Adjacent Subduction Zone Research Articles

Mantle Flow Induced by the Interplay of Downgoing Slabs Revealed by Seismic Anisotropy Beneath the Sula Block in Eastern Indonesia

AbstractThe North Sulawesi subduction zone is characterized by southward subduction of the Celebes Sea slab to a depth of ∼250 km, mainly overlying the Sangihe slab that subducts west from the Molucca Sea and penetrates the mantle transition zone. The palaeo‐subducted Sula slab dips northward and partially underlies both the Sangihe and Celebes Sea slabs. Adjacent subduction zones with horizontal overlapping subducting slabs in the upper mantle have unclear dynamic interactions. An extensive strike‐slip fault forms the western boundary of the active North Sulawesi subduction zone, providing an ideal setting to study mantle flow between overlapping slabs. We use local S‐wave and teleseismic S and SK(K)S waveform splitting analysis to measure seismic anisotropy in the northern Sulawesi region. Our observations reveal typical mantle wedge corner flow within the Sangihe subduction system. In the Gulf of Tomini, the observed trench‐oblique fast‐axis orientations above the Celebes Sea slab are likely a consequence of the interaction between two subducting slabs. The southernmost measurement with an E–W‐trending fast direction in the mantle wedge might be related to the subduction of the Sula slab. Furthermore, fault‐parallel fast‐axis orientations of anisotropy near the southern segment of the Palu‐Koro fault are attributed to large‐scale shearing across this lithospheric‐scale strike‐slip fault system. Overall, our observations suggest that the strain caused by lithospheric and asthenospheric deformation is mainly confined within the microplate, displaying a restricted flow pattern and localized effects due to the size of the plate boundaries, such as the Palu‐Koro fault.

Evolution of stress fields during the supercontinent cycle

We investigate the evolution of stress fields during the supercontinent cycle using the 2D Cartesian geometry model of thermochemical convection with the non-Newtonian rheology in the presence of floating deformable continents. In the course of the simulation, the supercontinent cycle is implemented several times. The number of continents considered in our model as a function of time oscillates around 3. The lifetime of a supercontinent depends on its dimension. Our results suggest that immediately before a supercontinent breakup, the over-lithostatic horizontal stresses in it (referring to the mean value by the computational area) are tensile and can reach −250 MPa. At the same time, a vast area beneath a supercontinent with an upward flow exhibits clearly the over-lithostatic compressive horizontal stresses of 50–100 МРа. The reason for the difference in stresses in the supercontinent and the underlying mantle is a sharp difference in their viscosity. In large parts of the mantle, the over-lithostatic horizontal stresses are in the range of ±25 MPa, while the horizontal stresses along subduction zones and continental margins are significantly larger. During the process of continent-to-continent collisions, the compressive stresses can approximately reach 130 MPa, while within the subcontinental mantle, the tensile over-lithostatic stresses are about −50 MPa. The dynamic topography reflects the main features of the supercontinent cycle and correlates with real ones. Before the breakup and immediately after the disintegration of the supercontinent, continents experience maximum uplift. During the supercontinent cycle, topographic heights of continents typically vary within the interval of about ±1.5 km, relatively to a mean value. Topographic maxima of orogenic formations to about 2–4 km are detected along continent-to-continent collisions as well as when adjacent subduction zones interact with continental margins.

Mantle Flow Patterns Beneath the Junction of Multiple Subduction Systems Between the Pacific and Tethys Domains, SE Asia: Constraints From SKS‐Wave Splitting Measurements

AbstractThe region around the Celebes Sea, SE Asia, is evolving within a convergent tectonic environment involving the Pacific plate to the east and the Indian‐Australian plate to the south. It is arguably one of the most tectonically complex regions in the world and serves as an ideal setting to study dynamic interactions between the Pacific and Tethys tectonic domains. The issue of which subducting plate plays a leading role in governing the regional mantle flow is not well understood. Mantle flow can be characterized by seismic anisotropy observations, providing clues for understanding regional tectonics. We conducted SKS‐wave splitting analysis by using data from seven seismic stations located around the Celebes Sea. Our results, when combined with previous observations, suggest the presence of various types of mantle flow in this area, including (a) corner flow in the mantle wedge above the westward‐subducting Molucca Sea (Sangihe) slab, (b) two‐layer anisotropy related to the eastward‐subducting Molucca Sea (Halmahera) slab, (c) deflected flow due to proximity to the Sangihe slab's edges, (d) trench‐normal mantle flow beneath southeastern Borneo due to the subduction of the Indian‐Australian plate, and (e) Northwest Borneo‐Palawan trough‐parallel mantle flow beneath northeastern Borneo. Various types of mantle flow indicate that the dynamic interactions of adjacent subduction zones played crucial roles in influencing the regional upper mantle dynamics between the Tethys and western Pacific domains since the breakup of the Gondwana supercontinent in the Mesozoic.

