Lithospheric Tear Research Articles

Comment on the article “Propagation of a lithospheric tear fault (STEP) through the western boundary of the Calabrian accretionary wedge offshore eastern Sicily (Southern Italy)” by Gallais et al., 2013 Tectonophysics

Abstract The oceanic lithosphere of the Ionian Sea is subducted under the Calabrian Arc, and it has been proposed that the Ionian lithosphere has been torn apart (STEP fault sensu Govers and Wortel, 2005) on its western side, close to the Malta escarpment, a major morphological feature inherited from the Mesozoic palaeogeography. In a recent paper Gallais et al. (Tectonophysics, 2013) have traced a right-lateral strike-slip fault that extends from south of the Alfeo Seamount all the way to the Tyrrhenian Sea. They also argue that the position of the STEP fault is distinct from, and located ca. 50 km east of, the Malta escarpment. Gallais et al. document with some additional details a fault segment that was already reported in the literature and that is located south of the Alfeo Seamount. However, Gallais et al. have little constraints to trace the STEP fault further to the north; in this sector a belt of surface deformation that can be related to the STEP fault has been mapped from a denser grid of seismic profiles, and is located west-ward, close to the Malta escarpment (Argnani and Bonazzi, 2005). This undermines the conclusion of Gallais et al. that the lithospheric tear is distinct from the Malta escarpment. It is therefore likely that surface deformation associated with the lithospheric tear fault is more complex and is not represented by a single strike-slip fault.

Late Cenozoic extension and crustal doming in the India‐Eurasia collision zone: New thermochronologic constraints from the NE Chinese Pamir

AbstractThe northward motion of the Pamir indenter with respect to Eurasia has resulted in coeval thrusting, strike‐slip faulting, and normal faulting. The eastern Pamir is currently deformed by east‐west oriented extension, accompanied by uplift and exhumation of the Kongur Shan (7719 m) and Muztagh Ata (7546 m) gneiss domes. Both domes are an integral part of the footwall of the Kongur Shan extensional fault system (KES), a 250 km long, north‐south oriented graben. Why active normal faulting within the Pamir is primarily localized along the KES and not distributed more widely throughout the orogen has remained unclear. In addition, relatively little is known about how deformation has evolved throughout the Cenozoic, despite refined estimates on present‐day crustal deformation rates and microseismicity, which indicate where crustal deformation is presently being accommodated. To better constrain the spatiotemporal evolution of faulting along the KES, we present 39 new apatite fission track, zircon U‐Th‐Sm/He, and 40Ar/39Ar cooling ages from a series of footwall transects along the KES graben shoulder. Combining these data with present‐day topographic relief, 1‐D thermokinematic and exhumational modeling documents successive stages, rather than synchronous deformation and gneiss dome exhumation. While the exhumation of the Kongur Shan commenced during the late Miocene, extensional processes in the Muztagh Ata massif began earlier and have slowed down since the late Miocene. We present a new model of synorogenic extension suggesting that thermal and density effects associated with a lithospheric tear fault along the eastern margin of the subducting Alai slab localize extensional upper plate deformation along the KES and decouple crustal motion between the central/western Pamir and eastern Pamir/Tarim basin.

Propagation of a lithospheric tear fault (STEP) through the western boundary of the Calabrian accretionary wedge offshore eastern Sicily (Southern Italy)

The rollback of a segmented slab of oceanic lithosphere is typically accompanied by vertical lithospheric tear fault(s) along the lateral slab edge(s) and by strike slip movement in the upper plate, defined as a STEP fault (Subduction Tear Edge Propagator). The Neogene evolution of the Central Mediterranean is dominated by the interaction between the slow Africa–Eurasia convergence and the SE-ward rollback of the Ionian slab, that leads to the back-arc opening of the Tyrrhenian Sea. Here, we present post-stack time migrated and pre-stack depth migrated Archimede (1997) multichannel seismic lines, that were acquired offshore eastern Sicily, at the foot of the Malta escarpment . First, we identify the recent deformation along the lateral ramp of the Calabrian accretionary wedge. Towards the east, the Calabrian wedge is formed by the accretion of the post-evaporitic sediments, above a decollement at the base of the Messinian evaporites . At the latitude of Syracuse, 50 km east of the Malta escarpment, a major N150°E trending crustal scale and vertical fault slices through the entire accretionary wedge. This fault cuts by several kilometers, through the pre-evaporitic Messinian sediments and into the basement. The vertical offset along this vertical fault decreases from north to south, and the fault is no longer observed on the seismic lines, 50 km SE of the Alfeo seamount . A previously published Moho depth isocontour map, offshore Sicily and the recent GPS data, combined with the presence of strike slip movements NE onshore Sicily, allow us to identify this 200 km long crustal-scale fault as the surface expression of a STEP fault. The presence of syntectonic Pleistocene sediments on top this crustal-scale fault suggests a recent lithospheric vertical movement of the STEP fault, in response to the rollback of the Ionian slab and to the SE-ward advance of the Calabria-Peloritan block. ► Depth migrated seismic reflection profiles from the Calabria/Sicily active margin. ► A N150°E striking, lithospheric scale fault, 50 km east of the Malta escarpment ► Interpreted to be a “Subduction Tear Edge Propagator” (STEP) fault ► Fault offset decreases to the S until termination (at the propagating tip) ► Deformed sediments suggest recent fault activity (related to Calabrian subduction).

