Graben-bounding Faults Research Articles

Readjustment of fluvial systems to Quaternary tectonic disruption in the Central Afar (Ethiopia)

The Afar region constitutes the diffuse triple junction adjoining the Red Sea, Gulf of Aden, and Main Ethiopian Rifts. Extension in the Afar is distributed between on-axis, magmatic crustal spreading, and off-axis, amagmatic extension, such as in Dobe graben. The timing of initiation and development of these amagmatic grabens is key to understanding the final stages of continental extension/breakup in central Afar. All major graben-bounding faults in central Afar cut through the Quaternary Afar Stratoid Series, constraining the ages of initiation to younger than approximately 1 million years. The Afar Stratoid Series presents a widespread thick lava unit that smoothed and infilled the topography created by the prior 30 Ma of deformation. The ∼1 Ma top surface of the Afar Stratoid Series serves as a regional marker predating most Quaternary deformation. The initiation of a particular fault (or location along a graben-bounding fault) begins a process of local lowering of base level – this is recorded in the landscape as a knickpoint that incises the paleo drainage in the footwalls of the grabens, connecting it to the basin floor with a deep canyon. The rate at which knickpoints retreat upstream permits a space-for-time approach to assessing initiation. This study develops and applies a landscape evolution model to estimate relative maturity and age of drainage basins by measuring the long profile response to tectonic forcings over intermediate timescales in central Afar. This is applied under the assumption that faulting postdates Afar Stratoid Series emplacement. The model is calibrated by analysis of stream longitudinal profiles and terrestrial cosmogenic nuclide (TCN) analysis of samples collected in the field. Longitudinal stream valley profiles are mapped through remote sensing and photogrammetry, and TCN concentrations are assessed to calculate the uplift rate along the graben-bounding faults. Results indicate that faulting of Dobe Graben in the central Afar initiated in the southeast and propagated to the northwest. Along the southern margin of Dobe Graben, ages of 503 ± 116 ka were calculated for fault initiation at Hann Canyon in the southeast, and 339 ± 80 ka at Ishyilu Canyon to the northwest. These timing constraints have implications for competing tectonics models for central Afar. The general approach developed for modeling knickpoint retreat has broader utility beyond Afar, allowing for application to extensional tectonic regimes elsewhere where similar geological and surface constraints exist.

Complex erosional response to uplift and rock strength contrasts in transient river systems crossing an active normal fault revealed by 10Be and 26Al cosmogenic nuclide analyses

AbstractUnderstanding the influence of bedrock lithology on the catchment‐averaged erosion rates of normal fault‐bounded catchments and the effect that different bedrock erodibilties have on the evolution of transient fluvial geomorphology remain major challenges. To investigate this problem, we collected 18 samples for 10Be and 26Al cosmogenic nuclide analysis to determine catchment‐averaged erosion rates along the well‐constrained Gediz Fault system in western Türkiye, which is experiencing fault‐driven river incision owing to a linkage event ~0.8 Ma and has weak rocks overlying strong rocks in the footwall. Combined with existing cosmogenic data, we show that the background rate of erosion of the pre‐incision landscape can be constrained as <92 mMyr−1, and erosion rates within the transient reach vary from 16 to 1330 mMyr−1. Erosion rates weakly scale with unit stream power, steepness index and slip rate on the bounding fault, although erosion rates are an order of magnitude lower than slip rates. However, there are no clear relationships between erosion rate and relief or catchment slope. Bedrock strength is assessed using Schmidt hammer rebound and Selby Rock Mass Strength Assessments; despite a 30‐fold difference in erodibility, there is no difference in the erosion rate between strong and weak rocks. We argue that, for the Gediz Graben, the strong lithological contrast affects the ability of the river to erode the bed, resulting in a complex erosional response to uplift along the graben boundary fault. Weak covariant trends between erosion rates and various topographic factors potentially result from incomplete sediment mixing or pre‐existing topographic inheritance. These findings indicate that the erosional response to uplift along an active normal fault is a complex response to multiple drivers that vary spatially and temporally.

