Variations In Crustal Structure Research Articles

Lateral variation in crustal and mantle structure in Bay of Bengal based on surface wave data

Surface waves generated by earthquakes that occurred near Sumatra, Andaman-Nicobar Island chain and Sunda arc are used to estimate crustal and upper mantle S wave velocity structure of Bay of Bengal. Records of these seismic events at various stations located along the eastern coast of India and a few stations in the north eastern part of India are selected for such analysis. These stations lie within regional distance of the selected earthquakes. The selected events are shallow focused with magnitude greater than 5.5. Data of 65, 37, 36, 53 and 36 events recorded at Shillong, Bokaro, Visakhapatnam, Chennai and Trivandrum stations respectively are used for this purpose. The ray paths from the earthquake source to the recording stations cover different parts of the Bay of Bengal.Multiple Filtering Technique (MFT) is applied to compute the group velocities of surface waves from the available data. The dispersion curves thus obtained for this data set are within the period range of 15–120s. Joint inversion of Rayleigh and Love wave group velocity is carried out to obtain the subsurface information in terms of variation of S wave velocity with depth. The estimated S wave velocity at a given depth and layer thickness can be considered to be an average value for the entire path covered by the corresponding ray paths. However, we observe variation in the value of S wave velocity and layer thickness from data recorded at different stations, indicating lateral variation in these two parameters. Thick deposition of sediments is observed along the paths followed by surface waves to Shillong and Bokaro stations. Sediment thickness keeps on decreasing as the surface wave paths move further south. Based on velocity variation the sedimentary layer is further divided in to three parts; on top lay unconsolidated sediment, underlain by consolidated sediment. Below this lies a layer which we consider as meta-sediments. The thickness and velocity of these layers decrease from north to south. The crustal material has higher velocity at the southern part compared to that at the northern part of Bay of Bengal indicating that it changes from more oceanic type in the southern part of the Bay to more continental type to its north. Both Moho and lithosphere − asthenosphere boundary (LAB) dips gently towards north. Thicknesses of both lithosphere and asthenosphere also increase in the same direction. The mantle structure shows complex variation from south to north indicating possible effect of repeated changes in type of tectonic activity in the Bay of Bengal.

Evidence of Dynamic Crustal Deformation in Tohoku, Japan, From Time‐Varying Receiver Functions

AbstractTemporal variation of crustal structure is key to our understanding of Earth processes on human timescales. Often, we expect that the most significant structural variations are caused by strong ground shaking associated with large earthquakes, and recent studies seem to confirm this. Here we test the possibility of using P receiver functions (PRF) to isolate structural variations over time. Synthetic receiver function tests indicate that structural variation could produce PRF changes on the same order of magnitude as random noise or contamination by local earthquakes. Nonetheless, we find significant variability in observed receiver functions over time at several stations located in northeastern Honshu. Immediately following the Tohoku‐oki earthquake, we observe high PRF variation clustering spatially, especially in two regions near the beginning and end of the rupture plane. Due to the depth sensitivity of PRF and the timescales over which this variability is observed, we infer this effect is primarily due to fluid migration in volcanic regions and shear stress/strength reorganization. While the noise levels in PRF are high for this type of analysis, by sampling small data sets, the computational cost is lower than other methods, such as ambient noise, thereby making PRF a useful tool for estimating temporal variations in crustal structure.

Lateral variations of crustal structure beneath the Indochina Peninsula

Crustal thickness (H) and Vp/Vs (κ) measurements obtained by stacking P-to-S receiver functions recorded at 32 broadband seismic stations covering the Indochina Peninsula reveal systematic spatial variations in crustal properties. Mafic bulk crustal composition as indicated by high κ (>1.81) observations is found to exist along major strike-slip faults and the southern part of the Peninsula, where pervasive basaltic magmatism is found and is believed to be the results of lithospheric thinning associated with the indentation of the Indian into the Eurasian plates. In contrast, crust beneath the Khorat Plateau, which occupies the core of the Indochina Block, has relatively large H values with a mean of 36.9 ± 3km and small κ measurements with an average of 1.74 ± 0.04, which indicates an overall felsic bulk composition. Those observations for the Khorat Plateau are comparable to the undeformed part of the South China Block. The laterally heterogeneous distribution of crustal properties and its correspondence with indentation-related tectonic features suggest that the Indochina lithosphere is extruded as rigid blocks rather than as a viscous flow.

