Anomalous Crust Research Articles

Cryptic Magma Chamber in the Deccan Traps Imaged Using Receiver Functions and Surface Wave Dispersion

AbstractAn anomalous crust and lithospheric mantle in the Deccan Volcanic Province are imaged below a 160 km long W‐E profile through the joint inversion of receiver functions and surface wave dispersion. The upper crust has an unusually low S‐wave velocity (Vs ∼ 3.3–3.5 km/s) at 8–17 km depth, underlying a 4 km thick high‐velocity layer (Vs > 3.8 km/s). The low velocity possibly represents the frozen magma reservoir, the source for the magma eruption at ∼65 Ma due to the interaction of the Reunion plume with India. The shallow, high‐velocity layer could be basaltic mafic intrusions responsible for the production of massive CO2 degassing. The Moho deepens beneath the west coast to ∼45 km due to 10–15 km of magma underplating. The mantle plume scar is seen as thinned lithosphere (80–100 km), with the presence of long‐lived low‐velocity layers in the shallow mantle attributed to the presence of sulfide melts.

Crustal structure beneath and surrounding the Appalachian Basin region of Eastern North America imaged by joint inversion of P wave receiver functions and surface wave dispersion measurements

SUMMARY To evaluate the plate flexure model for the formation of the Appalachian Basin, we investigate the extent to which crustal structure beneath and surrounding the basin was modified by the Palaeozoic orogenic events that created the basin. We jointly invert receiver functions and surface wave dispersion measurements to obtain 1-D crustal Vs profiles for 261 seismic stations located within and around the basin. The average crustal thickness for the region is 44 km, and the crust gradually thins to the east, consistent with previous studies. Four areas of anomalous crust are identified with respect to the eastward thinning of the crust. An area of thick crust is found along the Grenville Front on the western side of the Appalachian Basin where the crust thickens by ∼5–10 km. Moho depths of up to 54 km in this region likely result from suture-thickening. The crust is thinner beneath the Neoproterozoic Scranton rift by ∼5–7 km, coincident with a ∼40 mGal Bouguer gravity high. Across the Neoproterozoic Rome Trough, the crust thins by ∼4–5 km, coincident with a ∼10 mGal Bouguer gravity high. Density models for these rifts show that the rift-related crustal thinning is sufficient to explain the gravity anomalies. The Vs models obtained for stations in the rifts indicate little, if any, mafic layering in the mid-crust and only a modest amount of mafic layering in the lower crust. In the northwestern portion of the Appalachian Basin in northeastern Ohio and northwestern Pennsylvania within the Elzevir block, another area of anomalously thick crust (50–52 km) is found. This region is not associated with any known tectonic structures or boundaries or a gravity anomaly. The lower ∼5–10 km of the crust in this region is characterized by high (>3.9 km s−1) shear wave velocities and thus appears to be mafic. The origin of anomalous crustal structure in all four areas is best attributed to Precambrian tectonic events that predate the formation of the Appalachian Basin, indicating that the crystalline crust beneath and surrounding the basin was not significantly affected by the Palaeozoic basin-forming orogenic events, a finding which supports the use of plate flexure models for understanding basin formation.

Punctuated geochronology within a sustained high-temperature thermal regime in the southeastern Gawler Craton

