Plate Motion Vector Research Articles

Preliminary geodetic strain measurement from the South Fiordland Region of New Zealand using repeat GPS surveys: Implications for subduction on the Puysegur Trench

Comparing 1996 GPS data and data collected only one year earlier demonstrates that 50% of the instantaneous relative plate motion is accommodated between the west coast of the South Island and the eastern side of the Waiau Basin, a distance of about 75 km. The observed relative velocity vectors are nearly parallel to the inferred orientation of the plate boundary and rotated more than 30° clockwise compared to the relative plate motion vector, a difference that is significant at the 95% level of confidence. Modeling shows that the observed deformation rates are consistent with a model where nearly all the relative plate motion occurs on the plate boundary thrust as defined by the Benioff Zone associated with subduction on the Puysegur Trench.

Plate tectonic framework for the October 9, 1996, Cyprus Earthquake

We determine the centroid depths for the Mw=6.8, October 9, 1996, Cyprus earthquake and its largest after‐shock to 32 and 27 km, respectively, by modeling P and SH waveforms. These depths are consistent with shallow subduction as the African plate, west of Cyprus, penetrates beneath the Anatolian plate. We calculate the pole of rotation between Anatolia and Africa and determine their relative motion. The result indicates that, along the Cyprean Arc, convergence between the African and Anatolian plate, in agreement with the observed seismicity, decreases from west to east. The northeasterly striking nodal plane is suggested as the plane on which the Cyprus earthquake ruptured since its strike agrees well with the derived plate motion vector. The faulting geometry suggests the emergence of a strike‐slip tear fault within the African plate separating the continental Eratosthenes seamount south of Cyprus from the oceanic African lithosphere that subducts west of Cyprus.

Transpression and transtension zones

Abstract Transpression and transtension are strike-slip deformations that deviate from simple shear because of a component of, respectively, shortening or extension orthogonal to the deformation zone. These three-dimensional non-coaxial strains develop principally in response to obliquely convergent or divergent relative motions across plate boundary and other crustal deformation zones at various scales. The basic constant-volume strain model with a vertical stretch can be modified to allow for volume change, lateral stretch, an oblique simple shear component, heterogeneous strain and steady-state transpression and transtension. The more sophisticated triclinic models may be more realistic but their mathematical complexity may limit their general application when interpreting geological examples. Most transpression zones generate flattening ( k < 1) and transtension zones constrictional ( k > 1) finite strains, although exceptions can occur in certain situations. Relative plate motion vectors, instantaneous strain (or stress) axes and finite strain axes are all oblique to one another in transpression and transtension zones. Kinematic partitioning of non-coaxial strike-slip and coaxial strains appears to be a characteristic feature of many such zones, especially where the far-field (plate) displacement direction is markedly oblique (<20°) to the plate or deformation zone boundary. Complex foliation, lineation and other structural patterns are also expected in such settings, resulting from switching or progressive rotation of finite strain axes. The variation in style and kinematic linkage of transpressional and transtensional structures at different crustal depths is poorly understood at present but may be of central importance to understanding the relationship between deformation in the lithospheric mantle and crust. Existing analyses of obliquely convergent and divergent zones highlight the importance of kinematic boundary conditions and imply that stress may be of secondary importance in controlling the dynamics of deformation in the crust and lithosphere.

The impossible Galapagos connection: geometric constraints for a near-American origin of the Caribbean plate

Geometric constraints derived from the present plate configuration and from plate motion vectors of the Caribbean as well as the North and South American plates within a hotspot reference frame indicate that the thickened Caribbean oceanic crust was formed in a near-American position rather, than at the Galapagos hotspot. A lateral displacement of more than 1000 km between the Caribbean plate and the North and South American plates is related to differences in plate motion velocities during the Cenozoic era. The differential motion between the Caribbean and the American plates results from trench-parallel mantle flow as a response to the westward motion of the American plates.

