Normal Dip-slip Research Articles

Late Cretaceous coeval multi-directional extension in South Zealandia: Implications for eastern Gondwana breakup

Abstract Gondwana continental crust in South Zealandia was deformed by Cretaceous rift faults with multiple orientations. Our interpretation of 2D and 3D seismic reflection lines, tied to wells in the Canterbury Basin of South Zealandia, shows three coeval sets of faults with trends of NE-SW, E-W and NW-SE. Geometric and displacement analyses of segmented faults from each set suggests that they mainly accommodated normal dip-slip, producing multi-directional stretching . Growth strata indicate that the three fault sets were active synchronously over ~20 Myr from ~105 to ~85 Ma. The fault sets are present throughout Zealandia and roughly parallel to spreading centres produced during Gondwana breakup, primarily defining the present-day margins of Zealandia. Faults trending NE-SW and E-W are parallel to the mid-ocean ridge separating South Zealandia and western Antarctica, ~800–1000 km from the Canterbury Basin, while NW-SE rift faults are sub-parallel to the spreading centre between Australia and North Zealandia. Sub-parallelism between spreading centres and rift faulting is consistent with a model in which the multiple directions of extension produced mild ( 500 km) from these centres ceased or continued at much diminished rates.

Sequential modelling of the 2016 Central Italy earthquake cluster using multisource satellite observations and quantitative assessment of Coulomb stress change

SUMMARYA sequence of earthquake events consisting of three large shocks occurred in Central Italy from August to October in 2016 with the duration of almost 2 months. The preliminary study on the seismic mechanism suggests that the sequence of events is the result from the activity of the SW dipping Mt Bove–Mt Vettore–Mt Gorzano normal fault system. For investigation and understanding of the coseismic faulting of the seismogenic fault alignment, we collect a set of comprehensive satellite observations including the Sentinel-1A, ALOS-2/PALSAR-2 and GPS data to map the coseismic surface deformation and estimate the source models in this study. The derived faulting model for the first Amatrice event is characterized by two distinct slip asperities suggesting that it is a predominantly normal dip-slip motion with slight strike-slip component. The second event, Visso earthquake is almost a purely normal rupture. The third Norcia event is dominated by the normal dip-slip rupture of the seismogenic fault, and has propagated up to the ground with significant slip. The three faulting models are then utilized to quantify the Coulomb failure stress (CFS) change over the seismic zone. First, the CFS change on the subsequent two seismogenic faults of the earthquake sequence is estimated, and the derived positive CFS change induced by the preceding earthquakes suggests that the early events have positive effects on triggering the subsequent seismicity. We then explore the response relation of the aftershocks including 961 events with magnitudes larger than M 3.0 to the CFS change over the seismic zone. It suggests that the rupture pattern of the aftershocks is similar to the major shocks with predominantly normal dip-slip. To assess the risk of the future seismic hazard, we analyse quantitatively the spatial distribution of aftershock occurrence and CFS transfer at the seismogenic depth, indicating that the ruptures of the three major shocks do partly release the accumulated strain on the associated fault alignment as well as the dense aftershock, but the CFS increase zone with few aftershocks in the southwest of the eastern Quaternary fault alignment of Central Italy poses the potential of further rupture. In particular, the distribution of aftershock migration also suggests that the north extension of the Mt Bove fault is the potential zone with rupture risk.

Structural controls on Jurassic gold mineralization, and Cretaceous-Tertiary exhumation in the foreland of the southern Patagonian Andes: New constraints from La Paloma area, Deseado Massif, Argentina