The Yorke Peninsula region in the southeastern Gawler Craton forms the southern portion of the Olympic Cu-Au Province that hosts the giant Olympic Dam, Carrapateena and Prominent Hill deposits. The Yorke Peninsula hosts substantial Cu-Au mineralisation, with deposits at Hillside and in the Moonta–Wallaroo district that are broadly considered to have formed during the same event as Olympic Dam, the c. 1595–1575 Ma Hiltaba thermal event. Despite the region's economic significance, there is very little information on the nature and history of metamorphism in the Yorke Peninsula region, leaving a considerable gap in the knowledge of the tectonic controls on mineralisation in this area. New U–Pb LA-ICP-MS geochronology on metamorphic titanite, monazite and apatite from Wallaroo Group metasediments indicate the Yorke Peninsula region has a protracted thermal history, with three groupings of metamorphic ages at c. 1585 Ma, 1560–1550 Ma and 1530–1500 Ma, and apatite cooling ages that cluster around c. 1450 Ma. Phase equilibria modelling of andalusite-sillimanite bearing schists and cordierite-sillimanite bearing migmatitic gneisses indicate that metamorphism occurred between 1560 and 1500 Ma occurred at P–T conditions between 3.6 and 4.5 kbar and 660–750 °C. These high thermal gradient conditions (45–50 °C km−1) are recorded at least 20 million years after Hiltaba Suite magmatism; however, they occur within the thermo-chemically anomalous crust of the South Australian Heat Flow Anomaly, which predominantly consists of rock packages with heat production rates 60–100% greater than the global median for rocks of their age. One-dimensional thermal models constrained by the crustal architecture and thermal regime of the SE Gawler Craton suggest the metamorphic conditions recorded between 1560 and 1500 Ma can be achieved by burial of this heat producing element -enriched crust, without the influence of an advective heat source. Within this thermally-anomalous crustal regime in which high thermal gradients can be sustained, small changes in burial depth are considered sufficient to stimulate metal remobilisation in the metal-rich crust of the Yorke Peninsula region. This potential for burial-triggered, thermally-induced fluid liberation and subsequent metal remobilisation may be applicable to the entire eastern Gawler Craton, including the Olympic Cu-Au Province.

Crustal structure of the East African Limpopo margin, a strike-slip rifted corridor along the continental Mozambique Coastal Plain and North Natal Valley

Abstract. Coincident wide-angle and multi-channel seismic data acquired within the scope of the PAMELA Moz3-5 project allow us to reconsider the formation mechanism of East African margins offshore of southern Mozambique. This study specifically focuses on the sedimentary and deep-crustal architecture of the Limpopo margin (LM) that fringes the eastern edge of the Mozambique’s Coastal Plain (MCP) and its offshore southern prolongation the North Natal Valley (NNV). It relies primarily on the MZ3 profile that runs obliquely from the northeastern NNV towards the Mozambique basin (MB) with additional inputs from a tectonostratigraphy analysis of industrial onshore–offshore seismic lines and nearby or crossing velocity models from companion studies. Over its entire N–S extension the LM appears segmented into (1) a western domain that shows the progressive eastward crustal thinning and termination of the MCP/NNV continental crust and its overlying pre-Neocomian volcano-sedimentary basement and (2) a central corridor of anomalous crust bounded to the east by the Mozambique fracture zone (MFZ) and the oceanic crust of the MB. A prominent basement high marks the boundary between these two domains. Its development was most probably controlled by a steep and deeply rooted fault, i.e., the Limpopo fault. We infer that strike-slip or slightly transtensional rifting occurred along the LM and was accommodated along this Limpopo fault. At depth we propose that ductile shearing was responsible for the thinning of the continental crust and an oceanward flow of lower crustal material. This process was accompanied by intense magmatism that extruded to form the volcanic basement and gave the corridor its peculiar structure and mixed nature. The whole region remained at a relative high level during the rifting period and a shallow marine environment dominated the pre-Neocomian period during the early phase of continent–ocean interaction. It is only some time after break-up in the MB and the initiation of the MFZ that decoupling occurred between the MCP/NNV and the corridor, allowing for the latter to subside and become covered by deep marine sediments. A scenario for the early evolution and formation of the LM is proposed taking into account both recent kinematic and geological constraints. It implies that no or little change in extensional direction occurred between the intra-continental rifting and subsequent phase of continent–ocean interaction.

Seismicity, Metamorphism, and Fluid Evolution Across the Northern Cascadia Fore Arc

