demonstrates the clear connection and interaction between the deep structure of the lithosphere-asthenosphere system and near-surface tectonic features as seen today. It provides new and unexpected evidence for the entire Alpine tectonic evolution, a process which obviously changes significantly from west to east.

Reconstructing the Alps–Carpathians–Dinarides as a key to understanding switches in subduction polarity, slab gaps and surface motion

Palinspastic map reconstructions and plate motion studies reveal that switches in subduction polarity and the opening of slab gaps beneath the Alps and Dinarides were triggered by slab tearing and involved widespread intracrustal and crust–mantle decoupling during Adria–Europe collision. In particular, the switch from south-directed European subduction to north-directed “wrong-way” Adriatic subduction beneath the Eastern Alps was preconditioned by two slab-tearing events that were continuous in Cenozoic time: (1) late Eocene to early Oligocene rupturing of the oppositely dipping European and Adriatic slabs; these ruptures nucleated along a trench–trench transfer fault connecting the Alps and Dinarides; (2) Oligocene to Miocene steepening and tearing of the remaining European slab under the Eastern Alps and western Carpathians, while subduction of European lithosphere continued beneath the Western and Central Alps. Following the first event, post-late Eocene NW motion of the Adriatic Plate with respect to Europe opened a gap along the Alps–Dinarides transfer fault which was filled with upwelling asthenosphere. The resulting thermal erosion of the lithosphere led to the present slab gap beneath the northern Dinarides. This upwelling also weakened the upper plate of the easternmost part of the Alpine orogen and induced widespread crust–mantle decoupling, thus facilitating Pannonian extension and roll-back subduction of the Carpathian oceanic embayment. The second slab-tearing event triggered uplift and peneplainization in the Eastern Alps while opening a second slab gap, still present between the Eastern and Central Alps, that was partly filled by northward counterclockwise subduction of previously unsubducted Adriatic continental lithosphere. In Miocene time, Adriatic subduction thus jumped westward from the Dinarides into the heart of the Alpine orogen, where northward indentation and wedging of Adriatic crust led to rapid exhumation and orogen-parallel escape of decoupled Eastern Alpine crust toward the Pannonian Basin. The plate reconstructions presented here suggest that Miocene subduction and indentation of Adriatic lithosphere in the Eastern Alps were driven primarily by the northward push of the African Plate and possibly enhanced by neutral buoyancy of the slab itself, which included dense lower crust of the Adriatic continental margin.

Interpretation of tomographic images of uppermost mantle structure: Examples from the western and central alps

Abstract Three-dimensional seismic tomography results are normally calculated and displayed by use of reference models. The former is referred to as an initial reference model and the latter as a final reference model. Conventionally, these two radial or one-dimensional models are identical. The use of the initial reference model also for display purposes (as final reference model) is convenient and well-suited to test data and inversion procedure. For tectonic and petrological interpretation, however, the use of the initial model as a final reference model is sometimes a poor choice, as it may suppress reliable and important information of the actual three-dimensional velocity field. The PM2 one-dimensional model for Europe (Spakman et al., Phys. Earth Planet. Int. 79, 3–74, 1993) was derived by one-dimensional inversion of the same seismic data later used for three-dimensional tomographic inversion. The PM2 model differs from the global Jeffreys-Bullen (JB) reference model by a pronounced layer with reduced P-wave velocities associated with the asthenosphere. For mapping the lithosphere-asthenosphere boundary (LAB) the radial velocity distribution in the uppermost mantle is as important as the lateral velocity variation. To better image the lithosphere-asthenosphere system beneath the Alps, therefore, we re-display the three-dimensional tomographic results obtained by Spakman et al. (1993) relative to the JB model. The usage of this final reference model with a constantly increasing velocity in the depth range from 80 to 200 km leads to images with enhanced vertical velocity variation across the LAB. When defining the LAB as the depth where the vertical gradient of P-wave velocity assumes zero or negative values, the thickness of the lithosphere in the central European foreland outside the Rhinegraben is approximately 100 km. The deep-seated volume of high-velocity beneath the Po plain and northern Appennines found by several other studies, denotes the lithospheric slab connected with the European lithosphere in the central Alps and likely in the northern part of the western Alps. In contrast, the European lithosphere in the southern parts of the western Alps is disconnected from this slab. The overriding lithosphere has a thickness of about 100 km beneath Corsica and Sardinia and thins to only about 50 km beneath the Ligurian Sea. For the Adriatic lithosphere a thickness of about 90 km is obtained.