Constraints on the Fault Dip Angles of Lunar Graben and Their Significance for Lunar Thermal Evolution

Lunar grabens are the largest tensional linear structures on the Moon. In this paper, 17 grabens were selected to investigate the dips and displacement–length ratios (γ) of graben-bounding faults. Several topographic profiles were generated from selected grabens to measure their rim elevation, width and depth through SLDEM2015 (+LOLA) data. The differences in rim elevation (∆h) and width (∆W) between two topographic profiles on each graben were calculated, yielding 146 sets of data. We plotted ∆h vs. ∆W for each and calculated the dip angle (α) of graben-bounding faults. A dip of 39.9° was obtained using the standard linear regression method. In order to improve accuracy, large error data were removed based on error analysis. The results, 49.4° and 52.5°, were derived by the standard linear regression and average methods, respectively. Based on the depth and length of grabens, the γ value of the graben-bounding normal fault is also studied in this paper. The γ value is 3.6 × 10−3 for lunar normal faults according to the study of grabens and the Rupes Recta normal fault. After obtaining the values of α and γ, the increase in lunar radius indicated by the formation of grabens was estimated. We suggest that the lunar radius has increased by approximately 130 m after the formation of grabens. This study could aid in the understanding of normal fault growth and provide important constraints on the thermal evolution of the Moon.

Analogue modelling of basin inversion: implications for the Araripe Basin (Brazil)

Abstract. Basin inversion is a process that takes place when a sedimentary basin is subjected to compressional stresses resulting in the reactivation of pre-existing faults and/or the localization of deformation along new reverse faults. The Araripe Basin (NE Brazil) is an example of a Cretaceous intracontinental aborted rift, with its sedimentary infill found at ca. 1000 m altitude, 500 m above the host basement. Post-rift basin inversion has been proposed by previous authors as the cause of this topographic high, but how inversion affected this basin remains a matter of debate, with the following two end-member scenarios: reactivation of pre-existing normal faults leading to local uplift or regional uplift and differential erosion. Neither end-member fully explains the observations from seismic and field data. In this study, we, therefore, conducted analogue models to explore how basin inversion in the Araripe Basin could have taken place. We present two series of crustal-scale brittle–viscous experiments: (i) extension followed by compression without sedimentation, with a variation in divergence and convergence directions (orthogonal or 45∘ oblique); and (ii) extension with syn-rift sedimentation followed by compression, with the same variation in rifting and inversion directions. We found that orthogonal rifting without sedimentation forms throughgoing graben boundary faults, whereas oblique rifting initially creates en échelon faults that eventually link up, creating large graben boundary faults. Rift basins with syn-rift sedimentation evolved in a similar fashion; however, sedimentary loading resulted in increased subsidence. During both oblique and orthogonal inversion, most shortening was accommodated along new low-angle reverse faults. Significant intra-graben fault reactivation occurred in all models without syn-rift sedimentation. By contrast, orthogonal inversion of models with syn-rift sedimentation did not reactivate rift faults, whereas only a minor reactivation of rift faults took place during oblique inversion since the sediments strengthened the otherwise weakened basin, thus acting as a buffer during convergence. Based on our modelling results, we propose an alternative scenario for the evolution of the Araripe Basin, involving oblique inversion and the development of low-angle reverse faults, which better fits observations from seismic lines and field data from the region.

Andean evolution, orogenic deformation and uplift of the Western Cordillera and Altiplano of southern Peru, northern Bolivia and Chile: Eocene-Oligocene lithospheric delamination