Tectonostratigraphic model for underfilled peripheral foreland basins: An Alpine perspective

Advances in the development of quantitative models of foreland basin stratigraphy have outpaced the observational data used to constrain the input parameters in such models. Underfilled peripheral foreland basins comprise a broad threefold subdivision of depositional realms that translates into three stratigraphic units which are commonly superimposed during basin migration; these units are here termed the “underfilled trinity.” The three units of the trinity reflect (1) carbonate deposition on the cratonic margin of the basin (the lower unit), (2) hemipelagic mud sedimentation offshore from the cratonic margin of the basin (the middle unit), and (3) deep water turbiditic siliciclastic sedimentation toward the orogenic margin of the basin (the upper unit). Theoretical predictions of how such a complex basin fill initiates and evolves through time are not currently available; hence this study reviews the stratigraphy of underfilled peripheral foreland basins and provides a unique data set comprising rates of thrust advance and basin fill migration for the Tertiary foreland basin of the European Alps. The Paleocene to Oligocene Alpine foreland basin of France and Switzerland comprises a well-developed underfilled trinity that is preserved within the outer deformed margins of the Alpine orogen. Structural restorations of the basin indicate a decrease in the amount of basin shortening from eastern Switzerland (68%) to eastern France (48%), to southeastern France (35%). Structurally restored chronostratigraphic diagrams allow rates of basin migration to be calculated from around the Alpine arc. Paleogeographic restorations of the Nummulitic Limestone (lower unit) illustrate a radial pattern of coastal onlap on to the European craton. Time-averaged rates for northwestward coastal onlap of the underfilled Alpine basin across Switzerland were between 8.5 and 12.9 mm/yr. Time-equivalent westward to southwestward coastal onlap rates in France were between 4.9 and 8.0 mm/yr. The direction of migration of the cratonic coastline of the basin was parallel to the time-equivalent thrust motions, and oblique to the Africa-Europe plate motion vector. By comparing rates of thrust propagation into the orogenic margin of the basin to rates of coastal onlap of the cratonic margin of the basin, it is possible to suggest that the Alpine foreland basin of central Switzerland migrated with an approximately steady state geometry for at least 210 km northwestward over the European craton. The westward and southward decrease in the basin migration rate around the Alpine arc was associated with an increase in the degree of syndepositional normal faulting on the European plate; this is thought to relate to the opening of the Rhine-BresseRhone graben system.

Oblique convergence and the lobate mountain belts of western Pakistan

The thin-skinned structures of the Pakistani convergent margin have formed as a consequence of the relative motion between India and Eurasia. Most of the resultant motion is being accommodated along or near the current edge of the Eurasian plate: the southwest-northeast striking Chaman fault zone. It has been observed at oblique margins that the total plate motion is resolved into a component parallel to the margin, accommodated through strike-slip faulting, and a component normal to the margin taken up as contraction. However, the orientations of structures along the Pakistani convergent margin in and around the Sulaiman lobe and Sulaiman Range cannot be explained simply by resolving the plate motion vector into components normal and parallel to the plate boundary. Our modeling suggests that the complex juxtaposition of strike-slip faults with thrust faults of various orientations can be explained by the presence of a block centered upon the Katawaz basin that translates along the southwest-northeast structural barrier of the Chaman fault zone, moving with respect to both Eurasia and India. As this relatively rigid block moves northeastward relative to Asia, it causes deformation of the sedimentary cover and is responsible for much of the structural complexity in the Pakistani foreland. Our simple model explains several first-order features of this oblique margin, such as the eastward-facing Sulaiman Range, the strike-slip Kingri fault (located between the Sulaiman lobe and Sulaiman Range), and the reentrant at Sibi. This leads us to conclude that very complex structural and geometric relationships at oblique convergent plate boundaries can result from the accommodation of strain with simple initial geometric constraints.

The recognition of reactivation during continental deformation

Reactivation involves the accommodation of geologically separable displacement events (intervals >1 Ma) along pre-existing structures. The definition of a significant period of quiescence is central to this phenomenological definition and the duration of the interval chosen represents the resolution limit of reactivation criteria found in most ancient settings. In neotectonic environments, reactivation can be further defined as the accommodation of displacements along structures that formed prior to the onset of the current tectonic regime. This mechanistic definition cannot always be applied to ancient settings due to the uncertainties in constraining relative plate motion vectors. Four sets of criteria may be used to recognize reactivation in the geological record: stratigraphic, structural, geochronological and neotectonic. Some structural criteria may not be reliable if used in isolation to identify reactivated structures. Much of the previously published evidence cited to invoke structural inheritance is equivocal as it uses similarities in trend, dip or three-dimensional shape of structures. Numerous fault and shear zone processes can cause significant weakening both synchronously with deformation and in the long-term and may be invoked to explain reactivation. The collage of fault-bounded blocks forming most continents therefore carries a long-term architecture of inheritance which can explain much of the observed complexity of continental deformation zones.

