Summary The kinematic understanding of the relationship between relative plate motion and the structure of orogenic belts depends upon a knowledge of relative plate motion across the plate boundary system, the relative motion of small blocks and flakes within the system, an evaluation of orogenic body forces, and an understanding of the thermomechanical evolution of the upper part of the orogenic lithosphere in determining strength and detachment levels. We have built a preliminary model for the Cenozoic kinematic evolution of the western Mediterranean oceanic basins and their peripheral orogens that integrates (1) the motion of Africa relative to Europe based upon a new study of Atlantic fracture zones using SEASAT data and the Lamont-Doherty magnetic anomaly database, (2) a new interpretation of the rotation of Corsica/Sardinia and the opening of the Balearic and Tyrrhenian oceanic basins, (3) sedimentary facies sequences in the Apennines, Calabria, and Sicily, and (4) Apennine/Calabrian structure and structural sequence.

Kinematics of the western Mediterranean

Seismic tomography models of the three-dimensional upper mantle velocity structure of the Mediterranean-Carpathian region provide a better understanding of the lithospheric processes governing its geodynamical evolution. Slab detachment, in particular lateral migration of this process along the plate boundary, is a key element in the lithospheric dynamics of the region during the last 20 to 30 million years. It strongly affects arc and trench migration, and causes along-strike variations in vertical motions, stress fields, and magmatism. In a terminal-stage subduction zone, involving collision and suturing, slab detachment is the natural last stage in the gravitational settling of subducted lithosphere.

http://www.mantleplumes.org/WebDocuments/Wortel2000.pdf

Subduction and Slab Detachment in the Mediterranean-Carpathian Region

Geological atlas of western and central Europe

The new tectonic map of the Alps is based on the combination of purely structural data with criteria regarding paleogeographical affiliation and/or tectono-metamorphic evolution. The orogenic evolution of the Alps is discussed using a combination of maps and paleogeographical reconstructions. It is proposed that the Alps are the product of two orogenies, a Cretaceous followed by a Tertiary one. While the former is related to the closure of an embayment of the Meliata ocean into Apulia, the latter is due to the closure of the Alpine Tethys between Apulia and Europe. The along-strike changes in the overall architecture, as for example revealed by geophysical-geological transects, are by far more substantial than hitherto believed. It appears that the Alps are still far from being over-investigated, as is demonstrated by many surprising recent findings based on field geology, laboratory results and geophysical methods of deep sounding.

/pdf/tectonic-map-and-overall-architecture-of-the-alpine-orogen-23efh3jp0j.pdf

Tectonic map and overall architecture of the Alpine orogen

A correlation of tectonic units of the Alpine-Carpathian-Dinaridic system of orogens, including the substrate of the Pannonian and Transylvanian basins, is presented in the form of a map. Combined with a series of crustal-scale cross sections this correlation of tectonic units yields a clearer picture of the three-dimensional architecture of this system of orogens that owes its considerable complexity to multiple overprinting of earlier by younger deformations.

/pdf/the-alpine-carpathian-dinaridic-orogenic-system-correlation-2gm3qeefzn.pdf

The Alpine-Carpathian-Dinaridic orogenic system: correlation and evolution of tectonic units

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.