The Sudbury Huronian heat flow anomaly, Ontario, Canada

New measurements of the surface heat flux and radiogenic heat production in deep boreholes at 8 different sites are added to 10 older data to determine the heat flow field around the Sudbury basin, Ontario, Canada. This structure straddles the boundary between the Archean Superior Province to the North and the PaleoProterozoic Southern Province to the South, which terminates against the Grenville Front. The structure, that includes the Sudbury Igneous Complex (SIC), was created by a meteorite impact ca. 1.85Ga. The mean value of the heat flux (50mWm−2) is higher than that for the Superior Province (40mWm-2). Heat flow values are hardly perturbed by the presence of the SIC and require a regional enhancement of heat production in the upper crust with respect to the average Superior, due mostly to the enriched Huronian metasediments of the Southern Province. The average radiogenic heat production of SIC lithologies (1.2μW m-3) is higher than that of the local Archean basement, indicating that the impact melt sheet was partly derived from Huronian sediments. Local variations in heat flux can be related to heat production differences between the Abitibi upper crust and the Huronian metasediments as well as thrusting and tectonic thickening in the Southern Province. The higher than average values of heat flux and crustal heat production in the Southern Province imply elevated crustal temperatures and a weak rheology in the lower crust at the time of the meteorite impact and of the contemporaneous Penokean orogeny. Large variations of crustal thermal structure at the edge of the Superior Province may have led to along-strike differences in the style and extent of thrusting during the Grenville Orogeny.

Central Andean crustal structure from receiver function analysis

The Central Andean Plateau (15°–27°S) is a high plateau in excess of 3km elevation, associated with thickened crust along the western edge of the South America plate, in the convergent margin between the subducting Nazca plate and the Brazilian craton. We have calculated receiver functions using seismic data from a recent portable deployment of broadband seismometers in the Bolivian orocline (12°–21°S) region and combined them with waveforms from 38 other stations in the region to investigate crustal thickness and crust and mantle structures. Results from the receiver functions provide a more detailed map of crustal thickness than previously existed, and highlight mid-crustal features that match well with prior studies. The active volcanic arc and Altiplano have thick crust with Moho depths increasing from the central Altiplano (65km) to the northern Altiplano (75km). The Eastern Cordillera shows large along strike variations in crustal thickness. Along a densely sampled SW–NE profile through the Bolivian orocline there is a small region of thin crust beneath the high peaks of the Cordillera Real where the average elevations are near 4km, and the Moho depth varies from 55 to 60km, implying the crust is undercompensated by ~5km. In comparison, a broader region of high elevations in the Eastern Cordillera to the southeast near ~20°S has a deeper Moho at ~65–70km and appears close to isostatic equilibrium at the Moho. Assuming the modern-day pattern of high precipitation on the flanks of the Andean plateau has existed since the late Miocene, we suggest that climate induced exhumation can explain some of the variations in present day crustal structure across the Bolivian orocline. We also suggest that south of the orocline at ~20°S, the thicker and isostatically compensated crust is due to the absence of erosional exhumation and the occurrence of lithospheric delamination.

Crustal shear velocity structure in the Southern Lau Basin constrained by seafloor compliance