Fluid-rock interaction occurring within high-permeability conduits represented by fault-fracture meshes is well known to control the development of hydrothermal ore deposits. In this study we document the fundamental role of four sets of precursor joints, reactivated as dilational faults displaying variable normal dip-slip and strike-slip components of motion, in the formation of epithermal Au-Ag deposits of economic interest. Paleostress analysis unravels how all the resulting four sets of hybrid extension-shear fractures are kinematically compatible with an extensional stress field characterized by multiple permutations between the σ3 and the σ2 axes. Such permutations are interpreted to be associated with dominantly normal fault reactivation of suitably oriented early joint sets as a result of continued regional extension, and by orthogonal stretching produced by differential downthrow of the hanging-wall blocks of laterally-terminating Jurassic normal faults.Later fault reactivation appears to be associated with far field propagation of Andean stresses, marked by superposed strike-slip and reverse fault slip components recording roughly E-W horizontal compression. This late fault reactivation could be related with a late Early Cretaceous shortening event known to have affected the studied sector of the Deseado Massif. The subsequent tectonic evolution of the study area occurred within the general framework of a slow Cretaceous to Tertiary exhumation recorded by apatite fission track cooling ages. Our new thermochronometric results, recording steady-state cooling at a rate of c. 1 °C/Ma during the last 90 Ma, shed new light onto the post-Jurassic geodynamic evolution of the foreland of the southern Patagonian Andes.

Tectonic control on vein attributes and deformation intensity in fault damage zones affecting Natih platform carbonates, Jabal Qusaybah, North Oman

Understanding the factors that control fracture patterns in fault damage zones is fundamental to predicting fault zone permeability in the subsurface. In this contribution, we present outcrop data on vein attributes collected from 26 fault zones (10 strike-slip and 16 normal dip-slip) that cut Cretaceous Natih Formation platform carbonates exposed in the Jabal Qusaybah anticline, North Oman. Faulting occurred during the growth of the salt-cored anticline and progressed initially from dominant strike-slip faults (burial depth ∼3–4 km) to late normal dip-slip faults (burial depth <1–2 km). The displacements accommodated by both kinematic fault types range similarly from 0.1 to 100 m, and damage-zone width increases with displacement at the same rate for both types. Vein aperture (A), height (H), and spacing (S) were measured in vertical cross-sections (n = 10839 data) along fault-perpendicular, linear scanlines across fault damage zones. Data analyses indicate that, as the master slip surface is approached in each fault zone: (1) vein aperture and height generally increase; (2) vein spacing systematically decreases; and (3) deformation intensity, calculated as vein H/S ratio, increases. However, median H/S values calculated in each damage zone do not show a robust correlation with fault displacement. When analyzed collectively across-fault distributions, H/S ratios indicate that deformation intensity (i) in normal dip-slip fault damage zones is greater than in strike-slip fault damage zones; (ii) in strike-slip fault damage zones is symmetrical with respect to the master slip surface; and (iii) is locally asymmetrically distributed with greater deformation intensity in footwall blocks for normal dip-slip faults. Greater deformation in normal dip-slip fault zones is expressed by greater vein height, rather than smaller vein spacing. The main conclusion is that deformation intensity and vein attributes in fault damage zones are primarily controlled by burial stress and fault kinematics, and less importantly by rheological contrasts between layers, rather than local stress induced by fault displacement.

Coseismic Slip Vectors of 24 August and 30 October 2016 Earthquakes in Central Italy: Oblique Slip and Regional Kinematic Implications

AbstractAfter 24 August and 30 October 2016 central Italy earthquakes (Mw 6.0 and 6.5, respectively), photogrammetry and geodetic survey were performed at various sites along a 6‐km‐long portion of the rupturing Monte Vettore fault system, providing very high‐resolution georeferenced 3‐D point clouds and imagery of the 24 August rupture and a data set of bedrock fault scarp before/after the 30 October earthquake. The maximum coseismic displacement for both events occurs near Scoglio dell' Aquila with an average normal dip slip of 22 ± 4 and 184 ± 6 cm, respectively. Coseismic slip vectors and meter‐scale corrugation axis are oriented N280 ± 10° on the ~N140° striking main Vettore fault involving oblique normal slip with a right‐lateral component and N205 ± 10° on ~N170° fault strands implying oblique normal slip with a left‐lateral component. We quantify the near‐field coseismic displacements of the Monte Vettore fault for the 30 October event: the footwall has been translated horizontally by 42 ± 2 cm toward the ENE and uplifted by 11 ± 2 cm. The hangingwall has moved horizontally by 26 ± 2 cm, toward the NW, in a direction parallel to the fault plane and subsided by 116 ± 2 cm. The fault geometry and our determined surface coseismic slip vectors are mechanically compatible with a σ3 = N65 ± 15°. This stress regime is consistent with the 30 October 2016 focal mechanism and the relative motion between the Adriatic microplate and the Tyrrhenian coastal region. The 30 October surface rupture results from seismic slip at depth and thus has a tectonic origin.