AbstractWe invert traveltime data from regular seismicity and low‐frequency earthquakes (LFEs) on southern Vancouver Island to map fore‐arc structure and seismogenesis. Tomographic images reveal high Poisson's ratios associated with a previously mapped, dipping low‐velocity zone inferred to be overpressured, upper oceanic crust of the Juan de Fuca plate, where LFEs and other slow‐slip phenomena occur. Low Poisson's ratios (∼0.225) in the fore‐arc continental crust above the mantle wedge are accompanied by high Vp (∼6.65 km/s) and high levels of clustered microseismicity. This crustal anomaly is positioned above the slab, where focused expulsion of fluids is inferred based on independent evidence, suggesting an association between composition, fluids, and seismogenesis. We propose that the anomalous crust harbors a high percentage (≤15%) of quartz, characterized by extremely low Poisson's ratio of 0.1. Earlier studies have proposed that excess quartz in the crust is precipitated from fluids fluxed from the subducting slab. In contrast, we posit that quartz is produced and concentrated in situ by metasomatism in the fore‐arc crust catalyzed through focused ingress of slab‐derived fluids at high‐pore pressure. These fluids enable microseismic activation of high‐angle thrust faults through a fault‐valve mechanism that concentrates quartz via pore‐pressure cycling. Larger M ≥ 6 events like those recently proposed to occur along the Leech River Fault may nucleate deep in the brittle‐ductile transition as a result of these reactions. The fore‐arc crust in warm subduction settings may thus suffer extreme exposure to prolonged fluid flow, an inference with relevance to the genesis of orogenic gold deposits.

Crustal Architecture at the Collision Zone Between Rivera and North American Plates at the Jalisco Block: Tsujal Project

Processing and analysis of new multichannel seismic records, coincident with wide-angle seismic profiles, acquired in the framework of the TsuJal project allow us to investigate in detail the complex structure of the oceanic domain in the collision zone between Rivera Plate and Block Jalisco at its northern termination. The subducting Rivera Plate, which is overridden by the North American Plate–Jalisco Block, is clearly identified up to 21.5°N (just south of Maria Magdalena Island) as a two clear reflections that we interpret as the interplate and Moho discontinuities. North of the Tres Marias Islands the seismic images display a different tectonic scenario with structures that are consistent with large faulting and rifted margin. A two-dimensional velocity approach for the crustal geometry is achieved using joint refraction/reflection travel time tomography, the uncertainty of the results is assessed by means of Monte Carlo analysis. Our results show an average oceanic crustal thickness of 6–7 km with a moderate increase towards the Jalisco Block, an anomalous thick layers (~3.0 km) displaying a relatively low velocity (~5.5 km/s) underneath Maria Magdalena Rise, and an estimated Moho depth deeper than 15 km in the collision zone between Rivera Plate and Jalisco Block. We have also determined an anomalous crust on the western flank of the Tres Marias Islands, which may be related to the initial phases of continental breakup of the Baja California Peninsula and Mexico mainland. High-resolution bathymetry provides remarkable images of intensive slope instabilities marked by relatively large slides scars of more than 40 km2 extent, and mass-wasting deposits probably triggered by the intense seismicity in the area.

Аномальная земная кора Астраханского свода

Аномальная земная кора Астраханского свода

Expressions for the Global Gravimetric Moho Modeling in Spectral Domain

We apply a newly developed numerical method to improve the Moho geometry by the implementation of gravity data. This method utilizes expressions for the gravimetric forward and inverse modeling derived in a frequency domain. Methods for a spectral analysis and synthesis of the gravity field and crust density structures are applied in the gravimetric forward modeling of the consolidated crust-stripped gravity disturbances, which have a maximum correlation with the (a priori) Moho model. These gravity disturbances are obtained from the Earth’s gravity disturbances after applying the topographic and stripping gravity corrections of major known anomalous crust density structures; in the absence of a global mantle model, mantle density heterogeneities are disregarded. The isostatic scheme applied is based on a complete compensation of the crust relative to the upper mantle density. The functional relation is established between the (unknown) Moho depths and the complete crust-stripped isostatic gravity disturbances, which according to the adopted isostatic scheme have (theoretically) a minimum correlation with the Moho geometry. The system of observation equations, which describes the relation between spherical functions of the isostatic gravity field and the Moho geometry, is defined by means of a linearized Fredholm integral equation of the first kind. The Moho depths are determined based on solving the gravimetric inverse problem. The regularization is applied to stabilize the ill-posed solution. This numerical procedure is utilized to determine the Moho depths globally. The gravimetric result is presented and compared with the seismic Moho model. Our gravimetric result has a relatively good agreement with the CRUST2.0 Moho model by means of the RMS of differences (of 3.5 km). However, the gravimetric solution has a systematic bias. We explain this bias between the gravimetric and seismic Moho models by the unmodelled mantle heterogeneities and uncertainties in the CRUST2.0 global crustal model.

