High Crustal Velocities Research Articles

Decratonised lithosphere associated with mantle plume upwelling in the Singhbhum craton, eastern India based on geophysical and geodynamic perspective

The mode of crustal and lithospheric mantle evolution of cratonic blocks and their subsequent deformation and decratonisation due to regional plate tectonics and continued intraplate geodynamic process, are not well understood. This is especially true for the highly complex Eoarchean Singhbhum craton, located in the eastern part of the Indian shield, which underwent prolonged episodes of intraplate volcanism, mafic dyke intrusions, rifting, collision and widespread sedimentation lasting for over two billion years, leaving behind noticeable imprints in the crust and mantle lithosphere. Our detailed analysis of the geophysical characteristics indicates high crustal velocities and presence of a pronounced metasomatised zone between the depth 14 km and 30 km, which is characterised by drop in Vs from 3.93 km/s to 3.76 km/s. This zone is further followed by 14 km thick high velocity magmatic crust above the Moho. A substantial part of the upper crust has been eroded away from this craton due to thermal-induced uplifting and massive crustal exhumation, caused by the multiple hot mantle plume interactions during Precambrian period, in which mantle potential temperature and pressure reached as high as 1700 oC and 4.4 GPa respectively. This craton is conspicuously associated with highly positive gravity anomalies (up to +10mGal), which are well correlated with high elevation and the location of mafic volcanic suites. It is also characterised by an unusually thin lithosphere (∼ 60-128 km), high Moho temperatures (910°C) and high dose of heat flow (47mW/m2) emanating from the mantle, which conforms with the estimated low effective elastic thickness of ∼30 km. Transient effects of the paleo-thermal anomalies possibly still persist due to periodic rejuvenation of surrounding rift valleys and suture zones. Eventually, this craton now shows geophysical characters of more or less a mobile belt, not of a typical craton. Since, almost 150-200 km thick mantle lithosphere from the bottom has been eroded away from this region due to persistent thermo-geodynamic activities and mantle upwelling induced rise of mantle isotherms, our study does not favour the idea of cratonic stability and preservation of deep continental roots.

Regional characteristics of the lithospheric structure in the southeast of North China revealed by explosion seismology

Based on the blasting seismic detection data obtained in the southeast of North China in recent years, this paper comprehensively analyzes and studies the crust-mantle lithospheric structure and seismological characteristics of different tectonic regions, such as offshore basins, west Shandong uplift, Tanlu fault zone and Jiangsu-Shandong orogenic belt. The low-velocity Pg waves in Dongying depression and Northern Jiangsu basin reveal the unstable basement structure with extremely thick sediments. The travel time of Pg wave is characterized by relatively low propagation velocity and small crustal thickness of offshore continental margin; the first break time and high apparent velocity of Pg wave in west Shandong uplift indicate that the sedimentary basement is relatively thin. The Pm wave shows the characteristic of dominant wave in the first-order velocity discontinuity of the crust-mantle interface, which reflects the high crustal velocity and stable structure in west Shandong uplift. The Pm and Pl wave are obviously complicated, which can reflect the crust-mantle lithospheric structure of the transitional zone between Tanlu fault zone and Jiangsu-Shandong orogenic belt. The small time difference between Pn and PL waves can be regarded as the highly destructive seismological manifestation of Tanlu fault zone on the crust-lithosphere scale. Based on many geophysical phenomena such as electrical structure, density structure and terrestrial heat flow, it is believed that the lithospheric destruction degree of Tanlu fault zone and Jiangsu-Shandong orogenic belt was high during the destruction of the North China Craton.