Deep electrical resistivity structure of the northern Gibraltar Arc (western Mediterranean): evidence of lithospheric slab break-off

Terra Nova, 23, 179–186, 2011 Abstract Uncertainties about the lithospheric structure of the Gibraltar Arc have led to the proposal of several contradictory models to explain its geodynamic setting. Herein, we present a novel 3-D model of the lithospheric electrical resistivity distribution beneath the whole Betic Cordillera obtained by inverting both broad-band and long-period magnetotelluric data. The lithosphere–asthenosphere boundary under SW Iberia is shown to be deeper than under the Alboran Basin. In addition, the sensitivity tests confirm the presence of a N–S oriented low-resistivity anomaly at lithospheric mantle depths east of the 4°W meridian. It coincides with an area of low velocities without earthquake hypocenters and is interpreted as asthenospheric material intruded by the lateral lithospheric tearing and breaking-off of the E-directed subducting Ligurian slab under the Alboran Domain. This scenario suggests that the opening of the Alboran Basin is related to the westward rollback of this E-directed subducting slab.

Interactions between volcanism and tectonics in the western Aeolian sector, southern Tyrrhenian Sea

SUMMARY New high-resolution bathymetric and magnetic data from the western Aeolian sector, southern Tyrrhenian Sea, provide insights into structural and volcanic development of the area, suggesting a strong interaction between volcanism and tectonics. The analysis of these data combined with relocated earthquake distribution, focal plane solutions and strain rate evaluation indicates that the dextral strike-slip Sisifo-Alicudi shear zone is a complex and wide area of active deformation, representing the superficial expression of the deep seated lithospheric tear fault separating the subduction slab below Sicily and Calabria. Most of the observed volcanic features are aligned along a NW–SE trend, such as the Filicudi island-Alicudi North Seamount and Eolo-Enarete alignments, and are dissected by hundred-metre-high scarps along conjugate NNE–SSW trending fault systems. The magnetic field pattern matches the main trends of volcanic features. Spectral analysis and Euler deconvolution of magnetic anomalies show the existence of both deep and shallow sources. High-amplitude, high-frequency anomalies due to shallow sources are dominant close to the volcanic edifices of Alicudi and Filicudi, while the main contribution on the surrounding Eolo, Enarete, Alicudi North and Filicudi North seamounts is given by low-amplitude anomalies and/or deeper magnetic sources. This is probably related to different ages of the volcanic rocks, although hydrothermal processes may have played an important role in blanketing magnetic anomalies, in particular at Enarete and Eolo seamounts. Relative chronology of the eruptive centres and the inferred deformation pattern outline the Quaternary evolution of the western Aeolian Arc: Sisifo, Alicudi North and Filicudi North seamounts might have developed in an early stage, following the Late Pliocene–Early Pleistocene SE-ward migration of arc-related volcanism due to the Ionian subduction hinge retreat; Eolo, Enarete and Filicudi represent later manifestations that led volcanoes to develop during Mid-Late Pleistocene, when the stress regime in the area changed, due to the SSE-ward propagation of the subduction slab tear fault and the consequent reorientation and decrease of trench migration velocity. Finally, volcanic activity occurred in a very short time span at Alicudi, where an almost conical volcanic edifice emerged, suggesting negligible interactions with regional fault systems.