3D Geodynamic Models for HP‐UHP Rock Exhumation in Opposite‐Dip Double Subduction‐Collision Systems

AbstractThe dynamics of high pressure (HP) and ultra‐high pressure (UHP) rock exhumation in opposite‐dip double subduction‐collision systems remain enigmatic. Here, we present 3D thermo‐mechanical numerical models to study the geodynamics of continental‐margin subduction, collision, and crustal exhumation of HP‐UHP metamorphic rocks in oppositely dipping adjacent subduction zones. In our model setups, an initial transform‐fault separates a purely oceanic subduction zone from an adjacent oceanic subduction zone that in time transforms into continental‐margin subduction. The results show that most simulations cause continental‐margin deep subduction within the collisional belts. In contrast to the results of 2D and 3D single subduction‐collision systems, the slab‐slab interactions strongly modify the down‐channel Couette flow and the up‐channel Poiseuille flow in the interplate region when the plate edges are close to each other. This prevents the exhumation of UHP metamorphic rocks to the Earth's surface. However, far from the interplate region, rapid exhumation of UHP rocks occurs in Alpine‐type models with a low Moho temperature, strong lower crust, and fast upper‐plate divergence, whereas the Taiwan‐type models occur for a high Moho temperature, weak lower crust, fast convergence, fast erosion, and are likely to facilitate rapid dome‐like HP exhumation. Based on our results, we suggest that the main characteristics, such as deep crustal subduction and rapid exhumation of HP‐UHP metamorphic rocks during mountain building in Taiwan (∼8 Ma to present) and in the Western Alps (∼49–35 Ma), may be the result of opposite‐dip double subduction‐collision systems.

Complex Shear Partitioning Involving the 6 February 2012 MW 6.7 Negros Earthquake Ground Rupture in Central Philippines

A 75 km-long, generally NE-striking ground rupture associated with the 6 February 2012 MW 6.7 (Mb 6.9) Negros earthquake was mapped on the eastern side of Negros Island, Philippines. It closely follows a previously unmapped, pre-existing fault trace along the coast which is marked mostly by terrace-forming scarps. The dominance of vertical separation (west side up) is consistent with a west-dipping reverse fault, as indicated by focal mechanism solutions. The ground rupture map eliminates the ambiguity in the focal mechanism solution regarding the orientation, sense of motion, and location of the seismogenic fault plane, which are indispensable in the assessment of seismic hazards and the nature and distribution of deformation. This study uses the ground rupture map of the 2012 Negros earthquake in sorting out the mechanism of deformation in the Visayas Islands region. The ground rupture’s length is well within the aftershock area while its scarp heights are consistent with an earthquake of its magnitude and nature of movement. The 2012 Negros earthquake rupture’s pattern, scarp types, and offset of man-made structures are similar to those of recent reverse/thrust ground ruptures mapped globally and are distinct from those associated with erosion, landslide, and liquefaction. The onshore coseismic reverse fault of the Negros earthquake, which contradicts a model of coseismic slip on an offshore blind thrust fault by previous workers, represents the first thoroughly mapped ground rupture of its kind in the Philippines. The ground ruptures of the 2012 Negros and 2013 Bohol earthquakes, along with the Philippine Trench and the Philippine Fault Zone (PFZ), represent a complex shear partitioning mechanism in the Visayas Islands region. This departs from the current simple shear partitioning model for the region and is distinct from those for other regions along the PFZ and adjacent subduction zones. This study shows how an appreciation of morphotectonic features can lead to a better understanding of the distribution of deformation and the nature of earthquake hazards.