Combining seismically derived temperature with heat flow and bathymetry to constrain the thermal structure of oceanic lithosphere

The thermal structure and evolution of the oceanic lithosphere is revisited with the help of a global shear velocity model of the upper mantle. Seismic velocities of the lithospheric mantle are converted to temperatures, with a particular care dedicated to the estimation of final uncertainties. These are evaluated from a Monte Carlo error propagation, taking into account the uncertainties on velocities and those on the parameters related to the velocity–temperature relationship (mantle composition, thermoelastic properties and attenuation factor). The seismically derived temperature, averaged by age interval, serves to constrain the thermal structure of the lithosphere, together with surface heat flow and ocean depth. Using an experimentally determined thermal expansivity α, the Chablis model, which prescribes a constant heat flow at some isotherms, provides a much better fit to all data than the plate model, which imposes a constant basal temperature. Only a strongly reduced α (30% reduction) allows the latter model to achieve a joint fitting comparable to the Chablis model, and then with a fit to seismically derived temperature that remains inferior the latter model. The good fit of the plate model thus depends on a reduction of α down to the lower possible limit and relies mostly on ocean depth, whose behavior at old ages is considerably obscured by anomalous crust. The Chablis model therefore appears favored by this study, which should give new perspectives on various processes related to mantle convection and to the dynamics of the lithosphere.

Relocation of aftershocks of the 2001 Bhuj earthquake: A new insight into seismotectonics of the Kachchh seismic zone, Gujarat, India

Abstract In view of an anomalous crust–mantle structure beneath the 2001 Bhuj earthquake region, double-difference relocations of 1402 aftershocks of the 2001 Bhuj earthquake were determined, using an improved 1D velocity model constructed from 3D velocity tomograms based on data from 10 to 58 three-component seismograph stations. This clearly delineated four major tectonic features: (i) south-dipping north Wagad fault (NWF), (ii and iii) south-dipping south Wagad faults 1 and 2 (SWF 1 , SWF 2 ), and (iv) a northeast dipping transverse fault (ITF), which is a new find. The relocated aftershocks correlate satisfactorily with the geologically mapped and inferred faults in the epicentral region. The relocated focal depths delineate a marked variation to the tune of 12 km in the brittle–ductile transition depths beneath the central aftershock zone that could be attributed to a lateral variation in crustal composition (more or less mafic) or in the level of fracturing across the fault zone. A fault intersection between the NWF and ITF has been clearly mapped in the 10–20 km depth range beneath the central aftershock zone. It is inferred that large intraplate stresses associated with the fault intersection, deepening of the brittle–ductile transition to a depth of 34 km due to the presence of mafic/ultramafic material in the crust–mantle transition zone, and the presence of aqueous fluids (released during the metamorphic process of eclogitisation of lower crustal olivine-rich rocks) and volatile CO 2 at the hypocentral depths, might have resulted in generating the 2001 Bhuj earthquake sequence covering the entire lower crust.

Crust–mantle structure of the central North Island, New Zealand, based on seismological observations

Two geophysical features characterise the anomalous crust–mantle structure beneath central North Island of New Zealand: the apparent thinning of the felsic crust by a factor of at least 50%, and upper mantle P-wave velocities ( P n phase) about 10% less than normal (∼ 7.3–7.4 km/s). P n velocities increase slowly to a maximum of ∼ 7.8 km/s at a depth of about 80–100 km. Above a depth of 15 km seismic P-wave velocities ( V p ) are 6 km/s or less, representing what is likely to be a mix of the original greywacke crust that has been stretched, along with an unknown volume of intruded igneous rocks that are in various stages of cooling. Defining the Moho is problematic in this dynamic region. Below a depth of 15 km there is a continuous increase in V p such that velocities ∼ 7 km/s are reached at a depth as shallow as 20 km on a east west line and ∼7.4 km/s on a north–south line. We interpret these wave-speeds as a transition zone into low P n velocities, rather than high lower-crustal values of V p . P n wave-speeds as low as 7.4 km/s are detected by tomographic methods to depths of at least 65 km and laterally over an area of at least 10,000 km 2. A 1–4% partial melt in the mantle wedge beneath the central North Island is estimated, based on Q p , seismic anisotropy, low values of P n and amplitudes of deep reflections. These upper mantle features are more indicative of asthenosphere than lithosphere and, coupled with the geological evidence for rapid uplift here at 5 Ma, are consistent with convective removal of the mantle lithosphere from western and central North Island in late Miocene times.

