Midcrustal Low-velocity Zone Research Articles

Crust and upper mantle S wave velocity structure in eastern Turkey based on ambient noise tomography

Eastern Turkey is an ideal location to study continental collision, which is manifested here by the presence of extensive volcanism and rapid tectonic uplift. To better understand the relationship between these surface phenomena and underground structures, we collect continuous waveform data from 121 seismic stations in the region to investigate the S wave velocity structure of the crust and upper mantle using ambient noise tomography. We obtain Rayleigh wave dispersion curves for periods between 5 and 100 s based on frequency-Bessel (F-J) transform dispersion analysis. We then perform quasi-Newton inversion to calculate the S wave velocity structure between 0 and 200 km. The results indicate the presence of two high-velocity anomalies: one in the southern Black Sea that may represent stable and cold lithospheric mantle and another near Lake Van that may represent the subduction remnant of the southern branch of the Neo-Tethys Ocean. In addition, the results demonstrate that the overall velocity in the study area is lower than the global average and reveal extensive upper mantle low-velocity zones (UMLVZs) which represent asthenospheric materials, and mid-crustal low-velocity zones (MCLVZs). In this study, we propose two possible formation mechanisms of the MCLVZs: one is partial melting, and the other is the formation of low-velocity compositional layering due to complex refining and differentiation processes and subsequent continent reworking. Considering that the spatial distribution of the MCLVZs and UMLVZs is well correlated with regional volcanic activity, the MCLVZs and UMLVZs may provide the driving force and are the material sources for volcanism in the area. Furthermore, the extremely thin and weak lithospheric mantle lid and the exceedingly shallow lithosphere asthenosphere boundary (LAB) indicate that the lower part of the lithosphere has delaminated and that the asthenosphere intrudes into a very shallow location, which then supports regional tectonic uplift.

Mapping crustal structure across southern Australia using seismic ambient noise tomography

The rocks of southern Australia record over three billion years of Earth’s evolution, but the basement geology is veiled by sediment. Geophysical data are needed to unveil the geology. The 2018–2022 Lake Eyre Basin and 2020–2022 AusArray SA seismic arrays expand seismic coverage in South Australia. In conjunction with permanent and preceding temporary arrays, we extracted Rayleigh wave dispersion data from ambient noise recordings at a total of 501 seismic stations spanning the transition from Precambrian to Phanerozoic Australia. These data were used to develop Rayleigh wave phase velocity maps of southern Australia at periods 3–20s, and in turn, a shear wave velocity model to 20km depth. At upper-crustal depths, low velocity structure tracks Phanerozoic sedimentary accumulations. The Moyston Fault, regarded as the boundary between the Delamerian and Lachlan Orogens, has an intermittent expression in the shear velocity model: it has no obvious expression at depths shallower than ∼10km, but in the mid-crust is marked by a velocity contrast in southern Victoria and a velocity contrast tracing the southern edge of the Darling Basin in western New South Wales. An arcuate velocity contrast characterising the western edge of deep, sediment-filled troughs of the Darling Basin is a candidate for the transition from Precambrian to Phanerozoic crust. Fluid derived from neotectonic metamorphic devolatilization and/or remnant hydrated mantle is our preferred hypothesis for explaining seismicity and coincident seismic and conductivity anomalies in the mid-to-lower crust beneath the Ikara–Flinders Ranges. Our model suggests that the Olympic Dam and Carrapateena IOCG deposits reside above the margin of a mid-crustal low seismic velocity zone. We surmise that this might reflect past metalliferous fluid movement associated with the Olympic Cu-Au Province, akin to low reflectivity and low resistivity zones evident in 2D reflection seismic and magnetotelluric profiles, respectively.

