Multi-channel Seismic Reflection Survey Research Articles

Offshore Fault-related Fold along the Eastern Marginal Zone of Ishikari Lowland, Hokkaido, Japan

Distributions, characteristics, and activities of offshore fault-related folds which comprise the southern part of the Eastern Boundary Fault Zone of Ishikari Lowland (EBFIL) are clarified by conducting a high-resolution multi-channel seismic reflection survey. To determine termination point of the fault zone, we placed seven survey lines perpendicular to the fault-related folds with a total line length of 107.5 km. We choose a compact water gun (15 cubic inch) as the seismic source to obtain high-resolution imagery of the Late Quaternary and investigate activity of the fault zone. Based on seismic facies, we distinguished packages of offshore sediment (unit 2, 3 and 4, in descending order), which separated by downlap surfaces and correlated to the Middle to the Late Pleistocene. They are progressively deformed by the active fault and compose series of fault-related folds (Fo1, Fo2 and Fo3). Traces of these folds are figured out from the seismic sections obtained by this study and existing surveys. Deformation related to these folds are clearly recognized in the topography of the unit 4 base. The reliefs of strata indicate that the Fo2 fold should have uplifted seafloor at a rate of 0.09-0.20 m/ky since 300 ka. The structures, spatial distributions and growth-rates of the folds suggest that Fo1 and Fo2 should compose series of anticline developing at the front of EBFIL. Besides, Fo2 fold would compose the southern termination of the fault zone. This study has successfully demonstrated that densely operated high-resolution seismic survey can figure out distribution and activity of offshore active faults in the Late Quaternary, even at the outside of shelf.

Heat flow and 2D multichannel seismic reflection survey of the Devil's Hole geothermal reservoir in the Wagner basin, northern Gulf of California

The Wagner basin, located in the northernmost Gulf of California, hosts a large reservoir with geothermal potential documented in recent heat flow surveys. Although there is evidence of heat generation above the average value for an oceanic crust in the Wagner basin, it is unclear what the heat source is. To better understand the thermal structure in the northern Gulf of California, we acquired four 2D multichannel seismic reflection profiles and two systematic heat flow profiles across the Wagner basin. The survey targeted a geothermal anomaly, which we denominated “Devil's Hole” in a clear reference to pockmarks on the seafloor, and very high heat flow values. The heat flow profiles are ∼12 and ∼9 km long, are sub perpendicular to each other, have a nominal measurement spacing of ∼1 km, and were located along the transects of two of the four seismic reflection profiles. The two remaining seismic profiles are ∼34 and ∼40 km long, respectively, and are oriented SWW-NEE. To obtain seismic images, we used a conventional seismic processing workflow. After heat flow data correction, the minimum, maximum, and standard deviation of heat flow values for the Wagner basin are 295±42 and 10,894±114 mW/m2. We interpret the relatively high heat flow and large variability in the central part of the acquired profiles is caused by upward fluid flow at the Wagner fault zone. In contrast, the lower heat flow in the western flank of the basin suggest conductive heat transfer given the absence of faults. Based on these observations, we develop a geological model in which venting is not the product of a nascent spreading center as other authors have suggested. Instead, hydrothermal activity appears to result from isolated thermal plumes, rising from the base of the sedimentary column, captured by permeable fracture systems.

Possible fluid discharge associated with faults observed by a high-resolution dense-2D\xa0seismic reflection survey in Uchiura Bay off Numazu, Japan

Although fluid behavior along faults has often been associated with earthquakes and hydrocarbon accumulation, few observations of active, ongoing fluid discharge along the faults exist. We conducted a dense-2D multi-channel seismic (MCS) reflection survey in Uchiura Bay off Numazu, Japan, in 2018 to obtain a high-resolution image of active faults. As a result, active faults cutting the seafloor and possibly active faults cutting reflections immediately below the seafloor have been clearly imaged on MCS profiles. The spatial variation in their displacement on the seafloor has also been quantified. The seafloor displacement of the most obvious active fault, Fault A, changes by more than 2 m within a short horizontal distance of 50 m. On one of the MCS profiles, an enigmatic wavefield (EWF) is observed within the water column above the seafloor near the faults. From careful observations of the raw field data and shot records, the EWF shows three patterns that suggest fluid flow. Based on a velocity analysis of the seawater, the velocity of the EWF zone was somewhat higher than that of its surroundings. Because the seawater velocity is a function of temperature and salinity at the same depth, the effect of temperature is generally greater than that of salinity, so that the higher velocity would suggest a higher temperature. If temperature of the fluid flow is higher than that of its surroundings, the fluid would rise with its buoyancy and spread out along the sea surface. This scenario could well explain the three EWF patterns. From the appearance and non-appearance of the most obvious EWF on adjacent seismic lines, some fluid discharge might have been occurring intermittently from the seafloor near the faults in Uchiura Bay.