Geochemical and temporal evolution of Cenozoic magmatism in western Turkey: mantle response to collision, slab break-off, and lithospheric tearing in an orogenic belt

Abstract Post-collisional magmatism in western Anatolia began in the Eocene, and has occurred in discrete pulses throughout the Cenozoic as it propagated from north to south, producing volcano-plutonic associations with varying chemical compositions. This apparent SW migration of magmatism and accompanying extension through time was a result of the thermally induced collapse of the western Anatolian orogenic belt, which formed during the collision of the Sakarya and Tauride–Anatolide continental blocks in the late Paleocene. The thermal input and melt sources for this prolonged magmatism were provided first by slab break-off-generated aesthenospheric flow, then by lithospheric delamination-related aesthenospheric flow, followed by tectonic extension-driven upward aesthenospheric flow. The first magmatic episode is represented by Eocene granitoid plutons and their extrusive carapace that are linearly distributed along the Izmir–Ankara suture zone south of the Marmara Sea. These suites show moderately evolved compositions enriched in incompatible elements similar to subduction zone-influenced subalkaline magmas. Widespread Oligo-Miocene volcanic and plutonic rocks with medium- to high-K calc-alkaline compositions represent the next magmatic episode. Partial melting and assimilation-fractional crystallization of enriched subcontinental lithospheric mantle-derived magmas were important processes in the genesis and evolution of the parental magmas, which experienced decreasing subduction influence and increasing crustal contamination during the evolution of the Eocene and Oligo-Miocene volcano-plutonic rocks. Collision-induced lithospheric slab break-off provided an influx of aesthenospheric heat and melts that resulted in partial melting of the previously subduction-metasomatized mantle lithosphere beneath the suture zone, producing the Eocene and Oligo-Miocene igneous suites. The following magmatic phase during the middle Miocene (16–14 Ma) developed mildly alkaline bimodal volcanic rocks that show a decreasing amount of crustal contamination and subduction influence in time. Both melting of a subduction-modified lithospheric mantle and aesthenospheric mantle-derived melt contribution played a significant role in the generation of the magmas of these rocks. This magmatic episode was attended by region-wide extension that led to the formation of metamorphic core complexes and graben systems. Aesthenospheric upwelling caused by partial delamination of the lithospheric root beneath the western Anatolian orogenic belt was likely responsible for the melt evolution of these mildly alkaline volcanics. Lithospheric delamination may have been caused by ‘peeling off’ during slab rollback. The last major phase of magmatism in the region, starting c .12 Ma, is represented by late Miocene to Quaternary alkaline to super-alkaline volcanic rocks that show OIB-like geochemical features with progressively more potassic compositions increasing toward south in time. These rocks are spatially associated with major extensional fault systems that acted as natural conduits for the transport of uncontaminated alkaline magmas to the surface. The melt source for this magmatic phase carried little or no subduction component and was produced by the decompressional melting of aesthenospheric mantle, which flowed in beneath the attenuated continental lithosphere in the Aegean extensional province. This time-progressive evolution of Cenozoic magmatism and extension in western Anatolia has been strongly controlled by the interplay between regional plate-tectonic events and the mantle dynamics, and provides a realistic template for post-collisional magmatism and crustal extension in many orogenic belts.

Negligible convergence and lithospheric tearing along the Caribbean–South American plate boundary at 64°W

Prior studies of the Caribbean–South American plate boundary have suffered from poor constraint on the structure of the crust and uppermost mantle. We use a recent wide‐angle velocity model from the Broadband Ocean‐Land Investigation of Venezuela and the Antilles arc Region project to constrain new seismic reflection data and previously published line drawing interpretations of the Caribbean–South American plate boundary at 64°W. Though commonly characterized as obliquely convergent, we determine that convergence is negligible in our study area. Previous estimates of Miocene to present north‐south shortening onshore eastern Venezuela have commonly been 115 km or higher, but we constrain shortening to ∼35 km onshore, with an additional ∼30 km offshore. With such minor convergence, we conclude that uplift and basin subsidence in eastern Venezuela does not derive from typical collisional orogeny. Instead, the largely vertical tectonics likely result from mantle dynamics associated with an eastward propagating, near‐vertical tear in the lithosphere along the former passive margin.

Malta Escarpment fault zone offshore eastern Sicily: Pliocene‐Quaternary tectonic evolution based on new multichannel seismic data

The Malta Escarpment represents the dominant morphological feature offshore eastern Sicily, linking the deep Ionian basin to the east with the Hyblean carbonate platform to the west. Interpretation of purposely acquired multichannel seismic data allows division of the Malta Escarpment into two portions characterized by different tectonic structures. Along the segment south of Siracusa the Malta Escarpment is not affected by recent faulting and appears as a steep surface that flattens out toward the Ionian basin. A recent deformation, characterized by a broad area of uplift, occurs 20–30 km east from the slope, along a NNW‐SSE trend. The segment of the Malta Escarpment extending north of Siracusa, on the other hand, is characterized by the presence of NNW‐SSE, east dipping recent extensional faults and related sedimentary basins. The observed structural features support the occurrence of a lithospheric tear between the Ionian oceanic basin and the Hyblean plateau.

