Extinct Spreading Ridge Research Articles

Seismic structures of Zhenbei and Huangyan seamounts and their postspreading volcanic evolution

The Zhenbei and Huangyan seamounts located close to the extinct spreading ridge (ESR) in the South China Sea (SCS) have provided fundamental insight into postspreading magmatism. Long debates on the source and evolutional process of the postspreading seamounts are due to the lack of detailed structural crustal constraints. Here, we present three-dimensional (3D) crustal thickness and the Moho geometry of the Zhenbei and Huangyan seamounts and their surrounding area by forwarding and interpolating P-wave forward velocity structures along 14 seismic refraction and wide-angle profiles. We analyzed the variation in crustal velocity and thickness from the waning stage of seafloor spreading to the postspreading magmatism stage. Results show that the original oceanic crust, with an average thickness of 4.5 ± 0.7 km, is characterized by thin lower crust, thick upper crust, and low crustal velocity, compared with Atlantic oceanic crust, reflecting reduced magma supply and intense tectonic fracturing. Both the Zhenbei and Huangyan seamounts show high extrusion to intrusion ratio (2.6 and 2.7) and low P-wave velocities in the upper crust, indicating that extrusive processes are dominant during the postspreading volcanism in the SCS. The total magma volumes for building Zhenbei and Huangyan seamounts are estimated to be 4995 km3 and 3674 km3, respectively, with a related volume flux of ∼0.032 m3/s and ∼ 0.023 m3/s. These values are larger than those of Longnan seamount (2884 km3, 0.018 m3/s) but smaller than those of plume-derived seamounts. The relationship between average lower crustal velocity and crustal thickness also indicates that the postspreading magma may not be caused by the higher degree of mantle melting driven by elevated temperature, but by multiple lower degree of melting caused partly by the enriched mantle.

Forearc structures and deformation along the Manila Trench

The Manila Trench subduction zone is characterized by varying tectonic structures from north to south. Analyses of new seismic reflection data and bathymetric data indicate distinct morphological and deformational patterns in the forearc region. Differences in the nature of the subducting oceanic lithosphere (i.e. seafloor relief related to seamounts and ridges, sediment supply, reactivated features and faults associated with the South China Sea opening) cause along-strike heterogeneity in the Manila Trench and the Luzon forearc region. The northern segment is classified as an accretionary margin while the southern segment is mainly an erosive margin (with a narrow, steep, and often eroded frontal wedge). This sharp contrast is attributed to abundant sediment supply to the trench in the north and to the highly eroded frontal wedge in the south due to scarce sediment supply. The southern trench segment is prone to submarine slope failures and mass wasting processes. The 17°N latitude boundary also separates the forearc basin into the North Luzon Trough and the West Luzon Trough. Associated with this is the initiation of the Scarborough Seamount Chain (South China Sea extinct spreading ridge) subduction at 16°N latitude. A combination of forearc uplift and submarine mass movements attributed to subduction of bathymetric highs near and south of 17°N latitude produced the Stewart Bank. Seamount and other seafloor spreading features induced complex responses arising to diverse forearc architectures in the southern segment. Seamounts and other seafloor spreading-related features in the subducting slab induce slope steepening and significant vertical movement in the frontal wedge and the forearc region, respectively. Deformation associated with the subduction is overprinted by shearing related to the Philippine Fault Zone splay faults with the frontal wedge shortening associated with the ongoing subduction, further complicating the forearc development in the trench and marine forearc region.

Crustal velocity structure of the Northwest Sub-basin of the South China Sea based on seismic data reprocessing

