Wolfgang Frisch

Lateral extrusion in the eastern Alps, PArt 2: Structural analysis

The late Oligocene-Miocene tectonic style of the Alps is variable along strike of the orogen. In the Western and Central Alps, foreland imbrication, backthrusting, and backfolding dominate. In the Eastern Alps, strike-slip and normal faults prevail. These differences are due to lateral extrusion in the Eastern Alps. Lateral extrusion encompasses tectonic escape (plane strain horizontal motion of tectonic wedges driven by forces applied to their boundaries) and extensional collapse (gravitational spreading away from a topographic high in an orogenic belt). The following factors contributed to the establishment of lateral extrusion in the Eastern Alps: (1) a rigid foreland, (2) a thick crust created by indentation and earlier collision, (3) a decrease in strength in the crust due to thermal relaxation, (4) a crustal thickness gradient from the Eastern Alps to the Carpathians, and, possibly, (5) a disturbance of the lithospheric root. Northward indentation by the Southern Alps causes thickening in and in front of the indenter and tectonic escape. Gravitational spreading attenuates crustal thickness differences. Indentation structures occur in the western Eastern Alps and comprise folds, thrusts, and strike-slip faults. These structures pass laterally into spreading structures, which encompass transtensional and normal faults in the eastern Eastern Alps. The overall structural pattern is dominated by escape structures, namely, sets of strike-slip faults that bound serially extruding wedges. Structural complexity arises from (1) interference of major fault sets, (2) accommodation of displacement differences between the Eastern Alps and their fore- and hinterland, (3) displacement transfer from the Eastern Alps toward the Carpathians which act as a lateral unconstrained margin, and (4) crustal decoupling, which partitions extrusion into brittle upper plate and ductile lower plate deformation. The kinematics of lateral extrusion is approximated by an extrusion-spreading model proposed for nappe tectonics.

Extension in compressional orogenic belts: The eastern Alps

Extension in the eastern Alps encompasses the following: (1) Extension accompanying simple shear crustal stacking. Subhorizontal stretching occurs along flat-lying foliations and regionally consistent stretching lineations with associated high strains. On a local scale, stretching results from extrusion of ductile sediments squeezed between rigid basement blocks basement blocks during thrust-sheet imbrication. The overall geometric effect is crustal thickening. (2) Classical extensional tectonics. Extension first occurs as a consequence of accretion and underthrusting of continental and oceanic material and represents gravitational adjustment of an unstable orogenic wedge. Extension then occurs as a consequence of terminal continental collision and is accommodated by lateral extrusion of crustal of crustal blocks. The overall geometric effect is crustal thinning. Extension is generally oriented subparallel to the strike of the orogen.

A PLATE-TECTONIC MODEL FOR THE MESOZOIC AND EARLY CENOZOIC HISTORY OF THE CARIBBEAN PLATE

Abstract We present a model in which the Caribbean plate is an intra-American feature formed along the Caribbean spreading center as opposed to the current model that considers the Caribbean plate as a far-travelled crustal segment that formed in the Pacific region. Paleomagnetic data, which cover an age range from Jurassic through Paleocene, indicate the ophiolite complexes in Costa Rica and Panama formed in an equatorial position. The ophiolites did not fundamentally change their position relative to South America since their origin. Basaltic rocks of the lower part of the ophiolites are of mid-ocean ridge type suggesting formation at a spreading-center. They are interpreted to have formed as part of the proto-Caribbean crust at a spreading axis in an intra-American position during Jurassic and Early Cretaceous times. The upper part of the ophiolites is mainly built up by island arc and intraplate basalts. The island arc basalts evolved at the Central American landbridge which started in the Middle Cretaceous. The intraplate basalts are related to the Caribbean plateau basalt which thickened and stiffened the Caribbean crust in Middle Cretaceous to probably Campanian times. Changes in relative plate motions along the Middle American convergent plate margin are reflected by orientations and time sequences of paleostress tensors calculated from fault-slip data in southern Mexico and Costa Rica. Kinematic indicators from mylonite zones display sinistral movement along secondary shear zones within the southern Mexican crustal complexes and along the northern boundary of the Chortis block in the Motagua–Polochic fault system. This reflects the eastward drift of the Chortis block since the beginning of the Cenozoic. At this time the Chortis block became part of the Caribbean plate which moved more than 1000 km towards the east relative to the North and South American plates during the Cenozoic.