Reappraisal of Andean evolution in southern Peru, northern Bolivia and northern Chile provides support for models of orogenic cyclicity in which Andean crustal shortening in the Western Cordillera led to Eocene-Oligocene lithospheric delamination which triggered a renewed pulse of Andean uplift. The sedimentary fill of the Arequipa "rift" back-arc basin (Rhaetian-Neocomian) was controlled by two major graben-bounding fault systems: the Condoroma-Caylloma and Cincha-Lluta-Incapuquio; as well as the Cusco-Lagunillas fault system separating the back-arc region from the Cusco-Puno high. These thinned cortical inherited structures, were the most favourable places to undergo a positive tectonic inversion at the beginning of Andean compression (Albian), and then during Late Cretaceous-Paleocene shortening, they continued to develop as east-vergent crustal decollements which allowed crustal doubling and Western Cordillera thickening. The crustal thickening of >45 km took place along the Condoroma-Caylloma inverted graben where the lower crust, which was probably enriched in basalts, was subjected to high-pressure metamorphism leading to eclogitization, and became denser than the underlying mantle lithosphere and therefore delamination occurred. The subsequent asthenospheric upwelling led to lithospheric thinning, asthenospheric mantle magmatism of the first stage of Andahuaylas-Yauri batholith emplacement (∼48-43 Ma), which crystallized under temperatures of 1200–700 °C and pressures of 1.0–0.5 GPa (>40-30 km deep). This process was followed by the rapid surface uplift of the Western Cordillera that reached 3.1 km and fed the Kayra and Lower Moquegua basins, located on the Altiplano and in the forearc respectively. The delamination process was still active during 43–30 Ma, with second-stage magmatism of the Andahuaylas-Yauri batholith, emplaced continuously from deep mafic intrusions (40-30 km) to superficial porphyries (3 km). This indicates an important uplift of the Western Cordillera to 3.3 km, supported by mantle upwelling and the continuous flat-slab compression that prevented normal faults from developing, and rather produced the shortening of the delaminated crust. The Anta-Soncco foreland basins (43-30 Ma) of the Altiplano and Moquegua basin of the forearc, received the products of Cordilleran denudation at a rate of up to 700 m/My. The alkaline potassic magmatism (29–27 Ma) could represent episodes of post-delamination magmatism and the late stage of flat subduction.

Discovery of the Qinghe fault, the inner part of the Weihe graben, central China, and its geological significance

The Weihe graben is an active Cenozoic continental rift with frequent seismic activity. The previous work focused on the graben boundary faults, but less work has been conducted on the faults in the internal graben. The Weihe basin is an economically developed and densely populated area. Therefore, identifying these potential risks is significant for the evaluation of regional seismic hazards and to understand the evolution of continental rifts. Prior work used oil exploration methods to document a fault (we named it “Qinghe fault”) in the Qinghe river area of Sanyuan County, north of Xi’an. But the shallow structure and activity was not clear. Therefore, we carried out shallow seismic detection and borehole exploration, as well as a geological survey in the area. Based on seismic reflection profile and boreholes, the Qinghe fault is the main fault and with other secondary faults, together they constitute a typical Y-shaped structure, with a fracture zone width ∼4 km. Combined with the field survey, we provide evidence that the Qinghe fault is a Holocene active fault and capable of earthquakes of magnitude 5.2–5.5. The discovery of the Qinghe fault shows that the Weihe rift is active, and the faults inside the rift play an important role in tectonic deformation.

Rift propagation in south Tibet controlled by under-thrusting of India: a case study of the Tangra Yumco graben (south Tibet)

Active graben systems in south Tibet and the Himalaya are the surface expression of ongoing east–west extension, although the cause and spatiotemporal evolution of normal faulting is a still a matter of debate. We reconstructed the exhumation history driven by normal faulting in the southern Tangra Yumco graben using new thermochronological data. The Miocene cooling history of the footwall of the main graben-bounding fault is constrained by zircon (U–Th)/He ages (16.7 ± 1.0 to 13.3 ± 0.6 Ma), apatite fission track ages (15.9 ± 2.1 to 13.0 ± 2.1 Ma) and apatite (U–Th)/He ages (7.9 ± 0.4 to 5.3 ± 0.3 Ma). Thermo-kinematic modelling of the data indicates that normal faulting began 19.0 ± 1.1 Ma at a rate ofc.0.2 km myr−1and accelerated toc.0.4 km myr−1atc.5 Ma. In the northern Tangra Yumco rift, remodelling of published data shows that faulting startedc.5 myr later at 13.9 ± 0.8 Ma. The age difference and the distance of 130 km between these two sites indicates that rifting and normal faulting propagated northward at an average rate ofc.25 km myr−1. As this rate is similar to the Miocene convergence rate between India and south Tibet, we argue that the under-thrusting of India beneath Tibet exerted an important control on the propagation of rifts in south Tibet.Supplementary material: Figures S1, S2, and Table S1 with details on apatite fission track analysis are available athttps://doi.org/10.6084/m9.figshare.c.6198584