The deep crustal structure of the Northwest Sub-basin (NWSB) of the South China Sea (SCS) is of great importance for understanding the tectonic nature of the continent-ocean transition (COT) and magmatism in this oceanic basin. The 2-D wide-angle reflection/refraction seismic profile OBS2006-2 is almost parallel to the extinct spreading ridge (ESR) of the NWSB. In addition to the original data, we added the data of two reprocessed OBS stations, and carried out seismic phase re-picking and travel-time imaging to obtain the crustal velocity structure along this profile. Resolution tests demonstrate that the newly acquired velocity structure is more reliable than the prior interpretation. The depth of the Moho (23.5–11.8 km) and crustal thickness (20.5–6.5 km) systematically changes from continental crust of the Xisha Block to the oceanic crust within the NWSB. The COT zone has a width of ∼20 km and the depth of the Moho decreases from 15.0 to 11.0 km, corresponding to a ∼4 km decrease in crustal thickness (6–10 km). A high velocity layer (HVL, 7.2–7.4 km s−1) exists at the bottom of the crust at the location where the sharp lateral transition of the continental crust to the oceanic crust occurs. Age dating shows that the Double-peak Seamount was formed at ∼23 Ma, after the cessation of the NWSB seafloor spreading (∼32-25 Ma). The crust beneath the Double-peak Seamount is oceanic with a thickness of 9 km. We infer that this oceanic crust was formed by magmatic upwelling and decompression melting along a pre-existing zone of weakness.

Crustal underplating and effective elastic plate thickness of the Laxmi Ridge, northern Arabian Sea

Laxmi Ridge, a prominent basement high feature with negative gravity anomaly in the northern Arabian Sea, is generally considered as stretched continental crust. However, recently it is proposed that the ridge could be an extinct spreading ridge. Ship-borne and satellite-altimetry derived bathymetry, gravity and geoid data over the Laxmi Ridge are analysed and modelled to determine effective elastic plate thickness (Te) and crustal structure beneath the ridge. The estimation of Te and compensation mechanism of the ridge are examined based on 3D coherence and admittance analysis considering surface and subsurface loads which involve computation of coherence and admittance between Mantle Bouguer Anomaly (MBA) and sediment corrected bathymetry. The best fit model reveals that Te is about 3–4 km and amplitude of subsurface load at Moho is approximately equal to the surface load. Low value of Te implies elastically weak lithospheric plate which can be attributed to either weakening of lithosphere by re-heating and underplating caused by the then nearby Réunion Hotspot or volcanic emplacement on the young oceanic crust. Further, all along the length of the Laxmi Ridge, the MBA is negative and its magnitude reaches up to 115 mGal, which indicates either presence of thickened crust or less dense mantle beneath the ridge. 2D crustal models suggest that underplating material is prevalent all along the Laxmi Ridge. High amplitude (>400 nT) magnetic anomaly over the southern part of the ridge reveals several lineations in WNW-ESE direction suggesting magma outpouring along week zones. Geoid to topographic ratio over the ridge is negative (−1.98 m/km), possibly indicating shallow subsurface load (<20 km) with small wavelength (<200 km). Based on present study and available geophysical results, we infer that the Laxmi Ridge is a stretched continental sliver which is underplated, intruded by volcanics and isostatically compensated by Airy Model of Compensation.

Postseafloor Spreading Volcanism in the Central East South China Sea and Its Formation Through an Extremely Thin Oceanic Crust

AbstractP wave velocity models were obtained by forward and inverse modeling from 38 ocean bottom seismometers deployed in the central East subbasin of the South China Sea (SCS). Four types of crust have been defined: (a) thin oceanic crust (&lt;5 km), (b) typical oceanic crust (5–6 km), (c) thick oceanic crust hosting postspreading volcanoes (&gt;6 km) with significant intrusive roots, and (d) thick oceanic crust with enhanced spreading features (&gt;6 km) but without significant roots. Within the central East subbasin, the thin oceanic crust, only identified inside an 80 km wide zone, is located within an overall 150 km wide domain characterized by N055° seafloor spreading trends. The postspreading volcanoes were formed during a N‐S tensional episode around 6–10 Ma, several millions of years after seafloor spreading ceased in the SCS. Seafloor spreading (N055° and N145°) and postspreading (N000° and N090°) features are observed in the morphology of some of these volcanoes. The rupture of the brittle thin oceanic crust was focused where the crust was the weakest, i.e., at the intersection of the extinct spreading ridge with former fracture zones. From geological and geophysical arguments, we suggest that the postspreading volcanism might have been influenced by the Hainan plume activity through a buoyancy‐driven partial melting mechanism.

Alkalic marine tephra layers at ODP Site 1241 - Major explosive eruptions from an oceanic volcano in a pre-shield stage?