AbstractSeafloor morphology and crustal structure vary significantly in the Lau back‐arc basin, which contains regions of island arc formation, rifting, and seafloor spreading. We analyze seafloor compliance: deformation under long period ocean wave forcing, at 30 ocean bottom seismometers to constrain crustal shear wave velocity structure along and across the Eastern Lau Spreading Center (ELSC). Velocity models obtained through Monte Carlo inversion of compliance data show systematic variation of crustal structure in the basin. Sediment thicknesses range from zero thickness at the ridge axis to 1400 m near the volcanic arc. Sediment thickness increases faster to the east than to the west of the ELSC, suggesting a more abundant source of sediment near the active arc volcanoes. Along the ELSC, upper crustal velocities increase from the south to the north where the ridge has migrated farther away from the volcanic arc front. Along the axial ELSC, compliance analysis did not detect a crustal low‐velocity body, indicating less melt in the ELSC crustal accretion zone compared to the fast spreading East Pacific Rise. Average upper crust shear velocities for the older ELSC crust produced when the ridge was near the volcanic arc are 0.5–0.8 km/s slower than crust produced at the present‐day northern ELSC, consistent with a more porous extrusive layer. Crust in the western Lau Basin, which although thought to have been produced through extension and rifting of old arc crust, is found to have upper crustal velocities similar to older oceanic crust produced at the ELSC.

Crust and uppermost mantle structure of the Kyushu-Palau Ridge, remnant arc on the Philippine Sea plate

We acquired 27 wide-angle seismic profiles to investigate variation in crustal structure along the Kyushu-Palau Ridge (KPR), a 2600-km-long remnant island arc in the center of the Philippine Sea plate; 26 lines were shot across the strike of the KPR at 13°–31°N, and one was shot along the northernmost KPR. The derived P-wave velocity (Vp) models show that the KPR has a crustal thickness of 8–23 km, which is thicker than the neighboring backarc basin oceanic crusts of the West Philippine Basin to the west and the Shikoku and Parece Vela Basins to the east. While the KPR crust consists mainly of lower crusts with a Vp of 6.8–7.2 km/s, the thicker crust contains a thick middle crust with Vp of 6.0–6.8 km/s. In general, the KPR crust is thicker in the north than in the south. The uppermost mantle velocities just below the KPR bathymetric highs are lower than 8.0 km/s and are commonly associated with a slightly high Vp of 7.2 km/s at the base of the crust. Large amplitude reflection signals are sometimes observed at far offsets on several lines suggesting the existence of several reflectors at depths of 23–40 km in the mantle beneath the KPR. The characteristics of these reflections are similar to these observed beneath the Izu–Ogasawara (Bonin) island arc, the tectonically conjugate arc of the KPR before backarc basin spreading. Very thin crust and high Pn velocities characterize the transition between the KPR and the eastern basins, which is probably a relic of the initial stage of the rifting. West of the KPR, the crust varies in structure from north to south as a result of the different tectonic settings in which it evolved.

Local magnitude calibration of the Hellenic Unified Seismic Network

A new relation is proposed for accurate determination of local magnitudes in Greece. This relation is based on a large number of synthetic Wood-Anderson (SWA) seismograms corresponding to 782 regional shallow earthquakes which occurred during the period 2007–2013 and recorded by 98 digital broad-band stations. These stations are installed and operated by the following: (a) the National Observatory of Athens (HL), (b) the Department of Geophysics of the Aristotle University of Thessaloniki (HT), (c) the Seismological Laboratory of the University of Athens (HA), and (d) the Seismological Laboratory of the Patras University (HP). The seismological networks of the above institutions constitute the recently (2004) established Hellenic Unified Seismic Network (HUSN). These records are used to calculate a refined geometrical spreading factor and an anelastic attenuation coefficient, representative for Greece and surrounding areas, proper for accurate calculation of local magnitudes in this region. Individual station corrections depending on the crustal structure variations in their vicinity and possible inconsistencies in instruments responses are also considered in order to further ameliorate magnitude estimation accuracy. Comparison of such calculated local magnitudes with corresponding original moment magnitudes, based on an independent dataset, revealed that these magnitude scales are equivalent for a wide range of values.