Quaternary Activity of the Monastir and Grombalia Fault Systems in the North‒Eastern Tunisia (Seismotectonic Implication)

The Monastir and Grombalia fault systems consist of three strands that the northern segment corresponds to Hammamet and Grombalia faults. The southern strand represents Monastir Fault also referred to as the Skanes-Khnis Fault. These NW-trends are observed continuously in the major outcropping features of north-eastern Tunisia including both the Cap Bon peninsula and the Sahel domain. Along the Hammamet Fault, the north-eastern strand of Grombalia fault system, left lateral drainage offset of amount 220 m is found in Fawara valley. To the South, the left lateral movement is occurred along the Monastir Fault based on 180 m of Tyrrhenian terrace displacement. Field observations supported by satellite images suggest that the Monastir and Grombalia fault systems appear to slip mostly laterally with components of normal dip slip. Assuming the development of the stream networks during the Riss-Wurm interglacial (115000–125000 years) and the age of the Tyrrhenian terrace (121 ± 10 ka), the strike slip rates of the Hammamet and Monastir faults are calculated in the range of 1.5–1.8 mm/yr. There vertical slip rates are estimated to be 0.06 and 0.26 mm/yr, respectively. These data are consistent with the displacement rate in the Pelagian shelf (1–2 mm/yr) but they are below the convergence rate of African-Eurasian plates (8 mm/yr). Our seismotectonics study reveals that a maximum earthquake of Mw = 6.5 could occur every 470 years in the Hammamet fault zone and Mw = 6–every 263 years in the Monastir fault zone.

A transitional volume beneath the Sannio-Irpinia border region (southern Apennines): Different tectonic styles at different depths

In this paper we investigate the border between the Sannio and Irpinia seismogenic regions, a sector of the southern Apennine chain considered among the most active seismic areas of the Italian peninsula, to shed further light on its complex seismotectonic setting. We integrated recent seismicity with literature data. A detailed analysis of the seismicity that occurred in the 2013–2016 time interval was performed. The events were relocated, after manual re-picking, using different approaches. To retrieve information about the stress field active in the area, inversion of Fault Plane Solutions was also carried out. Hypocentral distribution of the relocated events (ML≤3.5), whose depth is included between 5 and 25km with the deepest ones located in the NW sector of the study area, shows a different pattern between the northern sector and the southern one. The computed Fault Plane Solutions can be grouped in three depth ranges: <12km, dominated by normal dip-slip kinematics; 12–18km, characterized by normal dip-slip and strike-slip kinematics; >18km, dominated by strike-slip kinematics. Stress field inversion across the whole area shows that we are dealing with an heterogeneous set of data, apparently governed by distinct stress fields. We built an upper crustal model profile through integration of geological data, well logs and seismic tomographic profiles. Our results suggest the co-existence of different tectonic styles at distinct crustal depths: the upper crust seems to be affected mostly by normal faulting, whereas strike-slip faulting prevails in the intermediate and lower crust. We infer about the existence of a transitional volume, located between 12 and 18km depth, between the Sannio and Irpinia regions, acting as a vertical transfer zone.

Postseismic deformation associated with the 2008 Mw 7.9 Wenchuan earthquake, China: Constraining fault geometry and investigating a detailed spatial distribution of afterslip

