Ambient noise tomography reveals basalt and sub-basalt velocity structure beneath the Faroe Islands, North Atlantic

© 2017 Elsevier B.V. Ambient noise tomography is applied to seismic data recorded by a portable array of seismographs deployed throughout the Faroe Islands in an effort to illuminate basalt sequences of the North Atlantic Igneous Province, as well as underlying sedimentary layers and Precambrian basement. Rayleigh wave empirical Green's functions between all station pairs are extracted from the data via cross-correlation of long-term recordings, with phase weighted stacking implemented to boost signal-to-noise ratio. Dispersion analysis is applied to extract inter-station group travel-times in the period range 0.5–15 s, followed by inversion for period-dependent group velocity maps. Subsequent inversion for 3-D shear wave velocity reveals the presence of significant lateral heterogeneity (up to 25%) in the crust. Main features of the final model include: (i) a near-surface low velocity layer, interpreted to be the Malinstindur Formation, which comprises subaerial compound lava flows with a weathered upper surface; (ii) a sharp velocity increase at the base of the Malinstindur Formation, which may mark a transition to the underlying Beinisvørð Formation, a thick laterally extensive layer of subaerial basalt sheet lobes; (iii) a low velocity layer at 2.5–7.0 km depth beneath the Beinisvørð Formation, which is consistent with hyaloclastites of the Lopra Formation; (iv) an upper basement layer between depths of 5–9 km and characterized by S wave velocities of approximately 3.2 km/s, consistent with low-grade metamorphosed sedimentary rocks; (v) a high velocity basement, with S wave velocities in excess of 3.6 km/s. This likely reflects the presence of a crystalline mid-lower crust of Archaean continental origin. Compared to previous interpretations of the geological structure beneath the Faroe Islands, our new results point to a more structurally complex and laterally heterogeneous crust, and provide constraints which may help to understand how continental fragments are rifted from the margins of newly forming ocean basins

Tectonic variation and structural evolution of the West Greenland continental margin

Because of its geographic extent of over 2500 km (1553 mi), the West Greenland margin provides a much understudied example of a divergent continental margin, both with respect to hydrocarbon exploration and academic studies. A seismic interpretation study of representative two-dimensional reflection profiles from the Labrador Sea, Davis Strait, and Baffin Bay was undertaken to identify sedimentary and structural components to elucidate the tectonic development of the margin. Nine horizons were interpreted from six representative seismic lines in the area. Margin-scale tectono-stratigraphy was derived from isochron maps, the geometry of mappable faults and their associated stratal architecture. Rifting began in Early to Late Cretaceous at ca. 145–130 Ma, which was followed by two pulses of volcanism in Eocene and Paleocene ages. The transition to the drift stage includes a typical subsidence phase but also erosion, uplift, and deposition of Neogene postrift packages. The shift in the position of depocenters in the Davis Strait and the Labrador Sea during Paleocene and Miocene times is evidence for structural modification of the basin bounding faults. Drift stage deformation suggests a possible anticlockwise rotation in the orientation of the spreading axis in Baffin Bay culminating in an ultraslow sea-floor spreading. Sea-floor spreading on the West Greenland margin started in the south at 70 Ma in the Labrador Sea and propagated northward into the Baffin Bay by 60 Ma. Prospective petroleum systems include thick Cretaceous age strata, with structural traps provided by grabens and inversion structures. Our structural model provides insight into a margin that is highly variable in its structural configuration, further modified by other processes such as magma-assisted rifting that may result in elevated regional heat flow, which has considerable impact on hydrocarbon maturation. Further constraining the implications of heat flow associated with volcanic activities in comparison to that associated with lithospheric stretching will be critical in future exploration

Insights into the structure and dynamics of the upper mantle beneath Bass Strait, southeast Australia, using shear wave splitting

