Forearc System Research Articles

Rheological Structure and Stress Triggered Megathrust Slip Constrained From the 2016 Mw 7.8 Kaikōura Crustal Earthquake

AbstractUnderstanding postseismic processes following the 2016 Mw 7.8 Kaikōura earthquake remains challenging due to the time‐dependent afterslip over the complex forearc system including crustal faults and megathrust, and the viscoelastic relaxation of the upper mantle. How the 2016 Mw 7.8 Kaikōura crustal earthquake interacts with the megathrust has yet to be better understood. Here we have derived the first 5‐year postseismic displacements from Global Positioning System (GPS) time series of 75 stations to study postseismic processes through a three‐dimensional viscoelastic finite element model. The optimal steady state viscosities of the crustal shear zone, megathrust shear zone, Australian upper mantle and Pacific upper mantle in the lowest‐misfit model among test models are 1018 Pa s, 4 × 1017 Pa s, 2 × 1019 Pa s and 1020 Pa s, respectively. The stress‐driven afterslip within the first 5 years after the earthquake is up to 80 cm over crustal faults, and up to 70 cm over the megathrust. A Kapiti slow slip sequence is probably promoted with a shorter interval by the 2016 earthquake, and is up to ∼11 cm within the first year after the earthquake. Afterslip over crustal faults and the megathrust are both required to reproduce the first‐order pattern of horizontal GPS observations. Coseismic rupture over the megathrust enhances shallow megathrust afterslip, which better fit the eastward postseismic displacement of sites near the rupture area. The southern end of Hikurangi megathrust may be activated during the 2016 earthquake and undergo continuous aseismic slip after the event.

Structural evolution of the southern Ecuadorian forearc in the Santa Elena Peninsula region

The southern Ecuadorian forearc system is related to the subduction of the oceanic Farallon/Nazca Plate beneath the continental South American Plate since the Late Cretaceous, and currently evolves with the dynamic of a tectonic block called North Andean Sliver. To explore the structural architecture and processes controlling the Upper Cretaceous-Cenozoic growth of the forearc, we built a ∼143 km-long onshore-offshore crustal-scale cross-section in the Santa Elena Peninsula region using seismic reflection profiles and well and field data. The structure of the Santa Elena Peninsula forearc system is controlled by imbrication of Upper Cretaceous-Palaeocene oceanic basement and Cenozoic sedimentary units, and underplating of distal Cenozoic sequences stacked at the trench zone. This led to the progressive construction of an accretionary wedge through time. The forearc substratum is mainly formed by the Upper Cretaceous-Palaeocene basement developed during the docking of oceanic terranes. It is later deformed by NW-trending landward-dipping, normal to strike-slip faults during the Middle Eocene, and renewed compression by inversion of inherited faults from the Oligocene onwards. Recent deformation consists in N-trending oceanward-dipping normal faults in the frontal slope domain and fault-controlled uplift of marine terraces along the coastal area. Therefore, the Upper Cretaceous to present-day structural evolution of the Santa Elena Peninsula forearc is controlled by the long-lasting subduction dynamics and structural inheritance of the upper plate.

Foraminifera assemblages from Fantangisña serpentinite mud seamount in the NW Pacific Ocean during the Pleistocene (IODP Expedition 366).

The Mariana forearc system, in the northwestern Pacific, is known as the only convergent margin setting with currently active serpentine mud volcanism. The Fantangisña serpentinite mud volcano lies 62 km west of the Mariana trench, within the influence of the North Equatorial Current (NEC). Cores recovered by International Ocean Discovery Program (IODP) Expedition 366 contain pelagic sediments overlying layered serpentinite mud deposits. At the bottom of the sequence, nannofossil-rich forearc deposits were recovered from under the seamount edifice. In this study, we investigated 47 samples from Site U1498A on the southern flank of the seamount for benthic and planktonic foraminifera assemblages. Statistical analyses on planktonic assemblages differentiated two sample groups related to the ratio between thermocline/mixed layer taxa, which indicate fluctuations in the depth of the thermocline (DOT) during the Pleistocene. Variations in the DOT reflect changes in the intensity of the NEC associated with El Niño/La Niña conditions. Mudflows do not influence the ecology of planktonic foraminifera but possibly enhance their preservation against dissolution, which was instead detected in the pelagic deposit as suggested by common Globigerinoides conglobatus. Benthic foraminifers were rare in serpentinite mud deposits as they are severely affected by mudflows. Conversely, they showed high diversity pre-/post-mud-volcanism, and indicate oligotrophic bottom-water conditions.

