Nikolaus Froitzheim

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.

Kinematics of Jurassic rifting, mantle exhumation, and passive-margin formation in the Austroalpine and Penninic nappes (eastern Switzerland)

The Austroalpine and Upper Penninic nappes in eastern Switzerland represent a passive continental margin and the adjacent ocean of Jurassic-Cretaceous age, imbricated by Late Cretaceous-Tertiary orogenic shortening. Well-preserved, rift-related faults allow reconstruction of the passive margin and ocean-continent transition zone and yield new information on the kinematics of rifting. Rifting evolved from pure-shear stretching to detachment-controlled, asymmetric stretching and resulted in complete exhumation of subcontinental mantle rocks at the sea floor. After precursory normal faulting in the Late Triassic, Jurassic rifting occurred in two phases. During the first rifting phase (Hettangian-Sinemurian), predominantly east-dipping normal faults developed in the upper crust; their dips decreased in the middle to lower crust, where they probably graded into anastomosing shear zones in the lower crust and mantle lithosphere. The resulting overall geometry approximated pure-shear stretching. During the second rifting phase (Toarcian-Middle Jurassic), a system of west-dipping detachment faults formed, penetrating the whole lithosphere and accommodating asymmetric extension. During progressive stretching, subcontinental mantle rocks were tectonically exhumed and exposed at the sea floor in two areas, represented by the Platta and Malenco nappes (Penninic). The intervening Margna and Sella continental nappes are interpreted as an extensional allochthon belonging to the Apulian margin. Finally, a mid-ocean ridge may have formed west of the Margna-Sella allochthon. The Austroalpine realm thus represents the lower-plate margin—and the Brianconnais, the upper-plate margin-of the Piemont-Liguria ocean. This scenario is in qualitative agreement with the subsidence histories of the two margins.

Extensional detachment faulting in the evolution of a Tethys passive continental margin, Eastern Alps, Switzerland

The Austroalpine nappes of Switzerland represent an exhumed and tectonically imbricated segment of the passive continental margin of the Jurassic Tethys. Within one of these nappes (Err nappe), part of an upper-crustal, extensional detachment is preserved, indicating that the thinning of the crust was achieved by non-uniform extension. The Mesozoic age of the detachment is shown by comparison between its associated cataclasites and identical cataclasites that are found as redeposited components in Middle(?) Jurassic sedimentary breccias. This low-angle detachment within the basement was kinematically linked to synsedimentary high-angle normal faults at the surface. The area of the Err nappe belongs, in terms of Jurassic paleogeography, to the most distal part of the continental margin, where both low- and high-angle normal faults dipped oceanward (that is, west to northwest in present-day coordinates). In the more proximal part of the margin, however, the high-angle normal faults dipped eastward toward the continent. Ammonite stratigraphy within the sediment prisms adjacent to the faults gives evidence for an Early Jurassic age of the faulting in the proximal part of the margin, whereas in the distal part, faulting occurred during latest Early to Middle Jurassic time. We therefore propose that the Jurassic extension of the crust, which finally led to the opening of the Piemont-Ligurian ocean, was achieved by two fault systems, which differ in geometry, fault orientation, and age. These fault systems were composed of a basal low-angle detachment with high-angle normal faults above. The orientation of the older, eastward-dipping detachment was prone for reactivation during early Alpine crustal shortening. The present-day thrust contact between the two major tectonic units of the area, the Lower Austroalpine and the Central Austroalpine nappe complex, therefore might correspond to an eastward-dipping, Jurassic low-angle normal fault.

The role of detachment faulting in the formation of an ocean-continent transition: insights from the Iberia Abyssal Plain