© 2019 Elsevier B.V. We investigate the structure of the upper mantle using teleseismic shear wave splitting measurements obtained at 32 broadband seismic stations located in Bass Strait and the surrounding region of southeast Australia. Our dataset includes ∼366 individual splitting measurements from SKS and SKKS phases. The pattern of seismic anisotropy from shear wave splitting analysis beneath the study area is complex and does not always correlate with magnetic lineaments or current N-S absolute plate motion. In the eastern Lachlan Fold Belt, fast shear waves are polarized parallel to the structural trend (∼N25E). Further south, fast shear wave polarization directions trend on average N25–75E from the Western Tasmania Terrane through Bass Strait to southern Victoria, which is consistent with the presence of an exotic Precambrian microcontinent in this region as previously postulated. Stations located on and around the Neogene-Quaternary Newer Volcanics Province in southern Victoria display sizeable delay times (∼2.7 s). These values are among the largest in the world and hence require either an unusually large intrinsic anisotropy frozen within the lithosphere, or a contribution from both the lithospheric and asthenospheric mantle. In the Eastern Tasmania Terrane, nearly all observed fast directions are approximately NW-SE. Although part of our data set strongly favours anisotropy originating from “fabric” frozen in the lithospheric mantle, a contribution from the asthenospheric flow related to the present day plate motion is also required to explain the observed splitting parameters. We suggest that deviation of asthenospheric mantle flow around lithospheric roots could be occurring, and so variations in anisotropy related to mantle flow may be expected. Alternatively, the pattern of fast polarisation orientations observed around Bass Strait may be consistent with radial mantle flow associated with a plume linked to the recently discovered Cosgrove volcanic track. However, it is difficult to characterise the relative contributions to the observed splitting from the lithospheric vs. asthenospheric upper mantle due to poor backazimuthal coverage of the data

New insights into North Sea deep crustal structure and extension from transdimensional ambient noise tomography

SUMMARY The deep crustal structure beneath the North Sea is poorly understood since it is constrained by only a few seismic reflection and refraction profiles. However, it is widely acknowledged that the mid to lower crust plays important roles in rift initiation and evolution, particularly when large-scale sutures and/or terrane boundaries are present, since these inherited features can focus strain or act as inhibitors to extensional deformation. Ancient tectonic features are known to exist beneath the iconic failed rift system of the North Sea, making it an ideal location to investigate the complex interplay between pre-existing regional heterogeneity and rifting. To this end, we produce a 3-D shear wave velocity model from transdimensional ambient seismic noise tomography to constrain crustal properties to ∼30 km depth beneath the North Sea and its surrounding landmasses. Major North Sea sedimentary basins appear as low shear wave velocity zones that are a good match to published sediment thickness maps. We constrain relatively thin crust (13–18 km) beneath the Central Graben depocentres that contrasts with crust elsewhere at least 25–30 km thick. Significant variations in crustal structure and rift symmetry are identified along the failed rift system that appears to be related to the locations of Laurentia–Avalonia–Baltica palaeoplate boundaries. We constrain first-order differences in structure between palaeoplates; with strong lateral gradients in crustal velocity related to Laurentia–Avalonia–Baltica plate juxtaposition and reduced lower crustal velocities in the vicinity of the Thor suture, possibly representing the remnants of a Caledonian accretionary complex. Our results provide fresh insight into the pivotal roles that ancient terranes can play in the formation and failure of continental rifts and may help explain the characteristics of other similar continental rifts globally.</jats:p

Seismic tomography of the North Anatolian Fault: New insights into structural heterogeneity along a continental strike-slip fault

