Underplated Bodies Research Articles

Seismic imaging evidence that forearc underplating built the accretionary rock record of coastal North and South America

AbstractThe submerged forearcs of Pacific subduction zones of North and South America are underlain by a coastally exposed basement of late Palaeozoic to early Tertiary age. Basement is either an igneous massif of an accreted intra-oceanic arc or oceanic plateau (e.g. Cascadia(?), Colombia), an in situ formed arc massif (e.g. Aleutian Arc) or an exhumed accretionary complex of low and high P/T metamorphic facies of late Palaeozoic (e.g. southern Chile, Patagonia) and Mesozoic age (e.g. Alaska). Seismic studies at Pacific forearcs image frontal prisms of trench sediment accreted to the seaward edge of forearc basement. Frontal prisms tend to be narrow (10–40 km), weakly consolidated and volumetrically small (∼35–40 km3/km of trench). In contrast, deep seismic imaging of submerged forearcs commonly reveals large volumes (∼2000 km3/km of trench) of underplated material accreted at subsurface depths of ∼10–30 km to the base of forearc basement. Underplates have been imaged below the southern Chile, Ecuador–Colombia, north Cascade, Alaska, and possibly the eastern Aleutian forearcs. Deep underplates have also been observed below the Japan and New Zealand forearcs. Seismic imaging of northern and eastern Pacific forearcs supports the conclusion drawn from field and laboratory studies that exposed low and high P/T accretionary complexes accumulated in the subsurface at depths of 10–30 km. It seems significant that imaged underplated bodies are characteristic of modern well-sedimented subduction zones. It also seems likely that large Pacific-rim underplates store a significant fraction of sediment subducted in Cenozoic time.

Nature of the crust in the northern Gulf of California and Salton Trough

AbstractIn the southern Gulf of California, the generation of new oceanic crust has resulted in linear magnetic anomalies and seafloor bathymetry that are characteristic of active seafloor-spreading systems. In the northern Gulf of California and the onshore (southeastern California, USA) Salton Trough region, a thick sedimentary package overlies the crystalline crust, masking its nature, and linear magnetic anomalies are absent. We use potential-field data and a geotherm analysis to constrain the composition of the crust (oceanic or continental) and develop a conceptual model for rifting. Gravity anomalies in the northern Gulf of California and Salton Trough are best fit with crustal densities that correspond to continental crust, and the fit is not as good if densities representative of mafic rocks, i.e., oceanic crust or mafic underplating, are assumed. Because extensive mafic underplated bodies would produce gravity anomalies that are not in agreement with observed gravity data, we propose, following earlier work, that the anomalies might be due to serpentinized peridotite bodies such as found at magma-poor rifted margins. The density and seismic velocities of such serpentinized peridotite bodies are in agreement with observed gravity and seismic velocities. Our conceptual model for the Salton Trough and northern Gulf of California shows that net crustal thinning here is limited because new crust is formed rapidly from sediment deposition. As a result, continental breakup may be delayed.

Crustal and uppermost mantle structure variation beneath La Réunion hotspot track

The Piton de la Fournaise basaltic volcano, on La Reunion Island in the western Indian Ocean, is one of the most active volcanoes in the world. This volcano is classically considered as the surface expression of an upwelling mantle plume and its activity is continuously monitored, providing detailed information on its superficial dynamics and on the edifice structure. Deeper crustal and upper mantle structure under La Reunion Island is surprisingly poorly constrained, motivating this study. We used receiver function techniques to determine a shear wave velocity profile through the crust and uppermost mantle beneath La Reunion , but also at other seismic stations located on the hotspot track, to investigate the plume and lithosphere interaction and its evolution through time. Receiver functions (RFs) were computed at permanent broad-band seismic stations from the GEOSCOPE network (on La Reunion and Rodrigues), at IRIS stations MRIV and DGAR installed on Mauritius and Diego Garcia islands, and at the GEOFON stations KAAM and HMDM on the Maldives. We performed non-linear inversions of RFs through modelling of P-to-S conversions at various crustal and upper mantle interfaces. Joint inversion of RF and surface wave dispersion data suggests a much deeper Mohorovici´iscontinuity (Moho) beneath Mauritius (∼21 km) compared to La Reunion (∼12 km). A magmatic underplated body may be present under La Reunion as a thin layer (≤3 km thick), as suggested by a previous seismic refraction study, and as a much thicker layer beneath other stations located on the hotspot track, suggesting that underplating is an important process resulting from the plume–lithosphere interaction. We find evidence for a strikingly low velocity layer starting at about 33 km depth beneath La Reunion that we interpret as a zone of partial melt beneath the active volcano. We finally observe low velocities below 70 km beneath La Reunion and below 50 km beneath Mauritius that could represent the base of the oceanic lithosphere.

