Description: The formation of Cenozoic mountain belts in the Mediterranean realm was preceded by tens of millions of years of subduction, forming volcanic arcs, and frontal contractional systems. In addition, subduction usually involves slab rollback and formation of oceanic backarcs. Although such structure must have influenced the orogeny of Mediterranean mountain belts, no active analog has been mapped with modern crustal-scale seismic methods. Here, we study the entire Calabrian subduction system to map the structure resulting from Tethys lithosphere subduction and slab rollback, in a process that must be akin to that operating during a phase of the formation of the Mediterranean orogenic belts. We present a crustal-scale cross section of the entire Calabrian subduction system obtained from on- and off-shore wide-angle seismic data. The 2D P-wave velocity section shows spatially abrupt (〈5 km of profile distance) structural and petrological transitions from the Ionian sedimentary wedge and Calabrian arc, to the rifted NW Calabrian margin, where the Quaternary Aeolian arc is emplaced. The margin, then, transitions northwards into the Marsili backarc region, where exhumed mantle and localized volcanism occurred during its formation. This complex structure implies rapid temporal and spatial changes between magmatic and amagmatic processes, and between compressional and extensional regimes during the evolution of this subduction system. We find that some terranes involved in the Alpine orogeny share petrological and tectonic similarities with some domains of the Calabrian subduction system. Based on the results of this study we propose the Calabrian Arc system as an analog for the subduction structuration that preceded the formation of Alpine orogenic systems.

Description: Published

Description: 116480

Description: 1T. Struttura della Terra

Description: JCR Journal

Description: Highlights • 2-D velocity models at the highest slip patch during the Chilean 2010 Mw 8.8 earthquake. • The highest slip patch correlates with large accretionary prisms. • The highest slip patch correlates with low continental slope angles. • A similar pattern is observed along the giant 1960 Mw 9.5 earthquake rupture area. Abstract Subduction megathrust earthquakes show complex rupture behaviour and large lateral variations of slip. However, the factors controlling seismic slip are still under debate. Here, we present 2-D velocity-depth tomographic models across four trench-perpendicular wide angle seismic profiles complemented with high resolution bathymetric data in the area of maximum coseismic slip of the 8.8 Maule 2010 megathrust earthquake (central Chile, 34°–36°S). Results show an abrupt lateral velocity gradient in the trench-perpendicular direction (from 5.0 to 6.0 km/s) interpreted as the contact between the accretionary prism and continental framework rock whose superficial expression spatially correlates with the slope-shelf break. The accretionary prism is composed of two bodies: (1) an outer accretionary wedge (5–10 km wide) characterized by low seismic velocities of 1.8–3.0 km/s interpreted as an outer frontal prism of poorly compacted and hydrated sediment, and (2) the middle wedge (∼50 km wide) with velocities of 3.0–5.0 km/s interpreted as a middle prism composed by compacted and lithified sediment. In addition, the maximum average coseismic slip of the 2010 megathrust event is fairly coincident with the region where the accretionary prism and continental slope are widest (50–60 km wide), and the continental slope angle is low (〈5°). We observe a similar relation along the rupture area of the largest instrumentally recorded Valdivia 1960 9.5 megathrust earthquake. For the case of the Maule event, published differential multibeam bathymetric data confirms that coseismic slip must have propagated up to ∼6 km landwards of the deformation front and hence practically the entire base of the middle prism. Sediment dewatering and compaction processes might explain the competent rheology of the middle prism allowing shallow earthquake rupture. In contrast, the outer frontal prism made of poorly consolidated sediment has impeded the rupture up to the deformation front as high resolution seismic reflection and multibeam bathymetric data have not showed evidence for new deformation in the trench region.