Geodynamic and fluid dynamic processes in formation of the anomalous crust beneath the Yurubchen-Tokhomo oil and gas accumulation zone of the Siberian platform

Recent geophysical studies carried out in hydro� carbon provinces have revealed spatial confinement of the oil and gas accumulation zone to areas with an anomalous crustal structure. In our opinion, the most significant results concerning this problem were obtained by A.V. Egorkin based on multichannel deep seismic profiling. Egorkin demonstrated a stable cor� relation between positions of deep (mantle and crustal) anomalous seismic domains and hydrocarbon accumulations in the sedimentary cover and the lack of anomalous patterns beyond localization areas of oil and gas condensate [1]. At the same time, despite the increasing quantity of such works, they do not con� sider processes that result in such a correlation. !"#$%&#$ ’ (’! #( (#)#" (’! ’ * ) ! * ) (’! ’ $ *# & +###*) # * # * ) ,#*"## #!#"#**) **)# -./ # *)$ * )$ *)*) ’ #*#*) &*#" $’ $# ’ " *) %"#*#*) %"#*#*) %"#*#*) ’ #") #") #") $ ) )! ’$ ! ’!*)#

Subsidence of normal oceanic lithosphere, apparent thermal expansivity, and seafloor flattening

Seafloor topography has been a key observational constraint on the thermal evolution of oceanic lithosphere, which is the top boundary layer of convection in Earth's mantle. At least for the first ~70 Myr, the age progression of seafloor depth is known to follow the prediction of half-space cooling, and the subsidence rate is commonly believed to be ~350 m Ma−1/2. Here we show that, based on a new statistical analysis of global bathymetry, the average subsidence rate of normal oceanic lithosphere is likely to be ~320 m Ma−1/2, i.e., ~10% lower than the conventional value. We define the ‘normal’ seafloor as regions uncorrelated with anomalous crust such as hotspots and oceanic plateaus, but the lower subsidence rate appears to be a stable estimate, not depending on how exactly we define the normal seafloor. This low subsidence rate can still be explained by half-space cooling with realistic mantle properties, if the effective thermal expansivity of a viscoelastic mantle is taken into account. In light of a revised model of half-space cooling, the normal seafloor unperturbed by the emplacement of anomalous crust exists for all ages, and the so-called seafloor flattening seems to be mostly caused by hotspots and oceanic plateaus.

The role of mineralogy, geochemistry and grain size in badland development in Pisticci (Basilicata, southern Italy)

AbstractMineralogical, geochemical and grain‐size composition of soil and pore‐water chemistry parameters were characterized on both eroded (south‐facing) and non‐eroded (north‐facing) clayey‐silt slopes in the Basilicata region (southern Italy).Only a few grain‐size parameters and clay mineralogy discriminate eroded from non‐eroded substrates. Compared with the latter, the former have fractions of over 63 µm and 1–4 µm lower and fractions 4–63 µm higher. Grain‐size characters of crusts did not discriminate with respect to substrate.Bulk rock mineralogy was not distinctive, but the clay mineral assemblage shows that the eroded slope is enriched in kaolinite, mixed layers (illite–smectite) and chlorite, whereas illite decreases, although overlaps are common.Chemical data enable discrimination between eroded and non‐eroded slopes. pH, SAR (sodium adsorption ratio), TDS (total dissolved salts) and PS (percentage of sodium) are distinctive parameters for both eroded and non‐eroded slopes. TDS increases in depth in the non‐eroded slope, whereas the maximum TDS is just below the crust in the eroded one. On average, eroded substrates are higher in pH, SAR and PS than non‐eroded ones. The ESP (exchangeable sodium percentage) of the eroded slope has a higher value than the non‐eroded one. Crusts are less dispersive than eroded substrates, and non‐eroded substrates behave as crusts. This suggests that the portion of the slope most severely exposed to weathering tends to stabilize, due to strong decreases in SAR, PS and ESP. Several diagrams reported in the literature show similarly anomalous crust samples on eroded slopes, compared with other samples coming from greater depths on eroded slopes. In the present case study, the exchangeable form of Na characterizes crusts more than the soluble form.This study describes the erosional mechanism, which involves morphological and geographic exposure and climatic elements, as well as grain size, mineralogy, chemistry and exchangeable processes of soils. Copyright © 2006 John Wiley & Sons, Ltd.