Seismic imaging of the eastern Algerian margin off Jijel: integrating wide-angle seismic modelling and multichannel seismic pre-stack depth migration

This study presents the results of a deep seismic survey across the north Algerian margin, based on the combination of 2-D multichannel and wide-angle seismic data simultaneously recorded by 41 ocean bottom seismometers deployed along a north-south line extending 180 km off Jijel into the Algerian offshore basin, and 25 land stations deployed along a 100-km-long line, cutting through the Lesser Kabylia and the Tellian thrust-belt. The final model obtained using forward modelling of the wide-angle data and pre-stack depth migration of the seismic reflection data provides an unprecedented view of the sedimentary and crustal structure of the margin. The sedimentary layers in the Algerian basin are 3.75 km thick to the north and up to 4.5-5 km thick at the foot of the margin. They are characterized by seismic velocities from 1.9 to 3.8 km s(-1). Messinian salt formations are about 1 km thick in the study area, and are modelled and imaged using a velocity between 3.7 and 3.8 km s(-1). The crust in the deep sea basin is about 4.5 km thick and of oceanic origin, presenting two distinct layers with a high gradient upper crust (4.7-6.1 km s(-1)) and a low gradient lower crust (6.2-7.1 km s(-1)). The upper-mantle velocity is constrained to 7.9 km s(-1). The ocean-continent transition zone is very narrow between 15 and 20 km wide. The continental crust reaches 25 km thickness as imaged from the most landward station and thins to 5 km over a less than 70 km distance. The continental crust presents steep and asymmetric upper- and lower-crustal geometry, possibly due to either asymmetric rifting of the margin, an underplated body, or flow of lower crustal material towards the ocean basin. Present-time deformation, as imaged from three additional seismic profiles, is characterized by an interplay of gravity-driven mobile-salt creep and active thrusting at the foot of the tectonically inverted Algerian margin.

Uplift prior to continental breakup: Indication for removal of mantle lithosphere?

Uplift or reduced subsidence prior to continental breakup is a key component of the rift-drift transition. This uplift causes lateral variations in the lithospheric potential energy, which can increase intraplate deviatoric tension, thereby facilitating continental rupture. There is a growing body of evidence that pre-breakup uplift is a global phenomenon characteristic of magmatic and magma-poor rifted margins. Evidence is provided by the subaerial extrusion of lava interpreted from drill logs, stratigraphic records, the presence of breakup unconformities, and the spatial extent of uplift associated with Afar (the Ethiopian-Somali plateau), which may be at the stage of rupture. Previously discussed mechanisms contributing to this uplift include phase transitions, dynamic uplift from mantle plumes, and magmatic underplated bodies. We show in this study that dynamic uplift resulting from passive upwelling asthenosphere below the rift is limited (∼200 m). Isostatic arguments suggest that removal of mantle lithosphere is a necessary and effective mechanism for uplift coincident with rupture. The combination of mantle phase transitions and a very thin mantle lid produces an excess potential energy state (as evidenced by a positive geoid anomaly) and leads to tensional forces favorable for rupture. These results underpin our proposed model for continental breakup where removal of mantle lithosphere by either detachment or formation of gravitational instabilities is a characteristic process. Observations of depth-dependent thinning and geochemical data support this model.