Continental Reworking in the Eastern South China Block and Its Adjacent Areas Revealed by F‐J Multimodal Ambient Noise Tomography

AbstractThe eastern South China Block (SCB) has experienced complex and drastic reworking processes since the early Neoproterozoic, the mechanisms of which remain unclear. To better understand the continental reworking mechanisms in the eastern SCB, we imaged the shear‐wave velocity structure of the crust and upper mantle in the eastern SCB and its adjacent areas with the F‐J multimodal ambient noise tomography method. The seismic ambient noise data were acquired by 652 seismic stations in this area. The shear‐wave velocity model presents widespread mid‐crustal low‐velocity zones (MCLVZs) and upper mantle low‐velocity zones (UMLVZs), the distribution of which shows a good correlation with regional tectonics. The MCLVZs formed due to compositional and structural layering during the late Neoproterozoic to the early Phanerozoic. The disrupted MCLVZs near the Qinling‐Dabie orogen can be linked to metamorphism caused by a continental collision between the SCB and the North China Craton during the Triassic Indosinian event. The thin crustal and lithospheric thicknesses east of the North‐South Gravity Lineament may be the result of asthenospheric upwelling due to the subduction of the Pacific plate since the late Mesozoic. In the areas near the Shaoxing‐Jiangshan‐Pingxiang fault zone and the Tanlu fault zone, upheaval UMLVZs indicate that these ancient fault zones were reactivated by the subduction of the Pacific plate, providing channels for asthenospheric upwelling. Well‐preserved MCLVZs north of the Qinling‐Dabie orogen can be used to assess compressions and extensions induced by the subduction of the Pacific plate.

Crustal S-Wave Velocity Structure Beneath the Northwestern Bohemian Massif, Central Europe, Revealed by the Inversion of Multimodal Ambient Noise Dispersion Curves

The northwestern Bohemian Massif and adjacent areas are a tectonically active region associated with complex geodynamic activities, that manifest as Quaternary volcanism, earthquake swarms in the upper and middle crust, degassing of CO2, and crustal fluid migration. The intricate tectonic evolution and activities of this region reflect the complexity of the crustal structure therein. However, the crustal models derived from previous studies in this area offer different, even contradictory information regarding the existence of a mid-crustal low-velocity zone (LVZ). In this study, we apply the frequency-Bessel transform (F-J) method to extract the fundamental-mode and up to five higher-mode Rayleigh wave dispersion curves from ambient seismic noise data recorded in the study area and perform multimodal ambient noise dispersion curves inversion. The addition of higher-mode dispersion curves enhances the vertical resolution of the velocity structure inversion results. Our models support the view that the general S-wave velocity level of the crust is high within the study area. We detect two S-wave LVZs beneath the study area that are distributed mainly in the middle crust rather than the lower crust, and these LVZs are separated by a high-velocity zone. Considering the results of previous studies in the area, we infer that these S-wave LVZs may be the consequence of crustal fluids, plastic deformation and even partial melting of the felsic middle crust at relatively high crustal temperatures. Furthermore, these S-wave LVZs could be responsible for the origin and foci depth distribution of earthquake swarms. S-wave low-velocity anomalies are also observed in the uppermost mantle beneath the study area. These S-wave models based on the joint inversion of multimodal dispersion curves can provide new references for understanding the tectonic activity and geodynamic evolution of the northwestern Bohemian Massif and adjacent areas.

Crustal thickness and magma storage beneath the Ecuadorian arc

The Northern Andes of Ecuador contain some of the most active volcanic systems in the Andes and extend over a broad region from the Western Cordillera to the Subandean Zone. While it is known that the arc straddles a range of basement compositions, from accreted mafic oceanic terranes in the west to silicic continental terranes in the east, the details of the crustal structure beneath the arc is unclear despite being critical for understanding magmatic and tectonic processes in this portion of the Andes. To gain insight into these processes, we create two 3D models of crustal and upper mantle seismic properties throughout the region. The first highlights the discontinuity structure using receiver functions, which allows for the recovery of crustal thickness beneath the Ecuadorian Andes. We observe a range from ~50 to 65 km under the high elevations, with thicker crust beneath the lower elevation Western Cordillera compared to the higher elevation Eastern Cordillera. This can largely be explained by density variations within the crust that are consistent with observed terranes at the surface, implying these terranes extend to depth. The second model combines our receiver functions with Rayleigh wave dispersion data from ambient noise measurements in a joint inversion to construct a 3-D shear wave velocity model. This model shows several mid-crustal (5–20 km below sea-level) low velocity zones beneath Ecuadorian arc volcanoes that contain a maximum of ~14% melt. These low velocity zones likely represent zones of long-term magma storage in predominantly crystalline reservoirs, consistent with “mush zones”. Furthermore, the depth of the inferred reservoirs below several of the volcanic centers (e.g., Chiles-Cerro Negro and Tungurahua) are in broad agreement with previous geobarometry and geodetic modeling. Our results provide new observations of possible long-term magma reservoirs below other less-studied volcanic systems in the Ecuadorian arc as well, and further contributes to a mounting number of observations indicating long-term magma storage at low melt percentages in the mid-crust beneath active arc systems.