Knowledge of the structure of continental strike-slip faults within the lithosphere is essential to understand where the deformation occurs and how strain localizes with depth. With the aim to improve the constraints on the lower crust and upper mantle structure of a major continental strike-slip fault, we present a high-resolution teleseismic tomography of the North Anatolian Fault Zone (NAFZ) in Turkey. Our results highlight the presence of a relatively high velocity body between the two branches of the fault and significant along-strike variations in the NAFZ velocity structure over distances of ~20 km. We interpret these findings as evidence of laterally variable strain focussing caused by preexisting heterogeneity. Low velocities observed in the crust and upper mantle beneath the NAFZ support the presence of a narrow shear zone widening in the upper mantle, where we constrain its width to be ~50 km.Major funding was provided by the UK Natural Environment Research Council (NERC) under grant NE/I028017/1. Equipment was provided and supported by the NERC Geophysical Equipment Facility (SEIS-UK). This project is also supported by Bogaziçi University Scientific ˘ Research Projects (BAP) under grant 6922 and Turkish State Planning Organization (DPT) under the TAM project, number 2007K120610

Structure of the crust and upper mantle beneath Bass Strait, southeast Australia, from teleseismic body wave tomography

© 2019 Elsevier B.V. We present new constraints on the lithospheric velocity structure of Bass Strait and the adjoining landmasses of mainland Australia and Tasmania in order to better constrain their geological and tectonic relationship. This is achieved by performing teleseismic tomography using data from fifteen deployments of WOMBAT and BASS transportable arrays, which span southeastern Australia. The starting model for the teleseismic tomography includes crustal velocity structure constrained by surface waves extracted from ambient seismic noise data and a Moho surface and broad-scale variations in 3-D upper mantle velocity structure from the Australian seismological reference Earth model (AuSREM). As a consequence, we produce a model with a high level of detail in both the crust and upper mantle. Our new results strengthen the argument for a low velocity upper mantle anomaly that extends down to ~150 km depth directly beneath the Newer Volcanics Province in Victoria, which is likely related to recent intra-plate volcanism. Beneath Bass Strait, which is thought to host the entrained VanDieland microcontinent, upper mantle velocities are low relative to those typically found beneath Precambrian continental crust; it is possible that failed rifting in Bass Strait during the Cretaceous, opening of the Tasman Sea, extension of VanDieland during Rodinian break-up and recent plume activity in the past 5 Ma may have altered the seismic character of this region. The data nevertheless suggest: (1) the velocity structure of the VanDieland microcontinent lacks continuity within its lithosphere; (2) the Moyston Fault defines an area of strong velocity transition at the boundary between the Cambrian Delamerian Orogen and the Cambrian-Carboniferous Lachlan Orogen; and (3) there is a rapid decrease in mantle velocity inboard of the east coast of Australia, which is consistent with substantial thinning of the lithosphere towards the passive margin

Post-Subduction Tectonics of Sabah, Northern Borneo, Inferred From Surface Wave Tomography

Abstract: We use two‐plane‐wave tomography with a dense network of seismic stations across Sabah, northern Borneo, to image the shear wave velocity structure of the crust and upper mantle. Our model is used to estimate crustal thickness and the depth of the lithosphere‐asthenosphere boundary (LAB) beneath the region. Calculated crustal thickness ranges between 25 and 55 km and suggests extension in a NW‐SE direction, presumably due to back‐arc processes associated with subduction of the Celebes Sea. We estimate the β‐factor to be 1.3–2, well below the initiation of seafloor spreading. The LAB is, on average, at a depth of 100 km, which is inconsistent with models that ascribe Neogene uplift to wholescale removal of the mantle lithosphere. Instead, beneath a region of Plio‐Pleistocene volcanism in the southeast, we image a region 50–100 km across where the lithosphere has thinned to <50 km, supporting recent suggestions of lower lithospheric removal through a Rayleigh‐Taylor instability

Post-Subduction Tectonics of Sabah, Northern Borneo, Inferred From Surface Wave Tomography