Ongoing mass wasting processes in the Traful lake area, Argentinean Patagonia

The Patagonian Andes are characterized by a high relief, with numerous confined lakes in deep glacial valleys that currently experience minor rock falls. Although, there is a poorly known historical and post-glacial register of mass wasting phenomena and a complete absence of monitoring for potential future processes. In particular, the area called Los lagos, in Argentina, which is located south of latitude 35° S, where the glacial erosion shaped several valleys on both sides of the Andean range. The Andean area between 35° and 42° S has been releasing almost 25% of the world's total seismic energy from the adjacent subduction zone. The objective of this work is to analyze, a sector, in the northern margin of the Traful Lake, called Bosque Sumergido. The name of Bosque Sumergido was given because of the presence of an area of submerged dead trees in life position where part of the wood can be found underwater. In this work, we evaluate the potential risk for the city of Traful, located in front of it. We described this area in the context of mass wasting processes, the coastal morphological changes that occurred in the last 22 years and even some underwater features and bathymetry and analyzed the potential risk of tsunamis due to material falling into Traful Lake.

P and S Wave Tomography Beneath the SE Tibetan Plateau: Evidence for Lithospheric Delamination

AbstractThe SE Tibetan Plateau is located between the eastern Himalayan syntaxis and the stable Yangtze craton; its tectonic evolution is important for understanding the plateau expansion. However, interactions between the plateau and its adjacent subduction zone and craton are still unclear. In this study, we determine updated high‐resolution P and S wave tomographic models of the crust and upper mantle beneath the SE Tibetan Plateau. We confirmed the existence of a high‐velocity layer in the upper mantle down to 200‐km depth beneath the Sichuan basin, the core of the Yangtze craton, representing the stable cratonic root. Another high‐velocity body is located beneath Burma, reflecting the subducting Indian slab in the upper mantle and possibly the remnant Burman slab in the mantle transition zone. A strong low‐velocity column exists above the high‐velocity body in the transition zone and reaches the Moho under the Tengchong volcano. In addition, we find high‐velocity fragments in the upper mantle beneath the Yangtze craton and northern Indochina, which may reflect delaminated lithosphere beneath the SE Tibetan Plateau. The lithospheric delamination may be closely related to mantle flow extruded from the high plateau and possible thermal upwelling from the lower mantle. Our results provide new insight into the deep interactions between the Tibetan plateau and its surrounding blocks near the SE plateau margin.

Divergence of cryptic species of Doryteuthis plei Blainville, 1823 (Loliginidae, Cephalopoda) in the Western Atlantic Ocean is associated with the formation of the Caribbean Sea

Although recent years have seen an increase in genetic analyses that identify new species of cephalopods and phylogeographic patterns, the loliginid squid of South America remain one of the least studied groups. The suggestion that Doryteuthis plei may represent distinct lineages within its extensive distribution along the western Atlantic coasts from Cape Hatteras, USA (36°N) to northern Argentina (35°S) is consistent with significant variation in a number of environmental variables along this range including in both temperature and salinity. In the present study D. plei samples were obtained from a large number of localities along the western Atlantic coasts to investigate the distribution of these possible species in a phylogeographic context. Phylogeographic analyses were performed using the mitochondrial Cytochrome Oxidase I gene and nuclear Rhodopsin gene. Divergence times were estimated using Bayesian strict clock dating with calibrations based on fossil records for divergence from the lineage containing Vampyroteuthis infernalis (162mya), the probable origins of the North American loliginids (45mya), and the European loliginids (20mya) and fossil statolith from Doryteuthis opalescens (3mya). Our results suggest a deep genetic divergence within Doryteuthis plei. The currently described specie consists of two genetically distinct clades (pair-wise genetic divergence of between 7.7 and 9.1%). One clade composed of individuals collected in northwestern Atlantic and Central Caribbean Atlantic waters and the other from southwestern Atlantic waters. The divergence time and sampling locations suggest the speciation process at approximately 16Mya, which is in full agreement with the middle Miocene orogeny of the Caribbean plate, ending up with the formation of the Lesser Antilles and the adjacent subduction zone, coinciding with a particularly low global sea level, resulting in the practical absence of continental shelves at the area, and therefore an effective geographic barrier for D. plei. Furthermore, this study also provides evidence of previously undocumented sub-population structuring in the Gulf of Mexico.