Magma-hydrothermal interactions at the Costa Rica Rift from data collected in 1994 and 2015

We use co-located CTD/transmissometry casts and multichannel seismic reflection surveys conducted at the Costa Rica Rift (CRR) to provide a better understanding of magma-hydrothermal processes occurring at an intermediate-rate spreading center. Water column observations reveal an ∼200 m thick plume head ∼650 m above the seafloor, which corresponds to a hydrothermal heat output of ∼200 ± 100 MW at the ridge axis. Assuming a hydrothermal vent temperature of 350°C and a discharge area between 104 and 105 m2, this heat output implies a mean crustal permeability within the discharge zone of between 2×10−14 and 6×10−13 m2, and a conductive thermal boundary layer thickness of ∼20 m. The volume of magma required to maintain the current hydrothermal heat output over the past two decades should result in an across-axis axial magma lens (AML) width between 270 and 1300 m, depending on the amount of cooling and crystallization. However, seismic reflection images, acquired in 1994 and 2015, while showing an apparent along-axis growth of the AML from 2.4 to 6.0 km between surveys, also suggest that, as of 2015, the AML has an apparent across-axis width of no more than 300 m, and that magma delivery at the intermediate spreading rate CRR may be episodic on time scales of tens of years. The data on magma-hydrothermal interactions at the CRR collected in 1994 and 2015 suggest that the hydrothermal system may have significantly cooled and crystallized the AML, primarily in the across-axis direction, and that this hydrothermal system may also episodically turn on and off. The current pattern of microseismicity supports this conclusion, with events not only mirroring the AML depth and location beneath the ridge axis, but also having a temporally varying focus.

MULTI-CHANNEL MARINE SEISMIC SURVEY AND PRELIMINARY RESULTS OFF-SHORE KARABURUN RIDGE (WESTERN SIGACIK GULF-AEGEAN SEA)

Sıgacık Gulf is located south of Izmir (West Anatolia), the third crowded city of Turkey. The gulf is restricted by two important ridges on land from west to east, Karaburun Ridge and Seferihisar Ridge, respectively. Recent geological and geophysical studies suggested that these ridges are dominated by active strike-slip faults both on land and off-shore. This manuscript presents the results of the high resolution multi-channel seismic reflection survey which carried out onboard K. Piri Reis, research vessel of Dokuz Eylül University, off-shore Karaburun Ridge (Aegean Sea) and surroundings in August-2005. Two main seismic units, lower unit and upper unit, were determined on seismic sections besides the active faults, buried faults and buried folds.

Active-source seismic survey on the northeastern Hawaiian Arch: insights into crustal structure and mantle reflectors

Seismic refraction and multi-channel seismic reflection surveys were conducted on the northeastern Hawaiian Arch to examine the effect of hotspot volcanism on the seismic structure of the crust and uppermost mantle. The crustal thickness deduced from the refraction data was typical for oceanic crust, which suggests that magmatic underplating does not occur, at least in our survey area, although the crustal seismic velocity may be influenced by flexure of the lithosphere on the arch. We identified high P-wave velocities (~ 8.65 km/s) in the uppermost mantle parallel to the paleo-seafloor spreading direction, which indicates that the shallower mantle structure immediately below the Moho preserves the original structure formed at a mid-ocean ridge. Moreover, we observed wide-angle reflection waves at large offsets in ocean-bottom seismometer records. The travel time analysis results showed that these waves were reflected from mantle reflectors at depths of 30–85 km below the seafloor, which are considered to represent heterogeneities consisting of frozen melts created during the cooling of the plate.