Partial detachment of a lithospheric root under the southeast Carpathians: Toward a better definition of the detachment concept

The southeast corner of the Carpathian Mountains (Romania) provides a unique opportunity to study the very final and shortlived stage of plate convergence when a piece of subducted lithosphere finally detaches from the overriding plate and begins to sink into the mantle. Research efforts commonly focus on trying to image the cold lithospheric slab (tomographic techniques) and on trying to understand the tearing process from the earthquakes it generates. Here this issue is tackled from a different perspective emphasizing that the southeast Carpathians are extremely low considering crustal thickness; the crust there is almost as thick as the crust of the Alps (50‐55 km), but the elevation is less than onehalf of the elevation of the Alps. It is interesting that the Apennines (Italy) are just the opposite; they are rebounding after slab breakoff to reach elevations higher than expected from their crustal thickness (30‐35 km like the crust of most passive-margin coasts). It is thus proposed that although seismologic studies indicate that the cold mantle body under the southeast Carpathians is already partly detached from the overriding lithosphere, it is still viscously coupled to the lithosphere and pulls it downward. By utilizing residual-topography analysis (observed topography minus the contribution of the crust to it), the subcrustal load is quantified and interpreted in terms of mantle-lithosphere thickness extending to a depth of ;300 km. A particularly interesting finding is that the seismically active region is shifted ;50 km east of the place of maximum pull-down, indicating that lithospheric tearing is now occurring at one side of the detaching root and not above it. This interpretation is consistent with the idea of lateral delamination of the lower lithosphere.

New insights on 3-D plates interaction near Taiwan from tomography and tectonic implications

Recent tomographic results are used to trace the South China Sea and Philippine Sea subducting slabs, south and northeast of Taiwan along the Manila and Ryukyu trenches, respectively. In particular, the 3-D plates interaction beneath Taiwan is discussed based on a close-up view of the tomographic sections together with earthquake hypocenters distribution. Our study indicates that: (1) the east-dipping South China Sea slab can be followed to the north, up to the latitude of Hualien, (2) the Eurasian plate subducts beneath most part of the Taiwan island down to the 670 km-depth discontinuity, (3) the north-dipping Philippine Sea slab can be followed slightly west of the longitude of Hualien. Both plates thus interact beneath northern Taiwan where the arc–continent collision is paroxysmal. (4) Slab detachment is envisaged at the northern edge of the subducted Eurasian plate beneath the Coastal Range of Taiwan, which may facilitate the northwestward motion of the Philippine Sea plate with respect to Eurasia. Slabs geometries obtained from tomographic sections allow us to reconstruct the Late Neogene plate kinematics and dynamics in this region. Our main conclusions are: (1) The size of the original South China Sea was about twice its present size. (2) The subducted part of the West Philippine Basin i.e. the largest oceanic basin of the Philippine Sea Plate, extends only 400 km north of the Ryukyu Trench. (3) Slab detachment might have occurred 3–5 my ago beneath the central and northern Ryukyu Arc along a weak zone that is aligned with the Gagua Ridge: an ancient plate boundary. (4) The Ryukyu Trench has propagated westward from 126°E of longitude (southeast of Miyako Island) to the locus of the present arc–continent collision, along a major lithospheric tear that cut through the continent–ocean boundary first, and then through the continental lithosphere. As a consequence, the southern Ryukyu margin developed progressively from east to west as a subduction zone during the last 8 my. It has been elaborated onto a passive margin which corresponded to the edge of the Eurasian continental shelf bordering the South China Sea prior to Pliocene. (5) The Southern Okinawa Trough rifted during the last 2±1 Ma following the propagation of the subduction zone. (6) Subduction of the Eurasian margin probably stopped recently beneath northern Taiwan and a flip of subduction from west to east is being initiated near Hualien.