Description: Highlights • A serpentinised peridotite basement is strongly supported by S-waves analysis • Depth dependent serpentinisation resembles to that observed at magma-poor margins. • Mantle exhumation was preceded by MOR-type magmatism and later intraplate volcanism. Summary The Tyrrhenian basin opened in the Neogene following the E–SE retreat of the Appenines–Calabrian subduction system and the subsequent back-arc extension of an orogenic crust. The resultant crustal structure includes a complex distribution of continental, back-arc magmatism, and mantle-exhumation domains. A clear example of this complex structure is found in the central and deepest part of the basin (i.e. Magnaghi–Vavilov sub-basin) where geophysical data supported that the bulk of the basement is composed of partially serpentinised peridotite representing exhumed mantle rocks, and intruded by basalts forming low ridges and volcanic edifices. However, those data sets cannot univocally demonstrate the widespread presence of serpentinised mantle rocks, let alone the percentage of serpentinisation. Here, we use S-wave arrivals and available geological information to further constrain the presence of mantle serpentinisation. Travel times of converted S-waves were used to derive the overall Vp/Vs and Poisson's ratio (σ), as well as S-wave velocity of the basement in the Magnaghi-Vavilov Basins. This analysis reveals Vp/Vs ≈ 1.9 (σ ≈ 0.3) that strongly supports a serpentinised peridotite forming the basement under the basins, rather than oceanic-type gabbro/diabase. P-wave velocity models is later used to quantify the amount of serpentinisation from fully serpentinised (up to 100%) at the top of the basement to 〈 10% at 5–7 km deep, with a depth distribution similar to continent–ocean Transition zones at magma-poor rifted margins. Seismic reflection profiles show normal faulting at either flank of the Magnaghi–Vavilov Basin that is potentially responsible for the onset of serpentinisation and later mantle exhumation. These results, together with basement sampling information in the area, suggests that the late stage of mantle exhumation was accompanied or soon followed by the emplacement of MOR-type basalts forming low ridges that preceded intraplate volcanism responsible for the formation of large volcanoes in the area.

Description: A temporary passive seismic network of 21 broad-band stations was deployed in Central Chile between 35°S to 36°S. The network recorded data prior to the magnitude Mw = 8.8 2010 Maule earthquake at a latitude of the major slip and surface deformation. The experiment was conducted to survey crustal and mantle structures and to assess the state of hydration of the mantle wedge. We present results of a teleseismic P receiver function study, supporting a continental Moho at approx. 38 km depth. Phase conversion at this boundary could be observed continuously from the intersection of the subducting slab with the continental Moho towards the Andes. The character of receiver functions indicated little evidence for infiltration of water from the subducting plate into the overlying mantle wedge, suggesting that only a small amount of water is released from the subducting plate. Aftershocks of the Maule earthquake and post-seismic slip reached depths of 50 km and hence slip spread down-dip of the continental Moho in the post-seismic phase. Co-seismic rupture, however, occurred updip of the continental Moho. Spare aftershock seismicity is observed at the intersection of the continental Moho with the subducting slab.

Description: Heat flow anomalies provide critical information in active tectonic environments. The Gulf of Cadiz and adjacent areas are affected by the plate convergence between Africa and Europe, causing widespread deformation and faulting. Active thrust faults cause lateral movement and advection of heat that produces systematic variations in surface heat flow. In December 2003 new heat flow data were collected during the research vessel Sonne cruise SO175 in the Gulf of Cadiz over two sites of recent focused research activity: (i) the Gulf of Cadiz sedimentary prism and (ii) the Marques de Pombal escarpment. Both features have also been discussed as potential source areas of the Great Lisbon earthquake and tsunami of 1755. Background heat flow at the eastern terminus of the Horseshoe abyssal plain is about 52–59 mW/m2. Over the Gulf of Cadiz prism, heat flow decreases from ∼57 mW/m2 to unusually low values of 45 mW/m2 roughly 120 km eastward. Such low values and the heat flow trend are typical for active thrusting, supporting the idea of an east-dipping thrust fault. Slip rates are 10 ± 5 mm per year, assuming that the fault dips at 2°. A fault dipping at 5°, however, would result into slip rates of 1.5–5 mm per year, suggesting that subduction has largely ceased. Based on seismic data, the Marques de Pombal fault is interpreted as part of an active fault system located ∼100 km westward of Cape San Vincente. Heat flow over the fault is affected by refraction of heat caused by the 1 km high escarpment. Thermal models suggest that the slip rate along the fault must either be small or shear stresses acting on the fault are rather high. With respect to other fault zones, however, it is reasonable to assume that the fault's slip rate is small.