We report a series of fourteen marine tephra layers that are the products of large explosive eruptions of Subplinian to Plinian intensities and magnitudes (VEI >4) from Cocos Island, Costa Rica. Cocos Island is a volcanic island in the eastern Central Pacific Ocean ~500km offshore Costa Rica, and is situated on the northwestern flank of the aseismic Cocos Ridge. Geochemical fingerprinting of Pleistocene (~2.4–1.4Ma) marine tephra layers from Ocean Drilling Project (ODP) Leg 202 Site 1241 using major and trace element compositions of volcanic glass shards demonstrates unequivocally their origin from Cocos Island rather than the Galápagos Archipelago or the Central American Volcanic Arc (CAVA). Cocos Island and the adjacent seamounts of the Cocos Island Province have alkalic compositions and formed on young (≤3Ma) oceanic crust from an extinct spreading ridge bounded by a transform fault against the older and thicker crust of the aseismic Cocos Ridge. Cocos Island has six times the average volume of the adjacent seamounts although all appear to have formed during the 3–1.4Ma time period. Cocos Island lies closest to the transform fault and we explain its excessive growth by melts rising from garnet-bearing mantle being deflected from the thick Cocos Ridge lithosphere toward the thinner lithosphere on the other side of the transform, thus enlarging the melt catchment area for Cocos Island compared to the seamounts farther away from the transform. This special setting favored growth above sea level and subaerial explosive eruptions even though the absence of appropriate compositions suggests that the entirely alkalic Cocos Island (and seamounts) never evolved through the productive tholeiitic shield stage typical of other Pacific Ocean islands, possibly because melt production rates remained too small. Conditions of magma generation and ascent resembled Hawaiian pre-shield volcanoes but persisted for much longer (<1m.y.) and formed evolved, trachytic magmas. Therefore Cocos Island may be a unique example for a volcanic ocean island that did not pass through the typical growth stages.

New Somali Basin magnetic anomalies and a plate model for the early Indian Ocean

The oldest portions of the Indian Ocean formed via the breakup of Gondwana and the subsequent fragmentation of East Gondwana. We present a constrained plate model for this early Indian Ocean development for the time period from Gondwana Breakup until the start of the Cretaceous Normal Superchron. The motions of the East Gondwana terranes are determined using new geophysical observations in the Somali Basin and existing geophysical interpretations from other coeval Indian Ocean basins. Within the Somali Basin, recent satellite gravity data clearly resolve traces of an east–west trending extinct spreading ridge and north–south oriented fracture zones. A thorough compilation of Somali Basin ship track magnetic data allows us to interpret magnetic anomalies M24Bn through M0r about this extinct ridge. Our magnetic interpretations from the Somali Basin are similar in age, spreading rate, and spreading directions to magnetic anomalies previously interpreted in the neighboring Mozambique Basin and Riiser Larsen Sea. The similarity between the two magnetic anomaly datasets allows us to match both basin's older magnetic anomaly picks by defining a pole of rotation for a single and cohesive East Gondwana plate. However, following magnetic anomaly M15n, we find it is no longer possible to match magnetic picks from both basins and maintain plausible plate motions. In order to match the post-M15n geophysical data we are forced to model the motions of Madagascar/India and East Antarctica/Australia as independent plates. The requirement to utilize two independent plates after anomaly M15n provides strong circumstantial evidence that suggests East Gondwana breakup began around 135Ma.

Crustal structure across the post-spreading magmatic ridge of the East Sub-basin in the South China Sea: Tectonic significance

The 140-km wide last phase of opening of the South China Sea (SCS) corresponds to a N145° direction of spreading with rift features identified on swath bathymetric data trending N055° (Sibuet et al., 2016). These N055° seafloor spreading features of the East Sub-basin are cut across by a post-spreading volcanic ridge oriented approximately E–W in its western part (Zhenbei–Huangyan seamounts chain). The knowledge of the deep crustal structure beneath this volcanic ridge is essential to elucidate not only the formation and tectonic evolution of the SCS, but also the mechanism of emplacement of the post-spreading magmatism. We use air-gun shots recorded by ocean bottom seismometers to image the deep crustal structure along the N–S oriented G8G0 seismic profile, which is perpendicular to the Zhenbei–Huangyan seamounts chain but located in between the Zhenbei and Huangyan seamounts, where topographic changes are minimum. The velocity structure presents obvious lateral variations. The crust north and south of the Zhenbei–Huangyan seamounts chain is ca. 4–6km in thickness and velocities are largely comparable with those of normal oceanic crust of Atlantic type. To the south, the Jixiang seamount with a 7.2-km thick crust, seems to be a tiny post-spreading volcanic seamount intruded along the former extinct spreading ridge axis. In the central part, a 1.5-km thick low velocity zone (3.3–3.7km/s) in the uppermost crust is explained by the presence of extrusive rocks intercalated with thin sedimentary layers as those drilled at IODP Site U1431. Both the Jixiang seamount and the Zhenbei–Huangyan seamounts chain started to form by the intrusion of decompressive melt resulting from the N–S post-spreading phase of extension and intruded through the already formed oceanic crust. The Jixiang seamount probably formed before the emplacement of the E–W post-spreading seamounts chain.

