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Rapid urban growth has led to large population densities in foreland basin regions, and therefore a rapid increase in the number of people exposed to hazard from earthquakes in the adjacent mountain ranges. It is well known that earthquake-induced ground shaking is amplified in sedimentary basins. However, questions remain regarding the main controls on this effect. It is, therefore, crucial to identify the main controls on earthquake shaking in foreland basins as a step towards mitigating the earthquake risk posed to these regions. We model seismic-wave propagation from range-front thrust-faulting earthquakes in a foreland-basin setting. The basin geometry (depth and width) and source characteristics (fault dip and source-to-basin distance) were varied, and the resultant ground motion was calculated. We find that the source depth determines the amount of near-source ground shaking and the basin structure controls the propagation of this energy into the foreland basin. Of particular importance is the relative length scales of the basin depth and dominant seismic wavelength (controlled by the source characteristics), as this controls the amount of dispersion of surface-wave energy, and so the amplitude and duration of ground motion. The maximum ground motions occur when the basin depth matches the dominant wavelength set by the source. Basins that are shallow compared with the dominant wavelength result in low-amplitude and long-duration dispersed waveforms. However, the basin structure has a smaller effect on the ground shaking than the source depth and geometry, highlighting the need for understanding the depth distribution and dip angles of earthquakes when assessing earthquake hazard in foreland-basin settings. [ABSTRACT FROM AUTHOR]

Density is a key driver of tectonic processes, but it is a difficult property to define well in the lithosphere because the gravity method is non-unique, and because converting to density from seismic velocity models, themselves non-unique, is also highly uncertain. Here we use a new approach to define the lithospheric density field of Australia, covering from 100?E to 165?E, from 5?N to 55?S and from the crust surface to 300 km depth. A reference model was derived primarily from the recently released Australian Seismological Reference Model, and refined further using additional models of sedimentary basin thickness and crustal thickness. A novel form of finite-element method based deterministic gravity inversion was applied in geodetic coordinates, implemented within the open-source escript modelling environment. Three spatial resolutions were modelled: half-, quarter- and eighth-degree in latitude and longitude, with vertical resolutions of 5, 2.5 and 1.25 km, respectively. Parameter sweeps for the key inversion regularization parameters show that parameter selection is not scale dependent. The sweep results also show that finer resolutions are more sensitive to the uppermost crust, but less sensitive to the mid- to lower-crust and uppermost mantle than lower resolutions. All resolutions show similar sensitivity below about 100 km depth. The final density model shows that Australia's lithospheric density field is strongly layered but also has large lateral density contrasts at all depths. Within the continental crust, the structure of the middle and lower crust differs significantly from the crystalline upper crust, suggesting that the tectonic processes or events preserved in the deep crust differ from those preserved in the shallower crust. The lithospheric mantle structure is not extensively modified from the reference model, but the results reinforce the systematic difference between the density of the oceanic and continental domains, and help identify subdivisions within each. The lithospheric static pressure field was resolved in 3D from the gravity and density fields. The pressure field model also highlights the fundamental difference between the oceanic and continental domains, with the former possessing lower pressure through most of the model. Overall pressure variability is large in the upper crust (60 MPa) but reduces significantly by -30 km elevation (20-30MPa). By -50 km elevation, thick lower-crust generates further disequilibria (25-35MPa) that are not compensated until -125 km elevation (10-20MPa). Beneath -125 km elevation higher pressure is observed in the continental domain, extending to the base of the model. This indicates a lithosphere that is to a large degree isostatically compensated near the base of the felsic-intermediate continental crust, and again near the theoretical base of mature oceanic lithosphere. [ABSTRACT FROM AUTHOR]

