Olivier Merle

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.

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

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.

Experimental study of caldera formation

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.

Scaled experiments of volcanic spreading

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.

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

Abstract Large-flank instability has been proposed by most recent authors as a major process in the dynamics of Mount Etna. However, several aspects of the models are still disputed, such as the boundaries of the moving area and the driving force for the instability. In this paper we present synthetic aperture radar interferometric data which allow us to identify two main sectors of active instability where the deformational process differs. An eastern sector is bounded to the north by the Pernicana–Provenzana fault, to the west by the North Rift Zone and the South Rift Zone and to the south by the Mascalucia–Tremestieri–Trecastagni fault system. Interferograms provide new data on the activity and extent of the southern boundary, which was previously underestimated. In this sector the structural features indicate an eastward sliding, driven by E–W regional extension, which can be interpreted as a result of the retreating slab of the Ionian subduction. A second collapsing sector exists along the southern flank of Mount Etna, where the interferograms show the presence of an active anticlinal ridge. This ridge is interpreted as the result of southward gravity spreading over a basal decollement between Etna and its Plio–Quaternary basement and the Hyblean platform. The two sectors first became active in summer 1996, after the beginning of a new cycle of eruptive activity at the summit. The activity was ongoing until January 1998, with the deformation rate ranging from 4–6 mm/yr for the Mascalucia–Tremestieri–Trecastagni faults to 12 mm/yr for the anticlinal ridge.

THE EFFECT OF VOLCANIC CONSTRUCTS ON RIFT FAULT PATTERNS

Volcanoes in rift situations often show distinctive “hour-glass” fault patterns, with increased fault throw as the volcano is approached. We present analogue models that show that this is caused by an interaction of the regional stress field with that set up by the volcano mass. For faults to be reorientated there must be a ductile layer below the volcano. This can be hot crust at mid-ocean ridges, continental rifts, or weak sedimentary strata. Increased volcano size or mass and lower brittle/ductile ratios lead to increased fault curvature, whereas an increased regional extension rate decreases the effect. Volcanoes on one side of a rift may capture it, forming the axis of a new rift, with curved or en echelon zones on either side. By concentrating extension, magma is more easily erupted, so volcanoes may erupt more magma of less-evolved composition. A positive feedback between increased extension and magma eruption rate will lead to rift narrowing, which can favor the formation of oceanic crust.

The Cenozoic evolution of the Roer Valley Rift System integrated at a European scale

The Roer Valley Rift System (RVRS) is located between the West European rift and the North Sea rift system. During the Cenozoic, the RVRS was characterized by several periods of subsidence and inversion, which are linked to the evolution of the adjacent rift systems. Combination of subsidence analysis and results from the analysis of thickness distributions and fault systems allows the determination of the Cenozoic evolution and quantification of the subsidence. During the Early Paleocene, the RVRS was inverted (Laramide phase). The backstripping method shows that the RVRS was subsequently mainly affected by two periods of subsidence, during the Late Paleocene and the Oligocene–Quaternary time intervals, separated by an inversion phase during the Late Eocene. During the Oligocene and Miocene periods, the thickness of the sediments and the distribution of the active faults reveal a radical rotation of the direction of extension by about 70–80j (counter clockwise). Integration of these results at a European scale indicates that the Late Paleocene subsidence was related to the evolution of the North Sea basins, whereas the Oligocene–Quaternary subsidence is connected to the West European rift evolution. The distribution of the inverted provinces also shows that the Early Paleocene inversion (Laramide phase) has affected the whole European crust, whereas the Late Eocene inversion was restricted to the southern North Sea basins and the Channel area. Finally, comparison of these deformations in the European crust with the evolution of the Alpine chain suggests that the formation of the Alps has controlled the evolution of the European crust since the beginning of the Cenozoic.

Experiments on the indentation process during cryptodome intrusions: New insights into Mount St. Helens deformation

Scaled experiments were carried out to study the deformation of a volcanic edifice by forcible intrusion of a cryptodome. As the magma analogue is injected vertically into a sand cone, asymmetric deformation is caused by the formation of a curved major shear fault, which dips inward from one side of the cone to the opposite edge of the intrusion. The path of the ascending silicone deviates to follow the trajectory of the fault, and a lateral bulge grows slowly from the footwall of the fault. The oblique push makes the bulge migrate outward, causing extension upslope to form an asymmetric graben in the hanging wall of the major shear fault. We suggest that the pattern of internal deformation within Mount St. Helens prior to the May 18, 1980, eruption was similar to that observed in our scaled models.

Shear criteria and structural symmetry

During the last decade, it has been shown that most relevant shear criteria within ductile rocks are asymmetric structures (e.g. pressure shadows, shear bands, CS structures, fabrics, tension gashes, folds, veins). The correspondence between coaxial or non-coaxial deformation, and symmetric or asymmetric particle velocity fields, respectively accounts for the use of structural symmetry as an indicator of strain history. The application of this symmetry concept to various field examples emphasizes that: (i) the degree of symmetry of a given structural pattern reflects the bulk strain regime irrespective of the size and the mechanical behaviour of the considered system; and (ii) the strain regime can also be inferred from the order of appearance and dominance of structures which contribute to the total deformation pattern, even where the progressive deformation results in a complex pattern which cannot be directly interpreted.

Extension induced by volcanic loading in regional strike-slip zones

Volcanoes and strike-slip pull-apart structures are commonly found together. We use theoretical predictions and analogue modeling to evaluate the effect of volcanic loading in strike-slip zones, and the converse effect of regional strike-slip faults on volcano structure. Theoretical predictions indicate that regional stress is reoriented by volcano loading to cause extension. Analogue models indicate that volcanoes in strike-slip zones develop extensional pull-apart structures. A graben typically forms normal to the minimum regional stress axis (σ3) at a cone. Cones emplaced onto preexisting faults increase that fault9s tensional component. If the substratum is weak enough to permit volcano spreading, thrusts form normal to the maximum stress axis (σ1). A feedback situation can arise, i.e., loading-related extension allows more magma influx, more eruption, and thus increased loading. This process can create large volcano-tectonic depressions. Volcano structures may also be used as indicators of regional strike-slip movements.