On the origin of the ultradeep East Barents Sea basin

Very large subsidence, with up to 20 km thick sediment layers, is observed in the East Barents Sea basin. Subsidence started in early Paleozoic, accelerated in Permo‐Triassic times, finished during the middle Cretaceous, and was followed by moderate uplift in Cenozoic times. The observed gravity signal suggests that the East Barents Sea is at present in isostatic balance and indicates that a mass excess is required in the lithosphere to produce the observed large subsidence. Several origins have been proposed for the mass excess. We use 1‐D thermokinematic modeling and 2‐D isostatic density models of continental lithosphere to evaluate these competing hypotheses. The crustal density in 2‐D thermokinematic models resulting from pressure‐, temperature‐, and composition‐dependent phase change models is computed along transects crossing the East Barents Sea. The results indicate the following. (1) Extension can only explain the observed subsidence provided that a 10 km thick serpentinized mantle lens beneath the basin center is present. We conclude that this is unlikely given that this highly serpentinized layer should be formed below a sedimentary basin with more than 10 km of sediments and crust at least 10 km thick. (2) Phase changes in a compositionally homogeneous crust do not provide enough mass excess to explain the present‐day basin geometry. (3) Phase change induced densification of a preexisting lower crustal gabbroic body, interpreted as a mafic magmatic underplate, can explain the basin geometry and observed gravity anomalies. The following model is proposed for the formation of the East Barents Sea basin: (1) Devonian rifting and extension related magmatism resulted in moderate thinning of the crust and a mafic underplate below the central basin area explaining initial late Paleozoic subsidence. (2) East‐west shortening during the Permian and Triassic resulted in densification of the previously emplaced mafic underplated body and enhanced subsidence dramatically, explaining the present‐day deep basin geometry.

The South Ecuador subduction channel: Evidence for a dynamic mega-shear zone from 2D fine-scale seismic reflection imaging and implications for material transfer

[1] Tectonic processes that control the transition from poorly consolidated sediment entering the subduction channel (SC) to the seismogenic zone are documented using seismic imaging. We applied pre-stack depth migration and a post-processing sequence to a seismic reflection line acquired across the Ecuador convergent margin to obtain a 2D-quantitative image of the first ∼24 km of the SC. Structural interpretation shows that the SC consists of a 630–1150-m-thick, low-velocity, continuous sheet of sediment that dips ∼6° landward and undergoes shear deformation. The long sheet is bounded at top and bottom by decollement thrusts, and developed over time Riedel shears and basal thrust faulting and folding downdip, pointing to a dynamic mega-shear zone. Modeling the strong uppermost and basal SC reflectors reveals that they are associated with 40–80-m-thick, 50–350 m/s, low-velocity perturbations layers inferred to be fluid rich and mechanically weak. A fine-scale velocity model shows two anomalously low-Vp areas in the long sheet, advocating patches of over-pressured fluids. Evidence for Vp variations along the upper-plate foundation suggests either underplated bodies or a fluid-damage zone. A simple temporal reconstruction indicates that underthrusting the long sheet initiated >450 kyr ago and interrupted ∼54 ± 13 kyr ago, when frontal accretion resumed. During this transient evolution, the SC boundaries revealed highly unstable as most of the SC was underplated while down going plate material may have been sheared off and incorporated to the SC.

The structure of the lower crust at the Argentine continental margin, South Atlantic at 44°S

It is well established that the Argentine passive margin is of the rifted volcanic margin type. This classification is based primarily on the presence of a buried volcanic wedge beneath the continental slope, manifested by seismic data as a seaward dipping reflector sequence (SDRS). Here, we investigate the deep structure of the Argentine volcanic margin at 44°S over 200 km from the shelf to the deep oceanic Argentine Basin. We use wide-angle reflection/refraction seismic data to perform a joint travel time inversion for refracted and reflected travel times. The resulting P-wave velocity-depth model confirms the typical volcanic margin structure. An underplated body is resolved as distinctive high seismic velocity ( v p up to 7.5 km/s) feature in the lower crust in the prolongation of a seaward dipping reflector sequence. A remarkable result is that a second, isolated body of high seismic velocity ( v p up to 7.3 km/s) exists landward of the first high-velocity feature. The centres of both bodies are 60 km apart. The high-velocity lower-crustal bodies likely were emplaced during transient magmatic–volcanic events accompanying the late rifting and initial drifting stages. The lateral variability of the lower crust may be an expression of a multiple rifting process in the sense that the South Atlantic rift evolved by instantaneous breakup of longer continental margin segments. These segments are confined by transfer zones that acted as rift propagation barriers. A lower-crustal reflector was detected at 3 to 5 km above the modern Moho and probably represents the lower boundary of stretched continental crust. With this finding we suggest that the continent–ocean boundary is situated 70 km more seaward than in previous interpretations.