Late Pleistocene eruptive recurrence in the post-collisional Mt. Hasan stratovolcanic complex (Central Anatolia) revealed by zircon double-dating

Mt. Hasan (3262 m) is the second highest stratovolcano in Central Anatolia, Turkey, and is considered active based on a ca. 9 ka explosive summit eruption that was arguably depicted in a Neolithic mural at the nearby Çatalhöyük archaeological site. Besides evidence for Holocene activity of Mt. Hasan, its eruptive chronostratigraphy and hazard potential remain poorly constrained. Here, we apply (U–Th)/He thermochronology in combination with U–Th–Pb crystallization ages (zircon double-dating), including a novel population-based approach to disequilibrium correction of (U–Th)/He data, in an attempt to determine the eruption ages of strategically sampled lavas and pyroclastic deposits and to refine the Late Pleistocene chronostratigraphy of Mt. Hasan.During the Late Pleistocene, Mt. Hasan erupted multiple interfingering andesitic lava flows along with block-and-ash-flows. Seven of these lava flows, typically the stratigraphically youngest in a particular segment around the two central vents of the volcano, yielded Late Pleistocene eruption ages between 91.9 ± 3.9 and 18.1 ± 2.4 ka (uncertainties stated at 1σ). An even younger block-and-ash-flow was deposited on the western flank of the volcano at 13.5 ± 1.5 ka, which together with a ca. 9 ka small-volume explosive summit eruption define the youngest activity at the main edifice of Mt. Hasan. Collectively, these data indicate a Late Pleistocene recurrence of at least one eruptive phase every ca. 5–15 ka. New volume estimates for the Quaternary edifice of ~130–180 km3 translate into an integrated long-term eruptive magma flux of ~0.3 km3/ka, which is just slightly higher than Late Pleistocene flux estimates based on lava flow geochronology and volumes determined by analysis of digital elevation models. This long-term eruptive flux estimate is similar to those of stratovolcanoes in continental arcs, indicating that eruptive productivity in post-collisional settings can match that of active continental margins. All Mt. Hasan eruptions, including the Holocene event, sampled zircon which crystallized coeval with earlier eruptive phases, indicating longevity of the magma plumbing system. Along with ongoing fumarole activity and evidence for mid-crustal low seismic velocity zones, this suggests that a subvolcanic magma reservoir remains active to the present day.

Crustal Scale Imaging of the Arabia‐Central Iran Collision Boundary Across the Zagros Suture Zone, West of Iran

AbstractWe calculate seismic velocity structure of the north Zagros suture zone, west Iran, to resolve the crustal features at the boundary of Arabian‐Central Iranian collision. We compute teleseismic receiver functions (RFs) for 46 stations along a transect crossing the suture. Through harmonic analysis and inversion of the RF data, we obtain information on the characteristics of the suture zone at depth. The RFs and their harmonics show a low angle NE dipping boundary between the overriding layer and a midcrustal low velocity zone, which corresponds to the suture zone. The overriding high velocity feature (Vs ~ 3.8 km/s) is interpreted as an intermediate depth crustal complex exhumed close to the surface through imbricate thrust faulting and enhanced by crustal buoyancy due to continental underthrusting. Significant anisotropy is found above and below the suture zone: We interpret it in terms of slow‐symmetry‐axis anisotropy and derive clues on the ongoing deformation processes.

