Plate Boundary Zone Research Articles

A comparison of GPS solutions for strain and SKS fast directions: Implications for modes of shear in the mantle of a plate boundary zone

The strain rate and vertical velocity fields for New Zealand are computed using GPS data from GEONET (NZ) collected during the past decade. Two domains for shear in the mantle are inferred by comparing the principal shortening direction with the fast direction of shear wave splitting. Beneath the central-southern part of the South Island the strains are low and its unclear if irrotational strain is taking place or if the splitting here is dominated by anisotropy in the asthenosphere. In contrast, data for the central and northern South Island suggest simple shear is dominant and distributed over a zone 200km wide. An analysis of the major strike-slip faults confirms that the strike of the major South Island fault systems makes a 60±15° angle with the shortening direction. A map of the vertical component of GEONET GPS velocities shows regions of surface uplift >5mm/y in both the central South and North Islands. While the pattern of uplift in central South Island is consistent with known geology, the rate of uplift in the central North Island is an order of magnitude higher than the geological rate estimated on a my time scale.

Deciphering tectonic phases of the Amundsen Sea Embayment shelf, West Antarctica, from a magnetic anomaly grid

The Amundsen Sea Embayment (ASE), with Pine Island Bay (PIB) in the eastern embayment, is a key location to understanding tectonic processes of the Pacific margin of West Antarctica. PIB has for a long time been suggested to contain the crustal boundary between the Thurston Island block and the Marie Byrd Land block. Plate tectonic reconstructions have shown that the initial rifting and breakup of New Zealand from West Antarctica occurred between Chatham Rise and the eastern Marie Byrd Land at the ASE. Recent concepts have discussed the possibility of PIB being the site of one of the eastern branches of the West Antarctic Rift System (WARS). About 30,000km of aeromagnetic data – collected opportunistically by ship-based helicopter flights – and tracks of ship-borne magnetics were recorded over the ASE shelf during two RV Polarstern expeditions in 2006 and 2010. Grid processing, Euler deconvolution and 2D modelling were applied for the analysis of magnetic anomaly patterns, identification of structural lineaments and characterisation of magnetic source bodies. The grid clearly outlines the boundary zone between the inner shelf with outcropping basement rocks and the sedimentary basins of the middle to outer shelf. Distinct zones of anomaly patterns and lineaments can be associated with at least three tectonic phases from (1) magmatic emplacement zones of Cretaceous rifting and breakup (100–85Ma), to (2) a southern distributed plate boundary zone of the Bellingshausen Plate (80–61Ma) and (3) activities of the WARS indicated by NNE–SSW trending lineaments (55–30Ma?). The analysis and interpretation are also used for constraining the directions of some of the flow paths of past grounded ice streams across the shelf.

Segmentation and morphology of the Central Indian Ridge between 3°S and 11°S, Indian Ocean

The segmentation pattern of 750km long Central Indian Ridge (CIR) between 3°S and 11°S latitudes in the Indian Ocean has been studied using multibeam bathymetry and magnetic data. Twelve ridge segments were identified that are separated by well defined transform faults and non-transform discontinuities. Magnetic model studies qualify the ridge as a slow spreading ridge with average full spreading rates varying from 26 to 38mm/yr. The disposition of the magnetic anomalies suggests that the plate opening direction has not changed during the last 0–4Ma period. Along axis variations in the magnetic anomalies, presence of axial volcanic ridges on the inner valley floor, variations in the depth and geometry of the rift valley, suggest distinct variati'ons in the accretionary processes along the ridge. Based on these characteristics and the segmentation pattern, we suggest that ten ridge segments are undergoing less magmatic phase of extension, while two segments have shown characteristics of magmatic accretion. The linear segments with narrow and shallow rift valley floor and near symmetric magnetic anomalies are identified as segments with magmatic accretion. The influence of diffuse plate boundary zone on the young seafloor fabric generated by the CIR has been examined. The seafloor topography different from the normal ridge parallel fabric observed at few places over the NE flank of the CIR is suggested to be the consequence of gradual and progressive influence of the distributed diffuse plate boundary (between the Indian–Capricorn plates), on the newly generated oceanic lithosphere. Further, we documented distinct ridge-transform intersection (RTI) highs, three of these RTI highs are identified as oceanic core complexes/megamullion structures. Megamullion structures are found to be associated with less magmatic sections. Increased seismicity has been observed at the less magmatic segment ends suggesting the predominance of tectonic extension prevalent at the sparsely magmatic sections.