Giant rhyolite lava dome formation after 7.3 ka supereruption at Kikai caldera, SW Japan

Kikai submarine caldera to the south of the Kyushu Island, SW Japan, collapsed at 7.3 ka during the latest supereruption (>500 km3 of magma) in the Japanese Archipelago. Multi functional research surveys of the T/S Fukae Maru in this caldera, including multi-beam echosounder mapping, remotely operated vehicle observation, multi-channel seismic reflection survey, and rock sampling by dredging and diving, provided lines of evidence for creation of a giant rhyolite lava dome (~32 km3) after the caldera collapse. This dome is still active as water column anomalies accompanied by bubbling from its surface are observed. Chemical characteristics of dome-forming rhyolites akin to those of presently active small volcanic cones are different from those of supereruption. The voluminous post-caldera activity is thus not caused simply by squeezing the remnant of syn-caldera magma but may tap a magma system that has evolved both chemically and physically since the 7.3-ka supereruption.

Variations in seismic velocity distribution along the Ryukyu (Nansei-Shoto) Trench subduction zone at the northwestern end of the Philippine Sea plate

The Ryukyu (Nansei-Shoto) island arc-trench system, southwest of Japan, is formed by the subduction of the Philippine Sea (PHS) plate. Among the subduction zones surrounding the Japan Islands, the Ryukyu arc-trench system is unique in that its backarc basin, the Okinawa Trough, is the area with current extensively active rifting. The length of the trench is around 1400 km, and the geological and geophysical characteristics vary significantly along the trench axis. We conducted multichannel seismic (MCS) reflection and wide-angle seismic surveys to elucidate the along-arc variation in seismic structures from the island arc to the trench regions, shooting seven seismic lines across the arc-trench system and two along-arc lines in the island arc and the forearc areas. The obtained P-wave velocity models of the Ryukyu arc crust were found to be heterogeneous (depending on the seismic lines), but they basically consist of upper, middle, and lower crusts, indicating a typical island arc structure. Beneath the bathymetric depressions cutting the island arc—for example, the Kerama Gap and the Miyako Saddle—the MCS record shows many across-arc normal faults, which indicates the presence of an extensional regime along the island arc. In the areas from the forearc to the trench, the subduction of the characteristic seafloor features on the PHS plate affects seismic structures; the subducted bathymetric high of the Amami Plateau is detected in the northern trench: the Luzon–Okinawa fracture zone beneath the middle and southern trenches. There are low-velocity (<~4.5 km/s) wedges along the forearc areas, except for off Miyako-jima Island. The characteristic high gravity anomaly at the forearc off Miyako-jima Island is caused not by a bathymetric high of a large-scale accretionary wedge but by shallower materials with a high P-wave velocity of ~4.5 km/s.Graphical Position of the seismic survey lines in the Ryukyu arc-trench system (right) and P-wave velocity models for the lines perpendicular to the trench axis (left). The contour interval is 2.5 km/s. The vertical blue line shows the position of the trench axis.

The TOMO-ETNA experiment: an imaging active campaign at Mt. Etna volcano. Context, main objectives, working-plans and involved research projects

&lt;p&gt;The TOMO-ETNA experiment was devised to image of the crust underlying the volcanic edifice and, possibly, its plumbing system by using passive and active refraction/reflection seismic methods. This experiment included activities both on-land and offshore with the main objective of obtaining a new high-resolution seismic tomography to improve the knowledge of the crustal structures existing beneath the Etna volcano and northeast Sicily up to Aeolian Islands. The TOMO ETNA experiment was divided in two phases. The first phase started on June 15, 2014 and finalized on July 24, 2014, with the withdrawal of two removable seismic networks (a Short Period Network and a Broadband network composed by 80 and 20 stations respectively) deployed at Etna volcano and surrounding areas. During this first phase the oceanographic research vessel “Sarmiento de Gamboa” and the hydro-oceanographic vessel “Galatea” performed the offshore activities, which includes the deployment of ocean bottom seismometers (OBS), air-gun shooting for Wide Angle Seismic refraction (WAS), Multi-Channel Seismic (MCS) reflection surveys, magnetic surveys and ROV (Remotely Operated Vehicle) dives. This phase finished with the recovery of the short period seismic network. In the second phase the Broadband seismic network remained operative until October 28, 2014, and the R/V “Aegaeo” performed additional MCS surveys during November 19-27, 2014. Overall, the information deriving from TOMO-ETNA experiment could provide the answer to many uncertainties that have arisen while exploiting the large amount of data provided by the cutting-edge monitoring systems of Etna volcano and seismogenic area of eastern Sicily.&lt;/p&gt;