Seismological imaging of ridge–arc interaction beneath the Eastern Lau Spreading Center from OBS ambient noise tomography

The Lau Basin displays large along-strike variations in ridge characters with the changing proximity of the adjacent subduction zone. The mechanism governing these changes is not well understood but one hypotheses relates them to interaction between the arc and back-arc magmatic systems. We present a 3D seismic velocity model of the shallow mantle beneath the Eastern Lau back-arc Spreading Center (ELSC) and the adjacent Tofua volcanic arc obtained from ambient noise tomography of ocean bottom seismograph data. Our seismic images reveal an asymmetric upper mantle low velocity zone (LVZ) beneath the ELSC. Two major trends are present as the ridge-to-arc distance increases: (1) the LVZ becomes increasingly offset from the ridge to the north, where crust is thinner and the ridge less magmatically active; (2) the LVZ becomes increasingly connected to a sub-arc low velocity zone to the south. The separation of the ridge and arc low velocity zones is spatially coincident with the abrupt transition in crustal composition and ridge morphology. Our results present the first mantle imaging confirmation of a direct connection between crustal properties and uppermost mantle processes at ELSC, and support the prediction that as ELSC migrates away from the arc, a changing mantle wedge flow pattern leads to the separation of the arc and ridge melting regions. Slab-derived water is cutoff from the ridge, resulting in abrupt changes in crustal lava composition and crustal porosity. The larger offset between mantle melt supply and the ridge along the northern ELSC may reduce melt extraction efficiency along the ridge, further decreasing the melt budget and leading to the observed flat and faulted ridge morphology, thinner crust and the lack of an axial melt lens.

Three‐dimensional mantle circulations and lateral slab deformation in the southern Chilean subduction zone

AbstractThe along‐strike variation in plate properties is a common feature in the subduction zones. The southern Chilean subduction is a notable example where the slab age and length, and the thickness of the overriding plate, vary substantially in the trench‐parallel direction. In this study the combined effects of these factors are examined using regional subduction zone models. In the models lateral slab deformation and along‐arc pressure gradient in the mantle occur in response to the differential slab retrograde motion due to variations in slab buoyancy and slab strength. The lower portion of the deep segments acts as a slab edge in part, while rollback of the neighboring, shallower segments influences the toroidal motion induced by the adjacent deep segments. These factors together give rise to complex flow patterns and lead to a great extent of trench‐parallel components in the mantle wedge and subslab mantle and significant upwelling in the back arc. Mantle circulations are characterized by variable length scales for both toroidal and poloidal motions extending over ~ 1500 km. Such three‐dimensional circulations may lead to complicated patterns of seismic anisotropy. The upwelling may result in decompression melting to cause the extensive Patagonian plateau basalts. Our model results indicate that certain segments of the Antarctic slab reach at least 100 km depth. The trajectories of passive tracers show intricate patterns such as helical streamlines and suggest that a recent input from the adjacent subduction zone for slab‐derived components of lavas from the Chilean Ridge cannot be ruled out.

GPS crustal deformation, strain rate, and seismic activity after the 1999 Chi‐Chi earthquake in Taiwan

Using data at 110 continuous GPS stations from 1 January 2003 to 31 December 2005, we characterized the surface deformation in Taiwan after the Mw = 7.6 Chi‐Chi earthquake of 21 September 1999. In continuous GPS (CGPS) data, the maximum coseismic deformation of the Chengkung earthquake and Ilan double earthquakes reached 165.5 ± 0.5 mm and 35.4 ± 0.5 mm in horizontal displacement and 181.7 ± 1.1 mm and 12.6 ± 1.5 mm in vertical displacement, respectively. With respect to Paisha station, S01R, the stations of the Coastal Range and Lanhsu showed an average displacement of 40.5–93.6 mm/yr with directions of 307°–333°. The stations in the Longitudinal Valley and Central Range revealed velocities in the range 19.0–49.3 mm/yr with directions of 285°–318°. In western Taiwan, the velocities in the inner fold‐and‐thrust belt range from 14.2 to 45.5 mm/yr with directions of 284°–304°. Extensional strain affects the Ilan and Pingtung plains near belt tips, revealing lateral extrusion toward the adjacent subduction zones. Extensional strain also affects the southern Central Range because of the rapid uplift related to the southward propagating collision process. Large and medium size earthquakes affect the strain pattern revealed by CGPS, albeit in different ways: regional extension and displacement were large and rotations were small regarding the Mw = 7.6 Chi‐Chi earthquake. In contrast, the limited size of the affected area, moderate displacement, and large rotations characterized the 2003 Mw = 6.8 Chengkung earthquake. The impact of smaller earthquakes such as the 2005 Mw = 5.9 Ilan double earthquakes and the 2005 Mw = 5.6 Hualien earthquake was locally significant but regionally minor. The CGPS data provide a snapshot of the deformation that is generally consistent with the long‐term history of the collision but should not be directly extrapolated because thrust deformation is migrating along the tectonic boundary. Regarding the Chi‐Chi earthquake, the new CGPS data show that the Chi‐Chi hanging wall is still recognizable as a kinematic block, whereas in the previous pattern the hanging wall was not discernable.