Modeling of dike-induced graben nucleation in the Elysium region, Mars: The role of planetary gravity

We have studied the processes of dike-induced graben nucleation through the analytical modeling of the failure sequence, i.e., the timing, location, and mode, of the discontinuities formed between the dike's upper tip and the surface. To this end we use as input parameters dike geometry and the elastic and frictional properties of a homogeneous medium. We applied this analytical model to three dike-induced graben in the Elysium region of Mars; Galaxias, Elysium, and Cerberus Fossae, some of which have been found to show active extensional tectonics. Firstly, we calculated dike aperture and depth from graben topography using an area balance technique. At each site we modeled with different combinations of input parameters which resulted in a total 27 models, of which 14 were found to be compatible with the inferred present-day graben-bounding faults. Based on these results we propose two conceptual models of dike-induced graben nucleation: 1) in a shallow dike intruding a low-compliance host rock, nucleation occurs through the linkage of near-surface mode I cracks and mode II discontinuities propagated from the dike tip, and 2) in a deep dike intruding a stiff host rock, shallow mode I cracks propagate to depth and collapse into normal faults in a dominantly tensile regime. A comparison of equivalent models under Martian versus Terrestrial conditions shows that in the former case failure is more likely to occur in a tension-dominated regime due to the greater weight of the dike induced stresses in a medium with a reduced lithostatic load. On Mars, dense networks of open fractures may have facilitated large fissure eruptions. On Earth, tensile failure occurs at very shallow depths whilst faults accommodate most of the deformation at depth. Our analytical modeling methodology explains intrusion experiments and observed dike-induced deformation. Therefore, the models provide plausible mechanisms of graben nucleation above intruding dikes. These need to be complemented by models which simulate long-term graben subsidence above dikes, which may be key to understand seismicity in tectonomagmatically active regions of Mars.

From metamorphic core complex to crustal scale rollover: Post-Caledonian tectonic development of the Utsira High, North Sea

The Utsira High (North Sea) records rift faulting that culminates with Jurassic-Cretaceous crustal-scale rollover towards the Viking Graben. This deformation is superimposed on Paleozoic fault-bounded basins on a substrate of Caledonian nappes. The latter contains multilevel crustal-scale shear systems that domed under the Utsira High during the Devonian, as indicated by mapping and interpretation of a large long-offset 2D seismic reflection dataset. Restorations show that isostatically driven doming from excision of overthickened crust caused uplift and erosion of basins and the underlying nappe stack. Doming took place above a crustal rollback system as a symmetrical metamorphic core complex formed. The dome discloses vertical flattening kinematics below bi-directional shear systems, evident by opposite shear-fabric kinematics on opposite dome flanks. A major detachment marks the upper boundary of a transition between upper and lower plate strain regimes, coinciding with low-reflective granitic units above strongly reflective Caledonian nappes. This detachment hosts transport-parallel corrugations that strike E-W to NE-SW. A series of deeply eroded half-grabens on top of the upper plate are bounded by pre-Permian faults exhibiting top-NE extensional kinematics. Faults sole out in the fundamental detachment level, locating the brittle-ductile transition, with all structures subsequently rotated during doming. A new localized detachment formed at a shallower level above the region of maximum crustal uplift, seen as upwards detachment migration driven by heating. Later faulting locates to the dome flanks, either rejuvenating rotated shear zones or cutting to deeper crustal levels while recording predominantly down-eastward transport. During Jurassic-Cretaceous rifting, deformation localized to the Viking Graben Boundary Fault, giving room for thick growth wedges in the Viking Graben and establishing the Utsira High as a crustal-scale rollover structure.