A late Carboniferous bimodal volcanic suite before closure of the North Tianshan Ocean at the southwestern margin of the Central Asian Orogenic Belt

The North Tianshan Ocean (NTO) was one of the youngest oceanic basins at the southwestern margin of the Central Asian Orogenic Belt. When, where and how the NTO was finally consumed and closed have not been well addressed. To better understand the tectonic evolution of the NTO, particularly before its closure, an integrated study of volcanic rocks in the Yamansu volcanic arc has been carried out. Two SHRIMP zircon U–Pb ages (317.6 ± 3.1 Ma and 310.7 ± 2.4 Ma) indicate that the previously identified Permian volcanic rocks erupted during the late Carboniferous instead. The late Carboniferous volcanics in the region are composed of a bimodal suite with a SiO2 compositional gap between 60 and 70%. The mafic volcanics could be further divided into LREE-poor basalts and LREE-rich basalt-andesites. The LREE-poor variety displays tholeiite and forearc-like characteristics with flat to depleted LREE patterns with (La/Yb)N < 1.5, which were likely derived from a moderately depleted mantle wedge metasomatized by a subducted fluid-like component. The LREE-rich suite is calc-alkaline and has enriched LREE patterns with high (La/Yb)N > 2. They were from a depleted mantle wedge metasomatized by subducted sediment-derived melts, although more or less suffered crustal contamination. The felsic volcanics are I-type rhyolites, derived from anatexis of Cambrian to Mesoproterozoic arc crustal rocks. Here, we suggest that the Kanggur Fault presumably represented the southernmost suture of NTO. The Yamansu volcanic arc was a forearc system of the Central Tianshan continental arc related to southward subduction of the NTO.

Subduction initiation of the western Proto-Tethys Ocean: New evidence from the Cambrian intra-oceanic forearc ophiolitic mélange in the western Kunlun Orogen, NW Tibetan Plateau

Abstract The supra-subduction zone ophiolite or ophiolitic mélange formed in the forearc setting is generally considered to be a key geological record for subduction initiation (SI) with petrological characteristics comparable to the SI-related rock sequence from forearc basalt (FAB) to boninite in the Izu-Bonin-Mariana subduction zone. Nevertheless, the standard FAB and boninite are generally difficult to observe in the forearc rocks generated during SI. Yet, a typical rock sequence indicating the SI of the western Proto-Tethys Ocean is reported for the first time in the Qimanyute intra-oceanic forearc system in the western Kunlun Orogen, Northwest Tibetan Plateau. The magmatic compositions, which range from less to more high field strength element (HFSE)-depleted and large ion lithophile element (LILE)-enriched, are changing from oceanic plagiogranites (ca. 494 Ma) to forearc basalt-like gabbros (FAB-Gs, ca. 487 Ma), boninites, and subsequent Nb-enriched gabbros (NEGs, ca. 485 Ma), which are thus consistent with the Izu-Bonin-Mariana forearc rocks as well as the Troodos and Semail supra-subduction zone-type ophiolites. The geochemical data from the chemostratigraphic succession indicate a subduction initiation process from a depleted mid-oceanic-ridge (MORB)-type mantle source with no detectable subduction input to gradual increasing involvement of subduction-derived materials (fluid/melts and sediments). The new petrological, geochemical, and geochronological data, combined with the regional geology, indicate that the well-sustained FAB-like intrusive magmas with associated boninites could provide crucial evidence for SI and further reveal that the SI of the western Proto-Tethys Ocean occurred in the Late Cambrian (494–485 Ma).