Comparisons of gravity anomalies at pseudofaults, fracture zones, and nontransform discontinuities from fast to slow spreading areas

Published mechanisms for rift tip propagation at spreading centers include extensional deformation and an initial period of slow spreading. We investigate whether the gravity signal and inferred crustal structure at pseudofaults formed in medium to superfast spreading environments resemble the gravity signal at fracture zones or nontransform discontinuities formed in slow spreading environments. We find that altimetry‐based gravity anomalies on the Mathematician, Bauer, Easter, Juan Fernandez, and northern Chile Ridge pseudofaults, located in 75–150 mm/yr (full rate) seafloor spreading environments, are similar in amplitude and form to Atlantic fracture zones with 20–30 mm/yr spreading rates. A 5–15 mGal positive mantle Bouguer anomaly is observed on the pseudofault bounding the eastern Juan Fernandez microplate, comparable to those at some similar age‐offset nontransform discontinuities in slow spreading environments. Our results suggest that the deeps associated with active propagating rift tips result from both a dynamic mantle component and anomalous crust, the latter of which remains frozen at pseudofaults. We predict that any pseudofaults with age offsets more than ∼1 m.y. and not coincident with hotspot volcanism will be associated with thin (and possibly unusually dense) crust, even in superfast seafloor spreading environments.

Current Seismicity and Tectonics Near the Gulf of Cambay: Evidences for the Khambat Plume Induced Activity

Abstract The Western Indian shield is seismically active as shown by both historical seismicity records and recently acquired instrumental data. Moderate to large earthquakes have occurred in the region but they do not clearly correlate with known tectonic features or faults. Current seismicity for this part of the shield for the period 1977-97. as evidenced from Gauribidanur Seismic Array (GBA) data, is found to be largely restricted to an area of about 80 km radius which accounts for almost 85 percent of the activity. The area is referred to as the Surat-Daman Seismic Zone (SDSZ) which has a general correspondence to the regional pattern of seismicity over the Western Indian shield established from both instrumental and non-instrumental data for a longer time span. The Indian shield crust is known to exhibit a sharp reduction in crustal thickness from 36 km below the Cambay Rift to about 20 km below the SDSZ within a distance of about 70 km. The thin shield crust is dissected into several faulted blocks that arc deformed to various degrees; some of these crustal blocks are seismically active. These features suggest that seismic activity in the region possibly relates to the fossil trace of the Khambat Plume along the Western continental margin near the Gulf of Cambay. The fossil plume appears to be located at least 100 km eastward onland. Bouguer anomalies over the fossil plume are largely positive with amplitudes upto 130 mgal, rendering further support to the presence of anomalous crust in the region. Available stress data from focal mechanism solutions indicate compressive stress in east-west to NNE-WSW direction. This suggests that stresses responsible for rift tectonics are not significant at the present stage; rather, the plume tectonics seems to be regionally predominant.