Description: EGU2011-12780 A temporary passive seismic network of 31 broad-band stations was deployed in the region around Talca and Constitución between 35°S to 36°S latitude and 71°W to 72.5°W longitude. The network was operated between March and October 2008. Thus, we recorded data prior the magnitude Mw=8.8 earthquake of 27 February 2010 at a latitude of the major slip and surface uplift. The experiment was conducted to address fundamental questions on deformation processes, crustal and mantle structures, and fluid flow. We present results of a teleseismic P receiver function study that covers the coastal region and reaches to the Andes. The aim is to determine the structure and thickness of the continental crust and constrain the state of hydration of the mantle wedge. The P-wave receiver function technique requires large teleseismic earthquakes from different distances and backazimuths. A few percent of the incident P-wave energy from a teleseismic event will be converted into S-wave (Ps) at significant and relatively sharp discontinuities beneath the station. A small converted S phase is produced that arrives at the station within the P wave coda directly after the direct P-wave. The converted Ps phase and their crustal multiples contain information about crustal properties, such as Moho depth and the crustal vp/vs ratio. We use teleseismic events with magnitudes mb 〉 5.5 at epicentral distances between 30° and 95° to examine P-to-S converted seismic phases. Our preliminary results provide new information about the thickness of the continental crust beneath the coastal region in Central Chile. At most of the stations we observed significant energy from P to S converted waves between 4 and 5 s after the direct P-wave within a positive phase interpreted as the Moho, occurring at 35 to 40 km. The great Maule earthquake of 27 February 2010 nucleated up-dip of the continental Moho. The rupture of this earthquake seems to have propagated down-dip of the Moho. The Moho reflection show a positive polarity, indicating that the mantle is either dry or only moderately hydrated. We observed converted energy from an intracrustal boundary at around 2 s that disappears near the coast. Further, positive polarity peaks occur that are possibly caused by the down going plate.

Description: The Chile Triple Junction (CTJ) is the place where the Chile Ridge (Nazca–Antarctic spreading center) is subducting beneath the continental South American plate. Sediment accretion is active to the south of the CTJ in the area where the northward migrating Chile Ridge has collided with the continent since 14 Ma. At the CTJ, tectonic erosion of the overriding plate narrows and steepens the continental slope. We present here a detailed tomographic image of the upper lithospheric Antarctic–South America subduction zone where the Chile Ridge collided with the continent 3–6 Ma off Golfo de Penas. Results reveal that a large portion of trench sediment has been scraped off and frontally accreted to the forearc forming a 70–80 km wide accretionary prism. The velocity–depth model shows a discontinuity at 30–40 km landward of the deformation front, which is interpreted as the contact between the frontal (poorly consolidated sedimentary unit) and middle (more compacted sedimentary unit) accretionary prism. The formation of this discontinuity could be related to a short term episode of reduced trench sedimentation. In addition, we model the shape of the continental slope using a Newtonian fluid rheology to study the convergence rate at which the accretionary prism was formed. Results are consistent with an accretionary prism formed after the collision of the Chile Ridge under slow convergence rate similar to those observed at present between Antarctic and South America (∼2.0 cm/a). Based on the kinematics of the Chile Ridge subduction during the last 13 Ma, we propose that the accretionary prism off Golfo de Penas was formed recently (∼5 Ma) after the collision of the Chile Ridge with South America.