Ambient noise tomography of local shallow structure of the southern segment of Tanlu fault zone at Suqian, eastern China

Comparable to San Andreas fault in western United States, the Tanlu fault zone (TLFZ) is a large-scale continental tectonic belt in eastern China and has played an important role controlling the regional tectonics and earthquake activity in eastern China. It is known there exists an earthquake gap in the southern segment around Suqian, Jiangsu province, where no earthquakes greater than M5.5 occurred. Fine fault zone structure could help to understand the fault activity and the potential earthquake hazards. However, only regional scale velocity model is available in this region, making it difficult to characterize detailed fault structures. In this study, we obtained local Vs model in the upper crust in this region by ambient noise tomography with a dense seismic array. For the Suqian segment, Cretaceous brittle normal faulting controlled the development of the eastern and western graben as well as the central horst between them. The graben-boundary faults are known simply as F1-F4 from east to west, and the NNE-striking fault F5 is between F1 and F2. The eastern boundary fault F1 and western boundary fault F4 separated by about 30 km are associated with intermittent high velocity anomalies in the shallow crust and are inactive. In comparison, faults F2, F5 and F3 are mainly associated with low velocity anomalies and are relatively active. These high velocity anomalies are very likely caused by the upwelling magma migrated along faults F1 and F4 in the crust, which on freezing could weld the fault zone and make it less active. Because faults F1 and F4 are hindered to move, the deformation could be transferred to nearby faults F2, F3 and F5. Our study sheds light on what controls fault activities along the TLFZ by obtaining local velocity structure in the Suqian segment.

Geometry, kinematics, and magnitude of extension across the Thakkhola Graben, Central Nepal Himalaya

The Thakkhola Graben has been a region of geologic inquiry for many decades. Although it is widely viewed to be in a class of structures that are important in accommodating the three-dimensional strain within the Himalayan thrust wedge, we still lack a detailed understanding of the total finite strain accommodated by graben-bounding faults, as well as their shape and cross-cutting relationships with structures deeper in the thrust wedge. Using geologic mapping and structural analysis, we show that a suite of pre-extensional shortening structures is offset by normal-oblique faults bounding the Thakkhola Graben that we use to define a piercing line. We calculate these faults to have accommodated 8.7 kilometers of vertical thinning, 7.2 kilometers of arc-perpendicular shear, and only 2.2 kilometers of arc-parallel extension. The magnitude of arc-parallel extension is quite low compared to extensional structures to the west in the Gurla Mandhata-Humla region. The cross-cutting relationships established in this study and timing constraints determined by previous works are consistent with a structural history of crustal thickening leading to foreland propagation of the locus of arc-perpendicular shortening contemporaneous with hinterland extension.

Tectonics of the Wysoka Kamieńska Graben (NW Poland) and implications for fault sealing potential

Reservoir confinement by faults is important for safe storage of liquid waste or hydrocarbons. Having access to 3D seismic and borehole data, we have interpreted the tectonic setting of the Wysoka Kamieńska Graben (WKG) in the NW part of the Polish Basin and subsequently made an interpretation of the sealing potential of the graben-bounding faults. The formation and development of the graben in the Late Triassic and Early Jurassic was controlled by mechanical decoupling in the salts of the Zechstein Group. The primary tectonic factor triggering the graben origin was dextral strike-slip movement along the regional fault zone in the Paleozoic basement, transtensional accommodation of which in the Zechstein-Mesozoic cover led to development of a horse-tail pattern of grabens. During the Late Cretaceous, the graben underwent minor tectonic inversion. Sealing potential analysis of the graben-bounding faults was performed for the Triassic–Jurassic sequence including juxtaposition seal and fault gouge seal components. Finally, we have focussed our interpretation on the Jurassic sequence where the best reservoirs have been recognized. Our results indicate good to moderate sealing potential of the Hettangian reservoir, poor to moderate sealing of the Pliensbachian reservoir and lack of sealing of the Bajocian reservoir. Hence, the Hettangian reservoir, characterized by large thickness, low clay content and a large regional extent, acts as a potential storage formation, being confined by the graben-bounding faults of the WKG