The depth distribution of mantle anisotropy beneath the Tonga subduction zone

Shear-wave splitting recorded by a temporary deployment of broadband seismometers located above the Tonga subduction zone yields strong constraints on the depth distribution of anisotropy in the mantle beneath the Tonga back-arc region. Splitting parameters obtained for local S and teleseismic SKS phases were modeled using a ray-based method that predicts shear-wave splitting on individual phase paths and determines the best-fitting values of anisotropic strength, orientation and depth extent. Splitting in teleseismic SKS phases is identical to that in S phases from local earthquakes that occur at the base of the transition zone, demonstrating that there is no significant splitting due to anisotropy in the lower mantle. Splitting times for local S phases do not vary significantly with depth, and for models in which the strength of anisotropy is laterally uniform, they rule out anisotropy in the transition zone. Finally, the observed splitting indicates that anisotropy of more than 0.8% exists in the upper mantle, with a fast symmetry axis at 60°W, roughly parallel to the absolute plate motion vector of the subducting Pacific lithosphere.

Intraplate deformation in the Japanese Islands: A kinematic study of intraplate deformation at a convergent plate margin

Relative motions within the deforming Japanese Islands with respect to the Sea of Japan are determined using earthquake records over the last 414 years, slip rates on Quaternary faults, and angular change rates obtained from triangulation in the last century. We use a least squares inversion method to match the strain rates on the surface of the Earth with continuous spline functions to recover the velocity gradient tensors. The directions of the principal strain axes obtained from seismic, geological, and geodetic data are in general agreement with each other, with the maximum shortening axis oriented in a WNW direction. Principal strain rate magnitudes obtained from the geodetic data are about 3–5 times larger than the principal strain rates obtained from the seismic and geological data. Intraplate deformation in southwestern Japan determined from the seismic data accommodates a velocity of5.5±2(l σ) mm/yr in a direction parallel to the Nankai trough, which is about 25% of the plate motion velocity component parallel to the Nankai trough between the Philippine Sea and Eurasian plates. In northern Honshu, the velocity vectors from the seismic data are nearly parallel to the plate motion vector at the Japan trench, and the intraplate deformation accommodates about 6% of the total Pacific‐Eurasian or North American plate motion velocity. The velocity vector at Kashima, northern Honshu, calculated from the geodetic data is about 9–12 mm/yr in a direction of N55°W‐N76°W, which is about half of the very long baseline interferometry velocity calculated in the Eurasian reference frame, suggesting that the Sea of Japan may not be rigidly attached to the assumed rigid Eurasian plate.

The rupture process and aftershock distribution of the 6 April 1992 MS 6.8 earthquake, Offshore British Columbia