Structural characteristics of the Bayonnaise Knoll caldera as revealed by a high-resolution seismic reflection survey

The Bayonnaise Knoll caldera is a conical silicic caldera located on the eastern part of the back-arc rift zone of the Izu-Ogasawara arc. Many geological and geophysical surveys have shown that the Bayonnaise Knoll caldera contains hydrothermal sulfide deposits. The Japan Agency for Marine-Earth Science and Technology conducted high-resolution multi-channel seismic reflection surveys across the Bayonnaise Knoll caldera to ascertain details of the crustal structure, such as the configuration of faults around the caldera. A reflection profile of excellent quality was obtained by high-density velocity analysis at about 150-m intervals. We applied prestack depth migration by using the results of the high-density velocity analysis and further analyzed this region. The depth-migrated profile shows many faults, which correspond to bathymetric lineations, on the eastern side of the Bayonnaise Knoll caldera. The velocity structure of the Bayonnaise Knoll caldera resembles that of the Myojin Knoll caldera, which has been well surveyed and is associated with the hydrothermal deposit. The depth-migrated profile shows a clear reflective zone that is distributed asymmetrically to the Bayonnaise Knoll caldera center. These data suggest that caldera formation was controlled by back-arc rifting activity in the Izu-Ogasawara arc. The hydrothermal fluid migration path in the Bayonnaise Knoll caldera is estimated to be the result of faulting and magmatic intrusion on the eastern side of the structure. It is assumed that these fluids formed the Kuroko-type sulfide deposit in the eastern part of the caldera structure.

Deformation of the Manazuru Knoll in Sagami Bay, central Japan, associated with subduction of the Philippine Sea plate

In January 2010, we conducted a multichannel seismic (MCS) reflection survey in Sagami Bay. As a result of this study, the deformation of the Manazuru Knoll, which is located near the plate boundary, was obtained. The Manazuru Knoll was formed by an asymmetric anticline, and the knoll has a geometry that is bent in a shape similar to that of a crank. The anticlinal axis, which was confirmed by MCS data, lies along the anticlinal axis shown on the bathymetric map, and the axis is bent first to the southeast and then to the east. It is estimated that the easternmost part of Manazuru Knoll has reached the vicinity of Miura Canyon. The offset of the strike of the anticline axis is approximately 7 km. A reverse fault related to the formation of Manazuru Knoll was identified in the southwestern side of the knoll. It is hypothesized that this reverse fault formed as a result of shortening of the structure, which occurred when the relative motion of the Philippine Sea plate was acting in a perpendicular direction close to the Manazuru Knoll. Therefore, it is estimated that the relative motion of the Philippine Sea plate was almost oblique or parallel to the anticlinal axis of Manazuru Knoll and that the eastern end of Manazuru Knoll was bent into a crank shape by strike-slip motion. This suggests that a part of Manazuru Knoll, located to the west of the plate boundary, moved to the northwest. Finally, it is assumed that the sediments of Miura Canyon and Sagami Knoll have been overlapping on the eastern end of Manazuru Knoll.

Distribution and migration of aftershocks of the 2010 Mw 7.4 Ogasawara Islands intraplate normal-faulting earthquake related to a fracture zone in the Pacific plate