Crustal velocity structure across the Main Ethiopian Rift: results from two-dimensional wide-angle seismic modelling

SUMMARY We present the results of velocity modelling of a recently acquired wide-angle seismic reflection/refraction profile across the Main Ethiopian Rift. The models show a continental type of crust with significant asymmetry between the two sides of the rift. A 2- to 5-km-thick layer of sedimentary and volcanic sequences is modelled across the entire region. This is underlain b ya 40- to 45-km-thick crust with a c. 15-km-thick high-velocity lowest crustal layer beneath the western plateau. This layer is absent from the eastern side, where the crust is 35 km thick beneath the sediments. We interpret this layer as underplated material associated with the Oligocene flood basalts of the region with possible subsequent addition by recent magmatic events. Slight crustal thinning is observed beneath the rift, where Pn velocities indicate the presence of hot mantle rocks containing partial melt. Beneath the rift axis, the velocities of the upper crustal layers are 5‐10 per cent higher than outside the rift, which we interpret as resulting from mafic intrusions that can be associated with magmatic centres observed in the rift valley. Variations in seismic reflectivity suggest the presence of layering in the lower crust beneath the rift, possibly indicating the presence of sills, as well as some layering in the proposed underplated body.

Perturbation to the lithosphere along the hotspot track of La Réunion from an offshore‐onshore seismic transect

A 250 km long NE‐SW lithospheric transect spanning the 40 km wide island of La Réunion and its submarine edifice is derived from lines of air gun shots at sea on either side, along the assumed hotspot trace. Seismic records were obtained from an array spanning the whole transect and including sea bottom and land receivers, providing a system of reversed and overlapping observations. Low seismic velocity, and hence density, is found on average for the whole edifice above the oceanic plate. We attribute high‐velocity anomalies within the edifice to an intrusive core confined under the central northern quarter of the island‐crossing segment. Unexpectedly, the main seismic interfaces, top and bottom of the prevolcanic crust, do not show significant flexural downwarping under the island. In addition, clear multipathing in the recorded wave field requires the presence of a body with a seismic velocity intermediate between the prevolcanic crustal material and the normal mantle. This lithospheric structure provides the first example where underplating occurs beneath an active volcanic island, suggesting a genetic relationship. The underplated body could represent residues of the evolution of primary picritic melts that yield erupted basalts. Evidence for reflectors deeper in the lithosphere may indicate further related heterogeneity. In the plate/hotspot model commonly assumed, the structural variation along the transect could be interpreted as a variation with time of the amount and physical state of underplated material.

