Structure of the Erdenet ore district according to gravimetric data

The purpose of the study is construction of a model of the upper crust structure of the ore region in Mongolia and the three-dimensional mapping of intrusive bodies with which copper-porphyry mineralization is associated. An areal gravity survey was carried out with an observation density of 1 point per 6 km2 with the measurement accuracy of ±0.8 mGal. As a result, it was found that copper-molybdenum ore occurrences of the area including the Erdenet ore district are confined to local gravitational minima, which are interpreted as thickening of the body of the Selenga granitoids. The latter are confined to local depressions of this body base. The spatial proximity of supply channels of small ore-bearing intrusions and large granitoid bodies of the Selenga complex has been established. Porphyry ore intrusions are confined to rather wide (about 10 km) zones located above the depressions of the base of all intrusions of the Selenga complex (both granitoid and diorite). Since the local base depressions of the granitoid intrusions correspond to the position of magma supply channels, ore-bearing small intrusions were introduced approximately in the same places where the supply channels of granitoid intrusions of the Selenga complex existed. Therefore, it can be assumed that this case is characterized by not only tectonic inheritance (confined to the same faults and their intersection points), but also by a genetic one, since residual melts of the same foci, in which intrusion magma of the Selenga complex was generated might be the sources of small intrusions. From this point of view, the expediency of distinguishing an independent Erdenet complex seems to be controversial. Geophysical data on the spatial proximity of specified intrusion supply channels permit only to raise the question of such expediency. The solution to this issue is possible on the basis of a comprehensive analysis of petrological and geochemical data.

On the Origin of Orphan Tremors and Intraplate Seismicity in Western Africa

On September 5–7, 2018, a series of tremors were reported in Nigeria’s capital city, Abuja. These events followed a growing list of tremors felt in the stable intraplate region, where earthquakes are not expected. Here, we review available seismological, geological, and geodetic data that may shed light on the origin of these tremors. First, we investigate the seismic records for parent location of the orphan tremors using a technique suitable when a single-seismic station is available such as the Western Africa region, which has a sparse seismic network. We find no evidence of the reported tremors within the seismic record of Western Africa. Next, we consider the possibility of a local amplification of earthquakes from regional tectonics, reactivation of local basement fractures by far-field tectonic stresses, post-rift crustal relaxation, landward continuation of oceanic fracture zones, or induced earthquakes triggered by groundwater extraction. Our assessments pose important implications for understanding Western Africa’s intraplate seismicity and its potential connection to tectonic inheritance, active regional tectonics, and anthropogenic stress perturbation.

Precambrian tectonic inheritance control of the NE Brazilian continental margin revealed by Curie point depth estimation

An investigation of Curie point depths (CPD) based on spectral analysis of airborne magnetic data was carried out in the NE Brazilian continental margin. The studied region represents a narrow hyper-extended margin with three sedimentary basins. Regional geothermal gradient and heat flow were also calculated. CPD results were integrated with interpretation of 2D deep seismic data and with estimated isostatic Moho depths. The results reveal that the narrow hyper-extended crust is 150 km wide in the southern sector and 80 km wide in the north, with a narrow ocean-continental transition (OCT) zone that varies from 50 km wide in the south sector to 30 to 20 km wide in the north. The CPD isotherm showed the strong influence of the three main continental blocks of Borborema ́s Shield in the tectonic evolution of the three marginal basins. The CPD analysis corroborated models provided by gravimetric data and successfully demonstrated the sharp control of basement compartments on the thermal properties of the marginal basins domains

Interplay between tectonic inheritance and salt tectonics in the tectono-sedimentary evolution of the Sahel basin (eastern Tunisia): implications for hydrocarbon prospectivity

<p>The Sahel basin in eastern Tunisia has been subject for hydrocarbon exploration since the early fifties. Despite the presence of a working petroleum system in the area, most of the drilled wells were dry or encountered oil shows that failed to give commercial flow rates. A better understanding of the tectono-sedimentary evolution of the Sahel basin is of great importance for future hydrocarbon prospectivity. In this contribution, we present integration of 2D seismic reflection profiles, exploration wells and new acquired gravity data. These subsurface data reveal that the Sahel basin developed as a passive margin during Jurassic-Early Cretaceous times and was later inverted during the Cenozoic Alpine orogeny. The occurrence of Triassic age evaporites and shales deposited during the Pangea breakup played a fundamental role in the structural style and tectono-sedimentary evolution of the study area. Seismic and gravity data revealed jointly important deep-seated extensional faults, almost along E-W and few along NNE–SSW and NW-SE directions, delimiting horsts and grabens structures. These syn-rift extensional faults controlled deposition, facies distribution and thicknesses of the Jurassic and Early cretaceous series. Most of these inherited deep-seated normal and transform faults are ornamented by different types of salt-related structures. The first phase of salt rising was initiated mainly along these syn-extensional faults in the Late Jurassic forming salt domes and continued into the Early and Late Cretaceous leading to salt-related diapir structures. During this period, the salt diapirism was accompanied by the development of salt withdrawal minibasins, characterized important growth strata due the differential subsidence. These areas represent important immediate kitchen areas to the salt-related structures. The later Late Cretaceous - Cenozoic shortening phases induced preferential rejuvenation of the diapiric structures and led to the inversion of former graben/half-graben structures and ultimately to vertical salt welds along salt ridges. These salt structures represent key elements that remains largely undrilled in the Sahel basin. Our results improve the understanding of salt growth in eastern Tunisia and consequently greatly impact the hydrocarbon prospectivity in the area.</p>

