Philippe Charvis

Are rupture zone limits of great subduction earthquakes controlled by upper plate structures? Evidence from multichannel seismic reflection data acquired across the northern Ecuador–southwest Colombia margin

[1]xa0Subduction of the Nazca plate beneath the Ecuador-Colombia margin has produced four megathrust earthquakes during the last century. The 500-km-long rupture zone of the 1906 (Mw = 8.8) event was partially reactivated by three thrust events, in 1942 (Mw = 7.8), 1958 (Mw = 7.7), and 1979 (Mw = 8.2), whose rupture zones abut one another. Multichannel seismic reflection and bathymetric data acquired during the SISTEUR cruise show evidence that the margin wedge is segmented by transverse crustal faults that potentially correlate with the limits of the earthquake coseismic slip zones. The Paleogene-Neogene Jama Quininde and Esmeraldas crustal faults define a ∼200-km-long margin crustal block that coincides with the 1942 earthquake rupture zone. Subduction of the buoyant Carnegie Ridge is inferred to partially lock the plate interface along central Ecuador. However, coseismic slip during the 1942 and 1906 earthquakes may have terminated against the subducted northern flank of the ridge. We report on a newly identified Manglares crustal fault that cuts transversally through the margin wedge and correlates with the limit between the 1958 and 1979 rupture zones. During the earthquake cycle the fault is associated with high-stress concentration on the plate interface. An outer basement high, which bounds the margin seaward of the 1958 rupture zone, may act as a deformable buttress to seaward propagation of coseismic slip along a megathrust splay fault. Coseismic uplift of the basement high is interpreted as the cause for the 1958 tsunami. We propose a model of weak transverse faults which reduce coupling between adjacent margin segments, together with a splay fault and an asperity along the plate interface as controlling the seismogenic rupture of the 1958 earthquake.

Western Hellenic subduction and Cephalonia Transform: local earthquakes and plate transport and strain

Abstract Focal parameters of local earthquakes in the region of the Ionian Islands of western Greece are constrained with a temporary dense array of three-component seismographs operated jointly offshore and onshore. Seismic deformation is documented to be confined to the east of the N20°E-striking steep continental slope west of Cephalonia island, the right-lateral Cephalonia Transform Fault, CTF, inferred from large earthquakes. The pre-Apulian continental material appears to be only deforming east of the transform fault, where it is in upper plate position to the Hellenic subduction. East of the transform fault, the transmission velocity tomography from local earthquakes, compared in depth-section with a previous marine reflection profile, provides evidence in support of a shallow landward dipping boundary around 12xa0km deep under the Ionian Islands along which they may override the lower plate. On either side of this interface local earthquakes occur with different focal mechanisms, in support with its interpretation as the interplate. Under Cephalonia island, reverse-faulting deforms the upper plate along NW–SE structures, which may also be affected by left-lateral bookshelf-faulting. Small earthquakes show normal faulting along the western coast of Cephalonia and its extension 20xa0km SSW, the trace of the CTF as inferred from the occurrence of the large strike-slip earthquakes. Another group of normal-fault earthquakes locates in the lower plate from under Cephalonia to Zante, just outboard of a possible change of interplate dip suggested from reflection seismics landward under the islands. These normal-fault earthquakes appear to coincide in position with that of the load imposed by the upper plate transported over them, rather than occurring in an outer rise, outboard the plate boundary and trench, as observed in other subductions and attributed to the control by the flexural bending of the lower plate under the pull of the sinking slab. Interpretation has to consider several peculiar features of plate interaction in western Greece with respect to a steady-state model for major subduction zones, in particular: a fast deformation of the upper plate in front of an orogenically overthickened crust and of the southwestward push of extruding Anatolia; its transport, which is the cause of the migration of the plate boundary rather than the roll-back of a slab which has been proposed to be detached; possibly a flat and ramp shape of the interplate; the geometrical complexity of the shear limit across the CTF between subduction and collision, and the nearby variation of the nature of the foreland crust.