Seismogenic structure in the source zone of the 1918 M7.5 NanAo earthquake in the northern South China Sea

As a typical rifted margin, the coastal region of the northern South China Sea has experienced many large crustal earthquakes in the past 400 years. For example, two destructive earthquakes with ~M7.0 in 1600 and ~M7.5 in 1918 occurred close to the NanAo Island. However, the mechanism of these earthquakes and seismotectonics of the region are still unclear. In this work we make the first study of the detailed three-dimensional crustal structure in the source zone of the 1918 NanAo earthquake (M7.5) using newly acquired wide-angle seismic reflection data. Our results show that current seismicity mostly occurred in a midcrustal low-velocity zone or its nearby high-to-low velocity transitional areas. We ascertain locations of the ENE-strike Littoral fault zone and the NW-strike Huanggangshui fault based on the seismic reflection profiles. These faults have caused a large-scale vertical dislocation of the sedimentary basement. The structure of the upper crust is very heterogeneous, containing many high or low velocity anomalies. However, the lower crustal structure is less heterogeneous, mainly characterized by low velocity mostly in the northern part of the study area and high velocity mainly in the southern part. The crust in the seismicity zone shows strong heterogeneity, reflecting highly cracked structural features possibly caused by destructive earthquakes. We infer that the midcrustal low-velocity zone and the intersection between the Littoral fault zone and the Huanggangshui fault could reflect weak structures in the crust and form a local stress buildup, which strongly affected or controlled the generation of intraplate seismicity. The 1918 NanAo earthquake very likely occurred within the current seismicity zone with a depth range of 15–25 km estimated from our tomographic images and previous geological and geophysical results.

Crustal anisotropy beneath northeastern Tibetan Plateau from the harmonic decomposition of receiver functions

SUMMARY A uniformly spaced linear transect through the northeastern Tibetan Plateau was constructed using 54 stations from ChinaArray Phase II. We used a set of colocated earthquakes to form receiver function beams that were then used to construct a 2-D image of main converting boundaries in our region and to investigate lateral changes in main impedance contrasts along the transect. The image revealed obvious mid-crustal low-velocity zones beneath the Qilian Orogen and the Alxa Block. We developed a new procedure that uses harmonically decomposed receiver functions to characterize seismic anisotropy, and that can determine both the orientations of symmetry axes and their type (fast or slow). We tested our technique on a number of synthetic models, and subsequently applied it to the data from the transect. We found that: (1) within the upper crust the orientations of slow symmetry axes are nearly orthogonal to the strike directions of faults, and thus anisotropy is likely caused by the shape preferred orientation of fluid-saturated cracks or fractures and (2) together with the low-velocity zones revealed from receiver functions stacks, anisotropic layers in the middle-to-lower crust could be explained by the crustal channel flow that was proposed for this region by previous studies. The shear within the boundary layers of crustal flow forms anisotropy with symmetry axes parallel to the flow direction.

Comparative tomography of reverse-slip and strike-slip seismotectonic provinces in the northern South Island, New Zealand

Actively deforming crust and upper mantle around the obliquely convergent Pacific-Australia plate boundary in the northern South Island of New Zealand have been investigated by seismic tomography using data from a temporary c. 50 station network plus GEONET. The Alpine-Wairau Fault (principal component of the plate boundary at the surface) transects the study area, separating the Buller-Nelson (BN) province of active compressional inversion involving steep reverse faulting to the northwest, from the Marlborough fault system (MA) dominated by dextral strike-slip faulting to the southeast. In the course of this study, MA hosted the 2013 Seddon earthquake sequence (M5.9, 6.6) and the 2016 M7.8 Kaikoura earthquake. Active fault structures and present seismicity are associated with a heterogeneous distribution of low-velocity and Vp/Vs anomalies. Elsewhere, there is a general association between crustal seismicity and low-velocity zones along major fault structures within both the MA and BN seismotectonic provinces along which high Vp/Vs anomalies are locally conspicuous. Areas of active crustal seismicity are also generally characterized by high Vp/Vs, for instance the hypocentre and aftershocks of the 2016 M7.8 Kaikoura earthquake overlie a low-velocity region with high Vp/Vs. The coincidence of anomalously low Vp and Vs and anomalously high Vp/Vs with zones of high electrical conductivity defined by a previous MT transect is consistent with the notion that upward migration of overpressured hydrothermal fluid from the subducting slab at depth leads to a heterogeneous distribution of overpressured fluid in and around the base of the crustal seismogenic zone, weakening the overlying crust and promoting seismic rupture along major fault systems. It seems possible that the Association of mid-crustal low-velocity zones with anomalously high Vp/Vs may diagnose rupture preparation zones where frictional strength is being lowered by the build-up of fluid overpressure concurrent with accumulating shear stress, so that eventual fault failure is ‘dual-driven’.

