Basement Geometry Research Articles

Gravity inversion for heterogeneous sedimentary basin with b-spline polynomial approximation using differential evolution algorithm

We have developed a MATLAB-based inversion program, b-spline polynomial approximation using the differential evolution algorithm (SPODEA), to recover the concealed basement geometry under heterogeneous sedimentary basins. Earlier inversion techniques used the discretized subsurface interface topography into a grid of juxtaposed elementary prisms to estimate the basement depth of a basin. Such discretization leads to the failure of the depth profile continuity and requires a higher number of inversion parameters for achieving the desired accuracy. The novel approach of SPODEA overcomes such limitations of earlier inversion techniques. SPODEA is based on the segment-wise b-spline optimization technique to estimate the basement depth by using high-order polynomials. Moreover, it can achieve an optimal misfit with minimal parametric information, which reduces the computational expense. Our inversion approach uses the differential evolution algorithm, which provides real parametric optimization and uses b-splines for accurate estimation of continuous depth profiles. The efficiency of our algorithm was determined with two complex synthetic sedimentary basin models comprised of constant and depth-varying density distributions. Furthermore, the uncertainty analysis of our inversion technique is evaluated by incorporating white Gaussian noise into the synthetic models. Finally, the utility of SPODEA is evaluated by inverting gravity anomalies for two different real sedimentary basins. It produces geologically reasonable outcomes that are in close agreement with basement structures from previously reported results.

Vp/Vs ratios in the Parnaíba Basin from joint active-passive seismic analysis – Implications for continental amalgamation and basin formation

The Phanerozoic intracontinental Parnaíba Basin in northeast Brazil lies atop crust composed of Archaean to Mesoproterozoic cratonic blocks and Neoproterozoic mobile belts. Recently, active and passive source geophysical surveys characterised the crustal structure beneath the basin. We use information from published active-source seismic and new, coincident receiver function (RF) data to obtain Vp/Vs ratios for sedimentary and crustal structure and make inferences about crustal compositions and tectonic evolution. In our approach, sedimentary and crustal Vp/Vs ratios are adjusted to match common conversion point (CCP) images of RFs and known Moho and basement geometry. We use a P-wave model from published wide-angle reflection/refraction (WARR) seismics, and structural features from a deep seismic reflection (DSR) profile. CCP images of the primary RF conversions were used to model the crust, whilst conversions of multiples were used for the sediment-basement interface. The maximum uncertainties in Vp/Vs are estimated to be 0.15 for the basin and 0.03 for the crust. Vp/Vs ratios in the basin were estimated between 1.7 and 2.2. Lower values correlate with the exposure of older units primarily in the east of the basin, whilst higher values coincide with exposed younger units of the Parnaíba Basin. The obtained crustal Vp/Vs ratios between 1.73 and 1.81 support the previously published segmentation of the crust. In particular, we identified three regions of elevated Vp/Vs ratios, which can be related to proposed Neoproterozoic suture zones underlying the Parnaíba Basin, as well as high velocity lower crust beneath. The high Vp/Vs ratios can be explained by mafic compositions, for example metamorphosed or intruded crust, or fluids and sedimentary rocks entrained into highly deformed crust, typical for modifications related to suture zones. These new deep geophysical models provide important and complementary evidence for crustal amalgamation and the formation of the Parnaíba Basin.

The exhumation detachment factory

Abstract Rifted margins include series of tectonic structures and basement geometries that record the thinning of continental crust and the birth of oceanic spreading centers. A consensus has been reached on the fact that the evolution is composite, including a phase of basement exhumation during which detachment faults play a major role. Rolling-hinge–type models are often advocated as the driving mechanism for basement exhumation, although the structural complexity of the exhumed regions frequently prohibits detailed analysis of the ways in which the faults form and evolve through time. Here, we place constraints on this process through an analysis of continental rifting numerical simulations, which we used to map fault network activity. The high-resolution models and mapping reveal a specific weak surface where detachments initiate at depth and terminate while migrating upward to the seafloor. This “detachment factory” model provides constraints on where, when, and how detachment faults originate in an exhumation system. Our model also predicts how abandoned detachments migrate off-axis with time, within the footwall and hanging wall, generating specific signals in geophysical data sets.