ANALYSIS AND PROCESSING OF ABNORMAL OBS DATA IN THE SOUTH CHINA SEA

AbstractProcessing and analysis of ocean bottom seismometer (OBS) data are of great importance to obtain the deep crustal structure. The deep seismic profile OBS2006‐2 was carried out in 2006 along the extinct spreading ridge of the northwest sub‐basin of the South China Sea. This survey is successful to provide high quality data whose seismic signals can be observed at the offset up to 120 km, but 2 OBSs’ recording data (OBS03 and OBS06) cannot be read correctly and were not used in later structure modeling. However, OBS data are very precious due to the high cost and arduous work. The price is more expensive when encountering severe weather during a survey. This paper is focused on reprocessing of these 2 OBSs’ data by use of the methods of checking data format, comparing signals with adjacent OBSs and resampling the data. Finally we acquire these 2 OBSs’ seismic record sections in which abundant seismic phases are clearly seen. We also obtain the seismic record section of OBS03 along the profile OBS973‐3 in the Nansha Island using the same methods above. The instrument OBS06 along the profile OBS2006‐2 and the instrument OBS03 along the profile OBS973‐3 are the same instrument confirmed by checking their logs recorded in 2006 and 2011, respectively. It demonstrates that the processing method for abnormal OBS data is reliable and effective. Then the ray‐tracing and travel‐time simulation were carried out for OBS06 and OBS03 using the interactive trial‐and‐error 2D ray‐tracing method based on the previous P‐wave velocity model of OBS2006‐2. The input of new travel‐time picks should provide stricter constraints for the deep structure beneath the profile OBS2006‐2. This research on abnormal data reprocessing does not only improve the reliability and resolution of the crustal structure, but also provide valuable experiences for OBS processing for other study areas in the future.

3D seismic structure of the Zhenbei–Huangyan seamounts chain in the East Sub‐basin of the South China Sea and its mechanism of formation

AbstractThe probable N055°‐oriented extinct spreading ridge of the East Sub‐basin is cut across by the post‐spreading volcanic ridge oriented approximately E–W in its western part and called the Zhenbei–Huangyan seamounts chain. A three‐dimensional ocean bottom seismometer survey was conducted in 2011 and covered both the extinct spreading ridge and the Zhenbei–Huangyan seamounts chain in the East Sub‐basin. The comprehensive seismic record sections of 39 ocean bottom seismometers underline the high‐quality data with clear and reliable P‐wave seismic first arrivals. The three‐dimensional tomographic results show that the crust is oceanic on each side of the Zhenbei–Huangyan seamounts chain. The seafloor spreading lineaments and associated fracture zones pattern provide channels for magma migration and eruption. Based on the ratio of volcanic extrusive and intrusive materials and the shape of the upward convex 6.4 km/s iso‐velocity curve beneath the Zhenbei and Huangyan seamounts, we conclude that the Zhenbei and Huangyan seamounts appear to be dominantly formed by intra‐crustal intrusion. Combining previous geochemical studies of post‐spreading volcanic ridge outcropping samples, the formation mechanism of the seamounts chain might be explained by a buoyancy‐driven decompression melting mechanism occurring as separately evolving partial melting bodies beneath each seamount. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Marine geophysics. New global marine gravity model from CryoSat-2 and Jason-1 reveals buried tectonic structure.