Radial anisotropy in the crust beneath the northeastern Tibetan Plateau from ambient noise tomography

Through analysis of Rayleigh wave and Love wave Green’s functions estimated from ambient noise tomography, we obtain radial anisotropy and shear wave velocity structure beneath the northeastern Tibetan Plateau. With two hundred and twenty three broadband seismic stations deployed by China Earthquake Administration, a collaborative seismic experiment of northern Tibet (ACSENT) experiment and northeastern Tibet seismic (NETS) experiment provide the unprecedented opportunity to resolve the spatial distribution of the radial anisotropy within the crust of the northeastern Tibetan Plateau. Discrepancies between Love (sh) and Rayleigh (sv) wave velocities show complex anisotropic patterns associated with the dynamic processes of the collision between the Indian and Eurasian plates: (1) In the upper crust, V sv>V sh anisotropy is dominant throughout the study area which probably reflects fossil microcracks induced by the uplift, folding and erosion geodynamic processes; (2) in the middle crust, V sh>V sv observed beneath the Songpan-Ganzi terrane and the northwestern Qilian orogen correlates well with a mid-crustal low velocity zone (LVZ); (3) at depths deeper than 40 km, V sh>V sv is still found in the Songpan-Ganzi terrane. This anisotropy could be caused by the sub-horizontal alignment of anisotropic minerals that has followed the collision between India and Eurasia. However, the northwestern Qilian orogen is associated with V sv>V sh anisotropy which may be related to the vertically aligned seismic anisotropic minerals caused by the crustal thickening.

Penetration of mid-crustal low velocity zone across the Kunlun Fault in the NE Tibetan Plateau revealed by ambient noise tomography

The NE Tibetan Plateau, composed of the Mesozoic accretions of Lhasa, Qiangtang and Songpan-Ganze Terranes, are bounded by the east Kunlun–Qaidam Block in the north with the boundary delineated by the Kunlun Fault. The NE Tibetan Plateau is at a nascent stage of plateau growth resulting from the collision between the Indian and Eurasian plate starting ∼50 million years ago, and is one of the best areas to study the growth mechanism of the Tibetan Plateau. In this study, we process continuous ambient noise data collected from ∼280 stations during 2007 and 2010 and generate Rayleigh wave phase velocity maps at 10–60 s periods with a lateral resolution of ∼30–50 km for most of the study region. By adopting a Bayesian Monte Carlo method, we then construct a 3-D Vsv model of the crust using the Rayleigh wave dispersion maps. Our 3-D model reveals that strong LVZs exist in the middle crust across the NE Tibetan Plateau; and the lateral distribution of LVZs exhibit significant west–east variations along the Kunlun Fault. In the west of 98°E, LVZs are confined to regions of the Kunlun Fault and the eastern Kunlun Ranges, but absent beneath the Qaidam Basin; while in the east of 98°E, LVZs extend and penetrate northward into the east Kunlun and Qinling Orogens over ∼100 km across the Kunlun Fault. The strong contrast of the LVZs distribution along the Kunlun Fault may be related to the distinct neighboring tectonic units in the north: a strong crust of the Qaidam Basin in the west blocking the penetration of LVZs, but a weak crust in the Qinling Orogens facilitating the extrusion of LVZs. The distribution of LVZs in the NE Tibetan Plateau is consistent with the crustal channel flow model, which predicts a branch of north-eastward mid-crustal channel flow. Our 3D model clearly delineates the north extent of the mid-crustal LVZs, probably reflecting the status of channel flow in the NE Tibetan Plateau.