West margin of North America — a synthesis of recent seismic transects

A comparison of the deep structure along nine recent transects of the west margin of North America shows many important similarities and differences. Common tectonic elements identified in the deep structure along these transects include actively subducting oceanic crust, accreted oceanic/arc (or oceanic-like) lithosphere of Mesozoic through Cenozoic ages, Cenozoic accretionary prisms, Mesozoic accretionary prisms, backstops to the Mesozoic prisms, and undivided lower crust. Not all of these elements are present along all transects. In this study, nine transects, including four crossing subduction zones and five crossing transform faults, are plotted at the same scale and vertical exaggeration (V.E. 1 : 1), using the above scheme for identifying tectonic elements. The four subduction-zone transects contain actively subducting oceanic crust, Cenozoic accretionary prisms, and bodies of basaltic rocks accreted in the Cenozoic, including remnants of a large, oceanic plateau in the Oregon and Vancouver Island transects. Rocks of age and composition (Eocene basalt) similar to the oceanic plateau are currently subducting in southern Alaska, where they are doubled up on top of Pacific oceanic crust and have apparently created a giant asperity, or impediment to subduction. Most of the subduction-zone transects also contain Mesozoic accretionary prisms, and two of them, Vancouver Island and Alaska, also contain thick, tectonically underplated bodies of late Mesozoic/early Cenozoic oceanic lithosphere, interpreted as fragments of the extinct Kula plate. In the upper crust, most of the five transform-fault transects (all in California) reflect: (1) tectonic wedging of a Mesozoic accretionary prism into a backstop, which includes Mesozoic/early Cenozoic forearc rocks and Mesozoic ophiolitic/arc basement rocks; and (2) shuffling of the subduction margin of California by strike-slip faulting. In the lower crust, they may reflect migration of the Mendocino triple junction northward (seen in rocks east of the San Andreas fault) and cessation of Farallon-plate subduction (seen in rocks west of the San Andreas fault). In northern California, lower-crustal rocks east of the San Andreas fault have oceanic-crustal velocity and thickness and contain patches of high reflectivity. They may represent basaltic rocks magmatically underplated in the wake of the migration of the Mendocino triple junction, or they may represent stalled, subducted fragments of the Farallon/Gorda plate. The latter alternative does not fit the accepted ‘slabless window’ model for the migration of the triple junction. This lower-crustal layer and the Moho are offset at the San Andreas and Maacama faults. In central California, a similar lower-crustal layer is observed west of the San Andreas fault. West of the continental slope, it is Pacific oceanic crust, but beneath the continent it may represent either Pacific oceanic crust, stalled, subducted fragments (microplates) of the Farallon plate, or basaltic rocks magmatically underplated during subduction of the Pacific/Farallon ridge or during breakup of the subducted Farallon plate. The transect in southern California is only partly representative of regional structure, as the structure here is 3-dimensional. In the upper crust, a Mesozoic prism has been thrust beneath crystalline basement rocks of the San Gabriel Mountains and Mojave Desert. In the mid-crust, a bright reflective zone is interpreted as a possible ‘master’ decollement that can be traced from the fold-and-thrust belt of the Los Angeles basin northward to at least the San Andreas fault. A Moho depression beneath the San Gabriel Mountains is consistent with downwelling of lithospheric mantle beneath the Transverse Ranges that appears to be driving the compression across the Transverse Ranges and Los Angeles basin.

Models of crustal anatexis in volcanic rifts: applications to southern Finland and the Oslo Graben, southeast Norway

Crustal anatexis in volcanic rifts is discussed in terms of a 1-D numerical model incorporating lithospheric extension, emplacement of hot mafic magma at crustal levels, various crustal compositions and pre-rift conditions. Only the volume of melt generated by anatexis is considered; crustal melt segregation, transport and emplacement at higher levels are not included. We find that under a broad range of conditions, influx of mafic magma to the crust is required for substantial anatexis to take place, and once this stops, silicic magmatism dies out rapidly. In many rifts the observed change in type of magmatism from predominantly mafic to silicic during rifting is probably more due to a diminishing potential for mafic magmas to ascend though the crust than to an actual halt in mafic melt generation at depth. This has important implications where melt production rates are used to analyse the formation of volcanic rifts and margins. When influx of mafic magmas takes place over a finite period of time, they may be focused in the depth region where the potential for anatexis is largest, and in some cases more anatexis can occur than by instantaneous emplacement of the entire mafic body. Our results support the idea that the 1.6 Gyr old rapakivi granites in southern Finland were formed by anatexis of an intermediate to felsic Svecofennian crust. Provided a mantle thermal anomaly of +200–250 °C existed beneath Finland, the seismically observed ∼10 km thick magmatic underplated body could have produced the exposed silicic rocks which are on average 2–3 km thick. In the Oslo Graben in southeast Norway, where the thickness of magmatic underplating is also about 10 km, the amount of rock produced by anatexis is poorly constrained, but appears to be between 1 and 2 km. Provided the pre-rift crustal thickness in the graben was 35–40 km or more, and heat was transported to crustal levels more efficiently than is feasible by conventional stretching models, the model predicts melt volumes in agreement with these estimates. We propose that secondary convection within the narrow graben could have contributed to the high heat transport efficiency. Finally, a conspicuous feature of the southern Finland magmatic region as well as the Oslo Graben is the large discrepancy between stretching factors derived from analysing dyke intrusions and faulting (Finland: β∼1.001; Oslo Graben: β<1.05) and by using variations in crustal thickness (Finland: β∼2; Oslo Graben: β≥1.7–2). A possible explanation for this difference may be that when large volumes of mafic rocks cause crustal anatexis, the partially molten crust becomes very weak and acts as a barrier for stress propagation, effectively isolating the upper crust from further deformations. This could add to the thermal weakening of the ductile lower crust due to the influx of hot mafic magmas. This hypothesis could be tested by investigating a large number of magmatic rifts, combined with development of new rheological models of the lithosphere including the effects of underplating and anatexis.