We present a new approach to measuring crustal thickness and bulk properties from teleseismic data. In contrast to the traditional H - k stacking approach, which involves receiver-function deconvolution followed by stacking along expected arrival-time curves, we eliminate the deconvolution step and generate synthetic transfer functions predicting the relationship between the vertical and radial components. Given a catalogue of precalculated transfer functions, we convolve the vertical component with each assumed transfer function and then calculate a misfit between the real and predicted radial component. As a single-layer crust is no longer a necessary assumption, we use the transfer-function approach to extend H - k analysis to models containing sedimentary basins. Applied to a data set in Minnesota, North Dakota and South Dakota, where the Archean Superior Province, Paleoproterozoic Trans-Hudson Orogen and Meso/Neoproterozoic Mid-Continent Rift are largely overlain by sediments of the Williston Basin, we find that the transfer-function approach is able to recover approximate sedimentary thickness, as well as remove the contaminating effects of the sedimentary layers from crustal thickness and basement P/S velocity ratio measurements. We find that the Superior Province has uniformly low P/S ratio, reflecting a highly felsic composition, but has substantial Moho topography, with substantial crustal thinning to the west possibly related to Trans-Hudson accretion. The Trans-Hudson has thick crust with higher (more mafic) P/S ratio; higher ratios in the Superior near the Trans-Hudson contact suggest that the Superior is overthrust on Trans-Hudson basement material. The vicinity of the Mid-Continent Rift shows thinned crust, elevated P/S ratio, and a localized zone of very high P/S ratio associated with a rift offset, possibly related to a former triple junction, that extends northwestward into the Superior following a recently detected linear mantle anomaly. [ABSTRACT FROM AUTHOR]

SUMMARY The young (< 5 Ma) Corinth Rift is an ideal natural laboratory to investigate rift deformation mechanisms by comparing extension rates determined by various methods spanning different depth and time ranges. Corinth Rift geodetic extension rates averaged over 5-100 yr have been interpreted to increase from ∼5 mm yr-1 or less in the east to >10-15 mm yr-1 in the west. We quantify total upper-crust and whole-crust extension on three profiles across the Corinth Rift. Whole-crust extension is greater across the central rift (∼11-21 km) than across the western part of the rift (∼5-13 km). This correlates with the overall rift morphology, which shows maximum basement subsidence, sediment accumulation, rift width and greatest summed Late Quaternary fault displacements in the central basin, but contrasts with the pattern of geodetic extension rates which are greater to the west of the central basin. The E-W increase in strain rates interpreted from geodetic data cannot have persisted over rift history to produce the observed rift morphology. We suggest the discrepancy between short-term and long-term extension patterns is related to shifts in the loci of maximum extension due to fault growth and linkage during Corinth Rift history, and is likely a characteristic of rift development in general. Total upper-crust and whole-crust extension estimates in the western rift, where extension estimates are best constrained, are within error. We propose that uniform pure-shear extension is a viable extension mechanism in the western rift and crustal extension estimates do not require the existence of a major active N-S dipping detachment fault. [ABSTRACT FROM AUTHOR]

Enhanced melt productivity as a consequence of buoyant upwelling and small-scale convection of the mantle during rifting may play an important role in determining the fundamental structure of igneous crust produced during and following continental breakup. This paper investigates the relationship between rift-related decompression melting and the influence of small-scale mantle convection and rift geometry on the subsequent production and distribution of melt-related crust. Extension of the lithosphere is modelled numerically using a 2-D plane-strain finite element method for viscous-plastic creeping flows. The evolving temperature and pressure fields within the model are coupled to an algorithm that predicts the amount and timing of decompression melting of upwelling mantle. Predicted melt fractions are converted to equivalent thicknesses of igneous crust, and the predicted crustal thicknesses for a series of models are compared to the observed crustal structure of rifted margins inferred from seismic data. Models characterized by small-scale mantle convection can to first order reproduce the general architecture of most volcanic rifted margins, that is, a relatively narrow band of thick (12–13 km) igneous crust (inferred to occur along strike of the margin), juxtaposed with thinner oceanic crust farther offshore. The variability in thickness (4–7 km) predicted for the later-stage thinner igneous crust is however difficult to reconcile with global observations of oceanic crustal thickness (7 ± 1 km). Also, the peak 13 km thickness of igneous crust predicted for models with convectively enhanced upwelling fails to match the great thicknesses (≥20 km) of igneous crust observed at many volcanic margins. Composite models that include both small-scale convection and a small increase to mantle potential temperature predict large pulses in initial magmatism and generation of 17 to 21-km-thick crust, followed by unstable production of thinner igneous crust. The results indicate that models with small-scale convection and no temperature anomaly may play a role in explaining the formation of volcanic margins with only moderately thick (11–15 km) igneous crust. Further, convection coupled with small increases to mantle temperature may be important during the initial phase of very thick igneous crust generation at some volcanic margins. Predicted distributions of igneous crust are moderately sensitive to asymmetric rifting of the lithosphere. Prior to breakup, igneous crust accretion is asymmetric; subsequent to breakup, symmetry in the thermal structure of the upwelling sublithospheric mantle is the dominant control on the final distribution of igneous crust. [ABSTRACT FROM AUTHOR]