Heat flow and thermal modeling of the Appalachian Basin, West Virginia

Recent heat flow studies indicate that the Appalachian Basin in West Virginia may represent an important location for high heat flow and future geothermal energy development. Currently, however, only limited one-dimensional (1-D) heat flow studies exist in this region, making it difficult to assess the potential for geothermal development. Here, we develop the first high resolution 2-D basin model for a portion of West Virginia. The model uses 2-D finite difference heat conduction, basin cross sections, equilibrium temperature, and oil and gas bottom-hole temperature data to quantify heat flow at the surface and at the base of the sedimentary basin. The temperature data show elevated temperature gradients in the eastern portion of the basin. A 2-D advection-diffusion model, created using available lithologic and structural data, was designed to test whether variations in crustal properties, structure, erosion, or fluid advection can account for the observed temperatures in the basin. Thermal properties were populated using measured values as well as published averages. A linear heat flow vs. heat production relationship was used to determine heat flow at the base of the model. The model constrains the heat flow at the base of the sedimentary basin to 49–55 mW/m2. Analysis of modeling results suggests that heat flow at the base of the sedimentary basin is nearly uniform. Variations in basin temperatures are most likely due to variations in sediment thermal properties, complex structures, and/or localized fluid advection.

CAN‐HK: An a Priori Crustal Model for the Canadian Shield

Online Material: Tables of the crustal model and Generic Mapping Tools‐compatible files, and color versions of figures. Crustal structure, which can vary greatly over relatively short length scales depending on the tectonic setting, has the potential to significantly influence the data used to infer the deeper features of the Earth. In particular, commonly implemented teleseismic methods are sensitive to crustal velocity structure but are invariably incapable of resolving it. Moho depth has a first‐order effect on travel‐time residuals of teleseismic body waves (>1 s variation; e.g., Waldhauser et al. , 2002); and, for typical Rayleigh‐wave periods of <150 s, the crust can contribute 50% or more to the surface‐wave‐derived velocity variations (Ritsema et al. , 2004; Artemieva, 2011). Crustal structure can also significantly affect the correct extraction of radial anisotropy in surface‐wave studies (Ferreira et al. , 2010; Panning et al. , 2010). The use of a high‐resolution crustal model thus has the potential to markedly enhance studies of the mantle. There are a number of global crustal models in circulation, for instance CRUST5.1 (Mooney et al. , 1998), CRUST2.0 (Bassin et al. , 2000), and CRUST1.0 (Laske et al. , 2013). These may lack the resolution required for detailed local seismic studies, especially where there are significant lateral variations in crustal structure across short length scales (e.g., at ocean–continent transitions; Marone and Romanowicz, 2007). Moreover, global compilations often require assumptions regarding local geology to extrapolate structure to regions of poor coverage. The goal of this contribution is to present a unified 3D crustal model of the Canadian shield. The new model presented here, CAN‐HK, utilizes new passive broadband …

Deep crustal structure of the North-West African margin from combined wide-angle and reflection seismic data (MIRROR seismic survey)

The structure of the Moroccan and Nova Scotia conjugate rifted margins is of key importance for understanding the Mesozoic break-up and evolution of the northern central Atlantic Ocean basin. Seven combined multichannel reflection (MCS) and wide-angle seismic (OBS) data profiles were acquired along the Atlantic Moroccan margin between the latitudes of 31.5° and 33° N during the MIRROR seismic survey in 2011, in order to image the transition from continental to oceanic crust, to study the variation in crustal structure, and to characterize the crust under the West African Coast Magnetic Anomaly (WACMA).The data were modeled using a forward modeling approach. The final models image crustal thinning from 36km thickness below the continent to approximately 8km in the oceanic domain. A 100km wide zone characterized by rough basement topography and high seismic velocities up to 7.4km/s in the lower crust is observed westward of the West African Coast Magnetic Anomaly. No basin underlain by continental crust has been imaged in this region, as has been identified north of our study area. Comparison to the conjugate Nova Scotian margin shows a similar continental crustal thickness and layer geometry, and the existence of exhumed and serpentinized upper mantle material on the Canadian side only. The oceanic crustal thickness is lower on the Canadian margin.