Dirivultidae (Copepoda: Siphonostomatoida) from hydrothermal vent fields in the Okinawa Trough, North Pacific Ocean, with description of one new species

AbstractTwo species of dirivultid copepod (Siphonostomatoida),Stygiopontius senckenbergiIvanenko &amp; Ferrari, 2013 andDirivultus kaikosp. nov., were discovered from hydrothermal vent fields in the Okinawa Trough, the western North Pacific. SinceS. senckenbergiwas originally described based on two adult males from the New Ireland Fore-Arc system, Papua New Guinea, the discovery here represents the second record. This species was attached on ventral setae covered with filamentous bacteria of the deep-sea squat lobsterShinkaia crosnieri(Decapoda: Munidopsidae). The female ofS. senckenbergiis described for the first time as well as the nauplius I and the copepodid IV. Sexual dimorphism is shown on several appendages (i.e. antennule, leg 2 and leg 5). The nauplius I shows typical lethithotrophic characteristics.Dirivultus kaikosp. nov., which was found on the tentacular crown of the siboglinid tubewormLamellibrachia columna, differs from its two congeners by the antennule, maxilliped and leg 5 of both sexes. These are the second and third dirivultid species to be reported from Japan.

Thermal structure and source rock maturity of the North Peruvian forearc system: Insights from a subduction-sedimentation integrated petroleum system modeling

Basin modeling is commonly used for the hydrocarbon potential evaluation of underexplored areas, taking into account the full basin history to assess its thermicity and fluid distributions. In the case of forearc basins, however, the influence of the subduction on the internal thermal structure is difficult to predict. The existing subduction thermal models usually focus on the present day lithosphere and do not consider the sediment infill history, while industrial modeling tools handle the thermal evolution of sedimentary basins but not the subduction process. In this paper, we propose a solution model in which the cooling engendered by the subduction is represented by the advective term of the heat conservation energy equation throughout the sedimentation. This study focuses on the Talara-Tumbes-Lancones petroleum province, part of the North Peruvian forearc system. Despite the long term exploration of the Talara Basin, the origin of its massive oil fields still remains questioned. The new subduction-sedimentation integrated petroleum system modeling presented here, calibrated by numerous structural and thermometric data, allows to better constrain the thermal structure and source rock maturity history of this forearc system. We show that the exposure of the sedimentary basin to the subducting cold lithosphere, the sedimentation rate and erosion are important factors impacting the maturity of the source rocks. Consequently, even in the same subduction setting, each depocenter of the North Peruvian forearc system presents a different thermal history and maturity timing, and each basin presents an independent petroleum system.

Origin of a double forearc basin: The example of the Tumaco - Manglares basin, Northestern Southamerica

The subsidence and uplift history of the forearc system of southwestern Colombia and northern Ecuador margin is complex and reveals several stages of deformation. The sequential stratigraphy of the forearc area shows the development of three megasequences (M1 to M3). The basal megasequence corresponds to the basement of the forearc, which was formed at the end of the Mesozoic and at the beginning of the Cenozoic and accreted against the Northwestern part of South America related to the accretion of the Late Cretaceous – Paleoceneoceanic plateau. This accretion occurred in a transpressional regime. The second megasequence is composed by deep water sediments, recording the transition between transpressional to compressional stages of the margin from the Late Eocene to the Middle Miocene. The third megasequence is characterized by shallow water sediments strongly constrained by the compressional stage of the margin and the uplift activity of the structural highs since the Late Miocene up to present. The structural geometry of the margin is characterized by basement thrusts that deformed the forearc crust. Westward, the forearc zone -according to the support of the overriding plate -is divided into mantle wedge and lower plate domains. The margin evolution suggests that the subducting plate geodynamical changes affect strongly the interplate coupling and mantle wedge and produce changes in the subsidence or uplift through the double forearc basin systems.

From accretion to forearc basin initiation: The case of SW Ecuador, Northern Andes