Isostatic constraint for 2D nonlinear gravity inversion on rifted margins

We have developed a nonlinear gravity inversion for simultaneously estimating the basement and Moho geometries, as well as the depth of the reference Moho along a profile crossing a passive rifted margin. To obtain stable solutions, we impose smoothness on basement and Moho, force them to be close to previously estimated depths along the profile and also impose local isostatic equilibrium. Different from previous methods, we evaluate the information of local isostatic equilibrium by imposing smoothness on the lithostatic stress exerted at depth. Our method delimits regions that deviate and those that can be considered in local isostatic equilibrium by varying the weight of the isostatic constraint along the profile. It also allows controlling the degree of equilibrium along the profile, so that the interpreter can obtain a set of candidate models that fit the observed data and exhibit different degrees of isostatic equilibrium. Our method also differs from earlier studies because it attempts to use isostasy for exploring (but not necessarily reducing) the inherent ambiguity of gravity methods. Tests with synthetic data illustrate the effect of our isostatic constraint on the estimated basement and Moho reliefs, especially at regions with pronounced crustal thinning, which are typical of passive volcanic margins. Results obtained by inverting satellite data over the Pelotas Basin, a passive volcanic margin in southern Brazil, agree with previous interpretations obtained independently by combining gravity, magnetic, and seismic data available to the petroleum industry. These results indicate that combined with a priori information, simple isostatic assumptions can be very useful for interpreting gravity data on passive rifted margins.

MagB_inv: A high performance Matlab program for estimating the magnetic basement relief by inverting magnetic anomalies

This paper presents a MATLAB based program, including an easy-to-use graphical interface, developed to estimate 3D geometry of magnetic basement interfaces from respective gridded magnetic anomalies by a rapid iterative procedure. The developed code uses an algorithm based on a relationship between the Fourier transforms of the magnetic data and the interface topography. Given the average depth to the magnetic interface and parameters related to the magnetization, the iterative procedure built up in the frequency domain results with accurate depth estimates in a very short computing time. The program is capable of handling large data sets. The convergence in the iterative process is improved by incorporating a high-cut filtering. The iterations stop when either the RMS error between two successive approximations at any step of the iteration has increased relative to the previous step or when the RMS is below a predefined threshold value, or after a specified maximum iterations number. Settings and updating of the input parameters as well as displaying and exporting the interpretation outputs can easily be managed with the interactive control skills supported by the graphical user interface (GUI) of the MagB_inv code. Applicability and efficacy of the algorithm are illustrated on synthetic data from three 3D interface models, the respective inverted depths match the actual depths even in the presence of noise. As a practical example, the algorithm was also applied to observed anomalies of the total magnetic field from NW-Germany. The calculated magnetic basement interface from the real data application is in good agreement with the previously published configurations of the basement of this study area.

Basement configuration and lineaments mapping from aeromagnetic data of Gongola arm of Upper Benue Trough, northeastern Nigeria

The petroleum prospect evaluation of the Gongola arm of the Upper Benue Trough in the northeastern part of Nigeria has been undertaken through the analyses of High Resolution Aeromagnetic data (HRAM) acquired over the basin, with a view to determining sections of the basin having basement geometry, sedimentary thickness and structural framework among others that could support generation and preservation of the hydrocarbon resource. Processed, transformed and enhanced Total Magnetic Intensity (TMI) data were subjected to various analyses to define terrain geology, extract lineaments and delineate basement geometry, which were subsequently employed to generate basement model and structural framework of the basin. Most of the extracted lineaments trend ENE – WSW direction while Analytic Signal - Euler Deconvolution depth-weighting generate solutions which range in sediment thickness from 50 m above to over 5 km below the Minna reference datum, thus presenting undulating basement topography. Three sub-basins having depths in excess of 3 km, separated from each other by intra-basin subsurface basement highs, were delineated. The coexistence of considerably thick (3–5.7 km) sedimentary rock units and underlying structurally active basement rocks, assured geothermal gradient for organic matter maturation and guaranteed trapping of generated oil and gas within reservoirs’ structural traps.