Lateral variation of crustal structure and composition in the Cathaysia block of South China and its geodynamic implications

In order to clarify lateral variations in the crustal structure and composition of the Cathaysia block in South China which was affected by violent Mesozoic and Cenozoic tectono-magmatic activities, we studied the crustal thickness and the Poisson’s ratio in this region using the H–κ stacking method of teleseismic receiver functions. Our results show that the Poisson’s ratio varies between 0.20 and 0.29 and the crustal thickness ranges from 26 to 34km in the Cathaysia block. The crustal thickness and Poisson’s ratio show considerable variations across the Lishui-Haifeng Fault. The Southeast Coast Magmatic Belt is characterized by Poisson’s ratios >0.25 and a crustal thickness of 28–33km, reflecting probably intermediate to mafic compositions of the crust. The southwestern part of the Cathaysia Fold Belt exhibits a crustal thickness of 27–30km and Poisson’s ratios <0.25, reflecting a more felsic crust. However, the northeastern part of the Cathaysia Fold Belt is characterized by an almost flat Moho at a depth of ∼31km and a strong variation of Poisson’s ratio from 0.22 to 0.27. Our results revealed an ESE-WNW trending boundary between the southwestern and northeastern parts of the Cathaysia Fold Belt, which is consistent with previous results of geochemical studies. The lateral variations in the crustal thickness and composition of the Cathaysia block may reflect not only the lithological variations of the crustal rocks but also significant effects of tectonism and magmatism on the interior of the Cathaysia block during the Mesozoic and Cenozoic.

Crustal structure of Precambrian terranes in the southern African subcontinent with implications for secular variation in crustal genesis

SUMMARY New estimates of crustal thickness, Poisson’s ratio and crustal shear wave velocity have been obtained for 39 stations in Angola, Botswana, the Democratic Republic of Congo, Malawi, Mozambique, Namibia, Rwanda, Tanzania and Zambia by modellingP-wave receiver functions using the H–κ stacking method and jointly inverting the receiver functions with Rayleigh-wave phase and group velocities. These estimates, combined with similar results from previous studies, have been examined for secular trends in Precambrian crustal structure within the southern African subcontinent. In both Archean and Proterozoic terranes we find similar Moho depths [38–39 ± 3k mSD (standard deviation)], crustal Poisson’s ratio (0.26 ± 0.01 SD), mean crustal shear wave velocity (3.7 ± 0.1 km s −1 SD), and amounts of heterogeneity in the thickness of the mafic lower crust, as defined by shear wave velocities ≥4.0 km s −1 . In addition, the amount of variability in these crustal parameters is similar within each individual age grouping as between age groupings. Thus, the results provide little evidence for secular variation in Precambrian crustal structure, including between Meso- and Neoarchean crust. This finding suggests that (1) continental crustal has been generated by similar processes since the Mesoarchean or (2) plate tectonic processes have reworked and modified the crust through time, erasing variations in structure resulting from crustal genesis.

Spatial and temporal variations in crustal production at the Mid‐Atlantic Ridge, 25°N–27°30′N and 0–27 Ma

AbstractWe use high‐resolution multibeam bathymetry, shipboard gravity, side‐scan sonar images, and magnetic anomaly data collected on conjugate flanks of the Mid‐Atlantic Ridge at 25°N–27°30′N and out to ~27 Ma crust to investigate the crustal evolution of the ridge. Substantial variations in crustal structure and thickness are observed both along and across isochrons. Along isochrons within spreading segments, there are distinct differences in seafloor morphology and gravity‐derived crustal thickness between inside and outside corners. Inside corners are associated with shallow depths, thin crust, and enhanced normal faulting while outside corners have greater depths, thicker crust, and more limited faulting. Across‐isochrons, systematic variations in crustal thickness are observed at two different timescales, one at ~2–3 Myr and another at &gt;10 Myr, and these are attributed to temporal changes in melt supply at the ridge axis. The shorter‐term variations mostly are in‐phase between conjugate ridge flanks, although the actual crustal thickness can be significantly different on the two flanks at any given time. We observe no correlation between crustal thickness and spreading rate. Thus, during periods of low melt supply, tectonic extension must increase to accommodate the full plate separation rate. This extension commonly is concentrated in long‐lived faults on only one side of the axial valley, resulting in strong across‐axis asymmetries in crustal thickness and seafloor morphology. The thin‐crust flank has few volcanic features and exhibits elevated, blocky topography with large‐offset, often irregular faults, while the conjugate thicker‐crust flank shows shorter‐offset, regular faulting, and common volcanic features. The variations in melt supply at the ridge axis most likely are caused either by episodic convection in the subaxial mantle or by variable melting of chemically heterogeneous mantle.