We describe the aftershocks of a Mw 7.4 intraplate normal-faulting earthquake that occurred 150 km east Ogasawara (Bonin) Islands, Japan, on 21 December 2010. It occurred beneath the outer trench slope of the Izu-Ogasawara trench, where the Pacific plate subducts beneath the Philippine Sea plate. Aftershock observations using ocean bottom seismographs (OBSs) began soon after the earthquake and multichannel seismic reflection surveys were conducted across the aftershock area. Aftershocks were distributed in a NW-SE belt 140 km long, oblique to the N-S trench axis. They formed three subparallel lineations along a fracture zone in the Pacific plate. The OBS observations combined with data from stations on Chichi-jima and Haha-jima Islands revealed a migration of the aftershock activity. The first hour, which likely outlines the main shock rupture, was limited to an 80 km long area in the central part of the subsequent aftershock area. The first hour activity occurred mainly around, and appears to have been influenced by, nearby large seamounts and oceanic plateau, such as the Ogasawara Plateau and the Uyeda Ridge. Over the following days, the aftershocks expanded beyond or into these seamounts and plateau. The aftershock distribution and migration suggest that crustal heterogeneities related to a fracture zone and large seamounts and oceanic plateau in the incoming Pacific plate affected the rupture of the main shock. Such preexisting structures may influence intraplate normal-faulting earthquakes in other regions of plate flexure prior to subduction.

日本海東部におけるマルチチャンネル反射法地震探査

日本海東部におけるマルチチャンネル反射法地震探査

Sedimentary evolution of Lake Van (Eastern Turkey) reconstructed from high-resolution seismic investigations

This paper presents results of a multi-channel seismic reflection survey at Lake Van and provides constraints on the sedimentary evolution of the lake. The geophysical data of the lake confirm the existence of three physiographic provinces: a shelf, a slope, and a deep, relatively flat basin. The most prominent features identified on the shelf and slope are clinoforms, submerged channels, as well as closely spaced lake floor depressions, reflecting a highly variable lake-level history. The morphological depressions are interpreted as resulting from subaquatic erosion by channelized, sediment-laden currents into horizontally bedded fan sediments. Submerged channels on the eastern shelf are interpreted as meandering-slope channels, probably as a consequence of a lake-level fall that exposed the shelf area. Clinoforms on the Eastern fan may represent relict deltas formed during stationary or slightly rising lake-level intervals. Merging subsurface imaging interpretation with morphological studies of exposed sediments reveals lake-level fluctuations of several hundreds of meters during the past ca. ~550 ka. The lake has three prominent basins (Tatvan, Deveboynu, and the Northern basin) separated by basement ridges (e.g., the Northern ridge). The seismic units in the Tatvan and Northern basins are dominated by alternations of well-stratified and chaotic reflections, while the Deveboynu basin subsurface consists mainly of chaotic units. The chaotic seismic facies are interpreted as mass-flow deposits, probably triggered by earthquakes and/or rapid lake-level fluctuations. The moderate-to-high-amplitude, well-stratified facies seen in the deeper parts of the basins are interpreted as lacustrine deposits intercalated with tephra layers. The occurrence of a clinoform in the deepest part of the lake suggests a major flooding stage of Lake Van more than ~400 ka ago. Seismic profiles from the deepest part of the lake basin show remarkably uniform and continuous stratigraphic units without any major erosional feature following the flooding event, indicating that the lake was never completely dry afterward and therefore significantly older than previously suggested.

Seismic reflection imaging of a Warm Core Ring south of Hokkaido

A multi-channel seismic reflection (MCS) survey was conducted in 2009 to explore the deep crustal structure of the Pacific Plate south of Hokkaido. The survey line happened to traverse a 250-km-wide Warm Core Ring (WCR), a current eddy that had been generated by the Kuroshio Extension. We attempted to use these MCS data to delineate the WCR fine structure. The survey line consists of two profiles: one with a shot interval of 200 m and the other with a shot interval of 50 m. Records from the denser shot point line show much higher background noise than the records from the sparser shot point line. We identified the origin of this noise as acoustic reverberations between the sea surface, seafloor and subsurface discontinuities, from previous shots. Results showed that a prestack migration technique could enhance the signal buried in this background noise efficiently, if the sound speed information acquired from concurrent temperature measurements is available. The WCR is acoustically an assemblage of concave reflectors dipping inward, with steeper slopes (~2°) on the ocean side and gentler slopes (~1°) on the coastal side. Within the WCR, we recognised a 30-km-wide lens-shaped structure with reflectors on the perimeter.