Velocity structure of the upper crust beneath the Jiaonan uplift and its adjacent areas, China

The Jiaonan uplift and its adjacent areas (JUAA) are the result of collision and amalgamation of the North China plate and Yangtze plate. In this area, the tectonic environment is complex and crustal deformation is strong. However, detailed and in-depth study of the upper crustal structure and medium properties in the JUAA has not previously been performed. The high-resolution three-dimensional crustal velocity structure of the JUAA is helpful to analyze the characteristics of the crustal structure in this area and is important for evaluating the tectonic environment and medium properties of the JUAA. We obtained the three-dimensional crustal velocity structure in the JUAA using the double-difference seismic tomography technique, and found that the Rizhao area and the sea areas to its southwest in the Jiaonan uplift are characterized by a high-velocity structure, and that most of the high-velocity anomaly area is located in the sea area. The crustal velocity values of the southern end of the Yishu fault zone are also high. The high crustal velocity anomaly areas in the JUAA are considered to be caused by the upwelling of mantle material. The velocity structure of the upper crust beneath the depression structures shows notable low-velocity anomalies, which are closely related to loose sediments in the depression structures. The existence of abnormally high-velocity and low-velocity structures in the Jiaonan uplift indicates that there are substantial differences in the properties of the crustal media in the Jiaonan uplift. Crust-mantle interaction in the Jiaonan uplift is mainly concentrated in the Rizhao area and sea areas to its southwest.

Crustal seismogenic structures and deformation styles along the Longmen Shan Fault belt in the eastern Tibetan Plateau inferred from ambient noise tomography

A seismic array comprising 80 broadband stations with ~10–20 km inter-station distances was deployed along the Longmen Shan fault belt (LMSF), the eastern boundary of the Tibetan Plateau. The recorded ambient noise data provided densely distributed inter-station cross-correlated surface waves. A new 3-D crustal S-wave velocity model for the LMSF was constructed by carrying out ambient noise tomography. The inverted model strongly improved data fitting and decreased data misfit compared to the reference (initial) model. The model highlights several crustal structure features. The Baoxing and Pengguan Massifs on the mountain side of the southern-to-middle LMSF exhibit relatively high crustal velocities, probably indicating strong crust. Low crustal velocities that may reflect weak, deformable brittle crust, exist mainly beneath the middle-to-northern segment of the LMSF and partly around the periphery of the Baoxing and Pengguan Massifs in its southern-to-middle segment. Two SW dipping low-velocity (weak) belts approximately perpendicular to the LMSF are imaged respectively around the Wenchuan-earthquake hypocenter in the south and Beichuan in the north. The low velocities in the two belts may focus movement of the eastern Tibetan Plateau relative to the Sichuan Basin (Yangtze Craton), and the uplift of the Tibetan Plateau over long time periods. Based on the velocity and tectonic structures, the young, high topography and thickened crust but low GPS shortening rates around the southern-to-middle LMSF may be due to the dominant effect of vertical crustal deformation caused by the existence of the strong Baoxing and Pengguan Massifs. This would then be in contrast to the characteristic lateral movements due to ductile crustal flow or weak, deformable brittle crust typical of the middle-to-northern LMSF.

Shear velocity structure beneath the central United States: implications for the origin of the Illinois Basin and intraplate seismicity

AbstractWe present new estimates of lithospheric shear velocities for the intraplate seismic zones and the Illinois Basin in the U.S. midcontinent by analyzing teleseismic Rayleigh waves. We find that relatively high crustal shear velocities (VS) characterize the southern Illinois Basin, while relatively low crustal velocities characterize the middle and lower crust of the central and northern Illinois Basin. The observed high crustal velocities may correspond to high‐density mafic intrusions emplaced into the crust during the development of the Reelfoot Rift, which may have contributed to the subsidence of the Illinois Basin. The low crustal VS beneath the central and northern basin follow the La Salle deformation belt. We also observe relatively low velocities in the mantle beneath the New Madrid seismic zone where VS decreases by about 7% compared to those outside of the rift. The low VS in the upper mantle also extends beneath the Wabash Valley and Ste. Genevieve seismic zones. Testing expected VS reductions based on plausible thermal heterogeneities for the midcontinent indicates that the 7% velocity reduction would not result from elevated temperatures alone. Instead this scale of anomaly requires a contribution from some combination of increased iron and water content. Both rifting and interaction with a mantle plume could introduce these compositional heterogeneities. Similar orientations for the NE‐SW low‐velocity zone and the Reelfoot Rift suggest a rift origin to the reduced velocities. The low VS upper mantle represents a weak region and the intraplate seismic zones would correspond to concentrated crustal deformation above weak mantle.