Mesoproterozoic plume‐modified orogenesis in eastern Precambrian Australia

Tectonic models for the latest Paleoproterozoic to earliest Mesoproterozoic evolution of eastern Australia (circa 1620–1500 Ma) are diverse and either emphasize plume or plate margin activity, neither of which satisfactorily explains all geological observations. The dichotomy is largely attributed to geochemical, spatial and temporal data that suggest voluminous A‐type felsic magmas are plume related, whereas distribution of arc‐related magmas and intense orogenic overprint suggest plate margin activity. The salient geological events include arc‐related magmatism at circa 1620–1610 Ma followed by a magmatic hiatus coincident with north‐south crustal shortening (1610–1590 Ma) and a magmatic flare‐up of A‐type felsic magmas throughout the Gawler Craton (circa 1595–1575 Ma). These magmas form the oldest component of a northward younging hot spot track that extends to the Mount Isa Inlier. At circa 1590–1550 Ma, arc magmatism resumed along the northern margin of the Gawler Craton and the rest of eastern Australia records a 90° shift in the regional shortening direction related to activity along the eastern margin of the Australian continent. A plume‐modified orogenic setting satisfies all of the spatial and temporal relationships between magma generation and orogenic activity. In this model, the Gawler Craton and the adjacent subduction zone migrated over a mantle plume (circa 1620–1610 Ma). Resultant flat subduction caused transient orogenesis (1610–1595 Ma) in the overriding plate. Slab delamination and thermal assimilation of the plume and the subducting slab caused a switch to crustal extension in the overriding plate, resulting in extensive mantle‐derived and crustal melting in the Gawler Craton (1595–1575 Ma).

Does extrusion occur at both tips of the Taiwan collision belt? Insights from active deformation studies in the Ilan Plain and Pingtung Plain regions

We analyse the present-day deformation in two key areas of the Taiwan collision belt, the Ilan Plain to the NW and the Pingtung Plain to the SW. Our approach is mainly based on consideration of horizontal displacement revealed by recent geodetic (GPS) surveys, derived strain rate tensors indicating horizontal deformation and three-dimensional seismotectonic stress regimes issued from inversion of focal mechanisms of earthquakes. We reconstruct a consistent, albeit complex, tectonic pattern involving non-rigid rotations (clockwise in NE Taiwan, anticlockwise in SW Taiwan), transitions from pure compression near the mountains to transtension near the sea and simple shear affecting the deforming domain (NW–SE left-lateral in NE Taiwan, NNE–SSW right-lateral in SW Taiwan). These tectonic patterns reflect lateral extrusion processes towards mechanically weak domains with respect to collision zone, i.e., adjacent subduction zones (Ryukyu to the NE, Manila to the SW). The extrusion of the Ilan Plain area occurs towards the SE, whereas that of the Pingtung Plain area occurs towards the SW. The extrusion in NE Taiwan is facilitated by the opening of the Okinawa Trough, so that velocity and deformation rates are higher than in SW Taiwan despite less active collision in this northern area. The symmetry of the extrusion patterns is altered by differences in relative positions and orientations of collision belt and adjacent subduction zones, which depend on the plate tectonic configuration and tectonic history of the Taiwan region.

Plate acceleration: The obduction trigger?

Obduction processes, though spectacular (dense oceanic ophiolites are emplaced on top of light, continental rocks along thousands of km), have been little elucidated since the advent of plate tectonics. Based on convergence velocities and blueschist formation, we show that the two recent large-scale, Upper Jurassic and Upper Cretaceous obductions coincided with periods during which velocities increased abruptly and more than doubled. The latter obduction also modified the interplate coupling across adjacent subduction zones. We critically propose a mechanism in which large-scale obductions are triggered by intraplate instabilities resulting from sharp plate accelerations, possibly in response to superplume events.