Tectonics of the New Guinea Region

The New Guinea region evolved within the obliquely and rapidly converging Australian and Pacific plate boundary zone. It is arguably one of the most tectonically complex regions of the world, and its geodynamic evolution involved microplate formation and rotation, lithospheric rupture to form ocean basins, arc-continent collision, subduction polarity reversal, collisional orogenesis, ophiolite obduction, and exhumation of (ultra)high-pressure metamorphic rocks. We describe the major onshore and offshore tectonic and geologic components, including plate boundaries, seismicity, faults, and magmatism, and we integrate these with emerging ideas about mantle dynamics to evaluate the Cenozoic tectonic evolution of New Guinea. Future research opportunities to resolve the mantle structure beneath New Guinea will enable mantle dynamics to be linked to lithospheric and surface processes. Virtually all plate tectonic and mantle processes have been active in the New Guinea region throughout the Cenozoic, and, as such, its tectonic evolution has global significance.

The Cenozoic tectonostratigraphic evolution of the Barracuda Ridge and Tiburon Rise, at the western end of the North America–South America plate boundary zone

The Barracuda Ridge and the Tiburon Rise, two major oceanic basement ridges, lie at the western end of the diffuse North America–South America plate-boundary zone, where they enter the subduction zone beneath the Lesser Antilles island arc. There is a large degree of uncertainty in the motion between the North American and South American plates predicted by kinematic models of plate motion for the region of these two ridges during the Cenozoic and Quaternary. From the analysis of new multibeam and seismic reflection profiles acquired in 2007, together with older geophysical and geological data, we provide new information on the geological history of this area, including the timing of the formation of the Barracuda Ridge and Tiburon Rise in their present-day configurations. The timing of the deformation in this region is now much better constrained through the correlation of several key seismic horizons with existing DSDP and ODP holes. The seafloor topography inherited from the process of formation of the crust at the mid-oceanic ridge, was buried by distal turbidites by the end of the Paleogene. Beginning in the Middle–Late Miocene and then the Pleistocene, the Tiburon Rise and Barracuda Ridge, respectively, were uplifted and acquired their present-day forms and elevation, which is much more recent than previously believed. In the Quaternary, the uplift was accompanied by the deposition of very large mass transport deposits. The causes of uplift and deformation of the ridges have been convergence between the North American and South American plates and the flexure of these plates as they enter the Lesser Antilles subduction zone, The zone of uplift and deformation migrated northward during the Neogene and Quaternary.

Paleomagnetic constraints on Cenozoic deformation along the northwest margin of the Pacific‐Australian plate boundary zone through New Zealand

New Zealand straddles the boundary between the Australian and Pacific plates, a zone of oblique continental convergence and transform motion. The actively deforming region offers a unique opportunity to study the dynamics of deformation, including vertical‐axis rotation of rigid blocks within a transcurrent plate boundary zone. We present and interpret paleomagnetic data from three new and three previously published sites from the NW part of the South Island (NW Nelson region), where sedimentary strata dated between 36 and 10 Ma overlie the crystalline Paleozoic basement assemblages of the Gondwana margin. Compared with reference directions from the Australian apparent polar wander path, none of the results provide evidence of post‐Eocene vertical‐axis rotation. This suggests that for the past 36 Myr NW Nelson has remained a strong, coherent block that has moved as a contiguous part of the Australian plate. This is in marked contrast to the strongly rotated nature of more outboard accreted terranes to the east. For example, the Hikurangi Margin in the North Island (NW of the Alpine Fault) and the Marlborough region in the NE of the South Island (SE of the Alpine Fault), have both undergone diverse clockwise rotations of up to 140° since the early Paleogene. The NW tip of the South Island seems to have acted as a rigid backstop relative to these more complex oroclinal deformations. We infer that, because of its relatively stiff bulk rheology, it has not been drawn into the distributed plate boundary rotational deformation associated with the New Zealand Orocline.