Geothermal potential of the eastern end of the Gediz basin, western Anatolia, Turkey revealed by three-dimensional inversion of magnetotelluric data

Three-dimensional models of the crustal electrical resistivity were derived from the inversion of magnetotelluric data in the eastern end of the Gediz basin, western Anatolia. In total, 179 MT sites measured in three different areas have been used in modeling. The study aims at providing information about the electrical resistivity distribution in the subsurface and its relations to structural setting and fluid pathways mainly controlling the development of geothermal reservoirs in the upper crust. The electrical resistivity models reveal a surficial low-medium resistivity layer associated with Neogene-Quaternary supradetachment sedimentary succession and a deeper high resistivity layer related to the Central Menderes Massif. The sedimentary cover over the metamorphic basement includes undulations, presenting an asymmetric extensional basin. The thickness of the sedimentary layer varies along strike from west to east and reaches approximately 3 km (max) in the graben. An anomalously conductive zone associated with a clay alteration zone was observed in this layer. The resistivity models also bring out two deep conductive zones along the southern margin of the basin: a conductive zone that could be interpreted as a geothermal reservoir at the easternmost end of the basin, however instead described as a part of a deep low resistivity zone with regional extent and another conductive zone interpreted as a region possessing an interconnected fracture network developed mainly by the extensional tectonic regime and that probably contains geothermal fluids. Moreover, normal faults exposing along the basin margins characterize the geometry of the basin, and particularly the main graben-bounding fault and Gediz detachment fault provide possible fluid pathways in the upper crust.

An integrated geophysical, hydrological, thermal approach to finite volume modelling of fault-controlled geothermal fluid circulation in Gediz Graben

Many high temperature geothermal fields generally occur at settings of recent active tectonism or volcanism accompanied by the active faults and fractures. It is well known that the structural controls such as topography, active faults, etc., have a major effect on fluid flow pathways in those systems. In this paper, a complete hydro-thermo-geophysical model is created for the first time in the Gediz Graben, Western Anatolia. The finite volume method is used for numerical simulations by implementing a finite volume code, ANSYS-Fluent. The thermal and physical rock properties used in the model are taken from previous studies. Fluid flow velocity vectors and resulted temperature patterns for the region are calculated and presented. Our simulations demonstrate that the low-angle Master Graben Boundary Fault (MGBF) has three dominant roles 1) transporting the meteoric water to the depths; 2) distributing the heated geothermal water into the basin with inner basin faults, 3) transmitting the heated water to the surface. The model in this work can be easily adopted and extended to explore the possible reservoir structures in other geothermal areas.

Extension and inversion of salt-bearing rift systems

Abstract. We used physical models to investigate the structural evolution of segmented extensional rifts containing syn-rift evaporites and their subsequent inversion. An early stage of extension generated structural topography consisting of a series of en-échelon graben. Our salt analog filled these graben and the surroundings before continued extension and, finally, inversion. During post-salt extension, deformation in the subsalt section remained focused on the graben-bounding fault systems, whereas deformation in suprasalt sediments was mostly detached, forming a sigmoidal extensional minibasin system across the original segmented graben array. Little brittle deformation was observed in the post-salt section. Sedimentary loading from the minibasins drove salt up onto the footwalls of the subsalt faults, forming diapirs and salt-ridge networks on the intra-rift high blocks. Salt remobilization and expulsion from beneath the extensional minibasins was enhanced along and up the major relay or transfer zones that separated the original sub-salt grabens, forming major diapirs in these locations. Inversion of this salt-bearing rift system produced strongly decoupled shortening belts in basement and suprasalt sequences. Suprasalt deformation geometries and orientations are strongly controlled by the salt diapir and ridge network produced during extension and subsequent downbuilding. Thrusts are typically localized at minibasin margins where the overburden was thinnest, and salt had risen diapirically on the horst blocks. In the subsalt section, shortening strongly inverted sub-salt grabens, which uplifted the suprasalt minibasins. New pop-up structures also formed in the subsalt section. Primary welds formed as suprasalt minibasins touched down onto inverted graben. Model geometries compare favorably to natural examples such as those in the Moroccan High Atlas.