Origin and evolution of the Kolbeinsey Ridge and Iceland Plateau, N-Atlantic

Variations in crustal structure along the 700 km long KRISE7 refraction/reflection and gravity profile, straddling 66.5°N across the Iceland Shelf, Iceland Plateau and western Norway Basin confirm that extinct spreading centers coexisted with the now extinct AEgir Ridge prior to the initiation of the Kolbeinsey Ridge at 26 Ma. The western 300 km of the profile, across the Iceland shelf, formed by rifting at the Kolbeinsey Ridge, whereas the eastern 400 km, across the Iceland Plateau and the western Norway Basin, formed by earlier rifting, possibly containing slivers of older oceanic or continental crust rifted off the central E-Greenland margin along with the Jan Mayen Ridge. Crustal thickness increases gradually across the Iceland shelf, from 12 to 13 km near the Kolbeinsey Ridge to 24–28 km near the eastern shelf edge, decreasing abruptly across the shelf edge, to 12–13 km. The Iceland Plateau has crustal thickness ranging from 12 to 15 km decreasing to 5–8 km across the western Norway Basin and 4–5 km at the AEgir Ridge. We suggest that high-velocity lower crustal domes and corresponding gravity highs across the Iceland plateau mark the location of extinct rift axes that coexisted with the AEgir Ridge. Similar lower crustal domes are associated with the currently active rift segments within Iceland and the Kolbeinsey Ridge.

Seismic velocities and composition of the Canadian crust

To investigate variations in crustal structure and composition, we analyze approximately 80,000 teleseismic P wave seismograms recorded at 343 broadband stations distributed about the Canadian landmass. These data are binned by horizontal slowness and simultaneously deconvolved into receiver functions. We apply stacking methods to retrieve estimates of the bulk crustal velocity ratio VP/VS and thickness H from this receiver function dataset under the assumption of locally 1D structure. These crustal parameters are analyzed together with a compilation of velocity and thickness estimates from previous active source experiments and global crustal thickness models to investigate competing models of crustal composition and evolution. Crustal thicknesses vary between ~25km and ~48km with an area-weighted average of 37km for all stations. VP/VS estimates range from 1.65 to 1.95 and display a systematic increasing trend from NW to SE across the Churchill, Superior and Grenville provinces. Analysis of intra-crustal conversions indicates little evidence for a well-defined, widespread Conrad discontinuity, although the lower crust appears to be more transparent to teleseismic converted waves than the upper crust. This observation runs contrary to that of P-reflectivity in many active source studies, and may be reconciled through a dominant scale-length of heterogeneity that is less than ~800m. The VP/VS observations and VP constraints from active source studies are compared with laboratory measurements for common crustal rocktypes and found to be consistent with bulk compositions falling between granite gneiss and diorite. Previously reported systematic variations in crustal thickness and VP/VS with age in a global data set are not supported by our data set.

Lateral variation of crustal structure in the Ordos block and surrounding regions, North China, and its tectonic implications

Crustal thicknesses and Poissonʼs ratios in the Ordos block and surrounding regions were estimated by the use of the H–k stacking method on teleseismic receiver functions. The data came from 353 temporary and permanent seismic stations in 2006–2011. Results show that the crustal thickness and Poissonʼs ratio gently vary within the Ordos block, with an average of 41.3 km and 0.265, respectively, consistent with a felsic composition of the crust. Crustal thicknesses predicted on the basis of Airy isostasy are consistent with the estimated thicknesses, implying that the topography is approximately compensated. The reactivated portion of the North China Craton that has undergone Mesozoic–Cenzoic lithospheric thinning is also characterized by the thinning crust, while the Ordos block maintains normal crustal thickness and average crustal velocity. Inferred higher densities in the lower crust and the anti-correlation between Poissonʼs ratio and crustal thickness in the Ordos block may be the result from underplating of mafic magmas in the Precambrian. Around the Ordos block, the Paleoproterozoic khondalite zone in the northern edge has higher Poissonʼs ratio and thickened crust, which is consistent with the lower crust being of more mafic composition. The Weihe–Shanxi graben in the southeastern edge has mid-high Poissonʼs ratio, high heat flow and thinning crust, which is consistent with the known transtensional tectonic setting. In the Liupanshan thrust belt, along the southwestern edge of the Ordos plateau, significant variations in crustal thicknesses and Poissonʼs ratios occur on two sides. Besides a thickened crust, a concaved Moho implies horizontal shortening of this edge of the Ordos block due to its collision with the northeastern Tibetan Plateau. The structural differences between the eastern and western edges of the Ordos block reflect that the Ordos block is in tectonic stress environment of the western compression and the eastern extension.