沈み込み帯の地震の発生機構

The mechanisms that generate the three main types of earthquake in subduction zones are discussed addressing their relations to geofluids. Studies on the spatial distribution of earthquakes and seismic velocity structure within the subducted slab provide evidence that strongly supports the dehydration embrittlement hypothesis for the generation of intermediate-depth intraslab earthquakes. Detailed imaging of the seismic velocity structure in and around plate boundary zones suggests that interplate coupling is mainly controlled by local fluid over-pressure. Seismic tomography studies show the existence of inclined sheet-like seismic low-velocity zones in the mantle wedge, not only in Tohoku but also in other areas in Japan, which perhaps correspond to the upwelling flow of the subduction-induced convection system. These upwelling flows reach the Moho directly beneath the volcanic areas, suggesting that those volcanic areas are formed by the upwelling flows. Aqueous fluids derived from the slab are probably transported up through the upwelling flows to the arc crust, where they might weaken the surrounding crustal rocks and finally cause shallow inland earthquakes. All of these observations suggest that geofluids expelled from the subducting slab play an important role in the generation of earthquakes in subduction zones.

Evidence for Quaternary convergence across the North America–South America plate boundary zone, east of the Lesser Antilles

New seismic and bathymetric data reveal that the plate boundary zone between the North American and South American plates, east of the Caribbean, has occupied a >200-km-wide zone of localized basement uplift and folding, and of faulting of the sedimentary cover, during the early Pleistocene to Holocene. This zone, which includes the Barracuda Ridge and Tiburon Rise, exhibits north-south compressional structures and continues eastward up to 500 km from the boundary of the Caribbean plate. A sequence of young turbiditedominated sediments fi lls the basin between these two ridges and shows synsedimentary deformation; the base of the sequence is of early Pleistocene age, dated by biostratigraphic correlation with Deep Sea Drilling Project Site 27. Recent deformation in the turbidite-fi lled trough north of the Barracuda Ridge is shown by uplift of the seafl oor at the crests of anticlines, across which the shallowest sediment layers thin or pinch out.

Lu–Hf garnet geochronology applied to plate boundary zones: Insights from the (U)HP terrane exhumed within the Woodlark Rift

High-pressure and ultra high-pressure (U)HP metamorphic rocks occur in many of the world's major orogenic belts, suggesting that subduction of continental lithosphere is a geologically important process. Despite the widespread occurrence of these rocks, relatively little is known about the timescales associated with (U)HP metamorphism. This is because most (U)HP terranes are tectonically overprinted and juxtaposed against rocks with a different history. An exception to this are the Late Miocene (U)HP metamorphic rocks found in active metamorphic core complexes (MCC) in the Woodlark Rift of southeastern Papua New Guinea. This region provides a rare opportunity to study the garnet Lu–Hf isotopic record of (U)HP metamorphism in a terrane that is not tectonically overprinted. In order to constrain the timing of garnet growth relative to the history of (U)HP metamorphism and the evolution of the Woodlark Rift, Lu–Hf ages were determined, in conjunction with measurements of Lu and major element zoning, for garnets from three metamorphic rocks. Garnets from the three samples yielded different ages that, instead of recording the spatial and temporal evolution associated with a single metamorphic event, provide information on the timing of three separate plate boundary events. The youngest Lu–Hf age determined was 7.1 ± 0.7 Ma for garnets in a Late Miocene coesite eclogite. The age is interpreted to record the time when a garnet-bearing partial melt of the mantle crystallized within subducted continental lithosphere at (U)HP conditions. The young Lu–Hf age from the coesite eclogite is in contrast to a 68 ± 3.6 Ma Lu–Hf age obtained on large (1–2 cm) garnet porphyroblasts, from within the Pleistocene amphibolite facies shear zone carapace bounding exposures of (U)HP rocks in the D'Entrecasteaux Islands. This older age records the growth of garnet in response to continental subduction and ophiolite obduction in the region north and east of Australia during late Mesozoic/early Cenozoic time. In order to relate the (U)HP metamorphic rocks to the larger-scale tectonic evolution of the Woodlark Rift, a third Lu–Hf age of 11.2 ± 2.1 Ma was determined for dynamically recrystallized garnet from the southeastern margin of the rift. While the tectonic implications of the Lu–Hf age from the southeastern-rifted margin are not fully known, this result suggests that the eastern and western parts of the Woodlark Rift might have a different metamorphic history. The garnet Lu–Hf results demonstrate that prior to rifting, the Woodlark Rift underwent a complex history of metamorphism involving garnet growth and recrystallization. The ability of the Lu–Hf system to date Late Miocene garnet growth, and at the same time preserve Latest Cretaceous garnets within Pleistocene shear zones in the Woodlark Rift demonstrate that Lu–Hf garnet geochronology is an essential component of investigations where the primary objective is documenting and understanding the full range of plate boundary processes recorded in metamorphic minerals.