We reconstruct two types of fault models associated with the 2008 Mw 7.9 Wenchuan earthquake, one is a listric fault connecting a shallowing sub-horizontal detachment below ∼20km depth (fault model one, FM1) and the other is a group of more steeply dipping planes further extended to the Moho at ∼60km depth (fault model two, FM2). Through comparative analysis of the coseismic inversion results, we confirm that the coseismic models are insensitive to the above two type fault geometries. We therefore turn our attention to the postseismic deformation obtained from GPS observations, which can not only impose effective constraints on the fault geometry but also, more importantly, provide valuable insights into the postseismic afterslip. Consequently, FM1 performs outstandingly in the near-, mid-, and far-field, whether considering the viscoelastic influence or not. FM2 performs more poorly, especially in the data-model consistency in the near field, which mainly results from the trade-off of the sharp contrast of the postseismic deformation on both sides of the Longmen Shan fault zone. Accordingly, we propose a listric fault connecting a shallowing sub-horizontal detachment as the optimal fault geometry for the Wenchuan earthquake. Based on the inferred optimal fault geometry, we analyse two characterized postseismic deformation phenomena that differ from the coseismic patterns: (1) the postseismic opposite deformation between the Beichuan fault (BCF) and Pengguan fault (PGF) and (2) the slightly left-lateral strike-slip motions in the southwestern Longmen Shan range. The former is attributed to the local left-lateral strike-slip and normal dip-slip components on the shallow BCF. The latter places constraints on the afterslip on the southwestern BCF and reproduces three afterslip concentration areas with slightly left-lateral strike-slip motions. The decreased Coulomb Failure Stress (CFS) change ∼0.322 KPa, derived from the afterslip with viscoelastic influence removed at the hypocentre of the Lushan earthquake, indicates that the postseismic left-lateral strike-slip and normal dip-slip motions may have a mitigative effect on the fault loading in the southwestern Longmen Shan range. Nevertheless, it is much smaller than the total increased CFS changes (∼8.368 KPa) derived from the coseismic and viscoelastic deformations.

Late Quaternary Activity of the Huashan Piedmont Fault and Associated Hazards in the Southeastern Weihe Graben, Central China

The Weihe Graben is not only an important Cenozoic fault basin in China but also a significant active seismic zone. The Huashan piedmont fault is an important active fault on the southeast side of the Weihe Graben and has been highly active since the Cenozoic. The well–known Great Huaxian County Earthquake of 1556 occurred on the Huashan piedmont fault. This earthquake, which claimed the lives of approximately 830000 people, is one of the few large earthquakes known to have occurred on a high–angle normal fault. The Huashan piedmont fault is a typical active normal fault that can be used to study tectonic activity and the associated hazards. In this study, the types and characteristics of late Quaternary deformation along this fault are discussed from geological investigations, historical research and comprehensive analysis. On the basis of its characteristics and activity, the fault can be divided into three sections, namely eastern, central and western. The eastern and western sections display normal slip. Intense deformation has occurred along the two sections during the Quaternary; however, no deformation has occurred during the Holocene. The central section has experienced significant high–angle normal fault activity during the Quaternary, including the Holocene. Holocene alluvial fans and loess cut by the fault have been identified at the mouths of many stream valleys of the Huashan Mountains along the central section of the Huashan piedmont fault zone. Of the three sections of the Huashan piedmont fault, the central section is the most active and was very active during the late Quaternary. The rate of normal dip–slip was 1.67–2.71±0.11 mm/a in the Holocene and 0.61±0.15 mm/a during the Mid–Late Pleistocene. As is typical of normal faults, the late Quaternary activity of the Huashan piedmont fault has produced a set of disasters, which include frequent earthquakes, collapses, landslides, mudslides and ground fissures. Ground fissures mainly occur on the hanging–wall of the Huashan piedmont fault, with landslides, collapses and mudslides occurring on the footwall.

Modeling ground deformation associated with the destructive earthquakes occurring on Mt. Etna's southeastern flank in 1984

Abstract. The Timpe Fault System is the source of very shallow but destructive earthquakes that affect several towns and villages on the eastern flank of Mt. Etna (Italy). In 1984, several seismic events, and specifically on 19 and 25 October, caused one fatality, 12 injuries and produced serious damage in the Zafferana and Acireale territories. This seismicity was mainly related to the activity of the Fiandaca Fault, one of the structures belonging to the Timpe Fault System. We inverted ground deformation data collected by a geodimeter trilateration network set up in 1977 at a low altitude along the eastern side of the volcano in order to define the Timpe Fault System faulting mechanisms linked to the seismicity in 1984. We have found that in the period May 1980–October 1984, the Fiandaca Fault was affected by a strike-slip and normal dip-slip of about 20.4 and 12.7 cm respectively. This result is kinematically consistent with field observations of the coseismic ground ruptures along the fault but it is notably large compared to displacements estimated by seismicity, then suggesting that most of the slip over the fault plane was aseismic. The results once again confirm how seismicity and its relation with ground ruptures and creep displacement represent a very high hazard to the several towns and villages situated along the Timpe Fault System.