Gravity and Magnetic Survey in Southwestern Part of Cuddapah Basin, India and its Implication for Shallow Crustal Architecture and Mineralization

ABSTRACT Gravity and magnetic surveys were carried out in southwestern part of Cuddapah basin (CB) covering an area of ~3660 km2. Southwestern part of CB gained lot of attention after discovery of second most important uranium province in India. Some non-metallic industrial minerals and base metal occurrence are also reported in the study area. Present study with high spatial resolution gravity-magnetic survey aims to decipher detail basin geometry, nature of sediments, along with possible mineral deposits in SW part of CB. The gravity survey comprising short and long wavelength anomalies brought out sedimentary characteristics and basement architecture underneath the sediments. The long wavelength features of the gravity map shows gneissic basement, which is characterized by both basic and acid magmatic intrusive. Residual gravity anomaly map shows good correspondence with the exposed high density basaltic rock units and also brought out occurrence of concealed high density litho-units, which have significance for mineral prospecting. The magnetic map shows that both sediments and underneath basement are non-magnetic in nature except SW part of the study area, where study suggests occurrence of concealed mafic lensoid body. Euler solutions and combined gravity-magnetic modelling further facilitated for understanding of structural feature and basement geometry. Based on the integrated gravity and magnetic study mineral prospecting zones have been delineated for further detailed study.

3D Tomographic Inversion of TEM Sounding Data

Abstract —One of the main objectives of geoelectric prospecting is mapping of the consolidated low-conductivity basement geometry. To resolve the issue, it will be equally important to delineate local structures at the bottom of the sedimentary complex and to estimate the misinterpretation of results due to the presence of areas with nonuniform conductivity in the stratified geologic cross section. Another relevant objective of EM sounding is to resolve the problem of delineation of anticlinal oil and gas traps. The horizontally layered model with a local inclusion of 3D abnormal areas is certain to fit for the above-mentioned objectives. The study is concerned with the technique of nonstationary EM sounding. When applied for the solution of structural problems, this technique considers the uniform distribution of fields within relatively large volume units of the space explored. Consequently, it results in the more efficient application of the perturbation technique (Born approximation) for the solution of the forward electrodynamics problem. The study presents the findings of 3D tomographic inversion with the use of synthetic and physical modeling data. They definitely allow us to acknowledge that the proposed mathematical apparatus for 3D inversion based on Born linearization of the forward problem has proved to be quite applicable.

Comparison of shallow basement thermal performance for different regions of Morocco using a three-dimensional heat transfer analysis

AbstractA three-dimensional numerical model was built to estimate the heat transfer between the soil and a shallow basement in four different climates (cold, temperate, semi-arid and arid climates) for, respectively, conditioned and unconditioned cases. The governing heat transfer equation in soil and basement was solved by the finite difference method using the alternating-direction implicit scheme (ADI). The air temperature for the case of conditioned shallow basement was maintained constant while it was computed for the case of unconditioned cellar using energy balance equation. The effects of the basement geometry, soil types and climatic conditions on the thermal behavior of the conditioned and unconditioned shallow basement were carried out. The heat losses and isotherms analysis showed that the heat flux is more significant through the walls than the basement floor and occurred mainly in the walls-floor edges. Furthermore, lowering the thermal diffusivity of the soil leads to a decrease in the shal...

Analytical investigation of tunnel deformation caused by circular foundation pit excavation

With the continuous progress of urbanization, excavation works near tunnel engineering projects are increasing. The stress relief caused by the excavation inevitably affects the deformation of a tunnel. Therefore, it is of great theoretical and practical significance to correctly evaluate the tunnel’s transverse and longitudinal deformation to ensure its safe and normal operation. Many studies have been conducted on the effect of quadrangle excavation on the deformation of adjacent subway tunnels. However, the influence of circular excavation on tunnel deformation is still not fully understood, and remains the main technical barrier toward optimal engineering design and construction. Based on Mindlin’s displacement solutions, an analytical model of tunnel deformation resulting from a circular excavation was developed, and the reliability of the solution was verified by comparison with existing centrifuge model test results and an analytical solution for a square excavation. On this basis, the influence of the basement geometry, relative position of the tunnel and basement, and soil’s Young’s modulus on tunnel deformation resulting from a circular excavation were analyzed. The results revealed that the analytical solutions proposed in this paper can better predict the maximum tunnel heave resulting from a circular excavation. Moreover, the maximum tunnel heave increased linearly as the diameter and depth of the circular basement excavation increased. However, the non-linearity decreased with the tunnel cover depth. The results obtained by this study can provide a simple calculation method to predict the maximum tunnel heave caused by a circular excavation, and can also be used to confirm the rationality of a numerical simulation.