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

Lateral extrusion encompasses extensional collapse (gravitational spreading away from a topographic high in an orogenic belt) and tectonic escape (plane strain horizontal motion of wedges driven by forces applied to their boundaries). In the Eastern Alps it resulted from (1) an overall northerly compression (Apulia against Eurasia), (2) a strong foreland (Bohemian massif), (3) lack of constraint along a lateral boundary (Carpathian region), and (4) a previously thickened, gravitationally unstable, thermally weakened crust (Eastern Alpine orogenic belt). Six indentation experiments reproduce lateral extrusion at lithospheric scale. The models have two to four lithospheric layers, with a Mohr/Coulomb rheology for the upper and a viscous rheology for the lower crust. The lithosphere rests upon a low-viscosity asthenosphere. A broad indenter, a narrow deformable area, and a weakly constrained eastern margin fullfill as closely as possible conditions in the Eastern Alps. Indentation produces both thickening in front of the indenter and escape of triangular wedges. Lateral variations in crustal thickness become attenuated by gravitational spreading. The overall fault pattern includes domains of reverse, strike-slip, oblique normal, and pure normal faults. Strike-slip faults in conjugate sets develop serially. The narrow width of the deformable area and the strength of the foreland determine the angles between the sets. Gravitational spreading produces a rhombohedral pattern of oblique and pure normal faults along the unconstrained margin. Opposite the unconstrained margin, the indenter front shows thrusts and folds intersecting with the conjugate strike-slip sets. A triangular indenter favors spreading. High velocity of indentation favors escape. High confinement limits lateral motion, inhibits spreading, and favors thickening. Lateral extrusion in the Eastern Alps is best modeled by (1) a weak lateral confinement, (2) a broad and straight indenter, (3) a narrow width of the deformable area, and (4) a rigid foreland. Crustal thickening, lateral escape, and gravitational spreading all contribute to the overall deformation.

Lateral extrusion in the eastern Alps, Part 1: Boundary conditions and experiments scaled for gravity

Scaled experiments have been carried out on caldera collapse mechanisms, using silicone as analogue magma and dry sand as analogue rock. Experiments were carried out in two and three dimensions using a range of roof aspect ratios (thickness/width 0.2 to 4.5) appropriate for caldera collapse. They reveal a general mechanism of collapse, only weakly dependent on the shape of the reservoir. For low roof aspect ratios (≤1), subsidence starts by flexure of the roof and the formation of outward dipping, reverse ring faults, which in turn trigger formation of peripheral inward dipping, normal ring faults. The subsidence always occurs asymmetrically. In cross section the reverse faults delimit a coherent piston, bounded on each side by an annular zone of inwardly tilted strata located between the reverse and normal ring fault sets. The surface depression consists of a nondeformed area (piston) surrounded by an annular extensional zone (tilted strata). For high aspect ratios (>1), multiple reverse faults break up the roof into large pieces, and subsidence occurred as a series of nested wedges (2-D) or cones (3-D). The extensional zone dominates the surface depression. In the case where preexisting regional faults do not play a major role, the collapse mechanics of calderas probably depends strongly on the roof aspect ratio. Calderas with low roof aspect ratios are predicted to collapse as coherent pistons along reverse faults. The annular extensional zone might be the source of the large landslides that generate intracaldera megabreccias. Collapse into magma reservoirs with high roof aspect ratios may be the origin of some funnel calderas where explosive reaming is not dominant.

Experimental study of caldera formation

Experiments were conducted to study the spreading of volcanic constructs. Volcanoes are simulated by a sand cone, and the volcanic substratum is simulated by a sand layer (brittle substratum) overlying a silicone layer (ductile substratum). Similarity conditions between natural volcanoes and experimental prototypes led to the definition of dimensionless π numbers. Experiments determine π values which predict whether or not spreading takes place. Of particular importance are the ratio between the thickness of the brittle substratum and the height of the volcano (π2) and the brittle/ductile ratio of the substratum (π3). π2 indicates that the volcano must be large enough to “break” the substratum before spreading occurs, whereas π3 controls the style of deformation. During spreading, these dimensionless numbers change with time, reaching values that tend toward those observed for stable configurations. Experimental values are compared with those from well-constrained natural examples. It is found that an essential requirement for volcanic spreading is the presence of a low-viscosity layer within the substratum. Flow of the weak layer away from the excess load is responsible for the spreading. The overlying edifice displays radial intersecting grabens, due to concentric stretching, dissected summit areas; concentric zones of thrusts and folds form in the substratum around the edifice, and diapirs of the ductile substratum rise within the fault zones.

Olivier Merle

Papers

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

Lateral extrusion in the eastern Alps, Part 1: Boundary conditions and experiments scaled for gravity

Experimental study of caldera formation

Scaled experiments of volcanic spreading

Active spreading and regional extension at Mount Etna imaged by SAR interferometry