Description: The Gulf of Cadiz and the passive continental margin of southern Iberia to the west of the Strait of Gibraltar locally accommodate the presently ongoing convergence between Africa and Eurasia by widespread, rather diffusive, seismic activity. Seismicity of the northern Gulf of Cadiz was derived from an amphibious seismological network, including 24 temporary marine offshore stations, besides the permanent networks in Portugal, Spain, and Morocco. During the 6 month of the offshore network operation, in total 86 local earthquakes were located at six or more offshore stations with the majority of earthquakes occurring to the southwest of Iberia and along the Algarve continental margin off southern Iberia. The distribution of events along the Algarve margin mimics features reported for the Atlantic passive continental margins of both South and North America. Focal mechanisms at the Portimão Bank support that seismically active areas are associated with compression. Similar stress patterns are reported for the east coast of South and North America. However, while earthquakes along the American east coast occur at crustal levels, earthquakes in the northern Gulf of Cadiz occur both in the lower crust and upper mantle, with the majority of events rupturing within the mantle, including a number of well-located earthquakes beneath crust forming the continent-ocean transition zone. The large number of earthquakes in the mantle might be caused by the unique geological setting, where deformation occurs in cool lithosphere of Mesozoic age. We suggest that seismicity along the Algarve margin is caused by re-activation of pre-existing margin-parallel faults rather than corresponding to newly formed structures related to a new developing plate boundary.

Description: Mud extrusion is frequently observed as a dewatering phenomenon in compressional tectonic settings such as subduction zones. Along the Middle American Trench, several of these features have been recently discovered. This paper presents a heat flow study of actively venting Mound Culebra, offshore Nicoya Peninsula, and is complemented by data from geophysical surveys and coring. The mud diapir is characterised by methane emission and authigenic carbonate formation at its crest, and is composed of overconsolidated scaly clays and clast-bearing muds. Compared with the conductive background heat flow, the flux through the mud dome is elevated by 10–20 mW/m2, possibly related to advection of heat by fluids rising from greater depth. Decreased chlorinity in the pore waters from gravity cores may support a deep-seated fluid origin. Geothermal measurements across the mound and temperature measurements made with outriggers on gravity corers were corrected for the effects of thermal refraction, forced by the topography of the mound. Corrected values roughly correlate with the topography, suggesting advection of heat by fluids rising through the mound, thereby generating the prominent methane anomaly over the dome and nurturing vent biota. However, elevated values occur also to the southeast of the mound. We believe that the overconsolidated clays and carbonates on the crest form an almost impermeable lid. Fluids rising from depth underneath the dome are therefore partially channelled towards the flanks of the mound.

Description: The 27 February, 2010 Maule earthquake (Mw=8.8) ruptured ~400 km of the Nazca-South America plate boundary and caused hundreds of fatalities and billions of dollars in material losses. Here we present constraints on the fore-arc structure and subduction zone of the rupture area derived from seismic refraction and wide-angle data. The results show a wedge shaped body ~40 km wide with typical sedimentary velocities interpreted as a frontal accretionary prism (FAP). Landward of the imaged FAP, the velocity model shows an abrupt velocity-contrast, suggesting a lithological change which is interpreted as the contact between the FAP and the paleo accretionary prism (backstop). The backstop location is coincident with the seaward limit of the aftershocks, defining the updip limit of the co-seismic rupture and seismogenic zone. Furthermore, the seaward limit of the aftershocks coincides with the location of the shelf break in the entire earthquake rupture area (33°S–38.5°S), which is interpreted as the location of the backstop along the margin. Published seismic profiles at the northern and southern limit of the rupture area also show the presence of a strong horizontal velocity gradient seismic backstop at a distance of ~30 km from the deformation front. The seismic wide-angle reflections from the top of the subducting oceanic crust constrain the location of the plate boundary offshore, dipping at ~10°. The projection of the epicenter of the Maule earthquake onto our derived interplate boundary yielded a hypocenter around 20 km depth, this implies that this earthquake nucleated somewhere in the middle of the seismogenic zone, neither at its updip nor at its downdip limit.