Abstract On 6 April 1992, a magnitude 6.8 (MS) earthquake occurred in the triple-junction region at the northern end of the Cascadia subduction zone. This was the largest earthquake in at least 75 yr to occur along the 110-km-long Revere-Dellwood-Wilson (RDW) transform fault and the first large earthquake in this region recorded by modern broadband digital seismic networks. It thus provides an opportunity to examine the rupture process along a young (<2 Ma) oceanic transform fault and to gain better insight into the tectonics of this triple-junction region. We have investigated the source parameters and the rupture process of this earthquake by modeling broadband body waves and long-period surface waves and by accurately locating the mainshock and the first 10 days of aftershocks using a well-located “calibration” event recorded during an ocean-bottom seismometer survey. Analysis of P and SH waveforms reveals that this was a complex rupture sequence consisting of three strike-slip subevents in 12 sec. The initial rupture occurred 5 to 6 km to the SW of the seafloor trace of the RDW fault at 50.55° N, 130.46° W. The dominant subevent occurred 2 to 3 sec later and 4.3 km beneath the seafloor trace of the RDW fault, and a third subevent occurred 5 sec later, 18 km to the NNW, suggesting a northwestward propagating rupture. The aftershock sequence extended along a 60- to 70-km-long segment of the RDW fault, with the bulk of the activity concentrated ∼30 to 40 km to the NNW of the epicenter, consistent with this interpretation. The well-constrained mechanism of the initial rupture (strike/dip/slip 339°/90°/−168°) and of the largest aftershock (165°/80°/170°) are rotated 15° to 20° clockwise relative to the seafloor trace of the RDW fault but are parallel to the Pacific/North America relative plate motion vector. In contrast, the mechanisms of the dominant subevent (326°/87°/−172°), and the long-period solution derived from surface waves aligns with the RDW fault. This suggests that small earthquakes (M < 6) in this area occur along faults that are optimally aligned with respect to the regional stress field, whereas large earthquakes, involving tens of kilometers of rupture, activate the RDW fault. For the mainshock, we estimate a seismic moment (from surface waves) of 1.0 × 1026 dyne-cm, a stress drop of 60 bars, and an average slip of 1.2 m. This represents only 21 yr of strain accumulation, implying that there is either a significant amount of aseismic slip along the RDW fault or that much of the strain accumulation manifests itself as deformation within the Dellwood and Winona blocks or along the continental margin.

Rates of deformation, uplift, and landscape development associated with active folding in the Waipara area of North Canterbury, New Zealand

Analysis of the geometry and ages of faulted and tilted late Quaternary fluvial terraces and their associated cover beds provide evidence of active folding at three localities in the Waipara area of North Canterbury, New Zealand, Terrace survey data, the occurrence of the approximately 22.6‐kyr‐old Aokautere Ash, and examination of soil profiles indicate that folding has continued into the late Holocene but that the amounts and rates of deformation are locally variable. Rates of uplift in the Waipara area are compared with those derived from marine terraces preserved at the Pacific coast, east of the study area. Results indicate that rates of measurable deformation reach a maximum along the Waipara range front, where bedrock deformation is most intense and shortening rates of up to 5.57±0.69%/100 kyr occur. Across the coastal ranges the average rate of shortening is 0.8±0.4%/100 kyr, which corresponds with an absolute shortening rate of 1.4±0.6 m/kyr and represents only a small proportion of the predicted plate motion vector in this region. Uplift rates range from 0–1.83 m/kyr for a late last glacial fluvial terrace and from 1.36–2.16 m/kyr for three marine terraces. Fluvial and marine terrace uplift rates vary in accord with the geometries of the folds in bedrock, and the spatial pattern of uplift directly reflects fold growth. The structure contour pattern of folded surfaces provides a first approximation to the spatial pattern of uplift. Differential uplift due to folding accounts for up to approximately 55–75% of the total uplift and has produced folds with structural relief of about 1300 m (i.e., amplitudes of 600–700 m). These folds have formed over the last 0.8±0.4 m.y. since the onset of Quaternary deformation in the Waipara region.

Ouachita-Appalachian Juncture: a Paleozoic Transpressional Zone in the Southeastern U.S.A.

The northern margin of Gondwana collided with the southern margin of the North American craton in the late Paleozoic (330-265 Ma). Near-normal compressional stresses, generated where the trend of the North American margin was nearly perpendicular to Gondwana's plate-motion vector (closure direction), caused extensive crustal shortening and decollement thrusting within the sedimentary section. The Appalachian and Ouachita fold belts are the erosional remnants of this collision on the North American margin. Transecting the Ouachita fold belt are several zones of late Paleozoic transcurrent faulting: the Val Verde, Ardmore-Anadarko, and Reelfoot zones. They occur where the precollision margin of the North American craton was oriented oblique or parallel to the closure direct on with Gondwana. Stresses generated within these zones during the collision were transpressional rather than simply compressional, and gave rise to high-angle faulting, high-amplitude vertical displacements, and the emplacement of positive flower structures. In the Val Verde and Ardmore-Anadarko basins, these features have proven to be major structural traps for oil and gas. On the basis of seismic and well data, this study identifies a zone of complex structures, including positive flower structures, in the subsurface Paleozoic section of Kemper County in east-central Mississippi. The region is considered to be another transpressional zone analogous to the Val Verde, Ardmore-Anadarko, and Reelfoot zones. The Kemper County zone separates and truncates the Appalachian Valley and Ridge fo d belt in the subsurface from the Mississippi Ouachita deformed belt to the west and is, therefore, also identified as the juncture between the Ouachita and Appalachian fold-belt systems in the southeastern U.S.A.