Velocity Structure of the Saint Elias, Alaska, Region from Local Earthquake Tomography

Abstract We use over 8000 regional and local earthquakes and ∼150,000 seismic arrivals recorded by a dense broadband seismic network in the St. Elias region, southern Alaska, and nearby regional stations to compute the 3D velocity structure down to a depth of ∼80 km. The tectonics of this region is dominated by a collision between the subducting Yakutat microplate and the North American continent. This model reveals slower velocities along the northern edge of the Yakutat microplate in the St. Elias coastal range that can be traced as deep as 30 km. This velocity anomaly contrasts with the offshore area in the south and the region north of the Chugach–St. Elias and Bagley faults and corresponds to tectonically off‐scraped and deformed rocks of the Yakutat sedimentary cover. The Chugach–St. Elias fault serves as a back stop for the deformed Yakutat sediments rather than the Bagley fault and represents a prominent structural boundary east of the Pamplona fault zone. The northward dipping Yakutat basement is imaged at depth and extends toward the Wrangell volcanoes. The region between the Bagley fault and the Wrangell volcanic field is characterized by higher crustal velocities, whereas the Wrangell volcanic field is characterized by slower velocities both near the surface and at depth. A strong low‐velocity anomaly corresponding to the Copper River sedimentary basin can be traced down to 20 km depth.

Ambient seismic noise tomography of Canada and adjacent regions: Part I. Crustal structures

AbstractThis paper presents the first continental‐scale study of the crust and upper mantle shear velocity (Vs) structure of Canada and adjacent regions using ambient noise tomography. Continuous waveform data recorded between 2003 and 2009 with 788 broadband seismograph stations in Canada and adjacent regions were used in the analysis. The higher primary frequency band of the ambient noise provides better resolution of crustal structures than previous tomographic models based on earthquake waveforms. Prominent low velocity anomalies are observed at shallow depths (<20 km) beneath the Gulf of St. Lawrence in east Canada, the sedimentary basins of west Canada, and the Cordillera. In contrast, the Canadian Shield exhibits high crustal velocities. We characterize the crust‐mantle transition in terms of not only its depth and velocity but also its sharpness, defined by its thickness and the amount of velocity increase. Considerable variations in the physical properties of the crust‐mantle transition are observed across Canada. Positive correlations between the crustal thickness, Moho velocity, and the thickness of the transition are evident throughout most of the craton except near Hudson Bay where the uppermost mantle Vs is relatively low. Prominent vertical Vs gradients are observed in the midcrust beneath the Cordillera and beneath most of the Canadian Shield. The midcrust velocity contrast beneath the Cordillera may correspond to a detachment zone associated with high temperatures immediately beneath, whereas the large midcrust velocity gradient beneath the Canadian Shield probably represents an ancient rheological boundary between the upper and lower crust.

Crust and upper mantle structure beneath the Pacific Northwest from joint inversions of ambient noise and earthquake data

We perform a joint inversion of phase velocities from both earthquake and ambient noise induced Rayleigh waves to determine shear wave velocity structure in the crust and upper mantle beneath the Pacific Northwest. We focus particularly on the areas affected by mid‐Miocene to present volcanic activity. The joint inversion, combined with the high density seismic network of the High Lava Plains seismic experiment and data from the EarthScope Transportable Array, provides outstanding resolution for this area. In Oregon, we find that the pattern of low velocities in the crust and uppermost mantle varies between the High Lava Plains physiographic province and the adjacent northwestern Basin and Range. These patterns may be due to the presence of the Brothers Fault Zone which separates the clockwise rotating northwest Basin and Range from the relatively undeformed areas further north. Further to the east, the Owyhee Plateau, Snake River Plain (SRP) and northeastern Basin and Range are characterized by high crustal velocities, though the depth extent of these fast wave speeds varies by province. Of particular interest is the mid‐crustal high velocity sill, previously only identified within the SRP. We show this anomaly extends significantly further south into Utah and Nevada. We suggest that one possible explanation is lateral crustal extrusion due to the emplacement of the high density mafic mid‐crustal sill structures within the SRP.