The Moho of North America: A brief review focused on recent studies

Abstract Recent national programs in Canada (Lithoprobe) and the U.S. (EarthScope) are providing vast quantities of data and many new scientific insights concerning crustal and lithospheric mantle structure and evolution. More modest but significant results are also being produced in and about Mexico. This paper primarily focuses on results published since 2005 and presents an updated Moho map of North America. This overview of crustal structure does reveal a number of interesting observations. For example, two buried distinct crustal blocks have been discovered in the past 20 years. One was caught in a Precambrian suture zone in Canada, and the other is buried under the Gulf Coast region and is still of unknown origin. The Precambrian assembly of the continent has been documented by the mapping of numerous suture zones, and shallow portions of subducted slabs have been preserved in several cases. Not surprisingly, the internal structure of these suture zones is complex and highly variable. Isostatic adjustment has been shown to be a complex process involving the lithospheric mantle not just crustal structure variations. Specifically, the correlation of crustal thickness with elevation is only approximate at best. Several crustal blocks that have been stable over very long periods of time possess unusually thick crust whose lower portion is usually fast (Vp > 7.0 km/s). Rifting has been shown to affect the crust in a variety of ways ranging from the massive magmatic modification that produced the Mid-continent rift system to the broad region of extension in the Basin and Range province. Crustal structure along the active modern western margin of North America is highly complex as would be expected as a result of transitions between subduction and transform faulting and variations in volcanism and intraplate magmatism. We clearly still have a lot to learn about the structure and evolution of this continent.

Precambrian crustal structure in Africa and Arabia: Evidence lacking for secular variation

We review the thickness and shear wave velocity structure of Precambrian crust in Africa and Arabia, where over 90% of the landmass is comprised of Archean and Proterozoic terranes, and examine the data for evidence of secular variation. The data come from many published 1D shear wave velocity profiles obtained by jointly inverting receiver functions and surface wave dispersion measurements, 35 new 1D shear wave velocity profiles for locations in eastern Africa, and a new map of crustal thickness for Africa and Arabia derived from modeling satellite gravity data. We find for both Archean and Proterozoic terranes a similar range of crustal thicknesses (~ 33–45 km), similar mean crustal shear wave velocities (~ 3.6–3.7 km/s), and similar amounts of heterogeneity in lower crustal structure, as reflected in the thickness of lowermost crust with shear wave velocities ≥ 4.0 km/s. There is little evidence for secular variation in crustal structure, indicating that there may have been few changes over much of Earth's history in the processes that form the continental crust. Post-formation tectonic events also may have modified many of the terranes to such an extent that secular trends arising from crustal genesis may be difficult to recognize.

Twin enigmatic microseismic sources in the Gulf of Guinea observed on intercontinental seismic stations

AbstractBased on studies of continuous waveform data recorded on broad-band seismograph stations in Africa, Europe and North America, we report evidences for two temporally persistent and spatially localized monochromatic vibrating sources (around 0.036 and 0.038 Hz, respectively) in the Gulf of Guinea, instead of just one source (0.038 Hz or 26 s) found 50 yr ago. The location of the 0.036 Hz source is close to the Sao Tome Volcano, therefore it may be related to volcano processes. However, the 0.038 Hz source cannot be explained with known mechanisms, such as tectonic or oceanic processes. The most likely mechanism is volcano processes, but there is no reported active volcano in source region. Such repetitive vibration sources may provide valuable tools for detecting temporal variation of crustal structure of the Earth.