Temporal and three-dimensional spatial analyses of the frequency-magnitude distribution near Long Valley Caldera, California

SUMMARY The 3-D distribution of the b value of the frequency‐magnitude distribution is analysed in the seismically active parts of the crust near Long Valley Caldera, California. The seismicity is sampled in spherical volumes, containing N=150 earthquakes and centred at nodes of a grid separated by 0.3 km. Significant variations in the b value are detected, with b ranging from b#0.6 to b#2.0. High b-value volumes are located near the resurgent dome, and at depths below 5 km at Mammoth Mountain. b values are found to be much lower south of the Long Valley Caldera. We interpret this to indicate that an active magma body has advanced from depths below 8 km to depths of 4 to 5 km beneath Mammoth Mountain in 1989, and that anomalous crust, either highly fractured or containing unusually high pore pressure, such as is the case in the vicinity of active magma bodies, exists north of the seismically active area beneath the resurgent dome at all depths. We also investigate the spatial distribution of temporal variations of the frequency‐magnitude distribution by introducing diVerential b-value maps. b values increased from b#0.8 to b#1.5 underneath Mammoth Mountain at the onset of the 1989 earthquake swarm and remained high thereafter. This suggests that an intrusion permanently altered the average distribution of cracks at 5‐10 km depth, or that the pore pressure permanently increased. We propose that high b values are a necessary (but not suYcient) condition near a magmatic body, and therefore spatial b-value mapping can be used to aid in the identification of active magma bodies.

Anomalous crust of the Bolivian Altiplano, central Andes: Constraints from broadband regional seismic waveforms

A one‐year deployment of broadband seismographs in the Bolivian Altiplano recorded numerous intermediate‐depth earthquakes at near‐regional distances. We modeled the associated broadband waveforms of two earthquakes to estimate an average crustal structure for the Altiplano. The resulting model is characterized by an anomalously low mean P velocity of 6.0 km/s, a low Poisson's ratio of 0.25, and a crustal thickness of 65 km. The combination of the low mean velocity and low Poisson's ratio can be explained only by a predominantly quartz‐rich, felsic bulk composition. This constraint precludes significant volumes of magmatic addition from the mantle contributing to the great thickness of the Altiplano crust, but is consistent with thickening by compressive shortening concentrated in a weak felsic layer.

Crustal structure of North Atlantic Fracture Zones

Seismic studies have established that large‐offset transforms along the slow spreading Mid‐Atlantic Ridge exhibit anomalous crustal structures that fall well outside the range typically associated with oceanic crust. Seismically, fracture zone crust in the North Atlantic is extremely heterogeneous in both thickness and internal structure. It is frequently quite thin (<1–2 km thick) and is characterized by low compressional wave velocities and the absence of a normal seismic layer 3. A more gradual crustal thinning can extend up to several tens of kilometers from these fracture zones. Anomalously thin crust has also been inferred from both seismic and gravity studies at smaller ridge axis discontinuities along the Mid‐Atlantic Ridge. The geological nature of the seismically anomalous crust found within Atlantic fracture zones, and how this crust forms, are still controversial. One interpretation consistent with available seismic observations is that the crust within North Atlantic fracture zones consists of a thin, intensely fractured, and hydrothermally altered basaltic section overlying ultramafics that are extensively serpentinized in places. Variations in apparent seismic crustal thickness along fracture zones may reflect different degrees of serpentinization of the upper mantle section or changes in the thickness of the igneous crust. The existence of a thinner crustal section in fracture zones can be explained by a reduced magma supply within a broad region near ridge offsets due to the three‐dimensional nature of upwelling beneath a segmented spreading center and by tectonic dismemberment of the crust by large‐scale detachment faults that form preferentially in the cold, brittle lithosphere near the ends of segments.

Seismic evidence for anomalous crustal structure beneath Mesozoic fracture zones in the Gambia Basin, eastern equatorial Atlantic

Five sonobuoy refraction profiles shot in the eastern equatorial Atlantic reveal the presence of thin and heterogenous crust in a zone at least 200 km wide near the southern margin of the Gambia Basin. Oceanic Layer 2 (V p ≈ 4.1–4.4 km/s) is detected on all profiles. On some sections it is underlain by crustal material with apparent seismic velocities between 5.7 km/s and 7.0 km/s; on others, Layer 2 directly overlies a zone with apparent velocities characteristic of normal upper mantle velocities ( ≈ 8 km/s). The formation of anomalous crust is attributed to a modification of accretion processes resulting from a close spacing of fracture zones during a period of slow spreading in the Early Cretaceous.