Characteristics of deformation structure around the 2007 Niigata-ken Chuetsu-oki earthquake detected by multi-channel seismic reflection imaging

Abstract A multi-channel seismic reflection (MCS) survey was conducted to investigate the tectonic structure off Niigata, which caused the 2007 Niigata-ken Chuetsu-oki earthquake, using the research vessel (R/V) KAIREI of the Japan Agency for Marine-Earth Science and Technology. Based on the results of data processing and interpretation of available data, three areas are identified according to seismic characteristics. The most deformed area is located on the continental shelf near the source region of the 2007 Niigata-ken Chuetsu-oki earthquake, i.e., the area east of the Yoneyama-Ogi Uplifts. A remarkable growth of folds, including fault-related folds, and a strong reflector dipping east is identifiable by localized strain concentrations. The second area is located between the Yoneyama-Ogi Uplifts and the Jouetsu Knoll in the Toyama Trough. Although the deformation of deposits in the second area was smaller than in the first area, folds are identified. The third area is located toward the west of the Jouetsu Knoll in the Toyama Trough. No significant deformed structures developed in this area. Based on the interpretation of stratigraphy obtained in previous studies, seismic characteristics, and well data, the development of an anticline was initiated by a compression field after about 3.6 Ma. In particular, the deformation of sedimentary layers by the compression field occurred rapidly after about 1.3 Ma. Folds have grown larger toward the east after about 1.3 Ma. In addition, subsidence of about 0.2 s in sedimentary layers can be seen at the western margin of the Yoneyama-Ogi Uplifts, suggesting that tectonic movement related to reverse faulting has advanced there very recently. From aftershock distribution on a depth section on line S-2 near the hypocentral region, most hypocenters were determined to be below the strong reflector. This result suggests that fold growth has accompanied past large earthquakes, such as the 2007 Niigata-ken Chuetsu-oki earthquake.

Significance of widespread low effusion rate eruptions over the past two million years for delivery of magma to the overlapping spreading centers at 9°N East Pacific Rise

Ever since the discovery that the mid-ocean ridge axis deepens at most axial discontinuities, there has been vigorous debate about whether ridge axis depth and shape reflect magma supply. Here we report evidence, from one of the most thoroughly studied ridge discontinuities on a fast-spreading mid-ocean ridge, that volcanic eruption effusion rates have been low relative to nearby segment centers throughout its two million year history. This pattern of low volcanic effusion rates is inferred from the fields of constructional volcanic mounds on the seafloor which are much more abundant in the off-axis discordant zone of the East Pacific Rise (EPR) 9°N Overlapping Spreading Centers (OSC) than on typical EPR seafloor. Detailed seafloor mapping in 2007 shows that these mound fields consist of concentrations of large pillow mounds that formed within the current OSC basin and ridge tips, rather than off-axis. Pillow lava, erupted at approximately an order of magnitude lower effusion rate than lobate or sheet flows, primarily builds mounds on the seafloor. Crustal thickness, a proxy for long-term magma supply, does not decrease in spite of the decreased lava effusion rates implied by widespread pillow eruptions. To resolve the problem of achieving the same magma emplacement rate with lower effusion rates, without violating our observations by requiring drastically increased eruption frequency or unreasonably long eruption durations, we propose that eruptions occur across the entire OSC basin. The current configuration of mid-crustal melt sills within the OSC as resolved by multichannel seismic reflection surveys have neither the correct location nor appropriate size to deliver magma to these eruptive vents in the OSC basin via vertically propagating dikes. Instead, we propose that magma delivery to eruptions occurs via laterally propagating dikes from robust mid-crustal melt sills outside of the overlap zone.

Deep structure, recent deformation and analog modeling of the Gulf of Cadiz accretionary wedge: Implications for the 1755 Lisbon earthquake