The SW of Ecuador offers a great opportunity to study the long-term behavior of an almost entire forearc system, from the external accretionary prism to the landward limit of the forearc basin. A combination of field observations, LA-ICP-MS U-Pb dating on zircon and interpretation of different vintages of unpublished industrial seismic records are used to study the evolution of the forearc system of SW Ecuador including the accretionary prism and the forearc depocenter. The youngest dates, obtained from U-Pb dating on zircon grains believed to be derived from the arc, define the best estimate for the age of sedimentation and permit a clear description of the temporal and spatial evolution of the accretionary, post accretionary and forearc basin series between 60 Ma and 10 Ma. This reinforces the idea that forearc sediments can be dated by U-Pb on zircon provided that the arc is active at the moment of sedimentation. The Azúcar Formation reflects the accretion of SW Ecuador against the South American continental margin from 61 Ma to 55 Ma. This event is post-dated by the formation of an accretionary prism which may have started at circa 55 Ma. Deformation of the accretionary prism may have been less active during sedimentation of the Ancón Group from ~54 Ma (?) to ~40 Ma which shows evidence of normal faulting and other extensional processes. The transition from the Azúcar Formation to the Ancón Group is marked by a significant decrease of the topographic profile, which in turn resulted in sedimentary series that shallow upwards. The forearc basin sensu strictu develops coevally with the uplift of the outer forearc high (Estancia Hills) and the sedimentation of the Zapotal Formation at 32–30 Ma; the latest marking a significant differentiation in the sedimentary style of the area to shallower and even continental environments. From 30 Ma to 10 Ma the forearc basin was defined by shallow water deposits, progressively influenced by tidal dynamics, most likely related to a protected triangular bay symbolizing the shape of the forearc basin. Evidence presented here suggests the existence of a stable outer forearc high limiting the forearc depocenter for at least 20 My and creating, at least in part, the accommodation necessary for an ~3 km thick forearc sedimentary package. The architecture of SW Ecuador is typical of a forearc basin setting including an accretionary prism and an outer forearc high such as those observed in the Kumano (Japan) and the Coast Range and Great Valley basins (USA). The Estancia Hills may correspond to one of the best preserved exposures of an ancient outer forearc high worldwide.

A viscoplastic shear-zone model for deep (15–50 km) slow-slip events at plate convergent margins

A key issue in understanding the physics of deep (15–50 km) slow-slip events (D-SSE) at plate convergent margins is how their initially unstable motion becomes stabilized. Here we address this issue by quantifying a rate-strengthening mechanism using a viscoplastic shear-zone model inspired by recent advances in field observations and laboratory experiments. The well-established segmentation of slip modes in the downdip direction of a subduction shear zone allows discretization of an interseismic forearc system into the (1) frontal segment bounded by an interseismically locked megathrust, (2) middle segment bounded by episodically locked and unlocked viscoplastic shear zone, and (3) interior segment that slips freely. The three segments are assumed to be linked laterally by two springs that tighten with time, and the increasing elastic stress due to spring tightening eventually leads to plastic failure and initial viscous shear. This simplification leads to seven key model parameters that dictate a wide range of mechanical behaviors of an idealized convergent margin. Specifically, the viscoplastic rheology requires the initially unstable sliding to be terminated nearly instantaneously at a characteristic velocity, which is followed by stable sliding (i.e., slow-slip). The characteristic velocity, which is on the order of <10−7 m/s for the convergent margins examined in this study, depends on the (1) effective coefficient of friction, (2) thickness, (3) depth, and (4) viscosity of the viscoplastic shear zone. As viscosity decreases exponentially with temperature, our model predicts faster slow-slip rates, shorter slow-slip durations, more frequent slow-slip occurrences, and larger slow-slip magnitudes at warmer convergent margins.

Deciphering the Late Cretaceous‐Cenozoic Structural Evolution of the North Peruvian Forearc System

AbstractThe link between plate tectonics and the evolution of active margins is still an ongoing task to challenge since the acceptance of plate tectonic paradigm. This paper aims at deciphering the structural architecture and uplift history of the North Peruvian forearc system to better understand the evolution and the mechanics that govern the Late Cretaceous‐Cenozoic building of this active margin. In this study, we report surface structural geology data, interpretation of seismic reflection profiles, apatite fission track data, and the construction of two offshore‐onshore crustal‐scale balanced cross sections. The structure of the North Peruvian forearc system is dominated by an accretionary style with northwestward propagation of thrust‐related structural highs involving continental/oceanic basement rocks and off‐scrapped sediments. The thrust systems bound thick thrust‐top forearc depocenters mainly deformed by crustal normal to strike‐slip faults and thin‐skinned gravitational instabilities. The sequential restoration of the margin calibrated with apatite fission track data suggests a correlation between uplift, shortening, and plate convergence velocity during Late Cretaceous and Miocene. Pliocene‐Quaternary shortening and uplift of the coastal zone is rather related to the subduction of asperities during convergence rate decrease. The development of crustal normal to strike‐slip faulting and subsidence zones might be the consequence of slab flexure, local basal erosion along subduction fault, and/or oblique subduction associated with sediment loading control. We conclude that the evolution of the North Peruvian forearc system was controlled by subduction dynamics, strong sediment accumulation, and recent ridge subduction, and it recorded the orogenic loading evolution of the Andes over the Cenozoic.