First epoch geodetic measurements with the Global Positioning System across the Northern Caribbean Plate Boundary Zone

The first geodetic survey across the northern Caribbean plate boundary zone with the Global Positioning System (GPS) was conducted in June 1986. Baseline vectors defined by the six station regional GPS network ranged from 170 to 1260 km in length. Repeatability of independent daily baseline estimates was better than 8 mm plus 1.3 parts in 108 of baseline length for horizontal components. The wet tropospheric path delay during the experiment was both high, sometimes exceeding 30 cm at zenith, and variable, sometimes exceeding 5 cm variation over several hours. Successful carrier phase cycle ambiguity resolution (“bias fixing”) could not be achieved prior to construction of a regional troposphere model. Tropospheric calibration was achieved with water vapor radiometers at selected sites, and with stochastic troposphere models and estimation techniques at remaining sites. With optimum troposphere treatment and single‐day orbital arcs, we resolved most biases on baselines up to about 550 km in length. With multiday orbital arcs we resolved most biases in the network regardless of baseline length. Our results suggest that constraints on plate boundary zone deformation in the Greater Antilles, and on the North America‐Caribbean relative plate motion vector, can be obtained with a series of GPS experiments spanning less than 10 and 15 years, respectively.

Interlinked system of ductile strike slip and thrusting formed by Caledonian sinistral transpression in northeastern Greenland

Research Article| May 01, 1991 Interlinked system of ductile strike slip and thrusting formed by Caledonian sinistral transpression in northeastern Greenland Robert E. Holdsworth; Robert E. Holdsworth 1Department of Geological Sciences, University of Durham, Durham DH1 3LE, England Search for other works by this author on: GSW Google Scholar Robin A. Strachan Robin A. Strachan 2Department of Geology, Oxford Polytechnic, Oxford OX3 3BP, England Search for other works by this author on: GSW Google Scholar Author and Article Information Robert E. Holdsworth 1Department of Geological Sciences, University of Durham, Durham DH1 3LE, England Robin A. Strachan 2Department of Geology, Oxford Polytechnic, Oxford OX3 3BP, England Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1991) 19 (5): 510–513. https://doi.org/10.1130/0091-7613(1991)019<0510:ISODSS>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Robert E. Holdsworth, Robin A. Strachan; Interlinked system of ductile strike slip and thrusting formed by Caledonian sinistral transpression in northeastern Greenland. Geology 1991;; 19 (5): 510–513. doi: https://doi.org/10.1130/0091-7613(1991)019<0510:ISODSS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The geometry and kinematic evolution of oblique convergence zones are poorly understood, especially in the more deeply eroded ductile roots of ancient orogenic belts. As a consequence, evidence for major lateral displacements may remain undetected. In Dronning Louise Land, northeastern Greenland, a major partitioned system of strike-slip and ductile thrust shear zones has formed in response to hitherto unrecognized Caledonian sinistral transpression. This system formed at mid-crustal depths (amphibolite facies), possibly due to oblique collision between Baltica and Laurentia during Ordovician to Early Devonian time. An early phase of low-angle strike slip is superseded by synchronous compressional thrusting and high-angle sinistral displacements. These are partitioned into shear zones arranged in a fashion similar to the fault patterns observed in the hanging walls of modern-day oblique convergent margins. Left-lateral displacements in the eastern Greenland Caledonides are likely to be tens to hundreds of kilometres. A direct correspondence between stretching lineations and Caledonian plate motion vectors is unlikely, although the strike-slip shear zone is probably parallel to the Laurentian paleo-plate margin. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Formation of interior basins associated with curved faults in Alaska