Seismicity and focal mechanisms of earthquakes in Egypt from 2004 to 2011

The earthquake activity and the state of stress in and around Egypt will provide an opportunity to evaluate the seismic hazard. The seismicity data were compiled from the Egyptian National Seismological Network database during the period from 2004 to 2011 in an attempt to identify the different seismic source regions. Thirteen seismic source regions have been identified in this study.The focal mechanisms for 36 earthquakes in and around Egypt are constructed for the same period using the waveform data recorded by the Egyptian National Seismological Network (ENSN) and the International Data Center (IDC) of the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO). These solutions are computed by joining P, SH, SV polarities and SV/P, SH/P and SV/SH amplitude ratios where the quality of each solution is evaluated. This set of solutions is considered as a completion of the Egyptian focal mechanism catalogue. It will be helpful in understanding the spatial variation in the stress field within Egypt. At northern Egypt, the dominated mechanisms reflect a normal dip-slip, sometimes with strike component except in Dahshour region where strike–slip mechanism dominates. Toward the south, in Aswan source region, the strike–slip mechanism is dominated reflecting that the local tectonics is important characterized by a major strike slip component. The orientations of the T axes appear to be changed from NE-SW in the Gulf of Suez to ENE-WSW in the Gulf of Aqaba and NNE-SSW in the remains of the Egyptian territory. It is clear that Egypt is mainly controlled by extensional stress field.

Quaternary faulting in the Tatra Mountains, evidence from cave morphology and fault-slip analysis

Abstract Tectonically deformed cave passages in the Tatra Mts (Central Western Carpathians) indicate some fault activity during the Quaternary. Displacements occur in the youngest passages of the caves indicating (based on previous U-series dating of speleothems) an Eemian or younger age for those faults, and so one tectonic stage. On the basis of stress analysis and geomorphological observations, two different mechanisms are proposed as responsible for the development of these displacements. The first mechanism concerns faults that are located above the valley bottom and at a short distance from the surface, with fault planes oriented sub-parallel to the slopes. The radial, horizontal extension and vertical σ1 which is identical with gravity, indicate that these faults are the result of gravity sliding probably caused by relaxation after incision of valleys, and not directly from tectonic activity. The second mechanism is tilting of the Tatra Mts. The faults operated under WNW-ESE oriented extension with σ1 plunging steeply toward the west. Such a stress field led to normal dip-slip or oblique-slip displacements. The faults are located under the valley bottom and/or opposite or oblique to the slopes. The process involved the pre-existing weakest planes in the rock complex: (i) in massive limestone mostly faults and fractures, (ii) in thin-bedded limestone mostly inter-bedding planes. Thin-bedded limestones dipping steeply to the south are of particular interest. Tilting toward the N caused the hanging walls to move under the massif and not toward the valley, proving that the cause of these movements was tectonic activity and not gravity.

Deep heterogeneity of the stress state in the horizontal shear zones

The formation structures of brittle destruction in a rock layer above an active strike-slip fault in the crystalline basement is studied. The problem is analyzed from the standpoint of loading history, when after the stage of pure gravitational loading, an additional strain state of uniform horizontal shear of both the layer and underlying basement develops, which is further followed by a vertically nonuniform shear caused by the activation of the deep fault. For the studied object, irreversible fracture deformations on macro- and microlevels arise as early as the initial stage of loading under the action of gravitational stresses. These deformations continue evolving on the megascopic level in the course of horizontal shearing that is quasi-uniform both along the depth and laterally. The final formation of the structural ensemble occurs after a long stage of horizontal displacement of the blocks of the crystalline basement—the stage of localized shear. The theoretical analysis of the evolution of the stress state and morphology of the failure structures established the presence of numerous fractures with the normal dip-slip components in the intermediate-depth part of the rock mass, which are formed at the stages of uniform and localized horizontal shearing. The fractures with a strike-slip component mainly arise in the upper and near-axial deep parts of the section.