Abstract The Iberia Abyssal Plain segment of the West Iberia margin was drilled during Ocean Drilling Program Legs 149 and 173 and has been extensively studied geophysically. We present new microstructural investigations and new age data. These, together with observed distribution of upper- and lower-crustal and mantle rocks along the ocean-continent transition suggest the existence of three detachment faults, one of which was previously unrecognized. This information, together with a simple kinematic inversion of the reinterpreted seismic section Lusigal 12, allows discussion of the kinematic evolution of detachment faulting in terms of the temporal sequence of faulting, offset along individual faults, and thinning of the crust during faulting. Our study shows that the detachment structures recognized in the seismic profile became active only during a final stage of rifting when the crust was already considerably thinned to c. 12 km. The total amount of extension accommodated by the detachment faults is of the order of 32.6 km corresponding to a β factor of about two. During rifting, the mode of deformation changed oceanwards. Initial listric faulting led to asymmetric basins, accommodating low amounts of extension, and was followed by a situation in which the footwall was pulled out from underneath a relatively stable hanging wall accommodating high amounts of extension. Deformation along the latter faults resulted in a conveyor-belt type sediment accumulation in which the exhumed footwall rocks were exposed, eroded and redeposited along the same active fault system.

Orogen-parallel extension in the Southern Carpathians

A structural study, including a kinematic analysis based on sense of shear criteria recorded in fault rocks, is combined with fission-track dating. A two-stage Alpine tectonic evolution is proposed for the major tectonic units which constitute the Southern Carpathians of the Parang mountains area. Upper Cretaceous top-to-the-SSE nappe stacking was followed by WSW‐ENE orogen-parallel extension in the Eocene. Top-to-the-ENE shearing in lower greenschist-facies mylonites from the eastern part of the Danubian window (Parang mountains) is associated with a low-angle detachment at the base of the brittlely deformed Getic nappe (the Getic detachment). Below the dome-shaped Getic detachment, east-dipping at the eastern termination of the Danubian window, the Danubian units were rapidly exhumed. Hence, the eastern part of the Danubian window represents a greenschist-facies core complex. As a working hypothesis, it is proposed that this orogen-parallel stretch was originally N‐S-oriented and that it formed when the Rhodopean fragment (which includes the Getic and Supragetic nappes) moved northward into an oceanic embayment, past the western margin of Moesia. This Eocene extension was part of a process of oroclinal bending in the area of the Southern Carpathians and their continuation into the Balkan mountains. Extension was followed by about 50o clockwise rotation of the Southern Carpathians, associated with dextral wrenching along the northern boundary of Moesia and compression in the Moldavides of the Eastern Carpathians.

Late Cretaceous, synorogenic, low-angle normal faulting along the Schlinig fault (Switzerland, Italy, Austria) and its significance for the tectonics of the Eastern Alps

Abstract The Schlinig fault at the western border of theOtztal nappe (Eastern Alps), previously interpreted as a west-directed thrust, actually represents a Late Cretaceous, top-SE to -ESE normal fault, as indicated by sense-of-shear criteria found within cataclasites and greenschist-facies mylonites. Normal faulting postdated and offset an earlier, Cretaceous-age, west-directed thrust at the base of theOtztal nappe. Shape fabric and crystallographic preferred orientation in completely recrystallized quartz layers in a mylonite from the Schlinig fault record a combination of (1) top-east-southeast simple shear during Late Cretaceous normal faulting, and (2) later north-northeast-directed shortening during the Early Tertiary, also recorded by open folds on the outcrop and map scale. Offset of the basal thrust of theOtztal nappe across the Schlinig fault indicates a normal displacement of 17 km. The fault was initiated with a dip angle of 10° to 15° (low-angle normal fault). Domino-style extension of the competent Late Triassic Hauptdolomit in the footwall was kinematically linked to normal faulting. The Schlinig fault belongs to a system of east- to southeast-dipping normal faults which accommodated severe stretching of the Alpine orogen during the Late Cretaceous. The slip direction of extensional faults often parallels the direction of earlier thrusting (top-W to top-NW), only the slip sense is reversed and the normal faults are slightly steeper than the thrusts. In the western Austroalpine nappes, extension started at about 80 Ma and was coeval with subduction of Piemont-Ligurian oceanic lithosphere and continental fragments farther west. The extensional episode led to the formation of Austroalpine Gosau basins with fluviatile to deep-marine sediments. West-directed rollback of an east-dipping Piemont-Ligurian subduction zone is proposed to have caused this stretching in the upper plate.