Crustal deformation at the southernmost part of the Ryukyu subduction (East Taiwan) as revealed by new marine seismic experiments

The southernmost part of the Ryukyu subduction, where the Philippine Sea Plate is subducting under the Eurasian Plate, is known to be a very seismically active region of transition from a north-dipping subduction along the Ryukyu subduction to an ~SE–NW collision along the Taiwanese orogenic wedge. In this paper, we will focus on the Ryukyu forearc area close to Taiwan where the deformation is paroxysmal. In order to decipher the nature of the seismic deformation in this region, a three month passive experiment, combining 22 Ocean Bottom Seismometers and 51 onland stations, has been led. Starting from an a-priori heterogeneous model, we have obtained 801 well-located earthquake hypocenters, a precise P-wave tomography model and 14 focal mechanisms. The seismicity along the Ryukyu forearc is mainly located not only in the vicinity of the Interplate Seismogenic Zone (ISZ) but also within both the subducting PSP and the overriding plate. Seismicity within the upper-plate is essentially localized east of Nanao basin where NW–SE extension occurs, and northwest of the Hoping basin where strike-slip dominates. As revealed by both the P-wave velocity structure and the newly derived seismicity, we argue that a sub-vertical step offsetting the subducting PSP around 10km may support the presence of a trench-parallel tear. The PSP also undergoes extension in its upper part that is probably caused by buckling and slab pull. The P-wave velocity structure reveals three other major features: (1) a continuity between the Central Range and the Ryukyu Arc with a shallower Moho (~30km depth) between ~122.3°N and ~122.5°N along the Ryukyu Arc, (2) high P-wave velocities along the eastern side of the Central Range and, (3) two bodies with similar high crustal velocities (6.5–7.0km/s) at 12–18km depths, embedded within the Ryukyu arc basement, just north of Hoping Basin and north of the Nanao Basin.

Deep crustal structure beneath large igneous provinces and the petrologic evolution of flood basalts

We present a review of seismological constraints on deep crustal structures underlying large igneous provinces (LIPs), largely from wide‐angle seismic refraction surveys. The main purpose of this review is to ascertain whether this seismic evidence is consistent with, or contrary to, petrological models for the genesis of flood basalt lavas. Where high‐quality data are available beneath continental flood basalt (CFB) provinces (Emeishan, Columbia River, Deccan, Siberia), high‐velocity structures (Vp ∼ 6.9–7.5 km/sec) are typically found immediately overlying the Moho in layers of order ∼5–15 km thick. Oceanic plateau (OP) LIPs exhibit similar layers, with a conspicuous layer of very high crustal velocity (Vp ∼ 7.7 km/sec) beneath the enormous Ontong‐Java plateau. These structures are similar to inferred ultramafic underplating structures seen beneath active hot spots such as Hawaii, the Marquesas, and La Reunion. Petrogenetic models for flood basalt volcanism based on hot plume melting beneath mature lithosphere suggest that these deep seismic structures may consist in large part of cumulate bodies of olivine and clinopyroxene which result from ponding and deep‐crustal fractionation of ultramafic primary melts. Such fractionation is necessary to produce basalts with typical MgO contents of ∼6–8%, as observed for the vast bulk of observed flood basalts, from primary melts with MgO contents of order ∼15–18% (or greater) such as result from hot, deep melting beneath the lithosphere. The volumes of cumulate bodies and ultramafic intrusions in the lowermost crust, often described in the literature as “underplating,” are comparable to those of the overlying basaltic formations, also consistent with petrological models. Further definition of the deep seismic structure beneath such prominent LIPs as the Ontong‐Java Plateau could place better constraints on flood basalt petrogenesis by determining the relative volumes of ultramafic bodies and basaltic lavas, thereby better constraining the overall process of LIP emplacement.