Study on the Focal Mechanism of the M5.1 Badong Earthquake in Hubei

Abstract: The focal mechanism solutions of the M5 . 1 Badong earthquake and subsequent 34 aftershocks at M L 2. 0 or more were calculated using the P-wave first motion method; the main earthquake was normal fault dip slip type, and the slip types of the seismogenic rupture surfaces of the subsequent aftershocks primarily include normal dip slip (14 times), reverse dip slip (9 times), normal strike slip (9 times) and reverse strike slip (2 times). The M 5. 1 Badong earthquake activities may be related to the stress adjustment caused by the rise of the groundwater level and the decrease of the frictional resistance betvveen structural planes of rock formations due to the effect of reservoir water penetration, and related to the joint activities of the NE-strike Gaoqiao fault and the near EW-strike Daping fault.

Superimposed basin formation during Neogene–Quaternary extensional tectonics in SW-Anatolia (Turkey): Insights from the kinematics of the Dinar Fault Zone

Abstract In the extensional province of SW-Anatolia, the cross-cutting relationship between the NW- and NE-oriented Neogene and Quaternary basins is an ongoing debate in the understanding of the tectonic evolution of this area. In order to contribute to this issue, we carried out a structural and kinematic study along the seismogenic NW-trending Dinar Fault Zone (DFZ). This structure was initially controlled by the sedimentary and tectonic evolution of the NE-oriented Neogene Baklan, Acigol and Burdur basins and, later, by the NW-oriented Quaternary Dinar Basin. On the basis of > 1000 structural and kinematic data, in conjunction with basin stratigraphy, the DFZ can be divided into three almost parallel and continuous bands, that are: (a) the Hangingwall where Quaternary sediments are deformed by normal faults with mechanical striations; (b) the Inner Zone, corresponding to the present Dinar fault scarp, where NW-trending normal faults with mechanical striations are dominant, and (c) the Outer Zone, located in the footwall of the structure comprising the area between the fault scarp and undeformed bedrock, where faults exhibit variable orientation and kinematics, from strike-slip to normal dip-slip. These kinematics are mainly indicated by calcite shear veins and superimposed mechanical striations, respectively. This suggests that the DFZ changed kinematics over time, i.e., the DFZ initiated as dominant dextral strike-slip to oblique-slip fault system and continued with a dominant normal movement. Therefore, we hypothesize that the NW-trending DFZ was initially a transfer zone during the late Miocene–Pliocene, coeval to the sedimentary and structural evolution of the NE-trending Baklan, Acigol and Burdur basins. During the Quaternary the DFZ, representing an already weakened crustal sector, played the role of a normal fault system providing the accommodation space for the Quaternary Dinar Basin. Hydrothermal circulation and volcanism at NE-/NW-trending faults intersection implies structurally-driven conduits channeling fluids from depth to surface.

Present-day tectonic stress regime in Egypt and surrounding area based on inversion of earthquake focal mechanisms

Stress field inversion is performed in Egypt on the basis of 219 focal mechanism solutions in the period from 1955 to 2007. For this purpose Egypt is divided into six seismotectonic zones: the northern part of the Gulf of Suez, southern Gulf of Suez, and Gulf of Aqaba, Cairo-Suez district, Dahshour zone and the Aswan Zone. The entire Gulf of Suez is currently under extensional stress field, with NE–SW trending horizontal extension. In the Gulf of Aqaba, the strike-slip regime predominates with sub-horizontal σ1 and σ3 axes trending NNW and ENE, respectively. A normal dip slip with small strike-slip component due to a nearly sub-vertical σ1 and sub-horizontal NNE striking σ3 characterizes Cairo-Suez district and Dahshour zone. Aswan seismic zone shows mainly strike-slip stress regime with a slight extension component (horizontal NW σ1 and NNE σ3). The stress field derived in this study indicates a prevailing tension stress (σ3 horizontal) which agrees well with the general tectonic frame of northeastern African, which is subjected to tensional stresses. Generally, extensional and/or extensional-strike slips are dominating the Egyptian territory. These regimes are compatible with the kinematics of the Red Sea–Gulf of Suez rift and Gulf of Aqaba transform plate boundary. Furthermore, the inferred stress in the present study (SHmin directed NNE–SSW) for the Cairo-Suez, Dahshour, and Aswan areas is similar to the East African Rift stress fields “Congo and Sudan” especially (Bosworth et al., 1992; Delvaux and Barth, 2010), whose origin is attributed to the far field effects of ridge push in the Atlantic and Indian Oceans (Zoback, 1992).