Spatial distribution of hotspot material added to the lithosphere under La Réunion, from wide-angle seismic data

Wide-angle seismic lines recorded by ocean bottom and land seismometers provide a pseudo three-dimensional investigation of the crust and upper mantle structure around the volcanically active hotspot island of La Reunion. The submarine part of the edifice has fairly low seismic velocities, without evidence for intrusives. An upper unit with a velocity-depth gradient is interpreted as made of material erupted subaerially then transported and compacted downslope. Between this unit and the top of the oceanic plate, imaged by normal incidence seismic reflection, a more homogeneous unit indicated by shadow zones on several wide-angle sections may correspond to lavas of a different nature, extruded underwater in the earlier phase of volcanism. Coincident wide angle and normal incidence reflections document that the oceanic plate is not generally downwarping toward the island but doming instead toward its southeastern part, with limited evidence for some intracrustal intrusion. Deeper in the lithosphere, the presence of a layer of intermediate velocity between the crust and mantle is firmly established. It is interpreted as resulting from the advection of hotspot magmatic products, possibly partially molten, and of a composition for which the crust is a density barrier. The extensive wide-angle coverage constrains the extent of this body. It does not show the elongated shape expected from plate drift above a steady hotspot supply. Alternative propositions can hence be considered, for example, that La Reunion is caused by a solitary wave of hotspot material or by a young hotspot. The size of the underplate, 140 km wide and up to 3 km thick, corresponds to less than half the volume of the edifice on top of the plate.

Crustal thickness constraints on the geodynamic evolution of the Galapagos Volcanic Province

We developed a simple quantitative framework based on crustal thickness estimations along the Carnegie, Cocos and Malpelo ridges, to place first-order constraints on the tectonic evolution of the Galapagos Volcanic Province and on the along-axis intensity of the Galapagos melt anomaly during the last 20 m.y. Our results suggest that the Cocos^ Nazca spreading centre has migrated northwards at 26 2 4 km/m.y. with respect to the Galapagos hotspot (GHS) during this period of time. At V20 m.y., the GHS was approximately ridge-centered, and thus the along-axis intensity of the melt anomaly at this time was the maximum. At V11.5 m.y. the hotspot was located 106 2 27 km north of the spreading center, and the along-axis intensity of the melt anomaly was 0.54 2 0.04 of that estimated at 20 Ma. At present day it is located at V190 km south of the spreading center and the along-axis intensity is only 0.19 2 0.03 of that estimated at 20 Ma. These results are used to reconstruct the relative position between the GHS and the Cocos^ Nazca Spreading Center. The spreading center passed over the GHS for the last time at 7.4 2 1.3 Ma. The Panama fracture zone was initiated at 8.9 2 1.6 Ma, leading to the separation between the Cocos and Malpelo ridges. The present configuration of the Galapagos Volcanic Province and the plate velocities are consistent with symmetric spreading with a mean full spreading rate of V60 km/m.y. along the CNSC during the last 20 m.y. A melt flux for excess crustal production of 9.4 2 5.1m 3 /s is obtained for the Galapagos melt anomaly at 20 Ma, implying that the maximum potential intensity of the Galapagos plume is similar to that of the Icelandic plume and twice smaller than

Deep structure of the southern Kerguelen Plateau (southern Indian Ocean) from ocean bottom seismometer wide-angle seismic data