Gravity models are powerful tools for mapping tectonic structures, especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment. We combined new radar altimeter measurements from satellites CryoSat-2 and Jason-1 with existing data to construct a global marine gravity model that is two times more accurate than previous models. We found an extinct spreading ridge in the Gulf of Mexico, a major propagating rift in the South Atlantic Ocean, abyssal hill fabric on slow-spreading ridges, and thousands of previously uncharted seamounts. These discoveries allow us to understand regional tectonic processes and highlight the importance of satellite-derived gravity models as one of the primary tools for the investigation of remote ocean basins.

High-resolution tectonic solutions

Marine Geophysics![Figure][1] CREDIT: SANDWELL ET AL. Detailed topographic maps are available for only a small fraction of the ocean floor, severely limited by the number of ship crossings. Global maps constructed using satellite-derived gravity data, in contrast, are limited in the size of features they can resolve. Sandwell et al. present a new marine gravity model that greatly improves this resolution (see the Perspective by Hwang and Chang). They identify several previously unknown tectonic features, including extinct spreading ridges in the Gulf of Mexico and numerous uncharted seamounts. Science , this issue p. [65][2]; see also p. [32][3] [1]: pending:yes [2]: /lookup/doi/10.1126/science.1258213 [3]: /lookup/doi/10.1126/science.1260459

Deep crustal structure in the eastern Gulf of Mexico

AbstractWe use air gun data recorded by ocean bottom seismometers to constrain the velocity structure along Gulf of Mexico Basin Opening Line 4, a profile extending from the northwestern Florida peninsula across the Florida Escarpment to the central Gulf of Mexico. Moderately thinned continental crust with a Moho depth of 32–33 km, average sediment thickness of 6 km, and an average crustal thickness of 27 km is interpreted on the northeast end of the profile offshore Florida. Thinned and intruded continental crust is identified over a horizontal distance of 225 km where the crustal layer thins from 25 km to 6–7 km; mean seismic velocities of the crust in this region increase from 6.55 km/s to 6.95 km/s from northeast to southwest and are evidence for increased magmatic input as rifting developed. Oceanic crust with an average thickness of 5.6–5.7 km is observed over a distance of 175 km on the southwest end of the profile, with an extinct spreading ridge with an axial valley morphology imaged on a coincident seismic reflection profile. Anomalously high upper oceanic crust velocities of 6.0–6.7 km/s are interpreted as massive basalt flows and could reflect increased temperatures during emplacement. Integrating well, seismic reflection and our seismic refraction data allow us to estimate a full‐spreading rate of 2.2 cm/yr for seafloor spreading along the profile; this indicates that oceanic crust was emplaced at a slow‐spreading center.

Neogene magmatism northeast of the Aegir and Kolbeinsey ridges, NE Atlantic: Spreading ridge–mantle plume interaction?

According to mantle plume theory the Earth's interior cools partly by localized large vertical mass transport, causing extensive decompression melting. The Iceland melt anomaly is regarded as a typical example of a mantle plume. However, there are centers of Miocene to recent magmatism in the Norwegian‐Greenland Sea not easily explained by the plume theory. Here we present new data to document diffuse late Miocene magmatic underplating of older oceanic crust located mostly north of the Aegir Ridge, an extinct seafloor spreading axis in the Norway Basin. There is also a region with similar magmatism northeast of the presently spreading Kolbeinsey Ridge north of Iceland. Intraplate magmatism in these locations is not easily explained by local plume models, edge‐driven convection, or by asthenosphere flow–lithosphere thickness interaction. On the basis of correlation between the magmatism and the active or extinct spreading ridges, we propose the mid‐ocean ridge basalt–capture model, in which this magmatism can be understood through plume–spreading ridge interaction: The asthenosphere flow out from Iceland captures deeper, low‐degree partially molten asthenospheric regions from underneath the spreading ridges and carry these across the terminating fracture zones, to subsequently underplate oceanic crust or to intrude and build seamounts. This model is similar to lithospheric cracking models for intraplate magmatism in requiring that low‐degree partial melt can be retained in the asthenosphere over time but differ in that the magma is extracted by internal magma movement processes and not by external tectonic forces.