EXTREME FRACTIONATION AND DEFORMATION OF THE LEUCOGRANITE - PEGMATITE SUITE AT RED CROSS LAKE, MANITOBA, CANADA. III. DESCRIPTION OF SHEARING AND MYLONITIZATION TEXTURES IN THE LEPIDOLITE PEGMATITES

Three types of subvertical leucogranites and pegmatites populate the Red Cross Lake pegmatite field, located in the Red Cross Lake greenstone belt of the Sachigo Subprovince, in the northwestern part of the Archean Superior Province of the Canadian Shield. The pegmatitic rocks are situated within, or closely adjacent to, the regional North Kenyon Shear Zone. Following consolidation and minor low-temperature alteration, the leucogranites and mainly the lepidolite pegmatites were subject to intensive shearing, which reduced the pegmatites to compositionally layered mylonite-gneisses. Predominant movement within the sheared pegmatite has been dextral strike-slip with minor normal dip-slip. The layered, lenticular and rodded units within the mylonites consist mainly of spodumene + quartz, albite + quartz + K-feldspar, and lepidolite + quartz + albite, and they represent deformed primary pegmatite assemblages. The mechanical response of individual mineral species grades from brittle to ductile; the sequence is from tourmaline through beryl, K-feldspar, amblygonite, pollucite, albite, spodumene, and quartz to micas. The most ductile minerals compose the groundmass of the sheared dikes, whereas the accessory minerals and K-feldspar are brittle and have undergone the least modification in grain size and shape.

Modeling of ALOS and COSMO-SkyMed satellite data at Mt Etna: Implications on relation between seismic activation of the Pernicana fault system and volcanic unrest

We investigate the displacement induced by the 2–3 April 2010 seismic swarm (the largest event being of Ml 4.3 magnitude) by means of DInSAR data acquired over the volcano by the Cosmo-SkyMed and ALOS radar systems. Satellite observations, combined with leveling data, allowed us to perform a high-resolution modeling inversion capable of fully capturing the deformation pattern and identifying the mechanism responsible for the PFS seismic activation. The inversion results well explain high gradients in the radar line of sight displacements observed along the fault rupture. The slip distribution model indicates that the fault was characterized by a prevailing left-lateral and normal dip–slip motion with no fault dilation and, hence, excludes that the April 2010 seismic swarm is a response to accommodate the stress change induced by magma intrusions, but it is due to the tectonic loading possibly associated with sliding of the eastern flank of the volcano edifice. These results provide a completely different scenario from that derived for the 22 September 2002M3.7 earthquake along the PFS, where the co-seismic shear-rupture was accompanied by a tensile mechanism associated with a first attempt of magma intrusion that preceded the lateral eruption occurred here a month later. These two opposite cases provide hints into the behavior of the PFS between quiescence and unrest periods at Etna and pose different implications for eruptive activity prediction and volcano hazard assessment. The dense pattern of ground deformation provided by integration of data from short revisiting time satellite missions, together with refined modeling for fault slip distribution, can be exploited at different volcanic sites, where the activity is controlled by volcano-tectonic interaction processes, for a timely evaluation of the impending hazards.

Source mechanisms of micro-earthquakes induced in a fluid injection experiment at the HDR site Soultz-sous-Forêts (Alsace) in 2003 and their temporal and spatial variations