Palinspastic reconstruction and topographic evolution of the Eastern Alps during late Tertiary tectonic extrusion

This paper presents a new palinspastic restoration of the Eastern Alps for Neogene time and an attempt to reconstruct the Neogene palaeogeology, palaeotopography and palaeohydrography in connection with the structural evolution. The Eastern Alps underwent radical horizontal displacement during the Neogene due to large strike-slip systems and formation of structural windows. Our palinspastic reconstruction considers: (a) the rearrangement of tectonic units dismembered during tectonic extrusion, (b) the tectonic denudation driven by displacement of the crystalline blocks, (c) geochronological arguments, and (d) the sedimentary record of the syn-extrusion basins. The rearrangement of tectonic blocks results in a remarkably good fit of highly dismembered zones both in crystalline and sedimentary areas and shows the pre-Miocene unstretched pattern of the Eastern Alps, reduced to 65% of its present-day E–W elongation. Using this structural frame and considering the sedimentary record, a set of palaeogeologic and palaeotopographic sketch maps with the palaeo-river systems is presented for three time slices (pre-, syn- and post-extension situation). In Late Oligocene and Early Miocene times, the western Eastern Alps were already mountainous, whereas the eastern part of the orogen formed lowlands or hilly areas. Enhanced block movement in the course of the extrusion process around the Early/Middle Miocene boundary led to the formation of intramontane sedimentary basins and a fault-induced reorientation of the drainage pattern, which forms the basis of the modern river system in the area east of the Tauern window. This region, where pre-Miocene land surfaces are preserved, probably became a mountainous area not before Late Miocene time and never reached the elevations of the areas further west.

Balancing lateral orogenic float of the Eastern Alps

Abstract Oligocene to Miocene post-collisional shortening between the Adriatic and European plates was compensated by frontal thrusting onto the Molasse foreland basin and by contemporaneous lateral wedging of the Austroalpine upper plate. Balancing of the upper plate shortening by horizontal retrodeformation of lateral escaping and extruding wedges of the Austroalpine lid enables an evaluation of the total post-collisional deformation of the hangingwall plate. Quantification of the north–south shortening and east–west extension of the upper plate is derived from displacement data of major faults that dissect the Austroalpine wedges. Indentation of the South Alpine unit corresponds to 64 km north–south shortening and a minimum of 120 km of east–west extension. Lateral wedging affected the Eastern Alps east of the Giudicarie fault. West of the Giudicarie fault, north–south shortening was compensated by 50 to 80 km of backthrusting in the Lombardian thrust system of the Southern Alps. The main structures that bound the escaping wedges to the north are the Inntal fault system (ca. 50 km sinistral offset), the Konigsee–Lammertal–Traunsee (KLT) fault (10 km) and the Salzach–Ennstal–Mariazell–Puchberg (SEMP) fault system (60 km). These faults, as well as a number of minor faults with displacements less than 10 km, root in the basal detachment of the Alps. The thin-skinned nature of lateral escape-related structures north of the SEMP line is documented by industry reflection seismic lines crossing the Northern Calcareous Alps (NCA) and the frontal thrust of the Eastern Alps. Complex triangle zones with passive roof backthrusts of Middle Miocene Molasse sediments formed in front of the laterally escaping wedges of the northern Eastern Alps. The aim of this paper is a semiquantitative reconstruction of the upper plate of the Eastern Alps. Most of the data is published elsewhere.

Slab in the wrong place: Lower lithospheric mantle delamination in the last stage of the Eastern Carpathian subduction retreat

A model with a new type of delamination of the lower lithospheric mantle is proposed to explain the Pliocene to recent tectonic evolution of the Eastern Carpathians. We suggest that, after the continental collision in middle Miocene time, break-off of the west-dipping subducting slab occurred at a depth of 70 km. Slab break-off propagated horizontally toward the east, inducing lithospheric delamination and movement of the Vrancea slab into its present position. Delamination was followed by rapid asthenospheric rise, resulting in magma generation and the explosive alkaline basaltic magmatism of the Persani Mountains. Contamination of the former subduction-related magmatic reservoirs with the alkalic basaltic material and differentiation processes produced the Harghita calc-alkaline and shoshonitic rocks. The asthenospheric rise induced crustal uplift, which is the triggering mechanism for extension and formation of the Brasov-Gheorghieni hinterland basins. The extension was accommodated by shortening and folding in the foreland. The vertical Vrancea slab appears in our model as a segment of delaminated lower lithospheric mantle that is seismically active due to the ongoing pull of the eclogitized oceanic crust. The model explains the displaced and shallow position of the slab relative to the Eastern Carpathian suture zone.

The Xigaze forearc basin: evolution and facies architecture (Cretaceous, Tibet)

Abstract The mid-Cretaceous to Eocene flysch deposits of the Xigaze forearc basin in southern Tibet were investigated in a 120 km segment along the Indus-Yarlung suture zone. The basin evolved south of the magmatic arc (Gangdise belt) of the Lhasa block on top of trapped oceanic or transitional crust. Remnants of shelf carbonates are preserved along the northern edge of the basin. The mapped segment was shortened by about 65%; metamorphism reached low-grade conditions along the northern margin of the basin. The flysch sequence reaches a thickness of at least 5 km and consists to a large degree of volcaniclastic (andesitic to dacitic) material shed from the magmatic-arc Gangdise belt. Particularly in the western part of the study area, plutonic and sedimentary rocks from deep erosion levels and/or more distal sources contributed to the basin fill. Rivers from the Lhasa block acted as point sources and fed five major deep-sea channel systems. Turbidity currents in the channels were directed towards the growing accretionary wedge of the subduction zone, thus indicating that the basin was continuously filled up to outer ridge level and gradually shallowing. The forearc flysch is subdivided into at least three megasequences, which begin with wide (up to several km) incised (10 to 50 m), coarse-grained channel fills and their associated fan deposits. The upper parts of the megasequences contain hemipelagic dark shales and marls (deposited above the calcite compensation depth). Lateral channel migration, channel-lobe switching, but also volcanic pulses generated a predominantly fining-upward, high-frequency cyclicity. After continental collision, the marine sedimentation in the forearc basin was replaced by fluvial deposits of the Eocene-Oligocene Qiuwu formation, which is time-equivalent to the Kailas and Indus molasses farther west and rich in coarse gravel derived from the Gangdise belt. Both forearc flysch and Qiuwu formation were deformed simultaneously during the Miocene. We assume that the molasse-type Qiuwu formation represents the final continental facies of the forearc basin filling.