Mapping crustal structure beneath southern Tibet: Seismic evidence for continental crustal underthrusting

Receiver function imaging along a temporary seismic array (ANTILOPE-2) reveals detailed information of the underthrusting of the Indian crust in southern Tibet. The Moho dips northward from ~50km to 80km beneath the Himalaya terrane, and locally reaches ~85km beneath the Indus–Yalung suture. It remains at ~80km depth across the Lhasa terrane, and shallows to ~70km depth under the Qiangtang terrane. An intra-crustal interface at ~60km beneath the Lhasa terrane can be clearly followed southward through the Main Himalaya Thrust and connects the Main Boundary Thrust at the surface, which represents the border of the Indian crust that is underthrusting until south of the Bangong–Nujiang Suture. A mid-crustal low velocity zone is observed at depths of 14–30km beneath the Lhasa and Himalaya terranes probably formed by partial melt and/or aqueous fluids.

Crustal Structure of the Northeastern Tibetan Plateau from the Southern Tarim Basin to the Sichuan Basin, China

We present a new crustal cross section of the northeastern Tibetan plateau based on active-source seismic data recorded along a 1600-km-long profile crossing the southern Tarim basin, the western flank of the South-Qilian Shan, the northeastern margin of Qaidam basin, East-Kunlun Shan, Songpan-Ganzi terrane, and Sichuan basin. The crustal P- and S-wave velocity structure and Poisson's ratio outline the seismic characteristics of the crust and provide constraints on the crustal composition. The derived crustal cross section shows several significant features. (1) The crustal thickness varies considerably along this profile, from 48km to 70km. (2) North of the Kunlun fault variations in total crustal thickness are mainly caused by variations in lower crustal thickness, whereas south of the Kunlun fault they are caused by thickness variations throughout the crust. (3) North of the Kunlun fault we detect a mid-crustal low-velocity zone that is not apparent south of the fault. (4) The Kunlun fault seems to act as a compositional boundary for the lower crust, with a Poisson's ratio of 0.29 north of the fault (Kunlun-Qaidam terrane) and 0.26 south of the fault (Songpan-Ganzi terrane). Measured Poisson's ratio and P-wave velocity values suggest that the lower crust throughout the Tibetan plateau (South-Qilian Shan, margins of the Qaidam Basin, East-Kunlun Shan, Songpan-Ganzi terrane) is of intermediate composition. Thus the NE Tibetan plateau along our profile is missing a mafic lower crustal layer.

Crustal structure of the Tethyan Himalaya, southern Tibet: new constraints from old wide-angle seismic data

SUMMARY A wide-angle seismic profile between Peigu Tso (85.5°E) and Pumoyong Tso (90.5°E) in southern Tibet, acquired under a joint Sino-French program in 1981, passes through the South Tibetan Detachment System (STDS, the eastern segment of the profile) and the central part of the Tethyan Himalaya (the western segment). We herein reinterpret this wide-angle seismic profile, which has a total length of ∼480 km, in order to improve our understanding of the crustal structure of the Tethyan Himalaya. We identify multiple P- and S-wave wide-angle reflections within the crystalline crust and from the Moho, but cannot identify any refracting waves beneath the Moho. We model the structure of P- and S-wave velocity throughout the whole crust, while acknowledging the significant uncertainties that are inherent in many places. A low-wave speed cover sequence may be identified with the Tethyan sedimentary cover (Vp < 5.8 km s−1), from the surface to a depth of (typically) 5–10 km, which is only absent where the profile crosses the Kangmar Dome. It may be presumed that these sedimentary rocks overlay a felsic upper-crustal basement (5.8 < Vp < 6.3 km s−1), which in places extends as far down as 35 km, and which itself overlays a 5–10-km-thick mid-crustal low-velocity zone. Between a depth of 30 and 45 km at the base of the low-velocity zone and the Moho at a depth of ∼73 km, the observed velocities gradually increase from 6.2 to 6.8 km s−1. The low-velocity layer (LVL) may consist of a partial melt due to the high geothermal gradients found in the felsic rocks, or of a fluid-rich shear zone as in the ductile channel of a channel flow model. The difference in depth of the LVL between the western (central Tethyan Himalaya) and eastern segments (nearly along the STDS) at the boundary of the Dingye-Mabjia fault, leads to a north–south dip estimate of the LVL of 8.5°, which is consistent with the estimated dip of the MHT obtained from the INDEPTH-I project, and can support the channel flow model. This north–south low dip angle of subduction slab, supports that the subducting Indian lithosphere endured a transition from steep subduction to low-angle subduction, and the subduction slab flattening may be attributed to the break-off of Indian lithosphere slab beneath the Indur-Zurpo suture.