Global catalogue, classification and tectonic origins of restraining- and releasing bends on active and ancient strike-slip fault systems

Abstract Restraining- and releasing bends with similar morphology and structure have been described by many previous studies of strike-slip faults in a variety of active and ancient tectonic settings. Despite the documentation of at least 49 restraining and 144 releasing bends along active and ancient strike-slip faults in the continents and oceans, there is no consensus on how these structural features are named and classified, or how their wide range of structures and morphologies are controlled by the distinctive strike-slip tectonic settings in which they form. In this overview, I have compiled published information on the strike-slip tectonic setting, size, basin and bend type, age, and models for active and ancient releasing and restraining bends. Examples of bends on strike-slip faults are compiled and illustrated from five distinctive active strike-slip settings: oceanic transforms separating oceanic crust and offsetting mid-oceanic spreading ridges; long and linear plate-boundary strike-slip fault systems separating two continental plates whose plate-boundary kinematics can be quantified for long distances along strike by a single pole of rotation (e.g. the San Andreas fault system of western North America); relatively shorter, more arcuate indent-linked strike-slip fault systems bounding escaping continental fragments in zones of continent–continent or arc–continent collision (e.g. the Anatolian plate); straight to arcuate trench-linked strike-slip fault systems bounding elongate fore-arc slivers generated in active and ancient fore-arc settings by oblique subduction (e.g. Sumatra); and cratonic strike-slip fault systems removed from active plate boundaries, formed on older crustal faults, but acting as ‘concentrators’ of intraplate stresses. By far the most common, predictable and best-studied settings for restraining and releasing bends occur in continental-boundary strike-slip fault systems, where arrays of two to eight en échelon pull-apart basins mark transtensional fault segments and single and sometimes multiple large restraining bends mark transpressional segments; fault areas of transtension versus transpression are determined by the intersection angles between small circles about the interplate pole of rotation and the trend of the strike-slip fault system. These longer and more continuous boundary strike-slip systems also exhibit a widespread pattern of ‘paired bends’ or ‘sidewall ripouts’, or adjacent zones of pull-aparts and restraining bends—that range in along-strike-scale from kilometres to hundreds of kilometres. En échelon arrays of pull-apart basins are also observed on active ‘leaky’ or transtensional oceanic transforms, but restraining bends are rarely observed. In indent-linked strike-slip settings, strike-slip fault traces bounding escaping continental fragments tend to be more arcuate, less-continuous, and more splayed – but paired bends are common. Trench-linked strike-slip fault patterns closely mimic the trends of the subduction zone; these strike-slip faults can vary from long and continuous to short and arcuate, depending on the trace of the adjacent subduction zone. Paired bends are also observed in this setting. Bends on active, cratonic strike-slip fault form isolated, seismically active structures that act as ‘stress concentrators’ for intraplate stress. Cratonic strike-slip faults are generally not associated with pull-apart basins, and therefore paired bends are not observed in this setting. The most likely geological models for the formation of releasing, restraining bends, and paired bends along boundary and trench-linked strike-slip faults include: progressive linkage of en échelon shears within a young evolving shear zone; this model is not applicable to older strike-slip fault traces that have accumulated significant, lateral fault offsets; formation of lenticular ‘sidewall ripout’ structures at scales ranging from outcrop to regional; ripouts are thought to form as a response to adherence or sticking along an adjacent and relatively straight strike-slip fault zone; this structural concept may help to explain the large number of paired bends embedded within strike-slip systems, sinusoidal curvature along the traces of many strike-slip faults, and the episodic nature of lateral shifts in the main strike-slip fault zone; interaction of propagating strike-slip faults with pre-existing crustal structures such as ancient rift basins. Propagation of new strike-slip faults and interaction with older structures may occur on plate boundary, indent-linked, and trench-linked strike-slip faults; and concentration of regional maximum compressive stress on pre-existing, basement fault trends in stable cratonic areas can produce active restraining-bend structures; periodic release of these bend-related stress concentrations is one of the leading causes of intraplate earthquakes within otherwise stable cratons.