Seismic imaging of the Amur–Okhotsk plate boundary zone in the Japan Sea

We determined high-resolution images of P and S wave velocity and Poisson’s ratio under the Japan Sea off Honshu Island using a large number of arrival-time data from local earthquakes to understand the interaction of the Amur and Okhotsk plates. Our new data collected from the crustal earthquakes under the Japan Sea relocated precisely with sP depth phase are crucial to make this work possible. Our results show that strong lateral heterogeneities exist in the crust and upper mantle under the Amur–Okhotsk plate boundary zone, which may have affected the seismotectonics in the region. The crustal velocity variations under the Japan Sea may reflect the complicated geologic structures that were produced during the opening of the Japan Sea and the present compressional stage of the Honshu arc associated with the collision of the Amur plate with the Okhotsk plate. Low-velocity zones in the mantle wedge are found to extend westward under the Japan Sea, rather than just confined under Honshu Island, suggesting that the back-arc magmatism and tectonics are part of the complex geodynamic system in East Asia.

New GPS constraints on active deformation along the Africa–Iberia plate boundary

We use velocities from 65 continuous stations and 31 survey-mode GPS sites as well as kinematic modeling to investigate present day deformation along the Africa–Iberia plate boundary zone in the western Mediterranean region. The GPS velocity field shows southwestward motion of the central part of the Rif Mountains in northern Morocco with respect to Africa varying between 3.5 and 4.0 mm/yr, consistent with prior published results. Stations in the southwestern part of the Betic Mountains of southern Spain move west–southwest with respect to Eurasia (∼ 2–3 mm/yr). The western component of Betics motion is consistent with partial transfer of Nubia–Eurasia plate motion into the southern Betics. The southward component of Betics motion with respect to Iberia is kinematically consistent with south to southwest motion of the Rif Mountains with respect to Africa. We use block modeling, constrained by mapped surface faults and seismicity to estimate the geometry and rates of strain accumulation on plate boundary structures. Our preferred plate boundary geometry includes one block between Iberia and Africa including the SW Betics, Alboran Sea, and central Rif. This geometry provides a good fit to the observed motions, suggesting a wide transpressive boundary in the westernmost Mediterranean, with deformation mainly accommodated by the Gloria–Azores fault system to the West and the Rif–Tell lineament to the East. Block boundaries encompass aspects of earlier interpretations suggesting three main deformation styles: (i) extension along the NE–SW trending Trans-Alboran shear zone, (ii) dextral strike-slip in the Betics corresponding to a well defined E–W seismic lineament, and (iii) right lateral strike-slip motion extending West to the Azores and right-lateral motion with compression extending East along the Algerian Tell. We interpret differential motion in the Rif–Alboran–Betic system to be driven both by surface processes related the Africa–Eurasia oblique convergence and sub-crustal dynamic processes associated with the long history of subduction of the Neotethys ocean lithosphere. The dextral slip identified in the Betic Mountains in Southern Spain may be related to the offshore fault that produced the Great 1755 Lisbon Earthquake, and as such may represent a significant seismic hazard for the West Mediterranean region.

Neotectónica de la región central de Costa Rica: frontera oeste de la microplaca de Panamá