Impact of Cocos Ridge (Central America) subduction on the forearc drainage system

Impact of Cocos Ridge (Central America) subduction on the forearc drainage system

付加型沈み込み帯浅部の地質構造：房総半島南部付加体-被覆層システム

付加型沈み込み帯浅部の地質構造：房総半島南部付加体-被覆層システム

Provenance signature of changing plate boundary conditions along a convergent margin: Detrital record of spreading-ridge and flat-slab subduction processes, Cenozoic forearc basins, Alaska

Cenozoic strata from forearc basins in southern Alaska record deposition related to two different types of shallow subduction: Paleocene–Eocene spreading-ridge subduction and Oligocene–Recent oceanic plateau subduction. We use detrital zircon geochronology (n = 1368) and clast composition of conglomerate (n = 1068) to reconstruct the upper plate response to these two subduction events as recorded in forearc basin strata and modern river sediment. Following spreading-ridge subduction, the presence of Precambrian and Paleozoic detrital zircon ages in middle Eocene–lower Miocene arc-margin strata and Early Cretaceous ages in lower Miocene accretionary prism–margin strata indicates that sediment was transported to the basin from older terranes in interior Alaska and from the exhumed eastern part of the Cretaceous forearc system, respectively. By middle-late Miocene time, diminished abundances of these populations reflect shallow subduction of an oceanic plateau and associated exhumation that resulted in an overall contraction of the catchment area for the forearc depositional system. In the southern Alaska forearc basin system, upper plate processes associated with subduction of a spreading ridge resulted in an abrupt increase in the diversity of detrital zircon ages that reflect new sediment sources from far inboard regions. The detrital zircon signatures from strata deposited during oceanic plateau subduction record exhumation of the region above the flat slab, with the youngest detrital zircon population reflecting the last period of major arc activity prior to insertion of the flat slab. This study provides a foundation for new tectonic and provenance models of forearc basins that have been modified by shallow subduction processes, and may help to facilitate the use of U-Pb dating of detrital zircons to better understand basins that formed under changing geodynamic plate boundary conditions.

Evidence for Hydrothermal Activity in the Earliest Stages of Intraoceanic Arc Formation: Implications for Ophiolite-Hosted Hydrothermal Activity

Forearcs within intraoceanic arcs preserve a record of earliest arc volcanism that took place at the initiation of subduction. In this study we have investigated the submarine section of the Bonin Ridge, which forms part of the Izu-Bonin intraoceanic arc and/or forearc system. The crustal stratigraphy of this forearc is found to have many similarities with suprasubduction zone ophiolite sequences, including the occurrence of sheeted dikes overlain by depleted tholeiites and/or boninitic magmatism. We propose that the Izu-Bonin forearc is potentially analogous to suprasubduction ophiolites. Hydrothermal alteration, together with sulfide disseminations, is exposed on some of the subaerial Bonin Islands. Based on geologic observations, the hydrothermal activity appears, in general, to have been contemporaneous with the boninite and boninitic andesite host volcanics. The sulfide mineral assemblage associated with the hydrothermal alteration is dominated by pyrite accompanied by minor chalcopyrite, sphalerite, and pyrrhotite. These sulfides are mainly found as disseminations in pillow breccias and hyaloclastite, or along the margins of dikes, but are also found as sulfide nodules in hydrothermal clay-rich zones. This forearc mineral assemblage contrasts with the Pb-rich mineralization associated with the Quaternary submarine hydrothermal deposits found along the main Izu-Bonin arc. Hydrothermal alteration in the Eocene Izu-Bonin forearc represents the style of hydrothermal activity generated by magmatism immediately after the initiation of intraoceanic subduction. Similarities in the style of hydrothermal activity and in the stratigraphy of the hosting volcanic sequence imply that the Bonin Ridge can be regarded as an in situ (i.e., not obducted or dismembered) analogue for a suprasubduction ophiolite including Cyprus-type massive sulfide deposits.