Oceanic-like axial crustal high in the central Red Sea

The Red Sea is an important example of a rifted continental shield proceeding to seafloor spreading. However, whether the crust in the central Red Sea is continental or oceanic has been controversial. Contributing to this debate, we assess the basement geometry using seismic reflection and potential field data. We find that the basement topography from seismically derived structure corrected for evaporite and other sediment loading has an axial high with a width of 70–100 km and a height of 0.8–1.6 km. Basement axial highs are commonly found at mid-ocean ridges affected by hotspots, where enhanced mantle melting results in thickened crust. We therefore interpret this axial high as oceanic-like, potentially produced by recently enhanced melting associated with the broader Afar mantle anomaly. We also find the Bouguer gravity anomalies are strongly correlated with basement reflection depths. The apparent density contrast necessary to explain the Bouguer anomaly varies from 220 kg m−3 to 580 kg m−3 with no trend with latitude. These values are too small to be caused primarily by the density contrast between evaporites and mantle across a crust of uniform thickness and density structure, further supporting a thickened crustal origin for the axial high. Complicating interpretation, only a normal to modestly thickened axial crust is predicted from fractionation-corrected sodium contents (Na8.0), and the basement reflection is rugged, more typical of ultra-slow spreading ridges that are not close to hotspots. We try to reconcile these observations with recent results from seismic tomography, which show modest mantle S-wave velocity anomalies under this part of the Red Sea.

Mineralogical, Rock-Magnetic and Palaeomagnetic Properties of Metadolerites from Central Western Svalbard

A combination of mineralogical, rock-magnetic and palaeomagnetic methods were employed in an attempt to shed a new light on the tectonism and paleogeography of Central Western Svalbard. The focus is on six metadolerite sites from the metamorphic Proterozoic–Lower Palaeozoic complex of south-western Oscar II Land (Western Spitsbergen). The primary mineral compositions of the metadolerites were strongly remineralized during Caledonian (sensu lato) greenschist-facies metamorphism although some younger tectonothermal modification is also apparent from the rock-magnetic studies. Rock-magnetic experiments supported by thin-section mineral identification and separation of Fe-containing fractions indicate that the main ferromagnetic carriers of the Natural Remanent Magnetization are represented by low-coercivity pyrrhotite and magnetite/maghemite. The investigated metadolerites are characterized by complex pattern of magnetization. The low-temperature palaeomagnetic components which demagnetized up to 250 °C, are characterized by high inclinations (~70–80°) potentially representing Mesozoic–Cenozoic remagnetization. The most stable middle-high temperature directions which demagnetized from 250 °C, were obtained from only two of six sites. Two Virtual Geomagnetic Poles calculated from two of the middle-high temperature site means do not correlate with the Laurussia reference path for syn- to post-Caledonian time. Two possible explanations of observed inconsistency are discussed. These are a modification of the Oscar II Land Caledonian basement geometry by listric faulting and/or tectonic rotations related to Daudmannsdalen–Protectorbreen high-strain (shear) zone. The results presented here suggest that post-Caledonian tectonic modification of the palaeomagnetic directions may be more a widespread feature of Western Svalbard.