Osbourn Trough: Structure, geochemistry and implications of a mid-Cretaceous paleospreading ridge in the South Pacific

Osbourn Trough is a key piece in an outstanding problem: do the Ontong Java, Manihiki and Hikurangi large igneous provinces represent a single ~100 million km3 magmatic pulse? Bathymetric mapping of a 145-km-wide swath across the ∼900-km-long Osbourn Trough revealed three segments offset by 23–35-km-long basins that strike perpendicular to the trough axis. Each segment comprises a 10–15-km-wide axial valley bounded by 300–500-m-high ridge mountains, has inside corner highs at its NW and SE margins that rise 1000–1200 m above the axial valley, and has a flanking set of subparallel abyssal hills. Dredging on steep escarpments successfully penetrated thick sediments and recovered Fe–Mn oxyhydroxide-encrusted volcaniclastic breccias. Lava clasts within the breccias have undergone variable degrees of marine weathering, leading to strong enrichment in most alkali elements and the light REE (except Ce). Nevertheless, their immobile element concentrations are consistently MORB-like and they plot within the MORB fields of tectonic discrimination diagrams. Isotope analyses indicate an affinity with Pacific MORB-source mantle.Both the morphology of Osbourn Trough and geochemistry of its lavas establish that it represents an extinct spreading ridge system. The trough is nearly equidistant (1750 km vs. 1550 km) from the Manihiki and Hikurangi Plateaus, which we interpret as remnants of a formerly contiguous Ontong Java–Manihiki–Hikurangi large igneous province. Inception of the Osbourn spreading ridge was coincident with reorganization of the former Pacific–Phoenix–Farallon spreading system and mega-plateau fragmentation at ∼118 Ma. Spreading across Osbourn Trough ceased when the Hikurangi Plateau collided with and blocked a southward-dipping subduction system developed along the Chatham Rise (eastern New Zealand) sector of the Gondwana margin at ∼86 Ma.

Introduction to the special volume on the Izu-Bonin - Mariana arc system

Introduction to the special volume on the Izu-Bonin - Mariana arc system

The Aegir Rift: Crustal Structure of an Extinct Spreading Axis

Two seismic refraction and gravity lines were obtained along and normal to the axis of the Aegir Rift, an extinct spreading centre in the Norway Basin. Velocity-depth solutions and crustal structure models are derived from ocean-bottom records using two-dimensional ray tracing and synthetic seismogram modelling techniques. Gravity data are used to generate models consistent with the lateral variations in thickness of the layers in the crustal models. The resulting models require considerable degree of lateral inhomogeneity along and perpendicular to the rift axis. Crust within the extinct spreading centre is found to be thinner and of low P-wave velocity when compared with the crust sampled off-axis. To explain reduced velocities of the lower crust we suggest that, due to the relationship between fracturing and seismic velocity, the decreasing spreading rate leading up to extinction let the mechanically strong layer thicken, so that faulting and fracturing extended to greater depths . Low velocities are also observed in the uppermost mantle underlying the extinct spreading ridge. This zone is attributed to hydrothermal alteration of upper mantle peridotites. Furthermore, after spreading ceased 32-26 my ago, ongoing passive hydrothermal circulation was accompanied by the precipitation of alteration products in open void spaces, thereby decreasing the porosity and increasing the velocity. Consequently the typical low velocities of layer 2 found at active mid-ocean ridges have been replaced by values typical of mature oceanic crust.

Tectonic implications of post–30 Ma Pacific and North American relative plate motions

The Pacific plate moved northwest relative to North America since 42 Ma. The rapid half rate of Pacific-Farallon spreading allowed the ridge to approach the continent at about 29 Ma. Extinct spreading ridges that occur offshore along 65% of the margin (Lonsdale, 1991) document that fragments of the subducted Farallon slab became captured by the Pacific plate and assumed its motion prior to the actual subduction of the spreading ridge. This plate-capture process can be used to explain much of the post–29 Ma Cordilleran North America extension, strike slip, and the inland jump of oceanic spreading in the Gulf of California. The Pacific and North American contact zone lengthened with each successive plate capture event, underpinning the parts of western North America directly inland with a strong plate undergoing Pacific relative motion. We suggest that much of the post–29 Ma continental tectonism is the result of the strong traction imposed on the deep part of the continental crust by the gently inclined slab of subducted oceanic lithosphere as it moved to the northwest relative to the overlying continent. The plate-capture hypothesis is distinctly different from theories involving shallow slab gaps. Kinematic problems associated with shallow slab-gap models cause us to question them. This conclusion is consistent with seismic refraction interpretations that suggest there is an inclined layer with high velocities like that of basalt or gabbro at the base of the continental crust beneath much of the Californian margin and the documented reduction of slab-pull forces and density associated with young subducting slabs. Thermal and rheologic modeling suggests that coastal California was a strong zone at all depths allowing it to be firmly linked to Pacific motion. Our model shows that deformed regions such as the basin and range and borderland provinces developed in predicted weak parts of the crustal section, but they have been incompletely linked to the deep plate across the ductile middle and lower crustal layer.