Origin of the Monte Rosa nappe in the Pennine Alps—A new working hypothesis

On the basis of lithological and structural evidence, a new solution is proposed for the classical problem of the restoration of folded continental and oceanic nappes in the Pennine Alps of Switzerland and Italy. According to this working hypothesis, the Monte Rosa nappe, a high-pressure metamorphic gneiss unit, represents basement of the northern European margin of Alpine Tethys. Its paleogeographic origin was formerly sought either in the Brianconnais microcontinent or in the southern Adriatic continental margin. The Monte Rosa nappe is enveloped by the lithologically heterogeneous and extremely deformed Furgg zone, interpreted here as a melange zone formed during subduction and collisional closure of the Valais ocean basin, the northern subbasin of Alpine Tethys. In a late stage of the Alpine orogeny (late Eocene), postdating closure of the Valais ocean basin, the European continental margin including the Monte Rosa basement was subducted southward under the Pennine nappe stack and reached eclogite facies depth. From there, the Monte Rosa nappe quickly ascended back toward the surface in an internal (southeastern) position, separated from the other Europe-derived units located farther northwest by the rootless, Brianconnais- derived Bernhard nappe system.

Tectonic aspects of the Alpine-Dinaride-Carpathian system

The Alps, Carpathians and Dinarides form a complex, highly curved and strongly coupled orogenic system. Motions of the European and Adriatic plates gave birth to a number of ‘oceans’ and microplates that led to several distinct stages of collision. Although the Alps serve as a classical example of collisional orogens, it becomes clearer that substantial questions on their evolution can only be answered in the Carpathians and Dinarides. Our understanding of the geodynamic evolution of the Alpine-Dinaride-Carpathian System has substantially improved and will continue to develop; this is thanks to collaboration between eastern and western Europe, but also due to the application of new methods and the launch of research initiatives. The largely field-based contributions investigate the following subjects: pre-Alpine heritage and Alpine reactivation; Mesozoic palaeogeography and Alpine subduction and collision processes; extrusion tectonics from the Eastern Alps to the Carpathians and the Pannonian Basin; orogen-parallel and orogen-perpendicular extension; record of orogeny in foreland basins; tectonometamorphic evolution; and relations between the Alps, Apennines and Corsica.

A new occurrence of microdiamond-bearing metamorphic rocks, SW Rhodopes, Greece

We describe a new locality with microdiamond-bearing, ultrahigh-pressure metamorphic rocks near the village Sidironero in the Rhodope Metamorphic Province in northern Greece, about 70 km west of the nearest known location at Xanthi. High- and ultrahigh-pressure metamorphic conditions are preserved in an intensely strained melange zone which is sandwiched between upper-greenschist to lower-amphibolite-facies rocks in the footwall (Pangaion-Pirin Complex) and upper-amphibolite-facies rocks in the hanging wall (Rhodope Terrane). The melange zone consists of various paragneisses, orthogneisses and metamafics. A strong overprint at upper-amphibolite-facies conditions associated with migmatisation in the orthogneisses and subsequent intense mylonitisation at lower-amphibolite-facies conditions almost obliterates peak-pressure assemblages. Relics of high-pressure conditions are preserved in mafic boudins and in garnet-kyanite-mica schists. Garnet in garnet-kyanite-mica schists displays inclusions of microdiamonds and swarms of non-oriented rods of rutile and quartz. The lithological and structural appearance of the melange zone resembles the exposure of ultrahigh-pressure metamorphic rocks further east at Xanthi, whereas the location at Kimi may occupy a higher structural level.

Exhumation of high- and ultrahigh-pressure metamorphic rocks by slab extraction

Exhumation of high- and ultrahigh-pressure metamorphic rocks in collisional orogens may be explained by upward extrusion of these rocks, erosion of their overburden, or extensional thinning of the overburden. Some high-pressure terranes, such as the Adula nappe in the Central Alps, fit none of these scenarios. We propose an additional way in which part of the overburden may be removed: it may sink off into the deeper mantle (slab extraction). Structural and metamorphic relationships in and around the Adula nappe indicate that the emplacement of this Alpine high- to ultrahigh-pressure nappe (to 3.2 GPa) in a pile of lower-pressure nappes resulted from the interaction of two subduction zones that accommodated the closure of two ocean basins, ultimately leading to the extraction of the intervening slab. In terms of mechanics, the cause of the exhumation is, in this case, not the buoyancy of the high-pressure rocks, but the negative buoyancy of the extracted slab.