Upper mantle structure beneath the Caribbean‐South American plate boundary from surface wave tomography

We have measured shear wave velocity structure of the crust and upper mantle of the Caribbean‐South American boundary region by analysis of fundamental mode Rayleigh waves in the 20‐ to 100‐s period band recorded at the BOLIVAR/GEODINOS stations from 2003 to 2005. The model shows lateral variations that primarily correspond to tectonic provinces and boundaries. A clear linear velocity change parallels the plate bounding dextral strike‐slip fault system along the northern coast of Venezuela, illustrating the differences between the South American continental lithosphere, the Venezuelan archipelago, and the Caribbean oceanic lithosphere. At depths up to 120 km beneath the Venezuelan Andes and the Maracaibo block, there is evidence of underthrusting of the Caribbean plate, but there is no other evidence of subduction of the Caribbean plate beneath the South American plate. In eastern Venezuela, linear crustal low velocities are associated with the fold and thrust belts whereas as higher crustal velocities are imaged in the Guayana shield lithosphere. The subducting oceanic part of the South American plate is imaged beneath the Antilles arc. The surface wave images combined with seismicity data suggest shear tearing of the oceanic lithosphere away from the buoyant continental South American plate offshore of northeastern Venezuela. The continental lithosphere south of the slab tear is bent down toward the plate boundary in response to the propagating tear in the lithosphere. We interpret a nearly vertical low‐velocity “column” west of the tear centered beneath the Cariaco Basin, with three‐dimensional asthenospheric flow around the southern edge of the subducting oceanic lithosphere, with the asthenosphere escaping from beneath continental South America and rising into the plate boundary zone. The complex plate boundary structure is best examined in three dimensions. We discuss the new surface wave tomographic inversion in the context of results from other researchers including local seismicity, teleseismic shear wave splits, and interpretations from active source profiling.

Evidence for mafic lower crust in Tanzania, East Africa, from joint inversion of receiver functions and Rayleigh wave dispersion velocities

SUMMARY The S-wave velocity structure of Precambrian terranes in Tanzania, East Africa is modelled by jointly inverting receiver functions and surface wave dispersion velocities from the 1994‐1995 Tanzania broad-band seismic experiment. The study region, which consists of an Archean craton surrounded by Proterozoic mobile belts, forms a unique setting for evaluating Precambrian crustal evolution. Our results show a uniform crustal structure across the region, with a 10‐15 km thick upper crust with VS = 3.4‐3.5 km s −1 ,o verlying a gradational lower crust with S-wave velocities up to 4.1 km s −1 at 38‐42 km depth. The upper-mantle lid displays uniform S-wave velocities of 4.5‐4.7 km s −1 to depths of 100‐150 km and overlays a prominent low-velocity zone. This low-velocity zone is required by the dispersion and receiver function data, but its depth interval is uncertain. The high crustal velocities within the lowermost crust characterize the entire region and suggest that mafic lithologies are present in both Archean and Proterozoic terranes. The ubiquitous mafic lower crust can be attributed to underplating associated with mafic dyke emplacement. This finding suggests that in East Africa there has been little secular variation in Precambrian crustal development.

Lithospheric structure of the Trans-Hudson Orogen from seismic refraction - wide-angle reflection studies

The Trans-Hudson Orogen (THO) is the world's largest Paleoproterozoic orogenic belt. Data from three refraction profiles are used to investigate its lithospheric structure in Saskatchewan and Manitoba. R1 crosses the orogen from the Hearne craton on the west to the Superior craton on the east; R2 and R3 are along the orogen. P-wave velocity structural models are generated using a ray-based technique. On line R1, higher crustal velocities in its eastern part coincide with rocks of the Flin Flon – Namew gneiss complex. Depth to Moho is in the 40–45 km range and equates to that from the reflection data, including a small crustal root below the Sask minicontinent. Along lines R2 and R3, depth to Moho varies from about 40 km up to 55 km at the north end of R2 and south end of R3. In general, variations in crustal velocity and depth to Moho do not correlate with the location and extent of geological domains; they appear to reflect the complex deformation and metamorphic history of the crustal rocks. Mantle velocities are high, ~8.2 km/s. However a limited area shows prominent velocity anisotropy, with values of 8.6 km/s along R2 and R3 and 8.1 km/s along R1. We speculate that the observed anisotropy represents an ~100-km-wide mantle suture zone resulting from the collision of Archean plates. The suture zone accommodated limited extensional deformation, associated with a counterclockwise rotation of the Superior plate, to generate the anisotropy. In this model, the lithospheric mantle of the THO internal domains and Sask craton are detached.