Implications for intraplate volcanism and back-arc deformation in northwestern New Zealand, from joint inversion of receiver functions and surface waves

SUMMARY We employ a joint inversion of teleseismic receiver functions and surface wave phase velocities to determine the shear wave velocity structure in the crust and upper mantle beneath northwestern New Zealand. Receiver functions primarily contain information on velocity contrasts, while surface waves are sensitive to the average shear velocity with depth. By performing a joint inversion we reduce the limitations of each method, resulting in a more robust shear wave model. Inversion results reveal regions of low shear wave velocity of ∼2.8 km s −1 in the mid-crust (10‐19 km depth) and ∼4.0 km s −1 in the upper mantle (70‐90 km depth) beneath Quaternary intraplate basalt fields. We infer that the mid-crustal low-velocity zones (LVZs) are bodies of partial melt, most likely rhyolite intrusions. We suggest that the upper mantle LVZ is caused by the melt-producing regions of the upper mantle and is a source for the basalts of the Auckland Volcanic Field. This is in agreement with models for a shallow upper mantle source rather than a deep-seated mantle plume for the Auckland volcanism. Average shear velocities for the upper crust are 3.4‐3.6 km s −1 , increasing to 3.6‐4.0 km s −1 in the lower crust. The Moho is interpreted to be 29 ± 1 km deep in the southern end of the array, shallowing to 26 ± 1 km towards the edge of the continental shelf of northern New Zealand. Low upper mantle shear velocities of 4.2 ± 0.1 km s −1 are observed, and are thought to represent a small percentage of partial melt in the upper mantle beneath this back-arc region. These low velocities extend to greater depth (∼90 km) beneath the southern stations, indicating that mantle deformation associated with the mantle wedge to the south may extend as far north as the southern end of our array. Near-surface low-velocity layers extend to 6 km depth beneath a station at the northern tip of New Zealand, likely representing the expression of a thick sequence of sediments from the Northland Allochthon and the more recent Northland Basin.

Multi-scale Tomography for Crustal P and S Velocities in Southern California

Seismic tomography is a viable tool in building depth-velocity models in the presence of strong lateral velocity variations. In this study 3-D P- and S-velocity models for the crust of southern California are constrained using more than 1,000,000 P-wave first arrivals and 130,000 S-wave arrivals from local earthquakes. To cope with the uneven distribution of raypaths, a multi-scale tomography is applied with overlapping model cells of different sizes. Within the 300 × 480 × 39 km3 model volume, the smallest cell size is 10 × 10 × 3 km3. During the iterations of velocity updating, earthquake hypocenters are determined using both P and S arrivals, and full 3-D ray tracing is implemented. Except near the edges and in the lower crust, the resultant models are robust according to various tests on the effects of reference models, resolution and signal-to-noise ratio. The tomographic velocities at shallow depths correlate very well with the regional geology of southern California. In the upper crust the P-wave and S-wave models exhibit slow velocities in major sedimentary basins and fast velocities in areas of crystalline rocks. Mid-crustal low velocity zones are present under the Coso Range, San Gabriel Mountains, and a large portion of the Mojave Desert. P- and S-velocity patterns maintain their similarity in the lower crust though the models are less reliable there.