Characterization of the depocenters and the basement structure, below the central Chile Andean Forearc: A 3D geophysical modelling in Santiago Basin area

AbstractSince the last century, several geological and geophysical studies have been developed in the Santiago Basin to understand its morphology and tectonic evolution. However, some uncertainties regarding sedimentary fill properties and possible density anomalies below the sediments/basement boundary remain. Considering that this is an area densely populated with more than 6 million inhabitants in a highly active seismotectonic environment, the physical properties of the Santiago Basin are important to study the geological and structural evolution of the Andean forearc and to characterize its seismic response and related seismic hazard. Two and three‐dimensional gravimetric models were developed, based on a database of 797 compiled and 883 newly acquired gravity stations. To produce a well‐constrained basement elevation model, a review of 499 wells and 30 transient electromagnetic soundings were used, which contribute with basement depth or minimum sedimentary thickness information. For the 2‐D modelling, a total of 49 gravimetric profiles were processed considering a homogeneous density contrast and independent regional trends. A strong positive gravity anomaly was observed in the centre of the basin, which complicated the modelling process but was carefully addressed with the available constrains. The resulting basement elevation models show complex basement geometry with, at least, eight recognizable depocenters with maximum sedimentary infill of ~ 500 m. The 3‐D density models show alignments in the basement that correlates well with important intrusive units of the Cenozoic and Mesozoic. Along with interpreted fault zones westwards and eastwards of the basin, the observations suggest a structural control of Santiago basin geometry, where recent deformation associated with the Andean contractional deformation front and old structures developed during the Cenozoic extension are superimposed to the variability of river erosion/deposition processes.

Modeling of the Foca-Uzunada magnetic anomaly and thermal structure in the gulf of Izmir, western Turkey

The Gulf of Izmir (GoI) is one of the largest gulfs in the Aegean Sea, Turkey. There is a large magnetic anomaly extending in the NE-SW direction between Foca and Uzunada (Uzun Island) in the gulf. Previously, Curie Point Depth (CPD), geothermal gradient, heat-flow and radiogenic heat production maps of the onshore part of the Aegean region were constructed from the aeromagnetic data. In this study, the same maps except radiogenic heat production map are presented for the offshore part and the largest magnetic anomaly in the northern part of the gulf is focused, particularly. As a result, the thermal structure of GoI is clearly defined and according to the results of this study, CPD values were found from 7 km in the NE of Foca to 10 km through the south of the gulf. The geothermal gradient values vary between 50 and 80 °C/km. Maximum heat flow values around the anomaly are calculated as 200 and 215 mW/m2 according to the thermal conductivity coefficients of 2.5 W m−1 K−1 and 2.7 W m−1 K−1, respectively. Although the anomaly is located in the Izmir Gulf; CPD, geothermic gradient, heat flow anomalies are shifted through the north of Foca and Aliaga towns in the Candarli Bay. This prominent anomaly in the Gulf of Izmir is associated with the magmatics that were encountered at 969 m in the Foca-1 well although it was drilled about 2 km away from the outermost closed contour of the magnetic anomaly. The anomaly is also modeled three dimensionally (3D) in this study. In the model map, the top of the causative body is completely located in the outer part of the gulf, and is very shallow at about 0.5 km while its bottom is inclined through the west of Cigli and Menemen. From this viewpoint, it is possible to suggest that the causative body is inclined through the Foca Peninsula. However, its closed contours are in the NE direction, through the Candarli Bay. Top depth of the causative body is also calculated from the basement horizon on the seismic sections crossing this anomaly. Depth calculations are consistent in these sections and confirm the top depths from the modeling study. The basement geometry in the seismic sections also reflects the shape of 3D model geometry, and bottom depth of the magmatics is also verified by the basement depth calculations in seismic sections.

Joint Audio-Magnetotelluric and Passive Seismic Imaging of the Cerdanya Basin

The structure of Cerdanya Basin (north-east of Iberian Peninsula) is partly known from geological cross sections, geological maps and vintage geophysical data. However, these data do not have the necessary resolution to characterize some parts of Cerdanya Basin such as the thickness of soft soil, geometry of bedrock or geometry of geological units and associated faults. For all these reasons, the main objective of this work is to improve this deficiency carrying out a detailed study in this Neogene basin applying jointly the combination of passive seismic methods (H/V spectral ratio and seismic array) and electromagnetic methods (audio-magnetotelluric and magnetotelluric method). The passive seismic techniques provide valuable information of geometry of basement along the profile. The maximum depth is located near Alp village with a bedrock depth of 500 m. The bedrock is located in surface at both sites of profile. The Neogene sediments present a shear-wave velocity between 400 and 1000 m/s, and the bedrock basement presents a shear-wave velocity values between 1700 and 2200 m/s. These results are used as a priori information to create a 2D resistivity initial model which constraints the inversion process of electromagnetic data. We have obtained a 2D resistivity model which is characterized by (1) a heterogeneous conductivity zone (<40 Ohm m) that corresponds to shallow part of the model up to 500 m depth in the centre of the profile. These values have been associated with Quaternary and Neogene sediments formed by silts, clays, conglomerates, sandstones and gravels, and (2) a deeper resistive zone (1000–3000 Ohm m) interpreted as Palaeozoic basement (sandstones, limestones and slates at NW and conglomerates and microconglomerates at SE). The resistive zone is truncated by a discontinuity at the south-east of the profile which is interpreted as the Alp-La Tet Fault. This discontinuity is represented by a more conductive zone (600 Ohm m approx.) and is explained as a combination of fractured rock and a fluid network. The result highlights that the support between different geophysical methods is essential in producing geophysical meaningful models.

