Receiver Functions

The Precambrian rocks of Madagascar were formed and/or modified during continental collision known as the Pan-African orogeny. Aborted Permo-Triassic Karoo rifting and the subsequent separation from Africa and India resulted in the formation of sedimentary basins in the west and volcanic activity predominantly along the margins. Many geological studies have documented the imprint of these processes, but little was known about the deeper structure. We therefore deployed seismic stations along an SE-NW trending profile spanning nearly all geological domains of southern Madagascar. Here we focus on the crustal structure, which we determined based on joint analysis of receiver functions and surface waves derived from ambient noise measurements. For the sedimentary basin we document a thinning of the underlying crystalline basement by up to ∼60% to 13 km. The crustal velocity structure demonstrates that the thinning was accomplished by removal or exhumation of the lower crust. Both the Proterozoic and Archean crust have a 10 km thick upper crust and 10–12 km thick midcrust. However, in contrast to the typical structure of Proterozoic and Archean aged crust, the Archean lower crust is thicker and faster than the Proterozoic one, indicating possible magmatic intrusions; an underplated layer of 2–8 km thickness is present only below the Archean crust. The Proterozoic mafic lower crust might have been lost during continental collision by delamination or subduction or thinned as a result of extensional collapse. Finally, the Cretaceous volcanics along the east coast are characterized by thin crust (30 km) and very large V P ∕V S ratios.

The Arizona Transition Zone is a narrow band that separates two of the main and most contrasting tectonic provinces in western US, namely the southern Colorado Plateau and the southern Basin and Range provinces. As such, the internal crustal structure and physical state of this transitional zone hold clues for understanding (i) the amalgamation of these provinces, (ii) the partitioning of deformation due to both past and present-day stress fields, and (iii) the role of thermal versus compositional effects in controlling surface observables. Here we employ and expand a novel multiobservable probabilistic inversion method and jointly invert fundamental mode Rayleigh phase velocities, receiver functions, surface heat flow, geoid height, and absolute elevation to obtain an internally consistent 3-D model of the temperature, density , V s , and V p of the Arizona Transition Zone and the southern portions of the Colorado Plateau and Basin and Range. Our results confirm a significant crustal thickening from 28 km in the SW of the Arizona Transition Zone and southern Basin and Range to 48 km beneath the southern Colorado Plateau. Inverted temperatures agree well with the location of recent volcanism and indicate that the lithosphere-asthenosphere boundary is not deeper than 70 km in most of the region. We find that major pre-Cambrian surface structures and/or shear zones separate crustal domains with distinct bulk properties, suggesting that the juxtaposed crustal blocks still retain, at least in part, their original characteristics. However, widespread intrusions of significant volumes of mafic magmas have affected these blocks at different depths, locally overprinting their original compositions and creating highly heterogeneous crustal sections. A dominant and large-scale internal crustal pattern of SW dipping planes/structures is evident in our models, coinciding with the orientation of deep faults previously inferred from earthquake focal mechanisms. While we cannot categorically corroborate the presence of melt or aqueous fluids within the crust, our results are compatible with these scenarios beneath some parts of the Basin and Range, the Mogollon-Datil, and Springerville volcanic fields.

The geological record of SE Canada spans more than 2.5 Ga, making it a natural laboratory for the study of crustal formation and evolution over time. We estimate the crustal thickness, Poisson's ratio, a proxy for bulk crustal composition, and shear velocity (V s) structure from receiver functions at a network of seismograph stations recently deployed across the Archean Superior Craton, the Proterozoic Grenville, and the Phanerozoic Appalachian provinces. The bulk seismic crustal properties and shear velocity structure reveal a correlation with tectonic provinces of different ages: the post-Archean crust becomes thicker, faster, more heterogeneous, and more compositionally evolved. This secular variation pattern is consistent with a growing consensus that crustal growth efficiency increased at the end of the Archean. A lack of correlation among elevation, Moho topography, and gravity anomalies within the Proterozoic belt is better explained by buoyant mantle support rather than ...

Seismic activity and non-volcanic tremors are often associated with fluid circulation resulting from the dehydration of subducting plates. Tremors in the overriding continental crust of several subduction zones suggest fluid circulation at shallower depths, but potential fluid pathways are still poorly documented. Using receiver function analysis in the Cascadia subduction zone, we provide evidence for a seismic discontinuity near 15 km depth in the crust of the overriding North American plate. This interface is segmented, and its interruptions are spatially correlated with conductive regions of the forearc and shallow swarms of seismicity and non-volcanic tremors. These observations suggest that fluid circulation in the overriding plate is controlled by fault zones separating blocks of accreted terranes. These zones constitute fluid escape routes that may influence the seismic cycle by releasing fluid pressure from the megathrust.

Receiver functions have a wealth of crustal information and are useful to study the lithosphere. They
require a powerful impulsive source like earthquakes and must be a teleseismic event from the receiver
station. Such requirements does not always guarantee a good azimuthal coverage from the station which is
essential to get good accurate results. Seismic interferometry is used to generate virtual impulsive sources
of energy and synthesize synthetic receiver functions which provides additional azimuthal coverage. Such
receiver functions can be stacked with other receiver functions having the same back azimuthal
characteristics to obtain better crustal information.