Numerical models of crustal scale convection and partial melting beneath the Altiplano–Puna plateau

We employ 2-D thermo-mechanical modelling to study possible mechanisms for generating large-scale crustal magmas in the Altiplano–Puna region of the Central Andes. The peak of ignimbrite activity in the late Miocene and Pliocene is associated in space and time with tectonic shortening and plateau uplift. A seismic low-velocity zone and other geophysical observations indicate that partial melting in the mid-crust is still present under the ignimbrite province today. We show that neither radiogenic heat production in a thickening crust, nor shear heating due to tectonic shortening, nor heat brought by intrusions of arc magmas into the mid-crust can heat the mid-crust to the degree and within the time frame suggested by the geologic, petrologic and geophysical observations. A viable mechanism to achieve high temperatures within the time constraints is convective heat and mass transfer by partially molten lower crust which itself was heated by enhanced mantle heat flow, possibly associated with delamination of the mantle lithosphere during tectonic shortening and intensified magmatic arc activity. The thermo-mechanical models explain the mid-crustal low-velocity zone and the high and strongly variable surface heat flow observed in the Altiplano. Convection by bulk flow of the crust appears when the middle and lower crust are mechanically weak (quartz-dominated rheology), the basal heat flow from the mantle is high (>60 mW/m2) and tectonic shortening is active. We argue that these conditions are satisfied in the Central Andes orogen.

Lithospheric Structure of the Marmara and Aegean Regions, Western Turkey

We have simulated the waveforms of three aftershocks of the Izmit (17 August 1999) earthquake, with magnitudes greater than M 5.0, to determine the lithospheric structure of the Gulf of Izmit, Marmara region. Using the discrete wavenumber technique (Bouchon, 1981), different layered crustal models have been tested for the simulation of the waveforms, and the best crustal model was determined by a best-fit criterion between the observed and simulated seismograms. Our results indicate that the average thickness of the crust is 32 km and that Pn and S velocities are 8.0 and 4.60 km/sec, respectively, in the Gulf of Izmit, Marmara region. We also computed synthetic waveforms of the 21 April 2000 Denizli (Honaz) and the 9 July 1998 Izmir (Doganbey) earthquakes in the Aegean in order to compare the lithospheric structures of the Marmara and the Aegean regions. The Aegean region has an average crustal thickness of 33 km and Pn and S velocities of 7.85 and 4.53 km/sec, respectively. Although thicknesses of the crusts are comparable and suggest an approximately equal amount of E-W stretching in the Marmara and N-S stretching in the Aegean regions, a patchy midcrustal low-velocity zone exists in the Aegean. The upper-mantle Pn velocity variation between the Marmara and the Aegean regions is interpreted as the effect of a thinning continental lithosphere toward the active Aegean arc and the establishment of a consequent upper-mantle temperature gradient, increasing from the north to the south. Additionally, the Black Sea oceanic lithosphere that steeply dips southward beneath the Marmara region, as evidenced by the seismic tomographic results obtained by Gulen and Kuleli (1995), can contribute to the advective cooling of the upper mantle that causes relatively high Pn velocities in the Marmara region. Manuscript received 20 August 2000.

Increase in melt fraction along a south-north traverse below the Tibetan Plateau: evidence from seismology

A mid-crustal low-velocity zone and crustal melt fraction contrast the Lhasa block in the Maxiang Yangbajain zone with the Tethyan Himalayas in the south. Evidence from wide-angle reflections is extended by vertical reflection and differential S to P wave teleseismic delays. The temperature and rheology implied allow phase transitions which may change crustal buoyancy and elevation (Le Pichon et al., 1997), and may allow lateral mass flow. In the uppermost mantle, particular path geometries to a tight temporary teleseismic array allow detection of a decrease in velocity northward through the Lhasa block. A larger relative variation in late teleseismic S with respect to P indicates an increase in Poisson's ratio, hence of partial melt fraction. This may be seen as evidence of a higher position of the mantle asthenosphere. Level Tibet hence appears to be underlain by variation in the structure within the crust across the Indus-Tsangpo suture, and in the structure within the mantle, further northward of the middle Lhasa block. These variations are most readily attributable to spatial variation in partial melt fraction, and hence temperature, which also induces phase transformations. Mode and amount of deformation and control of buoyancy on elevation may vary accordingly. The dynamics of the system is evidenced by seismic anisotropy which we relate to ductile flow.