Wide-angle seismic data collected during the Kerguelen ocean bottom seismometer experiment provide the first images of the deep structure of the southern Kerguelen Plateau and support a new interpretation of the origin of the plateau. Velocity models based on travel time inversions and reflectivity synthetic seismograms show a 22-km-thick crust composed of ∼1.6 km of sedimentary cover, ∼5.3 km of upper crust, ∼11.0 km of lower crust, and a 4- to 6-km-thick reflective zone immediately above Moho. Velocities in the upper crust (from 3.8–4.5 km/s at top to 6.0–6.5 km/s at bottom) are consistent with the basaltic nature of this layer, the top of which was sampled during the Ocean Drilling Program. Velocities in the lower crust increase continuously from 6.60 km/s at the top to 6.90 km/s at 19.5 km depth. The reflective zone at the base of the crust identified by wide-angle reflections is observed only along the NNW-SSE direction. It consists of alternating high- and low-velocity layers with an average velocity of 6.70 km/s in the NNW-SSE direction and ≥6.90 km/s in the perpendicular direction. Strong azimuthal anisotropy is also observed in the upper mantle with velocities of 8.60 and 8.00 km/s, in the NNW-SSE and E-W directions, respectively. The absence of high velocities at the base of the crust that characterizes many large-volume mafic provinces, the reflective lower crust, and anisotropy in upper mantle suggest that the southern Kerguelen Plateau represents a stretched continental fragment overlain by basaltic flows isolated from the Antarctic margin during the early opening of the Indian Ocean.

Vertical movements and material transport during hotspot activity: Seismic reflection profiling offshore La Réunion

The structure of the submerged part of the La Reunion hotspot island is determined by a grid of multichannel seismic reflection profiles. The submarine part of the edifice appears as a poorly stratified wedge of material lying above a significant thickness of preexisting sediments and the oceanic basement. The dense data coverage has allowed us to derive contour maps of the top of the basement and of the base of the volcaniclastic edifice, further constrained by coincident wide-angle profiles. The resulting isobath maps reveal new, unsuspected features that could not be deduced from observation along a single seismic line since the geometry of these horizons varies significantly from one radial profile to the next. Both maps show a large degree of heterogeneity in the topography, with no axial nor cylindrical symmetry, indicating that plate flexure is not dominant. A slight depression toward the island is observed only in the southern area, ahead of the hotspot trace. The lack of angular unconformity in the volcano-sedimentary pile that covers the oceanic basement firmly establishes the lack of significant vertical movement and flexure. The base of the edifice is roughly domed, centered on the island, with several topographic highs or lows superimposed. The submarine apron appears as a composite constructional body, spreading by slumping of its flanks. Superficial lenses of laterally transported material are observed on the seismic data south of the island, not only to the east of the active Piton de la Fournaise volcano. Oceanic sediments trapped beneath the apron seem undeformed.

Kerguelen plateau : a volcanic passive margin fragment ?

The 21–25-km-thick crust of the southern Kerguelen Plateau consists of three units: (1) a ≤ 2.3-km-thick sedimentary cover; (2) a 3–6-km-thick basaltic layer with velocities ranging from 4.5 to 6.2 km/s; and (3) a 15–17-km-thick lower crust with velocities from 6.6 to 6.9 km/s, including a 3–6-km-thick transition zone located at the base of the crust. The low-velocity transition zone has an average velocity of 6.7 km/s and exhibits several internal wide-angle reflections. The velocity-depth structure of the crust differs significantly from that of other hotspot-related oceanic plateaus and suggests that the southern Kerguelen Plateau may be a fragment of a volcanic passive margin composed of a thinned continental crust overlain by basalt flows.

Deep structures of the Ecuador convergent margin and the Carnegie Ridge, possible consequence on great earthquakes recurrence interval

[1]xa0The deep structure of the Ecuador subduction zone and adjacent Carnegie Ridge (CR) was investigated using on-shore off-shore wide-angle seimics. A crustal model obtained by 2-D inversion of traveltimes reveals the overthickened (14 km) oceanic crust of the CR that underthrusts the high velocity (>6 km/s) basement of the upper plate margin wedge, interpreted as part of the accreted oceanic terranes described on-shore. The plate interface dips 4° to 10° east from the trench to a depth of 15 km. Shadow zones observed on the margin OBS records are interpreted as a low velocity zone consisting of a thin layer of underthrust sediments, and the 3-km-thick CR layer 2, whose average seismic velocity of 5.1 km/s is slower than that of the margin wedge. Increased interplate coupling related to the subduction of the thick, buoyant CR may account for an apparent local increased recurrence interval between great interplate earthquakes.