We present new results on structure, thickness, and composition of the crust in southern Africa based on 6300 seismic receiver functions at 85 stations. Application of Hk-stacking to the entire SASE dataset and use of multi-frequency bands improve resolution substantially. We observe a highly heterogeneous crustal structure with short wavelength variations in thickness (H), Vp/Vs-ratio (composition), and Moho sharpness, which defines ~20 blocks that do not everywhere coincide with surface tectonic features. In the Zimbabwe Craton, the Tokwe block has H = 35–38 km and Vp/Vs = 1.74–1.79 whereas the thicker crust in the Tati block (H = 47– 51 km) may be related to deformation of the Archean crust along the cratonic margin. Two distinct crustal blocks with similar crustal thickness (42–46 km) but significantly different Vp/Vs-ratios are recognized in the Limpopo Belt. Extreme values of 1.90–1.94 at the dyke swarms in eastern Limpopo, and 1.84 at the Olifants River Dyke Swarm and easternmost Bushveld Intrusion Complex (BIC) indicate voluminous magmatic intrusions in the whole crust. We find no evidence for magmatic intrusions in the central (inferred) part of BIC, where the crust is thick (45–50 km) and Vp/Vs is low (1.68–1.70). This thick crustal root may have deflected rising magmas to form the two BIC lobes. Most of central Kaapvaal has thin (35–40 km) crust and Vp/Vs ~ 1.74. These characteristics are similar to the Tokwe block in Zimbabwe Craton and may indicate delamination of pre-existing lower crust, which is further supported by a very sharp Moho transition. The exposed cross-section in the Vredefort impact crater is non-representative of cratonic crust due to shallow Moho (34 km) and high Vp/Vs ~ 1.80 attributed to shock metamorphism. High Vp/Vs = 1.76 is typical of the Witwatersrand Basin, and anomalously low Vp/Vs = 1.66–1.67 marks the Kaapvaal–Kheis–Namaqua transition. Highly heterogeneous crust, both in thickness and Vp/Vs-ratio is typical of the Namaqua–Natal and Cape Fold Belts.

Keywords: Alps mantle transition zone receiver functions seismology slab break-off subducted Alpine Tethys Assessing the thermal state of the mantle is essential to understand mantle dynamics and provides important clues about present and past tectonic regimes. The tectonic processes related to the Cenozoic subduction of the Alpine Tethys likely affected the regional upper-mantle structure. We use receiver functions to determine the mantle transition zone thickness beneath the greater Alpine region as a proxy for its current thermal state. Compilation of a dataset of nearly 7000 high-quality receiver functions from 98 stations yields a reliable and stable image of the Alpine mantle transition zone. Significant signal from P-to-S conversions at the 410-and 660-km discontinuities is clearly observed. The 410-km discontinuity is found at a depth close to that predicted by the IASP91 global model; its seismic signature is generally strong except south of the Alps. Signal from the 660-km discontinuity is strong, continuous and emerges throughout the whole Alpine region at depths greater than predicted by the global model. The mantle transition zone is not or only slightly thickened (≤10 km) north of the Alps, but is up to 40 km thicker than normal south of the Alps and in the Western Alps outlining a roughly 300 km-wide and 800 km-long, ENE–WSW directed anomaly. Thickening, assuming conversions are due to olivine phase transformations, suggests that the Alpine mantle transition zone is at least 300 K colder than normal, if the thickness anomaly were caused by equal amounts of shallowing and deepening of the 410-and 660-km discontinuity, respectively. However, the thickness anomaly seems mainly due to depression of the 660-km discontinuity, which suggests that temperature at the bottom of the mantle transition zone could actually be up to 800 K cooler than ambient mantle while temperatures at 410-km depth may be close to normal. Both location and amount of suggested thermal anomaly agree with tectonic reconstruction and large-scale tomography models of the Euro-Mediterranean region. The cold material accumulated at the 660-km discontinuity is interpreted as remnants from the oceanic mantle lithosphere that detached from the Eurasian plate after closure of the Alpine Tethys.

In this study, the characterization of the depth of the Mohorovičić discontinuity under the crust of Funafuti island was determined by analyzing the 3 component seismograms from 54 different earthquake events recorded by the station between 2008 and 2012. These seismograms were from teleseismic earthquakes whose epicenter lay at distances greater than 3000 km from the station. The seismograms were iteratively deconvolved in the time domain to remove the un- wanted noise and then stacked to obtain better receiver functions. For analysis of the receiver functions, it was assumed that the range in which the Vp/Vs ratio would lie for the given region would be between 1.60 - 1.85 and the depth of the discontinuity was assumed to lie between 5 - 20 km. Analysis of the receiver functions showed that the Mohorovičić discontinuity was at a depth of 11 km and the Vp/Vs ratio was 1.75 for the region.