The time-dependent heat budget, thermal evolution and rheology of the lithosphere in the last 13 Myr, along a 120 km long profile in the foreland of the Eastern Carpathians bend is presented. The study area, a complex tectonic environment in the vicinity of the well-known intermediate-depth seismogenic Vrancea zone, includes the deepest sedimentary basin in Romania (∼18 km), with a recent rapid evolution in the last 13 Myr (8 km of sediments). Detailed high-resolution temperature logs in the depth ranges from 0–700 m to 0–2400 m in eight thermally stabilized boreholes, together with logging and geological information on structure, lithology and time evolution of the sedimentary pile have been used to model the thermal evolution by means of a 2-D finite element model which includes sedimentation history, sediment compaction, lateral and vertical variation of thermal properties of sediments and consolidated crust. Modelling results have been compared to measured temperatures, corrected for palaeoclimate effects, in a trial and error iterative approach. Sedimentation, palaeoclimate, undercompaction and heat refraction effects on one hand, and the heat generated in the upper crust, on the other, combine to explain the observed subsurface temperature field and in particular the pronounced curvature of the vertical temperature profiles. The Neogene-Quaternary sedimentation resulted in a rather uniform deficit of the surface heat flux of about 12 mW m−2 in the study area, in spite of the marked lateral variation of the sedimentary pile thickness. The palaeoclimate effect is responsible for a surface heat flux depression of 7–8 mW m−2, uniform along the study profile. Unlike older sedimentary basins, in the study area transient sedimentation thermal effects add to palaeoclimate effects in establishing the temperature field in the depth range of geothermal measurements. The lateral variation of the palaeoclimatically corrected surface heat flux from the centre of the Focsani Depression (40 mW m−2) to its margin and the foreland platform (70 mW m−2) is mainly the result of the lateral variation of the heat produced in the upper crust where thickness varies from 7 to 21 km, respectively. A heat production of 2.3 μW m−3 for the southeastern end of the profile and of 2.15 μW m−3 for the rest of it has been modelled. As a result of the sedimentation process, temperature variations as large as 70–100 °C occurred in the pre-Neogene sediments and the crystalline crust immediately under the sedimentary pile, with consequences on the metamorphic state and rheological behaviour of the foreland lithosphere. Progressively smaller variations were present to 40–50 km depth. Unlike flexural modelling, which indicate effective elastic thickness values of 10–15 km, our rheological analysis based on the derived temperature field indicates time-dependent lateral variations between 26–39 km, possibly favouring the recent evolution of the foredeep, characterized by depo-centre migration and basin expansion. [ABSTRACT FROM AUTHOR]