We have inverted the peak amplitudes of direct P waves of 45 micro-earthquakes with magnitudes between M = 1.4 and 2.9, which occurred during and after the 2003 massive fluid injection in the GPK3 borehole of the Soultz-sous-Forˆets Hot Dry Rock facility. These events were recorded by a surface seismic network of 15 stations operated by the Ecole et Observatoire des Sciences de la Terre, University of Strasbourg. The unconstrained moment tensor (MT) expression of the mechanism was applied, allowing the description of a general system of dipoles, that is, both double-couple (DC) and non-DC sources, as tensile fractures. The mechanisms of all but one event are dominantly DCs with a few per cent additional components at the most. We have checked carefully the reliability of the MT retrieval in bootstrap trials eliminating some data, by simulating the mislocation of the hypocentre and by applying simplified velocity models of the area in constructing Green’s functions. In some of the trials non-DC components amounting to several tens of per cent appear, but the F-test classifies them as insignificant. Even the only micro-earthquake with an exceptionally high non-DC mechanism cannot be classified unambiguously—the F-test assigns similar significance to the pure DC solution. The massive dominance of the DC indicates the shear-slip as the mechanism of the micro-earthquakes investigated. The mechanisms display large variability and are of normal dip-slip, oblique normal to strike-slip types. The T-axes are fairly stable, being concentrated subhorizontally roughly in the E–W direction. On the contrary, the P-axes are ill constrained varying in the N–S direction from nearly vertical to nearly horizontal, which points to heterogeneous stress in the Soultz injected volume. This is in agreement with the stress pattern from in situ measurements: the minimum stress axis is well constrained to E–W, whereas the maximum and intermediate stress values are close to one another, enabling the ambiguity of the P-axis direction. We found no significant dependence of source mechanisms either on magnitudes or depths. The time–space distribution of the events analysed suggests that the injection activated two segments of the natural faults existing in the area (I and II in our notation) showing different source mechanism patterns. The dip-slip regime is characteristic of fault segment I where the seismicity occurred during and also after injection, whereas the strike-slip regime prevails in segment II where the seismicity was triggered only after the injection shut in. This indicates that the tensile fractures, which are assumed to be created during injection, may have occurred on a smaller scale than the pure shear micro-earthquakes investigated.

Syncollisional extension along the India–Asia suture zone, south-central Tibet: Implications for crustal deformation of Tibet

Crustal deformation models of the Tibetan plateau are assessed by investigating the nature of Neogene deformation along the India–Asia suture zone through geologic mapping in south-central Tibet (84°30′E). Our mapping shows that the suture zone is dominated by a system of 3 to 4 ENE-striking, south-dipping thrust faults, rather than strike-slip faults as predicted by models calling upon eastward extrusion of the Tibetan plateau. Faults along the suture zone are not active, as they are cut by a system of NNW-striking oblique slip normal faults, referred to herein as the Lopukangri fault system. Fault-slip data from the Lopukangri fault system shows that the mean slip direction of its hanging wall is N36W. We estimate the net slip on the Lopukangri fault by restoring components of the thrust system. We estimate that the fault has accommodated ∼ 7 km of right-slip and ∼ 8 km of normal dip-slip, yielding a net slip of ∼ 10.5 km, and 6 km of horizontal east–west extension. The Lopukangri fault system is active and geomorphic offsets indicate right separations and westside-down dip-separation. The mapview curviplanar geometry and geomorphic expression of the Lopukangri fault system is similar to faults and rift basins to its east and west. These extensional faults are en echelon in map view and encompass a region that is 200 km long (east–west) and 95 km wide (north–south). Assuming our results for the Lopukangri fault are applicable to the entire system, we estimate a maximum of 18% extension across the zone. All active faults in the system terminate southward adjacent to the India–Asia suture zone. Because the individual rift geometries are similar and suggest a common kinematic relationship, we propose that the extensional system formed as a trailing extensional imbricate fan at the southern termination of the central Tibet conjugate fault zone. Alternatively, the extensional system may terminate to the north and represent a group of isolated crustal tears. Both kinematic interpretations imply a semi-smooth north–south variation in the magnitude of east–west extension in southern Tibet, with higher magnitudes in the north along the Bangong–Nujiang suture zone than in the south along the India–Asia suture zone. Our results from southern Tibet show that deformation between southern Tibet and the Himalayas is broad (95 km wide) and best described as a continuum possibly since the Late Miocene. Conversely, the structural boundary between western Tibet and the Himalayas, which is defined by the Karakoram fault, is presently a discrete boundary, and probably has been since the Middle Miocene. We think this variation in the displacement gradient and age of these structural boundaries within the interior of Tibet is best explained by the fault patterns and strain history describing wholesale E–W stretching and N–S shortening of the Tibetan crust.