Datos neotectonicos, de sismicidad y de sismotectonica son usados para definir el Cinturon Deformado delCentro de Costa Rica (CDCCR) como un sistema de fallas reciente, difuso y ancho localizado entre la placa Caribe y lamicroplaca de Panama, el cual es transversal a la region central de Costa Rica y localizado entre la fosa Mesoamericana(lado pacifico) y el Cinturon Deformado del Norte de Panama (sector caribe). En el antearco pacifico central, el sistemade fallas combina desplazamientos de rumbo siniestral y normal. En el arco interno existe: 1) Un sistema externo de fallas inversas, que tiene rumbo predominante WNW en la region norte del pais y 2) Un sistema interno de fallas que estaconstituido principalmente por fallas de desplazamiento de rumbo de tipos dextrales de direccion NW y siniestrales conrumbo entre ENE y NE. En la region trasarco, el sistema de fallas del CDCCR se une con el Cinturon Deformado delNorte de Panama, el cual incluye un sistema externo de fallamiento inverso y plegamiento de rumbo noroeste y otro in-terno de desplazamiento de rumbo dextral combinado con desplazamiento inverso de rumbo noroeste.Las fallas neotectonicas que constituyen el CDCCR son originadas por la colision del levantamiento del Coco con el surde Costa Rica. La indentacion tectonica generada por esta colision provoca el desplazamiento tectonico de la microplacade Panama con respecto a la placa Caribe, tanto hacia el sector este del Caribe como hacia el norte del pais. Este procesoha generado fallamiento extensional combinado con fallamiento transcurrente siniestral en la parte trasera oeste pacificadel limite difuso, lo que ha favorecido el desarrollo de cuencas marinas transversales al antearco. Por otro lado, el movi-miento delantero de la microplaca de Panama con respecto a la placa Caribe, ha originado un cinturon externo de fallasinversas y pliegues en la zona entre el arco interno y el trasarco. El sistema de fallas interno de desplazamiento de rumboque esta localizado dentro del arco interno permite movimientos traslacionales y rotacionales entre los diversos bloquestectonicos que constituyen la zona limite interplaca.

Large tectonic rotations in a wide zone of Neogene distributed dextral shear, northeastern South Island, New Zealand

The New Zealand plate-boundary zone in the northern part of South Island, New Zealand, comprises a series of active dextral strike-slip faults, referred to as the Marlborough Fault Zone (MFZ), which link oblique continental convergence farther south, along the Alpine Fault, to subduction of the Pacific plate along the Hikurangi margin. New paleomagnetic data are presented which, together with previous studies, suggest coherent Neogene clockwise rotation up to ~ 130° about a vertical axis of crustal blocks in the MFZ, starting at ~ 20 Ma. There is a striking correlation between the amount of observed rotation and the trend of Mesozoic basement structures, creating this part of the New Zealand orocline in a zone of distributed dextral shear, about 100 km wide, which has persisted for about 20 Ma. In the southern part of the MFZ, crustal blocks appear to be on a scale of 1–10 km, with boundaries that are subparallel to the basement strike. This way, deformation is accommodated by a combination of fault slip and rotation in a zone of more pervasive dextral shear, resulting in coherent bending of the basement structure. In the central part of the MFZ, this rotation appears to be that of elongate crustal blocks, ~ 50 km × 10 km in size, rotating ~ 80° clockwise at an average rotation rate ~ 4°/Ma. In detail, the rotation rate has decreased towards the present, from ~ 6°/Ma at ~ 20 Ma to < 2/Ma today. The overall effect of all this deformation is a ‘straightening-out’ of the major faults, so that the northern segments have evolved from ~ NW trending thrusts in the early Miocene to ~ NE dextral strike-slip faults today. A zone of higher shear strain farther north has resulted in an additional clockwise rotation 40–50°, with a total of ~ 130° clockwise rotation. The combination of rotation and fault displacement has acted as a hinge at the southern end of the Hikurangi margin, accommodating a drastic swing in trend from ~ NW in the early–middle Miocene to ~ NE trend.

Images of the Iberian Lithosphere from One Local Earthquake

Abstract On 12 August 2007, a magnitude M w 4.7 intraplate earthquake occurred near the center of the Iberian Peninsula, an area characterized by comparably simple crustal structure within the complex Iberia–Maghreb plate boundary zone, and characterized by complete azimuthal coverage with seismic broadband stations. We analyze regional intermediate-period (20- to 50-s) coda waveforms for this earthquake. They contain energy representing late-arriving surface waves that have been reflected laterally at lithospheric heterogeneities in or around Iberia, but complexity of the coda waveforms hampers a direct interpretation. We use coda recordings as the source for a back-propagating adjoint wave field and compute 3D Born sensitivity kernels for the dependence of least-squares waveform misfits of coda waves on wave speed variations. We hereby image the origin of single scattering recorded in the coda. Misfit kernels for P - and S -wave velocity show azimuth-dependent intensity variation as a result of source radiation, and an appropriate compensation significantly improves imaging quality, thereby revealing several clear lineaments. These are interpreted as surface-wave reflectors due to deep-rooted heterogeneity such as terrain boundaries or Moho topography, demonstrating the ability of the approach to unravel complex waveforms, and providing a new point of view on regional lithospheric structure.