The positions of certain nonmarine sedimentary basins in central and southern Alaska are spatially related to the geometry of adjacent curved faults. These interior basins occur along the northern, convex side of these fault systems which change trend from northwest in Canada and eastern Alaska to east‐west in central Alaska and to northeast in western Alaska. The basins probably initiated in early to middle Tertiary time and deposition within all basins continued through the Quaternary, broadly synchronous with Tertiary strike‐slip displacements along the Tintina, Denali, and Border Ranges fault systems. We examine the spatial and temporal relationship between basins and faults by using a mechanical model of curved strike‐slip faults based on boundary elements. We relate slip along the curved faults to Tertiary Pacific‐North American plate motions by using principal stress directions inferred from the plate motion directions. Locations of the predicted mean stress reductions around the fault bends caused by fault slip correlate well with positions of the interior basins. Our results suggest that basin subsidence and location were controlled primarily by the geometry of adjacent curved fault systems and the state of stress in Alaska during the Tertiary. Mechanical interaction between curved Alaskan fault systems increases the tendency for basin subsidence. Stresses inferred from early Tertiary plate motion vectors are not oriented correctly to drive right lateral slip along Kaltag, Farewell, and parallel faults in western Alaska. Furthermore, these stress states would have produced left lateral displacements, for which there is no evidence, along these western faults in their present orientations. We find that a combination of slightly rotated fault geometry and low friction along the faults appears to facilitate right lateral slip along the faults in western Alaska. Our results show that deformation well inside the continent can be explained in terms of the Pacific‐North American plate motions. Thus Tertiary plate motions have resulted in a broad zone of fault slip, uplift, and subsidence in the nonrigid North American plate, producing several large basins.

Sea Beam survey of an active strike‐slip fault: The San Clemente fault in the California Continental Borderland

The San Clemente fault, located in the California Continental Borderland, is an active, northwest trending, right‐lateral, wrench fault. Sea Beam data are used to map the major tectonic landforms associated with active submarine faulting in detail unavailable using conventional echo‐sounding or seismic reflection data. In the area between North San Clemente Basin and Fortymile Bank, the major late Cenozoic faults are delineated by alignments of numerous tectonic landforms, including scarps, linear trenches, benches, and sags. Character and spatial patterns of these landforms are consistent with dextral wrench faulting, although vertical offsets may be substantial locally. The main trace of the San Clemente fault cuts a straight path directly across the rugged topography of the region, evidence of a steeply dipping fault surface. Basins or sags located at each right step in the en echelon pattern of faults are manifestations of pull‐apart basin development in a right‐slip fault zone. Seismic reflection profiles show offset reflectors and a graben in late Quaternary turbidites of the Navy Fan, where the fault zone follows a more northerly trend. Modern tectonic activity along the San Clemente fault zone is demonstrated by numerous earthquakes with epicenters located along the fault's trend. The average strike of the San Clemente fault is parallel to the predicted Pacific‐North American relative plate motion vector at this location. Therefore we conclude that the San Clemente fault zone is a part of the broad Pacific‐North American transform plate boundary and that the southern California region may be considered as a broad shear zone.

North American—Pacific Relative Plate Motion in Southern California from Interferometry

Very long baseline interferometry measurements of baselines crossing the San Andreas fault zone in southern California have provided observational constraints on rates of elastic tectonic strain accumulation. The single site located near this fault (the Jet Propulsion Laboratory site) moves in a direction concordant with the Pacific plate motion vector but at approximately half the net rate relative to North America. This motion agrees approximately in amount with geologically determined displacement rates on the San Andreas fault alone but not with the local strike of the fault. When considered together with complementary geodetic data, these results suggest a complex relation between the short-term accumulation of elastic strain and its permanent accommodation on existing faults.

Lateral heterogeneity and azimuthal anisotropy of the upper mantle: Love and Rayleigh waves 100–250 s

The lateral heterogeneity and apparent anisotropy of the upper mantle are studied by measuring Rayleigh and Love wave phase velocities in the period range 100–250 s. Spherical harmonic descriptions of the lateral heterogeneity are obtained for order and degree up to 1 = m = 10. Slow regions are evident at the East Pacific Rise, northeast Africa, Tibet, Tasman Sea, southwestern North America, and triple junctions in the northern Atlantic and Indian oceans. Fast regions occur in Australia, western Pacific, and the southern Atlantic. These features are also found by a completely different analysis based on the Backus‐Gilbert method. The Backus‐Gilbert method also shows that the obtained phase velocities are averaged values within an area of about 2000‐km radius and the errors are about 1% of the phase velocity in the zeroth‐order spherically symmetric earth. Inversion for azimuthal dependence shows that for low angular order the fast phase velocity directions seem to correlate well with the plate motion vectors. However, resolution analysis by the Backus‐Gilbert method shows that the current data do not have enough resolution for everywhere on the globe. Only a few regions currently have adequate azimuthal coverage. Thus confirmation of the above correlation requires a more complete data set.