Earthquakes along the Northwestern Boundary of the Labrador Sea

Abstract As in much of northeastern Canada, earthquakes in the Labrador Sea occur predominantly along the passive margin. Geologically and geophysically, this region is complex and consists of an extinct spreading ridge and transform faults, both oceanic and continental crust, a possible hot spot trace, and magnetic and gravity anomalies. We have studied five recent earthquakes in the magnitude 4.5 to 5.5 range to determine their source properties and to better understand how they fit into the seismotectonic framework of the region. A combination of body and surface wave analysis techniques were used to determine the source parameters. LS69, LS89 and PB89 were well recorded teleseismically, and thus their source properties are better constrained than those for LS86 and LS87, for which only a few teleseismic records were available and whose source parameters were determined more from first motions than by modeling. The two events (LS69, LS89) that occurred near the intersection of the extinct spreading ridge (and associated transform) and Mesozoic rifted margin are noteworthy in that the former is of the thrust-fault and the latter of the normal-fault type. Local structure and possibly post-glacial rebound could be the causative factors for the occurrence of normal and thrust faulting in the same area. The two events (LS86, LS87) that occurred along the continent ocean transition zone have fairly similar fault-plane solutions, and are both thrust-faulting events. The fifth event (PB89), which occurred in Payne Bay, is also a thrust-faulting event and could be associated with the Ungava Transform fault to the northeast, or the Cape Smith fold belt to the northwest. The focal depths of all five events lie between 10 and 15 km, and may be dominated by thermal effects. The observation that the deviatoric compression axis of all five earthquakes lies in the northwest (or equivalently in the southeast) quadrant is consistent with recent modeling efforts of the tectonic stress field in this region.

Fracture zones in the North Atlantic from combined Geosat and Seasat data

We present a new tectonic fabric map of the northern and central North Atlantic Ocean floor from 10°S to 66°N based on combined Geosat and Seasat altimetry data. Short wavelength vertical gravity along‐track was derived by Hilbert transforming and high‐pass filtering Geosat and Seasat deflection of the vertical profiles. The along‐track gravity data were used in conjunction with magnetic anomaly data to locate a large number of fracture zones as well as extinct spreading ridges and V‐shaped structures. We traced many fracture zones more continuously and closer to adjacent continental margins than was previously possible. We also mapped a number of unnamed fracture zones and fracture zone segments, many of which were previously uncharted. We have evaluated the agreement between the mapped fracture zones and synthetic flow lines as predicted from recent plate models for the opening of the North, central North and South Atlantic. Most fracture zones in this area correlate quite well with synthetic flow lines, supporting the proposition that Geosat/Seasat altimetry data serve as a useful tool to map fracture zones, where insufficient bathymetric data are available. Many fracture zones, including some small‐offset fracture zones in the central North Atlantic, have been stable and follow tectonic flow lines for tens of millions of years. However, many other fracture zones do not reflect a ″railroad track″ pattern of plate motions. The geometry of many small‐ and medium‐offset fracture zones indicates that the ridge axis discontinuities that produce them can migrate along the ridge, causing off‐axis traces that deviate from tectonic flow lines. Velocity‐space diagrams constructed using a recent relative and absolute present day plate motion model indicate that some low‐angle V‐shaped troughs and ridges are likely formed by migration of the plate boundary relative to a mesospheric reference frame. Other low‐angle V‐shaped structures reflect slow migrations/oscillations of small‐offset ridge axis discontinuities that are distinct from both tectonic flow lines and V‐shapes reflecting “absolute” plate motions. High‐angle V‐shaped structures are the result of fast migrating ridge axis discontinuities (propagating rifts).