New species ofStygiopontius(Copepoda: Siphonostomatoida: Dirivultidae) from a deep-sea hydrothermal volcano in the New Ireland Fore-Arc system (Papua New Guinea)

A male of the new speciesStygiopontius senckenbergibelonging to the family Dirivultidae Humes &amp; Dojiri, 1980 (Copepoda: Siphonostomatoida) and endemic to deep-sea hydrothermal vents, is described from a raised fault block structure south of Edison seamount of the New Ireland Fore-Arc system (Papua New Guinea). The copepods were collected in by box-corer during cruise SO-133 of the RV ‘Sonne’ at a depth of 1610–1625 m, 3°19′S 152°35′E. The new species belongs to a group of eight species that are separate from 21 congeners on the basis of setation of legs 1 and 4: the coxa of leg 1 has an inner seta (absent on the others) and the third exopodal segment of leg 4 has three outer spines (instead of two spines). The new species shares withS. pectinatusHumes, 1987 a pectinate maxilliped but differs from it in lacking two pectinate, terminal claw-like setae on the endopod of the antenna.

Mariana Forearc Serpentinite Mud Volcanoes Harbor Novel Communities of Extremophilic Archaea

Extremophilic archaeal communities living in serpentinized muds influenced by pH 12.5 deep-slab derived fluids were detected and their richness and relatedness assessed from across seven serpentinite mud volcanoes located along the Mariana forearc. In addition, samples from two near surface core sections (Holes D and E) at ODP Site 1200 from South Chamorro were subjected to SSU rDNA clone library and phylogenetic analysis resulting in the discovery of several novel operational taxonomic units (OTUs). Five dominant OTUs of Archaea from Hole 1200D and six dominant OTUs of Archaea from Hole 1200E were determined by groups having three or more clones. Terminal-restriction fragment length polymorphism (T-RFLP) analysis revealed all of the dominant OTUs were detected within both clone libraries. Cluster analysis of the T-RFLP data revealed archaeal community structures from sites on Big Blue and Blue Moon to be analogous to the South Chamorro Hole 1200E site. These unique archaeal community fingerprints resulted from an abundance of potential methane-oxidizing and sulfate-reducing phylotypes. This study used deep-sea sediment coring techniques across seven different mud volcanoes along the entire Mariana forearc system. The discovery and detection of both novel Euryarchaeota and Marine Benthic Group B Crenarcheaota phylotypes could be efficacious archaeal indicator populations involved with anaerobic methane oxidation (AMO) and sulfate reduction fueled by deep subsurface serpentinization reactions.

P–Tevolution and timing of a late Palaeozoic fore-arc system and its heterogeneous Mesozoic overprint in north-central Chile (latitudes 31–32°S)

AbstractIn the late Palaeozoic fore-arc system of north-central Chile at latitudes 31–32°S (from the west to the east) three lithotectonic units are telescoped within a short distance by a Mesozoic strike-slip event (derived peakP–Tconditions in brackets): (1) the basally accreted Choapa Metamorphic Complex (CMC; 350–430°C, 6–9 kbar), (2) the frontally accreted Arrayán Formation (AF; 280–320°C, 4–6 kbar) and (3) the retrowedge basin of the Huentelauquén Formation (HF; 280–320°C, 3–4 kbar). In the CMC, Ar–Ar spot ages locally date white-mica formation at peakP–Tconditions and during early exhumation at 279–242 Ma. In a local garnet mica-schist intercalation (570–585°C, 11–13 kbar) Ar–Ar spot ages refer to the ascent from the subduction channel at 307–274 Ma. Portions of the CMC were isobarically heated to 510–580°C at 6.6–8.5 kbar. The age of peakP–Tconditions in the AF can only vaguely be approximated at ≥ 310 Ma by relict fission-track ages consistent with the observation that frontal accretion occurred prior to basal accretion. Zircon fission-track dating indicates cooling below ~ 280°C at ~ 248 Ma in the CMC and the AF, when a regional unconformity also formed. Ar–Ar white-mica spot ages in parts of the CMC and within the entire AF and HF point to heterogeneous resetting during Mesozoic extensional and shortening events at ~ 245–240 Ma, ~ 210–200 Ma, ~ 174–159 Ma and ~ 142–127 Ma. The zircon fission-track ages are locally reset at 109–96 Ma. All resetting of Ar–Ar white-mica ages is proposed to have occurred byin situdissolution/precipitation at low temperature in the presence of locally penetrating hydrous fluids. Hence syn- and postaccretionary events in the fore-arc system can still be distinguished and dated in spite of its complex heterogeneous postaccretional overprint.