Characterization of the causative fault system for the 2001 Bhuj earthquake of Mw 7.7

Precise hypocenters (ERH<0.5 km, ERZ<1.0 km) of 600 aftershocks ( M w 2.0–5.3) delineate a east–west trending blind thrust dipping (∼45°) towards south (named as North Wagad Fault, NWF), about 25 km north of Kachchh main land fault (KMF), as the causative fault for the 2001 Bhuj earthquake of M w 7.7. The aftershock zone involves a crustal volume of 60×40×35 km 3, lying between KMF and NWF. The waveform inversion of 45 aftershocks of M 3.0–5.3 suggest that the E–W trending south-dipping reverse faulting mainly characterizes the aftershock zone; however, some reverse faulting along NW–SE as well as NE–SW planes and some strike-slip faulting along NW–SE trending vertical plane are also noticed. The estimated P-axes point on an average towards N–S, while, T-axes orient in E–W agreeing well with the prevailing regional plate tectonic stress directions. The estimated velocity model from one-dimensional inversion of 8000 P and 5000 S travel times from 600 aftershocks deciphers a detailed crustal structure of the region. Upper 0–6 km, on an average suggests a low velocity zone characterizing the Jurassic and younger sediments. The depth range 6–42 km is characterized on an average by larger values of V p (6.31 to 6.98 km/s with an average of 6.71 km/s), V s (3.64 to 4.05 km/s with an average of 3.85 km/s) and V p/ V s (1.69 to 1.81 with an average of 1.75). The high crustal velocities can be attributed to the existence of a high velocity mafic intrusive/rift pillow structure beneath the region probably emplaced during the rifting time. This, brittle, competent high velocity zone beneath the epicentral area in response to the compression due to the northward movement of the Indian plate could induce sufficient local stress perturbation for generating large intraplate earthquakes of M w≥7.5 in the lower crust. Further within the overall higher velocity depth range a layer characterized by large values of V p (6.98 km/s), V s (3.854 km/s) and V p/ V s (1.81) is found in the depth range of 20.5–30 km, which is inferred to be fractured and saturated (high crack density and probably fluid-filled). Presence of numerous fractures in this layer is confirmed by a high b-value (0.75–1.1) in this layer. This layer might have facilitated the nucleation process of the 2001 Bhuj earthquake. The major rupture and 47% of the aftershocks have occurred within this layer.

Segment-scale variations in the crustal structure of 150-300 kyr old fast spreading oceanic crust (East Pacific Rise, ) from wide-angle seismic refraction profiles

Summary We have simultaneously inverted seismic refraction and wide-angle Moho reflection traveltimes for the 2-D crustal thickness and velocity structure of 150–300 kyr old crust along the East Pacific Rise (EPR) between the Siqueiros and Clipperton fracture zones (FZs). Our results show a strong correlation between ridge segmentation and upper- and mid-crustal seismic velocities, with higher velocities near segment centres and lower velocities near segment ends. Low crustal velocities at the Clipperton and Siqueiros FZs are interpreted as fracturing resulting from brittle deformation of the crust in the transform domain. A relict overlap basin left on the Pacific Plate by the 9°03′N overlapping spreading centre (OSC) as it propagated southward is associated with a large (∼1 km s−1), negative upper- and mid-crustal velocity anomaly. This anomaly is consistent with the presence of an unusually thick extrusive section within the basin and with tectonic alteration, fracturing and shearing arising from rotation of the basin as it was formed. The discordant zone left by this OSC on the Cocos Plate is characterized by moderately low crustal velocities, probably because of crustal fracturing as the OSC propagated into older crust. Higher crustal velocities near segment centres may reflect a higher ratio of dikes to extrusives in the upper crust, and lower-intensity tectonic alteration of the crust, than near segment ends. The mean crustal thickness along the EPR between the Siqueiros and Clipperton FZs is 6.7–6.8 km. The thickest crust is found beneath the Lamont seamounts (∼9 km), and in a southward-pointing, V-shaped band located just north of the off-axis trace of the 9°03′N OSC (7.3–7.8 km). The thinnest crust (<6 km) is found proximal to the Clipperton and Siqueiros FZs. The crust associated with the off-axis trace of the 9°03′N OSC is not anomalously thin, suggesting that magma supply beneath the OSC is similar to that of the northern and southern segments. We see a similar pattern of crustal thickness variation to that determined using multichannel reflection data, including a gradual thickening of the crust from north to south along the northern ridge segment, and the location of the thickest crust just north of the 9°03′N OSC. However, the magnitude of the along-axis crustal thickness variation we observe along the northern ridge segment between 9°50′N and 9°15′N(∼1.3–1.8 km, excluding the Lamont seamounts) is significantly less than the 2.3 km of variation previously reported, weakening the case for the existence of a low-density mantle diapir at 9°50′N inferred from gravity data. The band of thick crust located just north of the off-axis trace of the 9°03′N OSC suggests a close genetic link between this feature and the OSC. Thus we attribute the pattern of crustal thickness variations along the northern segment to the kinematics of the southward-propagating 9°03′N OSC over the past 0.5 Myr, and not to along-axis melt migration away from a mantle diapir as previously proposed.