Abstract: We use two‐plane‐wave tomography with a dense network of seismic stations across Sabah, northern Borneo, to image the shear wave velocity structure of the crust and upper mantle. Our model is used to estimate crustal thickness and the depth of the lithosphere‐asthenosphere boundary (LAB) beneath the region. Calculated crustal thickness ranges between 25 and 55 km and suggests extension in a NW‐SE direction, presumably due to back‐arc processes associated with subduction of the Celebes Sea. We estimate the β‐factor to be 1.3–2, well below the initiation of seafloor spreading. The LAB is, on average, at a depth of 100 km, which is inconsistent with models that ascribe Neogene uplift to wholescale removal of the mantle lithosphere. Instead, beneath a region of Plio‐Pleistocene volcanism in the southeast, we image a region 50–100 km across where the lithosphere has thinned to <50 km, supporting recent suggestions of lower lithospheric removal through a Rayleigh‐Taylor instability

Stage-Dependent Tolerance of the German Cockroach, Blattella germanica for Dichlorvos and Propoxur

Stage-dependent dichlorvos and propoxur tolerance in a field population of the German cockroach, Blattella germanica Linnaeus (Blatodea: Blattellidae), was investigated in the laboratory using a topical application bioassay. The results showed the 6 week-old nymphs were more tolerant to dichlorvos and propoxur than the other ages tested. LD50 values of dichlorvos and propoxur for the 6 week-old nymphs were 2.003 µµg per insect and 5.296 µµg per insect, respectively. Tolerance ratios of 18.55-fold and 4.98-fold for LD50 were obtained from 6-week-old nymphs compared to 4 week-old nymphs. The specific activity of acetylcholinesterase (AChE) from 1 week-old nymphs was the highest among all tested developmental stages of nymphs and adult males and females. The specific activity of AChE decreased significantly with increasing age. The sensitivity of AChE to dichlorvos was the highest with a ki value of 3.12××104 mol-1min-1 in the last nymphal stage of B. germanica (about 6 weeks-old). The AChE from 4 week-old nymphs was the most sensitive to propoxur, with the highest ki value being 2.63××105 mol-1min-1. These results indicated that the different developmental stages and sexes of B. germanica affected the inhibition of AChE by dichlorvos and propoxur

Cool deltas: Sedimentological, geomorphological and geophysical characterization of ice-contact deltas and implications for their reservoir properties (Salpausselkä, Finland)

Abstract: Sediments deposited by glacial meltwaters (for example, ice‐contact delta deposits) form permeable packages in the subsurface that can act as reservoirs for both water and hydrocarbons. They are also an important source of aggregate for the construction industry. As reservoirs they are challenging to characterize in terms of their structure, flow and storage properties due to their complex depositional history. In this study, ice‐contact deltas of Salpausselkä I and II end moraines in Southern Finland are studied using a combination of geomorphological mapping, sedimentological studies and near surface geophysical methods. Sedimentary logs from isolated outcrops were correlated to ground penetrating radar (GPR) profiles to unravel the internal structure and depositional history of these ice‐contact deltas. Subsequently, electrical resistivity tomography (ERT) and gravity data were analysed to estimate the depth to bedrock and to model porosity distribution within the sediments. Results of the study suggest that the delta deposits have a broad range of porosities (10 to 42%) with lowest values found in the bottomset beds. The most variable porosities are in the subaqueous ice‐contact–fan zone, and consistently high porosities occur in delta foreset/topset facies. Detailed sedimentary logging linked to the GPR data shows heterogeneities such as mud drapes on foresets and kettle holes which are below the resolution of ERT and gravity methods but significantly affect reservoir properties of the deltas. Moreover, oscillation of the ice‐margin may have introduced larger heterogeneities (for example, buried ice marginal ridges, or eskers) into the sedimentary sequence which are atypical for other Gilbert‐type deltas. Finally, subglacially sculpted, highly variable bedrock topography exerts a major control on sediment distribution within the delta making reservoir volume and quality less predictable. This work has implications for present‐day freshwater aquifers and low enthalpy geothermal energy in southern Finland and other deglaciated regions, as well as hydrocarbon exploration of analogous deposits in the subsurface from Pleistocene and pre‐Pleistocene glaciogenic sequences