Othmar-Adrian Pfiffner

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.

Collision tectonics in the Swiss Alps: Insight from geodynamic modeling

This paper compares results from two-dimensional finite element dynamic modeling with the kinematic evolution of the Swiss Alps during the collision phase. In particular, we investigate the role of inherited lateral strength heterogeneities on orogenesis. A number of first-order characteristics are directly comparable at crustal scales. In the models the entry of continental crust into the convergent margin marks the end of near-perfect subduction. Accretion of material of the subducting plate to the upper plate creates an orogenic wedge on the incoming (pro)side and initiates a retroshear zone (or model backthrust). The addition of material to the upper plate builds a bivergent orogen. Heterogeneities in the pro-crust focus shear and lead to the development of “nappe structures” The combined action of pro-shear (nappe stacking) and retroshear (backthrusting) uplifts a plug between the two shear zones. Subsequent focusing of shear along the retroshear zone results in rotation of the plug and overlying units, leading to crustal-scale backfolds as observed in the Swiss Alps. The model experiments predict features relevant to Alpine dynamics, including (1) similar crustal thicknesses and exhumation patterns to those observed in the Swiss Alps today for erosion rates comparable to natural ones (1 mm yr−1), (2) continued accretion and subduction of upper crustal fragments allowing high-pressure metamorphic conditions, (3) tilting and exhumation of lower crust when a midcrustal weak zone is present, and (4) “shunting” of material across the strong lower crustal wedge of the upper plate.

The relief of the Swiss Alps and adjacent areas and its relation to lithology and structure: topographic analysis from a 250-m DEM

Abstract In this paper we discuss the large-scale geomorphological characteristics of the Swiss Alps based on numerical analysis of a digital elevation model and compare these to an erodibility map constructed from a geotechnical map of Switzerland and regional geomorphological studies. Comparing the erodibility map with the large-scale morphometry shows an intimate relationship between mountain-scale erodibility and topography. On average, higher mean elevations and steeper mean slopes correlate with regions where rocks of low erodibility prevail. Areas with high peaks as well as the main water divides are controlled by the presence of bedrock with low to very low detachability. The drainage network of the Swiss Alps shows a close relationship to the lithological differences as well. Major longitudinal valleys follow easily erodible units. In the eastern and western part of the Swiss Alps, the highest values of local relief are located to the south of the main water divide, whereas in the central part, local relief is higher to the north of the main water divide. The large-scale geomorphic characteristics regarded in the framework of the geological history of uplift and denudation suggest that low and very low erodibilities lead to the development of areas of high elevations which are likely to persist over periods of 10–15 Ma. As the analysis of the Lepontine area shows, 20 Ma after cessation of exhumation, such high elevations are likely to be worn down and to manifest themselves as high relief only.

Deep seismic reflection profiling in the Swiss Alps: Explosion seismology results for line NFP 20-East

In September 1986, a 120-km-long seismic line was recorded through the Swiss Alps. The line traverses major units involved in thin- and thick-skinned Alpine tectonics. Our preliminary interpretation indicates that (1) lithologic boundaries such as basement-cover contacts, although severely deformed during the Alpine orogeny, can be identified on the seismic sections; (2) the top of the Aar massif, an external basement massif, extends deep into the Alpine orogen; (3) the upper and lower parts of the crust are more or less transparent, but are separated by a swarm of reflectors at mid-crustal level; (4) these mid-crustal reflectors might be related to trapped fluids from Alpine metamorphism; and (5) the Moho appears as a bright reflection that steepens from the north toward the south and terminates abruptly near the center of the Alpine chain, perhaps because of Alpine deformation.