Wide-angle seismic imaging across accreted terranes, southeastern Alaska and western British Columbia

This study addresses the question of crustal, Moho, and uppermost mantle structure across an accreted terrane, continental arc, and fold and thrust belt in southeastern Alaska and western British Columbia. The 186-km-long Portland Canal line of the ACCRETE wide-angle seismic dataset across the Coast Mountains is analyzed using a combination of travel-time analysis techniques, including delay-time tomography, turning ray tomography, reciprocal time analysis, intercept-time inversion, and forward ray tracing. Additional constraints on the model are obtained from a coincident vertical-incidence seismic section and from correlation with the mapping of geologic structures plunging into the seismic section. The study reveals moderately high crustal velocities, low-velocity gradient in the middle crust, and decreasing average crustal velocity and a north-deepening Moho. Termination of crustal reflectivity across a vertical zone (the Coast Shear Zone, CSZ) indicates that the CSZ is most likely a strike-slip fault associated with a transpressive tectonic regime. A mid-Cretaceous thrust system mapped on the surface to the west of the CSZ is imaged by several groups of mid- to lower crustal reflectors extending close to the Moho indicating it was a thick-skinned thrust system. NE-dipping fabric imaged within the Coast Mountains batholith (CMB) is associated with a ductile deformation during Early Eocene crustal extension. The crustal section under the CMB, which has an average velocity of 6.55 km/s and shallower than average crustal thickness of 31 km, can be considered as corresponding to the lower two thirds of an average crustal section which has been inflated by intrusions of high-velocity tonalite to gabbro sills.

Velocity structure of the coastal region of Southern Peru from seismic refraction/wide-angle reflection data

Refraction information from three separate experiments in southern Peru has been integrated to produce two reversed profiles and five unreversed profiles of travel-time data for the coastal region of the area. A fan of records NW of Atico, at ranges beyond the Pn crossover show azimuthal variability in their arrival times and waveform. These are interpreted to indicate local and large variations in the velocity structure in this region and impose limitations of the possible interpretation of the results. The two reversed profiles and one split profile sample about 1000 km of the coast, from Lima to the Chilean border. The data have been fitted with two-dimensional models with plane dipping interfaces, and have been found to be well represented by such models. Since profiles sample different azimuths, one obtains a quasi-three-dimensional picture of the region which includes gently dipping (about 5 degrees to the NE), interfaces. The reversed profiles are best fit by a model which includes: (a) a thin upper crustal layer with velocities of 5.4–6.0 km/s; (b) a fairly thick 6.5 km/s layer; (c) a relatively thick layer of unusually high crustal velocity, 7.3 km/s, which is commonly found among the first arrivals and may be interpreted from later arrivals in all the record sections; and finally, (d) a mantle velocity of 8 km/s.