Quaternary deformation in the Eastern Pamirs, Tadzhikistan and Kyrgyzstan

Active deformation in the eastern Pamir of Central Asia is concentrated on the margins of the orogen with minor deformation within the high terrain. Along the Trans-Alai mountain front at the northern perimeter of the orogen, Quaternary thrusting is documented by uplifted pediments, now at >500 m above the piedmont, Holocene fault scarps, and large earthquakes with N to NW oriented P axes. Seismicity in the interior of the orogen outlines a N–S belt that includes normal faulting events with E–W oriented T axes. N–S striking, active normal faults in the interior Lake Karakul region are compatible with these earthquakes; they define an asymmetric graben with a master fault at the western basin margin. In the southern Pamirs, dextral strike-slip faults root in the dextral Karakorum Fault, which bounds the Pamirs to the east. A mixture of dextral and reverse offsets totalling 135 m in Pleistocene terraces and 8 m in late Pleistocene/Holocene deposits demonstrates contemporary transpression, indicating average displacement rates of <1 mm/yr. The concentration of young thrusts along the Trans-Alai, the northward migration of thrusting, and the scarcity of other large-scale shortening features within the eastern Pamirs suggest that this part of the orogen moves northward en bloc and causes the progressive annihilation of the intermontane Alai Valley. Widespread dextral shear in the eastern Pamirs, both to the south and north of the extensional Karakul depression, and combined dextral strike-slip and normal faulting in the Muji-Tashgorgan graben of the Chinese Pamirs are interpreted as localized space accommodation phenomena, formed during progressive transfer of compressional deformation along a dextral strike-slip deformation zone with extensional stepovers.

Transpressional collision structures in the upper crust: the fold-thrust belt of the Northern Calcareous Alps

Abstract Two major structural events characterize the tectonic evolution of the Northern Calcareous Alps (NCA): (1) late-Early Cretaceous to Late Eocene NW-directed, dextral-transpressional stacking of nappes as an expression of the formation of the Austroalpine orogenic wedge; and (2) Miocene sinistral wrenching due to eastward lateral extrusion of crustal wedges along the central Eastern Alps. Three first-order detachment horizons are defined by differences of competence within areas of facies transition. Preexisting normal faults control the thrust architecture. Transpressional contraction in the NCA is indicated by: (a) NW-directed thrusting, oblique to both the long axis of the NCA and the edge of the orogenic foreland; (b) the occurrence of en-echelon arrays of thrusts and folds laterally displaced by dextral strike-slip faults; the thrusts and strike-slip faults are kinematically connected to each other and dissect the NCA into rhomboidal blocks; (c) NE-directed extension parallel to fold and ramp axes and the internal strike of the NCA; and (d) clockwise rotation (⩾ 30°) of the entire NCA around a vertical axis. Kinematic and dynamic analysis of mesoscale fault-striae data related to transpressional stacking indicates a NW-trend of contraction directions and, in general, a NE-trend of the extension directions, parallel to fold axes and branch lines of thrusts. Before rotation, the average contraction direction was probably parallel to the generally W-directed shear direction recorded in the crystal-plastically deformed central part of the Eastern Alps. Three balanced cross-sections across the NCA yield a minimum of 54–65% total shortening.

Kinematic evolution of the Romanian Carpathians

Abstract The regional pattern of contraction and extension directions and the evolution of the strain field from Paleogene to Neogene times enabled a reconstruction of the migration path of the Carpathian collision front. The Carpathian nappes were thrust around the Moesian Plate during Paleogene and Early Neogene times and protruded into a small oceanic embayment between the Moesian and European plates. The arc structure of the Carpathian fold-thrust belt was formed in Late Neogene times as a result of the eastward-escaping Tisza–Dacia block, due to N-directed convergence of the Adriatic plate and the retreating subduction of an oceanic slab. Brittle deformation structures in the Romanian Carpathians suggest three tectonic events related to major plate motions: (1) Paleogene to Middle Miocene NE to ENE contraction caused right-lateral curved strike-slip faults; (2) Middle Miocene to Pliocene fan-shaped orientations of contraction directions were caused by right-lateral oblique convergence in the Southern Carpathians, frontal convergence in the southern Eastern Carpathians and left-lateral convergence in the northern Eastern Carpathians; (3) Pleistocene to Holocene general E–W extension and N–S contraction in the Carpathian arc and local ESE–WNW contraction in the Vrancea area is related to the late roll back stage and break-off of the subducted slab in the bend area.