Geodynamics of flat subduction: Seismicity and tomographic constraints from the Andean margin

The cause and geodynamic impact of flat subduction are investigated. First, the 1500 km long Peru flat slab segment is examined. Earthquake hypocenter data image two morphologic highs in the subducting Nazca Plate which correlate with the positions of subducted oceanic plateaus. Travel time tomographic images confirm the three‐dimensional slab geometry and suggest a lithospheric tear may bound the NW edge of the flat slab segment, with possible slab detachment occurring down dip as well. Other flat slab regions worldwide are discussed: central Chile, Ecuador, NW Colombia, Costa Rica, Mexico, southern Alaska, SW Japan, and western New Guinea. Flat subduction is shown to be a widespread phenomenon, occuring in 10% of modern convergent margins. In nearly all these cases, as a spatial and temporal correlation is observed between subducting oceanic plateaus and flat subduction, we conclude that flat subduction is caused primarily by (1) the buoyancy of thickened oceanic crust of moderate to young age and (2) a delay in the basalt to eclogite transition due to the cool thermal structure of two overlapping lithospheres. A statistical analysis of seismicity along the entire length of the Andes demonstrates that seismic energy release in the upper plate at a distance of 250–800 km from the trench is on average 3–5 times greater above flat slab segments than for adjacent steep slab segments. We propose this is due to higher interplate coupling and the cold, strong rheology of the overriding lithosphere which thus enables stress and deformation to be transmitted hundreds of kilometers into the heart of the upper plate.

Depth and modes of Pliocene–Pleistocene crustal extension of the Apennines (Italy)

Since early Pliocene times the Apenninic chain has been dissected by normal faults propagating towards the Adriatic foreland. In the Tyrrhenian Sea extension involved deep crustal sections, whereas in the ‘Central Apennines Downfaulted Area’ it affected the shallow crust. The Tyrrhenian back‐arc domain is connected to the overall flexural retreat of the Adriatic–Apulia plate in front of the Apenninic collisional wedge. In the outer Apenninic belt thin‐skinned delamination and gravitational collapse occurred in the hanging wall of a thickened wedge, overthrusting the uplifted, buoyant crust of the Apulia foreland. Differential sinking velocity of the foreland plate results from the inherited competence contrast between the rigid Gargano–Apulia carbonate platform to the south, and the low‐competence lithology of the pelagic sequence of the Adriatic basin to the north. During late Messinian–early Pliocene times this palaeogeographical boundary acted as a lithospheric tear, separating segments of the Apulia plate subjected to different subduction modes.

Spherical shell tectonics: on the buckling of the lithosphere at subduction zones

The buckling phenomena of the subducting lithosphere due to the sphericity of the earth are studied by a nonlinear finite element method (FEM), and the effects of geometrical nonlinearity are examined. The subduction of the lithosphere is modelled by a hemispherical shell squeezed at its circumferential edge. In scaling the geometry of the subducting lithosphere, two parameters are employed; one is the thickness of the hemispherical shell (i.e., the thickness of the lithosphere) and the other is the length of its deformable portion (i.e., the length of the descending slab). To simulate the bending of the lithosphere at the trench, uniform inward load is applied to the free circumferential edge of the deformable portion, which corresponds to the leading edge of the subducting lithosphere. At the opposite edge of the deformable portion (i.e., at the trench), a built-in boundary condition is imposed; no displacements and no rotations are allowed. With a load greater than some critical value, the deformable portion of the shell buckles and undulates azimuthally. The investigation of this buckling phenomenon can be summarized as follows: 1. (1) Buckling with a shorter wavelength is associated with a thinner and shorter shell. Mechanical irregularities simulating lithospheric tearing or seamount collision do not alter the buckling wavelength very much. After appropriate scaling, the lengths of subduction zones coincide with the buckling wavelength calculated by F.E.M. 2. (2) Sharp cusps pointing outward from the shell appear during the postbuckling undulation without any irregularities in shell property and in the applied load. The shape of this cuspidal undulation is quite similar to that actually observed at an arc-arc junction. 3. (3) Buckling can take place in the stress environment likely to exist in the earth. These results strongly suggest that the arcuate shapes of subduction zones are in fact generated by buckling of the subducting lithosphere.

Seismological evidence for a lithospheric tearing beneath the Aleutian arc

Travel times have been studied from the Longshot underground explosion detonated on Amchitka Island on October 29, 1965. The travel-time residuals of P waves at stations in azimuths from N10°E to N30°E around the source, are, on the average, larger by 1 to 2 sec than those at stations in adjacent azimuths. This observation suggests the existence of a low-velocity region having a limited spatial extent; a P-velocity contrast of 0.3 to 0.6 km/sec is estimated. This low-velocity region can be explained in terms of a lithospheric tearing beneath the Aleutian arc and of a subsequent intrusion of low-velocity material from the underlying asthenosphere. The lithospheric tearing is considered to have been caused by bending of the oceanic lithosphere at a markedly curved portion of the Aleutian arc.