A magnetotelluric (MT) survey of the crust beneath sedimentary basins and thrust sheets along the southwestern edge of the São Francisco craton, central Brazil, reveals intricate electrical characteristics that are interpreted in the context of Proterozoic collision tectonics and horizontal transport of allochthonous rock units, emplacement of Cretaceous hypabyssals, lavas and diatremes of ultrapotassic–mafic composition, and occurrence of induced seismicity. The data exhibit strong distortions represented by 3-D induction effects and galvanic disturbances resulting from shallow structures, frequency and site dependence of electrical strike, and inhomogeneous anisotropic layers with smoothly varying phase split, conductance and azimuth of the highly conductive direction. Geoelectrical and orthogonal phase difference directions, 2-D inversion, and forward modelling characterize three distinct subhorizontal sections showing two anisotropic conductors within a highly resistive crust, laterally segmented into unique blocks. The model for the uppermost crust section has E–W geoelectric directions and a 15–30 S anisotropic zone at a depth of approximately 1–2 km along the entire profile. This conducting layer is interpreted to represent a brine-filled fracture layer possibly controlled by the present-day state of crustal stresses, as disclosed from reservoir-triggered quakes. The mid-crust section presents a deeper conducting zone located at depths below 10 km. It is defined by stronger MT responses having phase split directions oscillating from WNW–ESE to E–W beneath sites in the central-western area (Paraná basin and allochthonous cover units) and NNE–SSW in the northeastern region (autochthonous platform units and Sanfranciscana basin). Anisotropy is greater than an order of magnitude in the highly conductive direction, with conductance in the range of 250–400 S. Conjecturally, the source of this anomalous feature would come from interconnected grain-boundary phases and hypersaline fluids, exsolved and precipitated from upwelling Cretaceous magma. In the central-western area, favourable trapping of conductors was constrained along a nearly E–W direction, feasibly associated with relic structures inherited from Brasiliano/Pan African continental collisions. Along the northeast, however, the coincidence with superficial NNE–SSW structural directions suggests a localized direct causal relationship with the trend of extension related to magma emplacement. The lower crust has a highly resistive quasi–1-D section along the entire profile that prevails also at uppermost mantle depths. Thus, whereas the brittle crust would have reconciled subhorizontal strain with fluid percolation related to uplift and magma emplacement, a mechanically coupled and stronger lower crust/upper mantle would have controlled the deep magma generation during Cretaceous distention pulses. [ABSTRACT FROM AUTHOR]

A data assimilation approach is demonstrated whereby seismic inversion is both automated and enhanced using a comprehensive numerical sedimentary basin simulator to study the physics and chemistry of sedimentary basin processes in response to geothermal gradient in much greater detail than previously attempted. The approach not only reduces costs by integrating the basin analysis and seismic inversion activities to understand the sedimentary basin evolution with respect to geodynamic parameters—but the technique also has the potential for serving as a geoinfomatics platform for understanding various physical and chemical processes operating at different scales within a sedimentary basin. Tectonic history has a first-order effect on the physical and chemical processes that govern the evolution of sedimentary basins. We demonstrate how such tectonic parameters may be estimated by minimizing the difference between observed seismic reflection data and synthetic ones constructed from the output of a reaction, transport, mechanical (RTM) basin model. We demonstrate the method by reconstructing the geothermal gradient. As thermal history strongly affects the rate of RTM processes operating in a sedimentary basin, variations in geothermal gradient history alter the present-day fluid pressure, effective stress, porosity, fracture statistics and hydrocarbon distribution. All these properties, in turn, affect the mechanical wave velocity and sediment density profiles for a sedimentary basin. The present-day state of the sedimentary basin is imaged by reflection seismology data to a high degree of resolution, but it does not give any indication of the processes that contributed to the evolution of the basin or causes for heterogeneities within the basin that are being imaged. Using texture and fluid properties predicted by our Basin RTM simulator, we generate synthetic seismograms. Linear correlation using power spectra as an error measure and an efficient quadratic optimization technique are found to be most effective in determining the optimal value of the tectonic parameters. Preliminary 1-D studies indicate that one can determine the geothermal gradient even in the presence of observation and numerical uncertainties. The algorithm succeeds even when the synthetic data has detailed information only in a limited depth interval and has a different dominant frequency in the synthetic and observed seismograms. The methodology presented here even works when the basin input data contains only 75 per cent of the stratigraphic layering information compared with the actual basin in a limited depth interval. [ABSTRACT FROM AUTHOR]