Basement control on past ice sheet dynamics in the Amundsen Sea Embayment, West Antarctica

The development of landscapes and morphologies follows initially the tectonic displacement structures of the basement and sediments. Such fault zones or lineaments are often exploited by surface erosional processes and play, therefore, an important role in reconstructing past ice sheet dynamics. Observations of bathymetric features of the continental shelf of the Amundsen Sea Embayment and identification of tectonic lineaments from geophysical mapping indicate that the erosional processes of paleo-ice stream flows across the continental shelf followed primarily such lineaments inherited from the tectonic history since the Cretaceous break-up between New Zealand and West Antarctica. Three major ice flow trends correspond to different tectonic phases in east–west, northwest–southeast and north–south directions. East–west oriented basement trends correlate with coastline trends and overlay tectonic lineaments caused by former rift activities. Directional trends with northwest–southeast orientation are observed for the glacial troughs of the western embayment outer shelf, the western Pine Island Bay coastal zones, and the inner Pine Island glacial trough and are associated with a distributed southern plate boundary zone of the former Bellingshausen Plate. The north–south trend of the main Pine Island glacial trough and the north–northeast trend of the Abbot Ice Shelf trough on the outer shelf follow the predicted lineation trend of an eastern branch of the West Antarctic Rift System extending from the Thwaites drainage basin northward into Pine Island Bay. An understanding of this context helps better constrain the geometries and sea-bed substrate conditions for regional paleo-ice sheet models.

Receiver function imaging in strongly laterally heterogeneous crust: Synthetic modeling of BOLIVAR data

We resolve a large (∼20 km) discrepancy in Moho depth determined from PdS receiver functions (RFs) and from active source seismic profiling in the complex Caribbean-South American plate boundary zone in eastern Venezuela. As part of the BOLIVAR experiment 20 broadband stations were deployed along an active source profile to record teleseisms. Using the extremely heterogeneous crustal model obtained from active source data, we generated 2D finite-difference elastic wave synthetics and from them calculated receiver functions and CCP stacks. We compare the observations with synthetic sections that have been spatially sampled at 0.25 km to 40 km. The densely sampled synthetics show that several events in the field data that were originally interpreted as the Moho are multiple reflections within sedimentary basins. Where the Moho has the steepest dip under the plate boundary the CCP stacks fail to image the Moho well, regardless of the density of spatial sampling. A suitable spatial sampling criterion for clearly imaging the lower crust and Moho is to overlap Fresnel zones by 50% at Moho depth, which for the 1 Hz receiver functions examined here, requires an instrument spacing of 15–20 km, with the actual field data density ranging from 20 km to 100 km.

Correction to “Spatial and temporal patterns of exhumation across the Venezuelan Andes: Implications for Cenozoic Caribbean geodynamics”

[1] In the paper “Spatial and temporal patterns of exhumation across the Venezuelan Andes: Implications for Cenozoic Caribbean geodynamics” by Mauricio A. Bermúdez, Barry P. Kohn, Peter A. van der Beek, Matthias Bernet, Paul B. O'Sullivan, and Reginald Shagam (Tectonics, 29, TC5009, doi:10.1029/2009TC002635, 2010), there are corrections to Figure 9 caption and an additional reference. Figure 9. Possible kinematic configuration for the South American-Caribbean plate boundary zone during earliest Oligocene (Figure 9a), late Miocene (Figure 9b), and Pliocene times (Figure 9c). (a) Earliest Oligocene, widespread but relatively slow exhumation in the Venezuelan Andes, Eastern Cordillera of Colombia, and Perija range results from inversion of preexisting rift structures during regional compression, possibly related to initial subduction of Panama arc. (b) Late Miocene, rotation of the Maracaibo block (MB) is almost completed, left-lateral strike-slip displacement between the Maracaibo block and the South American plate is concentrated on the BFS, producing translation and minor rotation in the Trujillo block (TB) during its tectonic escape. Major exhumation of the SNB and SLCB occurs during this phase. (c) Pliocene, final closure of Panama arc is expressed by rapid localized exhumation in parts of the Venezuelan Andes (ECB) and Eastern Cordillera (Quetame Massif [Parra et al., 2009]). See text for more detailed discussion. Figures are modified from http://www.tectonicanalysis.com/site/caribbean/default.htm [see also Pindell and Kennan, 2009] with minor inputs from Montes et al. [2005], Corredor [2003], and Hoorn et al. [1995].