The Gulf of Cadiz spans the plate boundary between Africa and Eurasia west of the Betic–Rif mountain belt. A narrow east dipping subduction zone descends beneath the Gulf of Cadiz and the straits of Gibraltar. The deep crustal structure of the Gulf and the adjacent SW Iberian and Moroccan margins is constrained by numerous multi-channel seismic reflection and wide-angle seismic surveys. A compilation of these existing studies is presented in the form of depth to basement, sediment thickness, depth to Moho and crustal thickness maps. These structural maps image an E–W trending trough, with thin (< 10 km) crust beneath the Gulf of Cadiz. This trough is filled by an eastward thickening wedge of sediments, reaching a thickness of 10–15 km in the eastern Gulf. These sediments are tectonically deformed, primarily along a series of westward-vergent thrust faults and represent a 200–250 km wide accretionary wedge. The northern and especially the southern limits of the accretionary wedge are marked by sharp morphological lineaments showing evidence of recent deformation. These tectonic limits are situated in an internal position with respect to the Miocene deformation front (external Betic and Rif allocthons), which has been abandoned. At the western boundary of the accretionary wedge, near the adjacent Seine and Horseshoe abyssal plains, an E–W trending basement high (Coral Patch Ridge) can be seen indenting the deformation front in an asymmetric manner. Analog modeling is performed using granular materials accreted against a semicircular backstop (representing the basement of the Rif and Betic mountain belts). The modeling initially produces a symmetric, arcuate accretionary wedge. The ensuing collision of an oblique rigid indenter retards accretion on one side, resulting in an embayment and a locally steeper deformation front. The deformation pattern observed in morphology and high-resolution seismic profiles suggests the accretionary wedge and underlying subduction system is still active. The implications of active subduction for the source region of the 1755 Lisbon earthquake and the regional seismic hazard assessment are discussed.

Cenozoic crustal deformation of the offshore Burgos basin region (NE Gulf of Mexico). A new interpretation of deep penetration multichannel seismic reflection lines

Abstract Along northeastern Mexico close to the Texas-Mexico border, the Burgos basin and its extension offshore was developed and deformed from the Paleocene up to Present time. This is a key triple junction between the sub meridian dextral transtensive coastal plain of the Gulf of Mexico extending far to the south in Mexico, the NE Corsair fault zone offshore and the sinistral Rio Bravo fault zone, a reactivated segment of the Texas lineament. Offshore NE Mexico, in the main study area covered by available seismic profiles, we have evidenced below the main well known gravitational décollement level (5 to 7 s twtt → 6 to 8 km) a Cenozoic deep-rooted deformation outlined by a N010° W trending deep-seated reverse fault zone and crustal folding down to the Moho (11 s twtt → ~ 20 km). Based on extensive offshore 2D industrial multi-channel seismic reflection surveys, deep exploration wells and gravimetric data, we focus our study on the deep crustal fabric and its effects on the gravitational tectonics in the upper sedimentary layers: submeridian crustal transtensional normal faults and open folding of the identified Mesozoic basement were interpreted as Cenozoic buckling of the crust during a major phase of oblique crustal extension. This deformation has probably enhanced gravity sliding along N030° growth-faults related to salt withdrawal and halokinesis in the offshore Burgos basin. We have tentatively made a link between this crustal deformation episode and the Neogene tectonic inversion of the Laramide foredeep basin of the Sierra Madre Oriental. The latter is still affected by crustal strike slip faulting associated with basaltic volcanism observed into the gulf coastal plain. This study favours a dominant crustal Cenozoic tectonic activity along the gulf margin without any clear evidence of Mesozoic tectonic reactivation. We propose that the large gravity collapse of the gulf margin was triggered by subsequent crustal deformation.

Multi-channel seismic reflection survey in the northern Izu-Ogasawara island arc –KR07-09 cruise–

Multi-channel seismic reflection (MCS) experiments were carried out in KR07-09 cruise to investigate crustal struc- tures and deformations in the northern Izu-Ogasawara island arc, by using the R/V KAIREI of Japan Agency for Marine-Earth Science and Technology (JAMSTEC). Swath bathymetry, geomagnetic, and gravity data are also simultaneously observed dur- ing this cruise. Five MCS survey lines (Lines IBr5, IBr9, KT01, KT04, and KT05) with 2001.6km of total line length were obtained in this survey. An airgun array with a total capacity of 12,000 cubic inches (eight airguns with 1,500 cubic inches each) was shot with shot spacing of 50 m, air pressure of 2000 psi, and towing depth of 10 m. These airgun signals are record- ed by a 204-channel hydrophone streamer cable with group interval of 25 m. The record length and sampling interval were 18 sec (Lines IBr5 and IBr9) or 15sec (Lines KT01, KT04, and KT05) and 4 msec, respectively. The MCS data has sufficient quality to know the crustal deformation of the arc-backarc transition zone. The preliminary interpretations from the onboard processing result indicated unique features of the basement and sediment in the northern Izu-Ogasawara island arc.