Three-component analysis of regional phases at NORESS and ARCESS: Polarization and phase identification

Abstract Particle motion characteristics of short-period three-component (3-C) data are compared for various seismic phases at NORESS and ARCESS, and their usefulness for phase identification is evaluated. Continuous recordings at the arrays of 3-C elements were processed during routine operation of the Intelligent Monitoring System (IMS). The data set used in this study consists of 3822 arrivals extracted from the IMS database and covers a period of about 2.5 months. First, polarization attributes and azimuth of the dominant linear motion are compared for local/regional phases (Pn, Pg, Sn, Lg) at the two arrays. P-type arrivals have larger angles of incidence at ARCESS than at NORESS, on average, for similar ranges of distance and signal-to-noise ratio (SNR). This can be partly explained by higher crustal velocities under the ARCESS array. Also, at ARCESS the ratio of horizontal to vertical power is similar for Sn and Lg, on average, while at NORESS it is larger for Sn, Sn and Lg azimuths at ARCESS (and Lg azimuth at NORESS, with more scatter) provide good estimates of backazimuth (with a 180° ambiguity) and indicate predominance of SH motion at ARCESS. In the second part of this study, multivariate data analysis is performed to obtain phase identifications (with associated confidence), using polarization attributes as predictors. P- and S-type phases are distinguished with a success rate of 82% at NORESS and 89% at ARCESS. The performance is even better for P-type arrivals with 3-C SNR &amp;gt; 2 (96 and 98%, respectively). Sn and Lg are correctly identified for 74% of S-type phases at NORESS and 64% at ARCESS. Contamination of Lg by Sn coda at shorter ranges and dominant SH motion for both S-type phases at ARCESS affect the performance. This study shows the importance of evaluating each 3-C station individually.

Effects of Canary hotspot volcanism on structure of oceanic crust off Morocco

Analysis of over 6400 km of multichannel seismics (MCS) and 50 sonobuoy reflection and refraction experiments reduced both in the domain of X‐T and tau‐p shows that a region within the Jurassic Quiet Zone off Morocco underwent dramatic changes as a result of the passage of the lithosphere over the Canary hotspot commencing approximately 60 Ma. A seismic unit (UCF), interpreted as volcanic in origin, is observed within the sediments in a region characterized by a broad bathymetric swell. It shows diffractions from its upper surface and an internally chaotic seismic facies and pinches out between sedimentary units of continuous, subparallel facies. A velocity inversion is noted between the UCF (4.7km/s) and the underlying sediment (3.1 km/s). The UCF is time transgressive; it lies near the Cretaceous/s Tertiary boundary in the northern portion of the study area and is younger to the south. Kinematic studies of the movement of the Canary hotspot relative to Africa show that the hotspot first appeared off NW Africa about 60 Ma and was located beneath oceanic crust in the region where the UCF is observed. Depth‐to‐basement measurements in areas not effected by the hotspot show a consistent linear trend of increased depth with age. In areas effected by the hotspot the thermal rejuvenation is evident as basement depths shoal with increased proximity to the present hotspot. The reheating of the crust resets the thermal age of the lithosphere with many of the properties of crust of a younger age. Subsidence curves of the reheated crust off Morocco show good correlation to subsidence curves of other reheated crust on a global basis. A zone characterized by high crustal velocities, (7.1–7.4 km/s) and greater crustal thicknesses (by ∼1–2 km) is observed in an area that corresponds to the bathymetric swell, the region of the UCF, and the reelevated basement. The high velocities and increased crustal thickness are interpreted to be the result of underplating and assimilation of existing oceanic crust caused by the Canary thermal anomaly. The presence of high crustal velocities coupled with a thickened crustal section has been noted on various passive margins of the world. They have generally been attributed to the thermal processes associated with continental rifting. Off Morocco, we believe that similar, thermally induced phenomena have occurred but that here; the heat anomaly was the midplate volcanism associated with the Canary hotspot.

Estimation of crustal structure from observed group velocities ofPL-waves in Japan

Forward modelling of the crustal structure of the eastern Honshu Island, Japan, was made based on the group velocities ofPL-waves in the period range of 20–30 s. The observed values of group velocity were obtained by appling the multiple filter technique to the seismograms for earthquakes with the epicentral distance ranging from 500 to 1000 km. The theoretical values were calculated using Oliver and Major's method to find the best fit dispersion curve in the least-squares sense. The obtained structural model has considerably high crustal velocities compared to other previous models. It was shown that thePL-wave group velocity in the period range of interest was most sensitive to seismic velocities of the center of the crust. Numerical experiments confirmed the applicability of the approximation methods employed to obtain both observed and theoretical group velocities.