How seismicity relates to lithospheric heterogeneity in the Alps

<p>The Alps mountains and its forelands consist of a heterogeneous lithosphere, comprised of a multitude of tectonic blocks from different tectonic provinces with different thermo-physical properties. Patterns of seismicity distribution are also observed to vary significantly throughout the region. However, the relationship between seismicity and lithospheric heterogeneity has been often overlooked in previous studies. We present an overview of recent results that have attempted to address these questions through the use of integrated 3D modelling techniques, thereby including: (i) a gravity and seismic data constrained, 3D, density structural model of the lithosphere; (ii) a 3D thermal model constrained against available wellbore temperature data; and,  (iii) a 3D rheological model of the long-term lithospheric strength and effective viscosities. Our models support the existence of a first-order correlation between the distribution of seismicity (laterally and with depth) and the strength of the lithosphere, with the former being clustered mainly within weaker domains. Beneath the Alps, observed upper-crustal level (i.e., unimodal) seismicity correlates with a weaker lithosphere where plate strength is controlled by the thick crustal root. Whereas in the southern foreland, weaker zones are found preferentially around the stronger Adriatic indenter while in the northern foreland they are located in the crust beneath the the Upper Rhine Graben (URG). We found that this correlation is primarily controlled by resolved thermal gradients and is a function of the tectonic inheritance setting (e.g., UGR), crustal architecture (e.g., thickness of sediments, upper and lower crust) and LAB depth. Sediment thickness and topographic effects controls the shallow thermal filed (0 – 10 km) whereas the deeper thermal field is controlled by the thickness of felsic upper crust (higher radiogenic heat contribution), the mafic lower crust (less radiogenic heat contribution) and basal thermal boundary condition from LAB depth. Seismicity is bounded by specific isotherms, 450 <sup>o</sup>C in the crust and < 600 <sup>o</sup>C in the mantle, except in regions where slabs are imaged by seismic tomography models. This is in contrast to the recent proposition that convergence velocity is a first-order factor controlling seismicity in an orogen rather than its architecture. Fast convergence rates (e.g., Himalayas) have been related to the subduction of the cold crust to deeper crustal depths thereby leading to a deepening of the brittle  domain and to a bimodal (i.e., upper and lower crust) seismicity character. In contrast, slow convergence (e.g., Alps) is thought to lead to a hotter ductile lower crust thus limiting brittle deformation within the upper crust. We therefore end our contribution by opening a discussion on the relative role of convergence rates and lithospheric heterogeneities, inherited and/or developed during orogenesis, in controlling the seismicity. In doing so we carry out a comparison between observed seismicity and lithospheric architecture in the other mountain ranges of the western Alpine-Himalayan collision zone where  convergence velocities are of a similar order of magnitudes as Alps, i.e., the Betics, the Pyrenees and the Apennines but where seismicity is observed to occur both at upper and lower crustal levels.</p>

Evolution of strain patterns in deforming upper plates in subduction zones: the case study of Cretaceous extension in the Iranian plateau