On the Use of Aeromagnetism for Geological Interpretation: 2. A Case Study on Structural and Lithological Features in the Northern Vosges

AbstractIn many grabens of the West European Rift System, several drilling projects are targeting fluid conductive faults to produce electricity with geothermal energy. In order to reach the adequate temperatures, the targeted faults are located in the basement rocks or at the basement/cover interface. Unfortunately, the basement rock facies and structural data useful for reservoir modeling are often poorly constrained because of the lack of boreholes deeper than 2,000 m and seismic information on the deep subsurface. With the aim to outline the basement structure in the Upper Rhine Graben, two aeromagnetic surveys were conducted where the basement outcrops in the Northern Vosges, with different flight parameters and types of magnetometers. An interpolator using equivalent sources for gridding these two surveys is described by Gavazzi et al. (2020). Here, we describe the geological interpretation of these surveys. We demonstrate the consistency between the total magnetic field intensity and the outcropping lithological units organized in ENE‐WSW subvertical bands. In detail, three main blocks were delineated, separated by N‐S faults linked to the graben formation and ENE‐WSW‐inherited Hercynian faults. The latter faults, together with more local NE‐SW and NW‐SE faults, compartmentalize the N‐S‐oriented basement rocks. This is relevant for geothermal prospection where the N‐S faults are the main targets because fluid flow through other fault systems is deemed restricted. With a focus on the interface between the Northern Vosges and the Upper Rhine Graben, we also demonstrate considerable segmentation on the main graben boundary faults.

The 6 November 1774 MI 6 Bolaños Graben Earthquake (Southern Basin and Range Province, West-Central Mexico): Macroseismic Observations and Neotectonic Implications

AbstractThe continental part of west-central Mexico is characterized by the active extensional tectonic regimes of the Trans-Mexican volcanic belt and the adjacent southern Basin and Range Province. The deformation of the latter is distributed over several topographically very pronounced grabens and half-grabens (width 10–20 km, length ≤200 km; throw 1–2 km), including the Aguascalientes, Juchipila, Tlaltenango, and Bolaños grabens. Here, an A.D. 1774–1775 earthquake series in that area is documented based on numerous contemporary sources. The 6 November 1774 mainshock caused moderate-to-severe damage in several communities of the Bolaños graben, including the silver mining town of Bolaños, and moderate damage to communities in the Tlaltenango graben, such as the administrative center of Colotlán. Based on the macroseismic intensity distribution, the epicenter was in the Bolaños graben. The preferred magnitude of the mainshock is ∼6.0±0.5. No major historical earthquake had been reported previously from this region. Existing ground-shaking hazard models may, therefore, give a false sense of security.In the Bolaños graben, motion along the graben-bounding faults and the observed tilting of the graben shoulders has to be mostly younger than the 19.9 Ma age of the youngest basalt of the graben-shoulder stratigraphy. Its correlation across the western master fault indicates a 1300 m throw and a vertical long-term slip rate of 0.07 mm/yr. The observations of alluvial fan deposits juxtaposed against the footwall ignimbrites along the western master fault of the Bolaños graben, the displacement of alluvial fan deposits along secondary faults within the graben, and the existence of hot springs along the western boundary fault all are indicative of active deformation, and so is evidently the A.D. 1774 earthquake.