Investigation of the earthquake sequence off Miyagi prefecture with historical seismograms

Abstract We investigated the old seismograms of several earthquakes which occurred offshore of Miyagi prefecture in 1933 (June 18, 21:37 UT), 1936 (Nov. 2, 20:45 UT), 1937 (July 26, 19:56 UT), 1978 (June 12, 08:14 UT), and 2005 (Aug. 16, 02:46 UT). A characteristic earthquake model for the sequence of the 1936, 1978, and several other earlier events is the basis of the recent forecast by the Earthquake Research Committee (2003) that “The probability of the occurrence of another similar earthquake in the next 30 years is 99%”. To assess the validity of the characteristic earthquake model, we compared the waveforms, size and other characteristics of these earthquakes recorded at Pasadena, DeBilt, Abuyama, Aso, Weston, Strasbourg, and Christchurch. We conclude that (1) The 1978 event is 3 to 4.5 times larger (in seismic moment) than the 2005 event; (2) The 1936 and the 2005 events are about the same size and are fairly close in location; (3) The 1937 event is smaller than the 1936 event, and is significantly deeper, possibly as deep as 90 km. In contrast, the 1933 event is significantly shallower than the 1936 event. The differences between these events are too large to justify the use of a simple characteristic earthquake model for the probabilistic forecast. The seismic slip rate in this area and along the adjacent subduction zone to the south is about 1/4 of the plate convergence rate, which has an important implication for the long-term seismic hazard in this area.The 2005 West Off Fukuoka Prefecture earthquake caused serious damage to and on Genkai Island as well as to downtown Fukuoka City. There were no strong motion instruments on the island, therefore no one knows how the strong ground motion occurred during the mainshock. The ground motion simulation on Genkai Island is very important to our understanding of earthquake damage at the near-source region. We have conducted an aftershock observation on the island in order to verify site amplification due to steep topography and to record aftershocks for reproducing ground motion during the mainshock by the empirical Green’s function method. The observed records of aftershocks show small variations in the input motions in the island, indicating that the amplification due to the topography seems to be small below 2 Hz. We first estimated the strong motion generation area for the mainshock using the observation records at stations surrounding the source region. We then carried out broadband ground motion simulation on Genkai Island by using the aftershock records as empirical Green’s functions. The simulated ground velocities exceed 1 m/s with a dominant period of 1–2 s due to the forward rupture directivity, and the instrumental seismic intensity reaches 6.6.

Internal deformation of the Gorda Plate observed by hydroacoustic monitoring

On August 29, 1991, the National Oceanic and Atmospheric Administration VENTS Program began monitoring the northeast Pacific Ocean plate boundaries using continuously recorded data from the U.S. Navy's Sound Surveillance System (SOSUS) hydrophone arrays. Routine detection and location of small (2.4 ≤ M ≤ 3.5) earthquakes revealed a NNE‐SSW trending band of seismicity through the center of the Gorda Plate. The majority of the earthquakes in the band were not detected by land‐based seismic networks. This band of microearthquakes was active from the initiation of SOSUS monitoring in August 1991 through July 1992, at a rate of ∼20 events per week. Since August 1992, however, the activity has effectively ceased. Some of the microearthquakes in the band are likely aftershocks to three large (MW > 6) mid Gorda Plate earthquakes that occurred 1–2 months prior to the beginning of SOSUS monitoring. The hydroacoustically detected events in the band, however, do not occur in the proximity of the MW > 6 events, and the activity level increased several months afterward, suggesting the microseismicity band represents a more general pattern of deformation within the Gorda Plate. The cessation of the midplate activity follows the occurrence of a large earthquake sequence in the adjacent Cascadia Subduction Zone in April 1992. It is proposed that the band of microseismicity and the three large midplate events reflect an accumulation of stress in the Gorda Plate, with the eventual termination of this pattern due to stress reduction associated with movement in the adjacent subduction zone. Stress field modeling of the region yields results consistent with this interpretation.