Structure and development of a microcontinent: Elan Bank in the southern Indian Ocean

Microcontinents appear to commonly form on young continental margins close to hot spots, but difficulties in understanding their geology and evolution have inhibited assessment of their global distribution and significance. Thick volcanic accumulations in areas affected by hot spot magmatism only complicate the issue. Elan Bank, a large western salient of the Kerguelen Plateau, is a microcontinent that originally lay between India and Antarctica in Gondwana. Recent regional plate tectonic reconstructions suggest that during Gondwana breakup, Elan Bank and India initially separated from Antarctica, and Elan Bank became isolated in the Southern Ocean via a ridge jump to the north between Elan Bank and India. In Albian time (∼108 Ma), voluminous magmatism attributed to the Kerguelen hot spot overprinted and radically altered the original microcontinent and its surroundings. Recent ODP investigations, deep seismic reflection data, and a wide-angle seismic line on Elan Bank allow us to gain the first insight into the features integrated crustal structure and geological evolution and the adjacent continent-ocean transition zone. Our analysis shows that Elan Banks crust is at least 16 km thick. The upper igneous crust consists of a 2–3 km thick layer with seismic velocities ranging from 4.4 to 5.9 km/s that can be interpreted as the result of accumulation of lava flows originating from the Kerguelen hot spot. Seismic velocities at the base of the crust are as low as 6.6 km/s, which is consistent with a fragment of thinned continental crust ∼14 km thick. A high velocity body, located at depths of 5 to 10 km, could be interpreted as plutonic rocks emplaced during the major regional magmatic episode. On the basis of deep seismic reflection data, we interpret extensional structures beneath the volcanic flows. In Albian time, when the area was affected by the Kerguelen hot spot, volcaniclastic material and lava flows accumulated in faulted grabens and basins both on the bank and within the continent-ocean transition zone to the south, creating the appearance of flat, unstructured basement. The seismic structure and inferred composition of Elan Bank revealed by this study contribute to our understanding of microcontinent formation as well as provide a template for identifying microcontinents in accreted terranes and mountain belts.

The 2010 Haiti earthquake: A complex fault pattern constrained by seismologic and tectonic observations

After the January 12, 2010, Haiti earthquake, we deployed a mainly offshore temporary network of seismologic stations around the damaged area. The distribution of the recorded aftershocks, together with morphotectonic observations and mainshock analysis, allow us to constrain a complex fault pattern in the area. Almost all of the aftershocks have a N‐S compressive mechanism, and not the expected left‐lateral strike‐slip mechanism. A first‐order slip model of the mainshock shows a N264°E north‐dipping plane, with a major left‐lateral component and a strong reverse component. As the aftershock distribution is sub‐parallel and close to the Enriquillo fault, we assume that although the cause of the catastrophe was not a rupture along the Enriquillo fault, this fault had an important role as a mechanical boundary. The azimuth of the focal planes of the aftershocks are parallel to the north‐dipping faults of the Transhaitian Belt, which suggests a triggering of failure on these discontinuities. In the western part, the aftershock distribution reflects the triggering of slip on similar faults, and/or, alternatively, of the south‐dipping faults, such the Trois‐Baies submarine fault. These observations are in agreement with a model of an oblique collision of an indenter of the oceanic crust of the Southern Peninsula and the sedimentary wedge of the Transhaitian Belt: the rupture occurred on a wrench fault at the rheologic boundary on top of the under‐thrusting rigid oceanic block, whereas the aftershocks were the result of the relaxation on the hanging wall along pre‐existing discontinuities in the frontal part of the Transhaitian Belt.