Interaction of crustal heterogeneity and lithospheric processes in determining passive margin architecture on the southern Namibian margin

Abstract The influence of pre-rift crustal heterogeneity and structure on the evolution of a continental rift and its subsequent passive margin is explored. The absence of thick Aptian salts in the Namibian South Atlantic allows imaging of sufficient resolution to distinguish different pre-rift basement seismic facies. Aspects of the pre-rift basement geometry were characterized and compared with the geometries of the Cretaceous rift basin structure and with subsequent post-rift margin architectural elements. Half-graben depocentres migrated westwards within the continental synrift phase at the same time as basin-bounding faults became established as hard-linked arrays with lengths of c. 100 km. The rift–drift transition phase, marked by seaward-dipping reflectors, gave way to the early post-rift progradation of clastic sediments off the Namibian coast. In the Late Cretaceous, these shelf clastic sediments were much thicker in the south, reflecting the dominance of the newly formed Orange River catchment as the main entry point for sediments on the South African–Namibian margin. Tertiary clastic sediments largely bypassed the pre-existing shelf area, revealing a marked basinwards shift in sedimentation. The thickness of post-rift megasequences does not vary simply according to the location of synrift half-graben and thinned continental crust. Instead, the Namibian margin exemplifies a margin influenced by a complex interplay of crustal thinning, pre-rift basement heterogeneity, volcanic bodies and transient dynamic uplift events on the evolution of lithospheric strain and depositional architecture.

Integrated interpretation of overlapping AEM datasets achieved through standardisation

Numerous airborne electromagnetic surveys have been acquired in Australia using a variety of systems. It is not uncommon to find two or more surveys covering the same ground, but acquired using different systems and at different times. Being able to combine overlapping datasets and get a spatially coherent resistivity-depth image of the ground can assist geological interpretation, particularly when more subtle geophysical responses are important. Combining resistivity-depth models obtained from the inversion of airborne electromagnetic (AEM) data can be challenging, given differences in system configuration, geometry, flying height and preservation or monitoring of system acquisition parameters such as waveform. In this study, we define and apply an approach to overlapping AEM surveys, acquired by fixed wing and helicopter time domain electromagnetic (EM) systems flown in the vicinity of the Goulds Dam uranium deposit in the Frome Embayment, South Australia, with the aim of mapping the basement geometry and the extent of the Billeroo palaeovalley. Ground EM soundings were used to standardise the AEM data, although results indicated that only data from the REPTEM system needed to be corrected to bring the two surveys into agreement and to achieve coherent spatial resistivity-depth intervals.

Physical experiments of land subsidence within a maar crater: insights for porosity variations and fracture localization