<p><span><span>Existing plate tectonic models rely on two essential features: (1) rigid tectonic plates and (2) very narrow plate boundaries where all deformation is localized. On the world geological map, plate boundaries are materialized by lines. Subduction plate boundaries, however, affect domains several hundred kilometers wide. In the upper plate of subduction zones, this deformation can result in the formation of orogenic-like compressive structures or extensional back-arc basins. In both cases, the respective contributions of slab movements, far-field stresses (i.e., boundary conditions) and tectonic inheritance in localizing strain in the upper plate are not yet well understood.</span></span></p><p><span><span>Located in the upper plate of the Late Triassic to Oligocene Neotethys subduction, the Iranian plateau records a long-lived convergence history, with numerous episodes of intraplate deformation. We herein focus on the Cretaceous back-arc opening (e.g., formation of the Nain-Baft marginal basin), whose possible triggers include a change in internal slab dynamics and/or regional-scale convergence dynamics (e.g., kinematics of the Neotethyan subduction, ridge subduction, opening of peripheral basins such as the Caspian Sea).</span></span></p><p><span><span>The Iranian plateau is part of a composite continental lithosphere made of blocks detached from Gondwana during the Paleozoic. It preserves evidence for structures inherited from the Precambrian Panafrican orogeny, as well as thinning and shortening during the opening and closure of the Paleotethys (during the Devonian and Late Triassic, respectively). Important lateral contrasts are observed after the Neotethys Permian rifting: the southwestern part (Sanandaj-Sirjan Zone) was thinned and filled with volcanic products, whereas the northeastern part (Kopeh-Dag and Yadz block) was thickened during the Late Triassic Cimmerian event. From NW to SE, deformation was also likely partitioned across large-scale strike-slip faults such as the Doruneh fault. These imprints make it difficult to assess the nature and extent of lateral heterogeneities in the crust, and in particular the variation of Moho depths prior to the Cretaceous extension. </span></span></p><p><span><span>In order to determine which parameters controlled the deformation of the Iranian upper plate, ultimately leading to localized back-arc extension along the Nain-Baft basin (i.e., SE of the Doruneh fault), we designed a parametric numerical study using the thermo-mechanical code pTatin2D, in which metamorphic reactions were implemented to model the subduction process realistically. Model results are evaluated based on the evolution of strain in the upper plate, in particular the characteristic size (~500 km) and duration of back-arc deformation (~30 Ma of extension prior to closure of this domain). The importance of structural inheritance is assessed by imposing either (1) a prexisiting crustal scale fault, (2) a partially thickened (3) or thinned crust. Those different tests allow to propose tentative geodynamic scenarios for the deformation of the upper plate Iranian plateau during the Cretaceous</span></span><span><span>.</span></span></p>

The role of tectonic inheritance during multiphase rifting: insights from analogue model experiments

<p>Rift systems worldwide are influenced by pre-existing crustal or lithospheric structures. Here, we use brittle-viscous analogue models to examine the role of tectonic inheritance on fault evolution during two non-coaxial rift phases. In our experiments the tectonic inheritance is a linear crustal weakness zone consisting of two offset and parallel linear segments connected by a central oblique linear segment. The first phase of rifting is either orthogonal and followed by a second phase of oblique rifting or vice versa.</p><p> </p><p>The experiments reveal that the tectonic inheritance localizes initial faulting during early rifting, with faults in the domains away from it forming later. The nature and orientation of early faults depends on first-phase rift obliquity, with a progressive switch from dip-slip dominated faulting to strike-slip dominated faulting with increasing obliquity, even resulting in local transpressional structures at very high rift obliquities. First-phase rift structures, in particular those above the tectonic inheritance, exert an important control on the overall fault geometry during the second phase of rifting. Our experiments show that two-phase rifting results in fault patterns evolving by the formation of second-phase new faults and the reactivation of first-phase faults.  Irrespective of the order of the applied two phases of non-coaxial rifting and the difference in rift obliquity angle between the two phases, a major rift (master rift) forms above the tectonic inheritance, underlining its strong control on fault evolution despite markedly different multiphase rift histories.</p><p> </p><p>Nevertheless, close inspection of the master rift reveals differences related to the relative order of the two rift phases: (i) Oblique rifting superseding orthogonal rifting results in a major master rift, whose rift-boundary faults are not reactivated during second-phase rifting. Instead, first-phase intra-rift normal faults are being reactivated with an important strike-slip component of displacement.</p><p>Above the oblique segment of the tectonic inheritance, first-phase en echelon intra-rift normal faults are mostly reactivated and propagate along strike reorienting their tips into high angles to the local principal stretching direction (ii) Orthogonal rifting overprinting oblique rifting, on the other hand, produces first-phase strike-slip faults that link up and trend (sub)-parallel to later formed rift-boundary faults and intra-rift normal faults.</p><p> </p><p>Away from the tectonic inheritance faults have more freedom to evolve in response to the regional rift obliquity, and although they may reactivate, propagate sideways and slightly reorient their fault tips during the second phase of rifting, their trend at the end of the second-phase of rifting with respect to the orientation of the master rift reflects whether first-phase rifting was orthogonal or oblique. Our model results can be used to assess the influence of tectonic inheritance on faulting, the relative order of rifting and the relative difference in obliquity in natural settings that have undergone two phases of rifting.</p>