Tectonic evolution and sequence stratigraphy of the Central Pacific margin of Costa Rica

On the Central Pacific of Costa Rica, more than 1600 km of seismic reflection data, and potential field data, located offshore and onshore, joined to the surface geology data have been analyzed in order to clarify the structural configuration, tectonic evolution and characteristics of the sedimentary infill. Two main depocenters are recognaized in the area: the Nicoya basin comprising about 3.5 km of Cretraceous-Tertiary sediments in the offshore region of the Nicoya Gulf and extending toward the noretheast on the onshore region; and the Parrita basin located offshoreand onshore with a sedimentary fill of more 4.5 km. Both basins are limited on the south by a shallow uplift of the basement which represents an extension of the outer arc over the platform.It is interpreted that in the Cretaceous, the area now occupied by the Nicoya and Patarrita basins, was part of an extense forearc system that spanned along the Pacific margin of Costa Rica encompassing the Tempisque and Terraba basins. The present structural configuration of the Parrita and Nicoya Basins initiates in the Middle Eocene, relatedto shear stresses caused by the rotationin clockwise direction of the southern portion of the Costa Rica territory along a main transcurrent fault zone. Such movement displaced the Tempisque and Terraba basins on the south and north respectively, developing the half-graben configuration and the extense system of strike-slip faults of the transtension and the subsidence along this series of faults, transverse to the coast, which are in some cases, still active.The seismic stratigraphy analysis defines five seismic sequences and a equal number of time structure maps, besides the top of basement. This analysis has been tied up to the surface geology information and discussed under the light of sequence stratigraphy concepts. In general, the sediment in depocenters of this central Pacific region of Costa Rica, can be characterized as a shallowing upward megasequence, including deposits ranging from slope to shallow marine.

Structure, evolution and tectonic activity of the eastern Sunda forearc, Indonesia, from marine seismic investigations

Forearc structures of the eastern Sunda Arc are studied by new multichannel reflection seismic profiling. We image a high along-strike variability of the subducting oceanic plate, the interface between subducting and overriding plate, the accretionary wedge, the outer arc high and forearc basins. We highlight ongoing tectonic activity of the entire outer arc high: active out-of-sequence thrust faults connecting the plate interface with the seafloor, slope basins showing tilted sedimentary sequences on the outer arc high, vertical displacement of young seafloor sediments, and tilted sedimentary sequences in the Lombok forearc basin. While frontal accretion plays a minor role, the growth of the outer arc high is mainly attributed to oceanic sediments and crustal fragments, which are attached to the base of the upper plate and recycled within the forearc. We image ongoing large-scale duplex formation of the oceanic crust. The incoming oceanic crust is dissected by normal faulting into 5–10 km wide blocks within a 50–70 km wide belt seaward of the deep sea trench. These blocks determine the geometry and evolution of duplexes attached to the base of the overriding plate landward of the trench. Long-lasting and ongoing subsidence of the Lombok Basin is documented by distinct seismic sequences. In the Lombok Basin we image mud diapirs, fed from deeply buried sediments which may have been mobilized by rising fluids. We propose a wrench fault system in the eastern Lombok forearc basin that decouples the subduction regime of the Sunda Arc from the continent–island arc collision regime of the western Banda Arc. The observed tectonic activity of the entire forearc system reflects a high earthquake and tsunami hazard, similar to the western part of the Sunda Arc.