Age constraints on crustal recycling to the mantle beneath the southern Chile Ridge: He‐Pb‐Sr‐Nd isotope systematics

Basalts from the four southernmost segments of the subducting Chile Ridge (numbered 1–4 stepping away from the trench) display large variations in Sr, Nd, Pb, and He isotope and trace element compositions. Klein and Karsten [1995] showed that segments 1 and 3 display clear trace element evidence for recycled material in their source (e.g., low Ce/Pb). The uniformly mid‐ocean ridge basalt (MORB)‐like 3He/4He and modest variations in Pb, Sr, and Nd isotopes of segment 1 (nearest the trench) suggest recent (<20 Ma) introduction of a contaminant into its source, consistent with recycling of material from the adjacent subduction zone. In contrast, segment 3 lavas display a dramatic southward increase in enrichment, extending to highly radiogenic Pb and Sr isotopic compositions (e.g., 206Pb/204Pb = 19.5) and the lowest 3He/4He yet measured in MORB (3.5RA). The segment 3 variations are most readily explained by ancient (∼2 Ga) recycling of terrigenous sediment and altered crust, but we cannot rule out more recent recycling of material derived from a distant continental source. The similarity in isotopic signatures of segment 4 lavas to Indian Ocean MORB extends the Dupal anomaly to the Chile Ridge. Like Indian Ocean MORB, the segment 4 isotopic variations are consistent with contamination by anciently recycled pelagic sediment and altered crust and require a complex history involving at least three stages of evolution and possibly a more recent enrichment event. Southern Chile Ridge MORB reflect the extensive degree of heterogeneity that is introduced into the depleted upper mantle by diverse processes associated with recycling. These heterogeneities occur on a scale of ∼50–100 km, corresponding to transform‐ and propagating‐rift‐bounded segmentation, and attest to the presence of distinct chemical domains in the mantle often bounded by surficial tectonic features that maintain their integrity on the scale sampled by melting.

Komatiite–basalt–rhyolite volcanic associations in Northern Superior Province greenstone belts: significance of plume-arc interaction in the generation of the proto continental Superior Province

Archean greenstone belts from the northern Superior Province, ranging in age from 3.0 to 2.9 Ga, comprise an association of komatiite–tholeiite sequences, some of which were erupted onto continental crust, intercalated with felsic volcanic rocks. Komatiites from the Uchi subprovince are characterised by variable LREE enrichment and negative Nb anomalies consistent with contamination of Munro-type Al-undepleted komatiites by a felsic melt. The majority of komatiites display variable HREE patterns and positive normalised V–Sc anomalies. Tholeiitic flows intercalated with the komatiites typically have flat rare earth element (REE) patterns with variable Nb anomalies and high Th/Ce ratios, consistent with minor degrees of contamination by sialic crust. Two suites of felsic volcanic rock, Type 1 and Type 2, have been recognised intercalated with the komatiite–tholeiite sequences. Both types display LREE enriched patterns and negative Nb and Ti anomalies, but Type 1 has strongly fractionated HREE patterns, whereas Type 2 HREE are generally flat and have elevated Ni and Cr contents. The predominant Type 1 rhyolite is directly comparable to southern Superior Province felsic volcanic rocks associated with oceanic arc sequences and inferred to be the products of oceanic slab melting. Type 2 rhyolite geochemical signatures may result from mixing of Type 1 rhyolites with tholeiitic magmas, or a contribution from mantle wedge sources located above the garnet stability field. The intercalation of komatiites and arc-type felsic volcanic rocks is best explained by interaction between an ascending mantle plume and adjacent subduction zones.

Ocean-ridge basalts with convergent-margin geochemical affinities from the Chile Ridge

COLLISIONS between active spreading centres and subduction zones have occurred frequently throughout Earth history1. Of the few sites of ridge subduction active today, the southern Chile Ridge has the simplest tectonic history2. We report here that basalts recovered from the Chile Ridge adjacent to the Chile Trench have geochemical characteristics unlike those of mid-ocean-ridge basalts sampled elsewhere, and show affinities with arc volcanics. The observed chemical variations are consistent with contamination of depleted sub-oceanic mantle by marine sediments and altered oceanic crust, which may have been derived from the adjacent subduction zone. Although some ocean islands show evidence of crustal recycling3–5, the Chile Ridge lavas are the first ocean-ridge basalts to exhibit such extreme geochemical characteristics, and may thus offer the opportunity to study the development of one type of mantle heterogeneity as it occurs. The discovery of ocean-ridge basalts with convergent-margin chemical characteristics suggests that caution is warranted in using trace-element systematics to identify the provenance (mid-ocean ridge or supra-subduction zone) of ophiolites6,7.