The effect of active extensional tectonics on the structural controls and heat transport mechanism in the Menderes Massif geothermal province: Inferred from three-dimensional electrical resistivity structure of the Kurşunlu geothermal field (Gediz Graben, western Anatolia)

The current study in the Menderes Massif geothermal province aims to assert a crustal-scale geoelectrical evidence showing the role of extensional tectonics on the flow of geothermal fluids through both high-angle and low-angle normal (detachment) faults and on the heat transport mechanism in the convection dominated-amagmatic geothermal play systems, on the basis of three-dimensional electrical resistivity structure of the Kurşunlu geothermal field, Gediz Graben, western Anatolia. The electrical resistivity structure was derived from the three-dimensional inversion of the magnetotelluric data acquired from 74 sites in the period range from 0.001 s to 1000s. The resistivity model brings out two different types of reservoirs, namely (i) a shallow low resistivity reservoir (secondary reservoir) corresponding to the Kurşunlu hot spring in the sedimentary layer of the Gediz Graben and (ii) a deep more resistive reservoir (primary reservoir) beneath a prominent highly conductive (≤ 10 ohm-m) hydrothermal clay alteration zone. The circulation of geothermal fluids in reservoirs is dominantly controlled by high-angle normal faults including the main graben-bounding fault and low-angle Gediz detachment fault. The possible heat source for the geothermal system has been attributed to high heat arising from upwelling asthenosphere through the mantle window in the northward subducting and retreating African slab, which has been accompanied by crustal extension resulting in a thin crust and shallow mantle, and the deeply penetrating main graben-bounding fault acting as a conduit through which heat is transported from the deeper parts of the crust to near surface probably controls heat transport in the area.

Full-waveform event location and moment tensor inversion for induced seismicity

Locating microearthquake events below complex heterogeneous overburden requires robust location methodologies that can honor multipathing in the seismic wavefield. We have developed two full-waveform event location methods that form a complementary solution for locating earthquakes and simultaneously deriving focal mechanisms via moment tensor inversion. The methods are based on the application of 3D elastic wavefield modeling, which is used to generate waveforms and extract wavefield attributes, for comparison to the observed field data. Events are located and focal mechanisms are derived via a multiparameter inversion, which minimizes the differences between synthetic and observed data. The results have been applied to the induced seismicity observed within the giant Groningen gas field, onshore Netherlands, where recorded earthquakes are triggered by stress changes, induced in the reservoir through pressure depletion. Locating events below the field is compounded by the presence of strong guided waves, which are trapped in the lower velocity reservoir interval. This complex multivalued wavefield is problematic for traditional event location methods, which assume a single traveltime arrival. We overcome this limitation by using all event arrivals in a wave-based solution to improve the accuracy of locating earthquakes and overcome the ambiguity of solving for location and the focal mechanism simultaneously. The event location methods have been applied to shallow and deep monitoring networks, and 150 events have been located with high accuracy. The interpretation of the earthquake activity indicates that the events studied originate from the movement of larger graben bounding faults, which are oriented in a north-northwest–south-southeast direction.

EVOLUTION OF THE NASIELSK–DĘBE GRABEN (EASTERN CENTRAL POLAND) DURING CRETACEOUS TIMES

The paper portrays the geological evolution of the Nasielsk–Dębe tectonic graben during Cretaceous times. The analysis is based on data from the Dębe 1, 2, 5, 6, and 7 and Nasielsk 1 and 2 boreholes. Only well logs, archived borehole materials (drill core description) and published data have been used because the drill cores are no longer available. The graben’s Cretaceous evolutionary stages have been identified based on thickness analysis, and correlated to depositional cycles. The analysis shows that greater thickness gradients are observed at the north-eastern graben-bounding fault, which indicates its higher activity compared to the south-western fault throughout nearly the entire Cretaceous. The upper part of the Cretaceous succession reveals the presence of at least two discontinuity surfaces (possibly even hardgrounds): approximately at the Campanian/Maastrichtian boundary, and near the lower Mastrichtian/upper Maastrichtian boundary. It is suggested to distinguish two separate lowest-order cycles within the lower-order cycle K4-IV: cycle K4-IVa correlated approximately with the late early Maastrichtian, and cycle K4-IVb corresponding roughly to the early late Maastrichtian.