Crustal structure of the central part of the Vøring Basin, mid-Norway margin, from ocean bottom seismographs

Regional Ocean Bottom Seismograph (OBS) data acquired in the central and northern part of the Vøring Basin, mid-Norway margin, have been modelled by use of two-dimensional (2-D) ray-tracing. The regional dataset comprises thirty OBSs deployed along seven 100–170 km long profiles. The deeper part of the Vøring Basin is difficult to map using multichannel reflection data due to the presence of sills at intermediate sedimentary levels (2–5 km below sea-floor), but the modelling of the OBS-data reveals that this technique provides a reliable estimate of structures and seismic velocities from the sea-floor to the upper mantle. The shallow and intermediate sediments (to 5 km below sea-floor) are characterized by a vertical increase in velocity due to increased confining pressure. There is also considerable lateral variation in the velocities within sedimentary layers at all levels. The OBS-data confirm that intrusions of sills are important at intermediate and deep sedimentary levels (2–10 km below sea-floor) in most parts of the area. The sills seem to vary in lateral extent from about 20 km to more than 100 km, and their thickness is generally inferred to be about 200 m. The low velocity in the upper crystalline crust (6.2 km/s) confirms that the crust in the Vøring Basin is of continental origin. In most parts of the area the velocity of the lower crust is as high as 7.3–7.6 km/s. This high-velocity layer is interpreted as a magmatic underplated body with strong lateral variations in thickness. The base of the 7.3 km/s layer is interpreted as the Moho, and the upper mantle velocity is estimated to 8.2 km/s.

Comparison of deep structure along three transects of the western North American Continental Margin

Similarities in geology and potential field data that have in the past been noted among the regions of southern Alaska, southern Vancouver Island, and central California are now seen to be accompanied by similarities in deep crustal structure. A number of tectonic elements have been identified in the deep structure along transects in these three regions, although not all elements are present along each transect. These elements are (A) an actively subducting oceanic plate and (B) an overriding continental plate that consists of (1) a Cenozoic accretionary prism, (2) a Mesozoic accretionary prism, (3) a backstop to the Mesozoic prism, (4) a tectonically underplated body of oceanic rocks, and (5) a crustal root. The Mesozoic prism is in some cases an underthrust body (type 2a) but in other cases forms the principal component of a landward verging tectonic wedge (type 2b). The technically underplated body of oceanic rocks extends landward from the fault contact between the Cenozoic and Mesozoic prisms to a point beneath the backstop. The crustal root lies beneath the backstop and landward of the underplated body. All of these elements are interpreted to be present along the Alaskan and Vancouver Island transects. In Alaska the underplated body is interpreted to be fragments of the Kula plate; the same may be true at Vancouver Island. These two transects appear to differ in that, in Alaska, the Mesozoic prism, in one interpretation, is the principal component of a tectonic wedge (type 2b), whereas at Vancouver Island, it is an underthrust body (type 2a). Along the central California transect, active subduction is no longer taking place, and the San Andreas fault has removed the Cenozoic prism from this region of the North American plate. On the North American plate (i.e., east of the San Andreas fault), the Mesozoic prism, interpreted as the main component of a tectonic wedge (type 2b), and the backstop to the Mesozoic prism are present. There is, however, no clear evidence of tectonically underplated oceanic rocks, and the crust is thin (no root). In both Alaska and Vancouver Island, the Mesozoic prisms above the underplated bodies experienced low‐pressure/high‐temperature metamorphism at about the time of tectonic underplating; no such metamorphism is currently exposed in California. The metamorphism may have been caused by the underplating of young, hot oceanic crust, or, alternatively, by subduction of an oceanic ridge. The presence of a tectonic wedge (type 2b) in Alaska and California and the absence of such a wedge at Vancouver Island could arise either from the fact that in the former two locations the Mesozoic prisms were more voluminous, owing to either more rapid trench sedimentation or more rapid convergence, or to the possibility that at the latter location the Mesozoic prism was juxtaposed with the backstop primarily by strike‐slip faulting.