Inversion of a symmetric basin: insights from a comparison between analogue and numerical experiments

Abstract We use both analogue and numerical experiments to study the inversion by shortening of a symmetric sedimentary basin. The combination of the two modelling techniques uses the strengths of each method to provide insight into basin-inversion processes. The experiments start with a pre-existing basin filled, in part, with weak layers simulating weak sedimentary rocks. Both footwall and hanging wall can deform freely. The physical properties of the materials used in the analogue experiments (sand and microbeads) and the numerical experiments are appropriately scaled to represent upper crustal rocks. We present a systematic study of the effects of basin infill, basin width and basin location and a sensitivity analysis to understand the effects of the boundary conditions. The results of both methods show that the graben fill accommodates most shortening. Weak layers play an important role in localising shortening with limited reactivation of pre-existing (but weakened) faults. In general, forward thrusts and back thrusts nucleate at the lateral contrast of strong and weak materials and cut across the graben-bounding faults. Weak basal detachments are required to transfer shortening to the basin region. The overall evolution of the analogue and numerical models is encouragingly similar.

Numerical models for the control of inherited basin geometries on structures and emplacement of the Klippen nappe (Swiss Prealps)

Cover nappes commonly deform above a shallow basal detachment surface located above rigid crystalline basement rocks. Inherited basin geometries are presumed to control the kinematic evolution and detachment of cover nappes in an accretionary wedge. We compare results from two-dimensional finite element modelling with the structural style of a natural cover nappe, the Klippen nappe, which was detached from its basement along an evaporitic detachment horizon and thrust over a distance of roughly 100 km. First-order characteristics of the Klippen nappe paleo-basin include a complex distribution of weak detachment rocks and various changes in the sediment composition and layer thicknesses; two types of structural style can be distinguished: an imbricate fan at the rear end of the nappe (Prealpes Medianes Rigides) and a fold-dominated region at the front (Prealpes Medianes Plastiques). The model results resemble the first-order characteristics of the Klippen nappe. The model experiments suggest that the formation of imbricates and fault-related folds are controlled by the discontinuity of detachment horizons. Discontinuities are locations where thrust ramps are triggered and layer heterogeneities control the locations for the initiation of detachment folds. Basement horsts may lead to the formation of recumbent folds, which develop a melange zone on the highly sheared inverted limb. The experiments also suggest that the thickness of the basal detachment horizon of the Medianes Plastiques decreases gradually from the foreland to the hinterland.

Numerical models of Alpine-type cover nappes

Abstract We investigate the dynamics of Alpine cover nappes derived from basins with inherited geometries using a two-dimensional finite element code. Model studies are based on a natural analogue, the Klippen nappe in the Swiss Alps, which detached from the basement over a basal evaporite horizon and was transported onto the foreland in a thin-skinned tectonic style. Finite-element models are used to examine the influence of specific features on the structural style developing in the models. Experiments dealing with the influence of wedge geometry and mechanical properties show modifications in wavelength patterns and structures in detail but do not change the general trends. The control that different layer thicknesses, multilayered rocks and lateral heterogeneities in the underlying basement and within the sediments have on the style of deformation is investigated. Results suggest that the development of fold versus thrust structures in a cover nappe is controlled by the thickness of the weak detachment horizon and the relative thickness of individual layers (thickness ratio n=thickness of weak/thickness of strong layer). If n>1, folds with variable and irregular wavelengths form, if 0.5

Crustal structure and reflectivity of the Swiss Alps from three‐dimensional seismic modeling: 2. Penninic Nappes

Surface geological mapping, laboratory measurements of rock properties, and seismic reflection data are integrated through three-dimensional seismic modeling to determine the likely cause of upper crustal reflections and to elucidate the deep structure of the Penninic Alps in eastern Switzerland. Results indicate that the principal upper crustal reflections recorded on the south end of Swiss seismic line NFP20-EAST can be explained by the subsurface geometry of stacked basement nappes. In addition, modeling results provide improvements to structural maps based solely on surface trends and suggest the presence of previously unrecognized rock units in the subsurface. Construction of the initial model is based upon extrapolation of plunging surface structures; velocities and densities are established by laboratory measurements of corresponding rock units. Iterative modification produces a best fit model that refines the definition of the subsurface geometry of major structures. We conclude that most reflections from the upper 20 km can be ascribed to the presence of sedimentary cover rocks (especially carbonates) and ophiolites juxtaposed against crystalline basement nappes. Thus, in this area, reflections appear to be principally due to first-order lithologic contrasts. This study also demonstrates not only the importance of three-dimensional effects (sideswipe) in interpreting seismic data, but also that these effects can be considered quantitatively through three-dimensional modeling.