Abstract. We present the results of a series of physical models aiming to reproduce rapid subsidence (at least 25 m in 30 years) observed in the sediments of a maar crater caused by extraction of groundwater in the interconnected adjacent aquifer. The model considered plausible variations in the geometry of the crater basement and the measured rate of groundwater extraction (1 m per year in the time interval from 2005 to 2011) in 15 wells located around the structure. The experiments were built within a rigid plastic bowl in which the sediments and rocks of the maar sequence were modeled using different materials: (a) plasticine for the rigid country rock, (b) gravel for the fractured country rock forming the diatreme fill and, (c) water saturated hollow glass microbeads for the lacustrine sedimentary fill of the crater. Water table was maintained initially at the surface of the sediments and then was allowed to flow through a hole made at the base of the rigid bowl. Water extraction provoked a sequence of gentle deformation, fracturing, and faulting of the surface in all the experiments. Vertical as well as lateral displacements were observed in the surface of the experiments. We discuss the results of 2 representative models. The model results reproduced the main geometry of the ring faults affecting the crater sediments and helps to explain the diversity of structures observed in relation with the diatreme geometry. The surface of the models was monitored continuously with an optical interferometric technique called structured light projection. Images collected at nearly constant time intervals were analyzed using the ZEBRA software and the obtained interferometric pairs permitted to analyze the full field subsidence in the model (submilimetric vertical displacements). The experiments were conducted at a continuous flow rate extraction and show a also a linear subsidence rate. Comparison among the results of the physical models and the fault system associated to subsidence in the maar show that fault geometry in the sedimentary sequence imitates closely the geometry of the volcanic basement.

Crustal framework of the northwest Paraná Basin, Brazil: Insights from joint modeling of magnetic and gravity data

The Transbrasiliano Lineament (TBL) is a major Proterozoic strike-slip shear zone crossing the Paraná Basin, Brazil, yet the relationship between the TBL and basin structures is poorly understood. Magnetic and gravity data are available from the whole northwestern region of the Paraná Basin and provide insights into crustal partitioning and basement geometry. Joint 2D modeling of these data was performed in three strategic sections of the study area. Two smaller sections were constrained by available borehole data and receiver function analysis. Modeling of these sections facilitated modeling of the third, larger NW-SE section, which crosses the basin in an area with significant lack of data. These three sections together form the basis for a preliminary tectonic model of the northern Paraná Basin. The TBL includes strong N30°E-trending gravity and magnetic gradients represented by the Serra Negra fault zone. This structure separates two crustal tectonic domains, the Paranapanema Block to the east, and the Amazon paleocontinent to the west. The tectonic domain northeast of the fault is represented by the Arenópolis Magmatic Arc. The N60°–70°E-trending General Carneiro fault, also reflected in gravity anomalies, forms another important magnetic contact. Our tectonic model shows a region of crustal thinning between the two fault zones, where structures associated with possible reactivation form small Cretaceous basins. The Baliza fault may accommodate significant vertical displacements of Devonian and Carboniferous sections of the Paraná Basin. The main structures revealed in the sections are associated with grabens and horsts, interpreted as reactivated structures that controlled Paraná Basin sedimentation. The northwestern portion of the basin exhibits basement asymmetry caused by dip- and strike-slip faulting along the TBL.

Effects of irregular basement structure on the geometry and emplacement of frontal thrusts and duplexes in the Quebec Appalachians: Interpretations from well and seismic reflection data

Irregular basement geometry may affect thrust propagation in foreland fold–thrust belts creating a perturbation in structural continuity of hydrocarbon fields. Here we investigate how the irregular pattern of normal faults, along with the presence of uplifts and transverse faults in the Grenvillian basement has influenced the geometry and emplacement of frontal thrusts and duplexes in the parautochthonous domain of the southern Quebec Appalachians during the middle–late Ordovician Taconian orogeny. Integration of data from surface geology, wells, and 2D and 3D seismic reflection surveys into a regional-scale structural model is used to reconstruct the 3D geometry and emplacement history of one- and two-horse duplexes in the Joly–Saint-Flavien gas storage area. The normal hinterland to foreland sequence of thrusting in this area is complicated by the differential emplacement of tectonic slices along strike of the orogenic front, starting in the SW and developing subsequently to the NE within each structural level. The shortening related to duplex emplacement decreases laterally over the distance of 20km from −49% in the SW (Joly area) to −31% in the NE (Saint-Flavien area). Duplex emplacement resulted from the differential forward propagation, deflexion and vertical-axis rotation of the roof thrust (Logan's Line), which, in turn, has been induced by the presence of frontal uplift and transverse faults in the basement. The structural–lithological analysis of tectonic slices and restoration of their initial location allows us to consider the lower slice of the Joly duplex as a structural trap comparable to the fractured reservoir in lower Ordovician dolomites of the Saint-Flavien duplex.