Microaftershock survey of the 1978 Bermuda Rise earthquake

On March 24, 1978, a magnitude 6.0 intraplate earthquake occurred 380 km southwest of Bermuda near magnetic anomaly M4 (≈118 m.y.B.P.). A catalog of seismicity for the Bermuda rise indicates that this is an area of significant intraplate seismicity in the western North Atlantic Ocean. The fault plane solution for the 1978 event is of thrust type and strikes 340°, in an intermediate direction to the trends of major fracture zones (300°) and abyssal hill topography (035°) in the area. The P axis of this mechanism is nearly horizontal and trends 259°, subparallel to the absolute plate motion vector for North America. Aftershock activity was detected teleseismically for approximately 8 months after March 24, and the entire sequence is best described as a prolonged mainshock‐aftershock series. During June 18–28, 1978, we conducted a microaftershock survey of the area using ocean bottom hydrophones and recorded 250 events (0 &lt; mb &lt; 2). These smaller events are located in a tight cluster 5–10 km northeast of the NEIS mainshock location. SP‐P data from microaftershocks recorded during this survey indicate a relatively narrow band of activity (about 10 km wide), smaller than would be inferred based on NEIS aftershock locations alone. Many of the recorded microaftershocks exhibited prominent water wave phases, which have been used to constrain further the epicentral locations. The depths of these events, while not well deter mined, appear to be shallow, i.e., less than 5 km below the seafloor. Estimates of the b value, determined from two instruments for the 10‐day recording period, are 1.10 ±0.30 to 1.50 ±0.60. Overall, both the fault plane solution and subsequent aftershock locations of the 1978 event suggest that faulting did not occur along a major oceanic fracture zone but, rather, along a smaller‐scale feature within the M11‐M4 spreading discontinuity. The spatial clustering of seismicity near the perimeter of the rise suggests that variations of crustal thickness associated with the Bermuda rise may be responsible for triggering intraplate seismicity in this region.

Northeast Quadrant Mapping, Circum-Pacific Map Project: ABSTRACT

The Northeast Quadrant panel, comprising geoscientists from Mexico, Canada, U.S.A., Central America, and Hawaii, was originally organized under the direction of C. A. Burk in 1973. He was succeeded by K. J. Drummond in 1976. For the Northeast Quadrant, compilation of all maps at a scale of 1:10,000,000 is under way with the cooperation of all countries involved. Published maps ready for display include the Geographic map and the Plate-Tectonic map. The latter shows the active lithospheric plates, their boundaries, and associated geologic phenomena. Present day movements are illustrated by seismicity, vulcanism, and plate motion vectors. These, with spreading axis and magnetic lineations, demonstrates how the present configuration of continental masses and oceanic basins evolved through interaction of the various plates. Preliminary color proofs of other maps are also ready to be displayed. These include Geologic, Mineral Resources, and Energy Resources maps. The Mineral Resources map has been prepared under the direction of Phil Guild, U.S. Geological Survey, with major contributions from Ken Dawson, Geological Survey of Canada, and Bill Sala , Consejo de Recursos Minerales, Mexico. This map depicts the geographic distribution of mineral deposits by symbol which show the class, size, and age of mineralization, overprinted on a geologic/tectonic background. The Geologic map attempts to show the relationship between continental and oceanic regimes. Numerous elements for the Tectonic and Geodynamics maps have been compiled and work copies of this material can be viewed. These include preliminary tectonic maps, gravity and magnetic compilations, geothermal and heat flow data, crustal thickness, etc. The Northeast Quadrant panel welcomes suggestions and corrections from all interested geoscientists. End_of_Article - Last_Page 965------------