GPS Medium-Range Kinematic Positioning for the Seafloor Geodesy of Eastern Taiwan

To realize the plate motion of the Philippine Sea Plate (PSP) and the characteristics of crustal deformation in the plate boundary zone between the PSP and Eurasia Plate, three seafloor geodetic arrays using a combination of GPS kinematic positioning and acoustic ranging techniques have been established off eastern Taiwan since 2008. Each array is composed of three transponders deployed on the ocean bottom in a triangular shape and has been observed at least two times since 2009. The GPS kinematic positioning in the relative distance ranging from 80 to 120 km off the eastern coast of Taiwan plays a main role in the whole seafloor geodetic deformation system. Seven stations from Taiwan Continuous GPS Array are taken as reference sites and three or four rover GPS units are set up on the vessel or buoy. Both on-land reference and onboard rover receivers record data in sampling rates of 1 and 5 Hz to determine the instantaneous positions of transducer onboard which transmits and receives the acoustic signal to and from seafloor transponders and the attitude of vessel or buoy in kinematic mode. We compare the results of medium-range kinematic positioning between the on-land reference stations and rovers onboard by the GrafNav and Bernese V5.0 software, respectively. In addition, we determine the attitude at all times by way of computing the inter-distance of rover receivers onboard. Hence there are two positioning results can be estimated which are direct (by short relative distances from onboard) and indirect (by medium-range kinematic mode for each GPS unit from on-land continuous stations) methods, and the difference reveals in decimeter level.

Overview of results from continuous GPS observations in Iceland from 1995 to 2010

Iceland is a natural laboratory for a variety of processes associated with crustal deformation, such as earthquakes, magmatic events, tectonic plate motions, and glacial load changes. Continuous GPS (CGPS) measurements started in Iceland in 1995, and since then data from the network have helped to shed light on many different active deformation processes. The number of CGPS sites in Iceland tripled during 2006–2008, as a result of an international collaborative effort coordinated by Icelandic scientists. By early 2010 the number of CGPS stations in Iceland had reached 64, located primarily around and within the North-American–Eurasian plate boundary zone. Since its initiation, the CGPS network has played an important role in monitoring volcanoes and seismogenic areas, most notably during the 2009–2010 Eyjafjallajökull volcano unrest. Plate spreading of up to 2 cm per year usually dominates the horizontal motion observed at the CGPS sites, while uplift is observed at many of the stations due to recent retreat of the Icelandic ice caps. Co-seismic and post-seismic deformation of the largest earthquakes in 2000 and 2008 in the South Iceland Seismic Zone were captured by the network, and high-rate (1 Hz) CGPS observations helped to identify two magnitude 6 mainshocks in 2008 that were separated in time by only 2–3 seconds. The CGPS network has thus enabled us to monitor deformation occurring over days to months caused by migration of magma or fluids, post-seismic transients, rapid deformation caused by earthquakes and eruptions, as well as the long term plate spreading signal.

Gravity changes observed between 2004 and 2009 near the Tokai slow-slip area and prospects for detecting fluid flow during future slow-slip events

Slow-slip events (SSEs) have been observed in many plate-boundary zones along the circum-Pacific seismic belt. Previous studies have revealed that high-pressure fluids supplied from the subducted oceanic plate can generate SSEs. However, the behavior of these fluids during an SSE has not been fully elucidated. This paper discusses possible fluid migration along the plate boundary on the basis of spatiotemporal gravity changes observed by absolute and relative gravimeters during a long-term SSE in the Tokai district, Japan. Relative-gravity data are sometimes unreliable because of limited observation accuracies and possible noise produced by groundwater. Nevertheless, the observed gravity changes show a systematic pattern of spatial changes over the slow-slip area. This pattern can be explained by a poroelastic model assuming fluid migration along the plate interface, for which an inversion indicates a permeability of about 10−15 m2. This lies within the range of permeability values inferred by other studies in slow-slip areas. Long-term SSEs have occurred repeatedly in the Tokai district. If the permeability remains greater than 10−15 m2 during a future SSE, it will be possible to detect fluid migration by improving the observation accuracy to the 1-μGal level and accurately evaluating groundwater-related noise.