Dextral Strike-slip Displacement Research Articles

Trans-Saharan Belt Provenance: A Potential Source for the Ordovician Succession of the Balkan Terrane (Svoge Unit) - Clues from LA-ICP-MS Detrital Zircon Dating Analysis

Two sandstone samples from the upper and lower part of the Ordovician succession of Svoge Unit were analyzed in order to determine their detrital zircon U–Pb age spectra using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). One sample was collected from the Hirnantian siliciclastic glaciomarine deposits of the Sirman Formation. The other sample is amiddle Darriwilian sandstone from the lower half of the Grohoten Formation. Additionally, micropetrographic studies were performed. They are composed predominantly of detrital monocrystalline quartz grains with uniform or rarely undulose extinction and only sporadically presented feldspar grains, muscovite flakes and single rock fragments. The heavy mineral assemblages are also characterized mostly by detrital zircon. As suggested from previous studies of the Hirnantian glaciomarine deposits of Sirman Formation, the provenance of siliciclastic material was most probably associated with sedimentary recycling of maturesands deposited across the North Gondwana Platform. Multi-dimensional scaling statistical technique allowed a reliable objective identification of the potential source areas in Northern Africa and palaeogeographic reconstructions were made. Both samples are close to the Trans-Saharan Belt provenance, which is the most probable source for the detrital component. Our data supportthe idea proposed by previous researchers that the present-day position of some of these terranes implies significant dextral strike-slip displacement, probably due to the movement on the Pangea megashear during the Carboniferous and Permian.

Paleoseismologic trenching confirms recent Holocene activity of the major Algeciras fault system in southern Colombia

The Algeciras Fault System (AFS) in Colombia, which is characterized by its predominant dextral strike-slip movement, is part of the major transcurrent fault system known in Ecuador, Colombia and Venezuela as the Eastern Frontal Fault System, that disconnects the North Andean Block (NAB) from the South American plate in a process of tectonic escape. It is responsible for some of the most important seismic events in the seismic history of Colombia that occurred in 1827 with Mw 7.1 that destroyed large part of all settlements in the Huila Department and caused considerable damage and loss of life in the capital of Bogota. More recently another event of Mw 7.0 took place in 1967 that also caused damage in the capital, and in December 2019 an event of Mw 6.0 caused minor damage in various settlements in the Eastern foothills of the Eastern Cordillera. So far, mapping studies based on data obtained from satellite imagery, airphotos and structural field data have outlined the complexity of the AFS and have identified the existence of at least six tectonic basins occurring along the main fault of the system. In 2016, within the framework of activities of the Space Geodesy Research Group of the Geohazards Directorate of the Geological Survey of Colombia (SGC) a program of morphotectonic field studies was carried out in and around the Algeciras pull-apart basin in the Huila Department that led to the opening of two exploratory paleoseismologic trenches. This paper presents the results of the first of these two trenches, located in the center of the Algeciras valley at a place named Lagunillas. It is situated at the rim of a shallow fault-dammed lake formed by an uphill facing scarp generated by the dextral strike-slip displacement of a shortcut fault that traverses the alluvial fill of the basin along its longitudinal axis. The results of the paleoseismologic study prove the occurrence of seven seismic events with magnitudes Mw 7.0 during the Holocene with an average recurrence interval of 1300–1400 years. This constitutes a significant contribution to the study of seismic hazard assessment of the AFS that complements, in a quantitative way, the evidence provided by the data of historic seismicity of this major fault and points out the considerable danger that the fault presents to the country as a whole and in particular to the metropolitan area of Bogota.

The Balkan terranes: a missing link between the eastern and western segments of the Avalonian–Cadomian orogenic belt?

ABSTRACT The Alpine–Himalayan collision zone involves a number of crustal fragments that originated in the Neoproterozoic to Cambrian Avalonian–Cadomian belt of northern Gondwana. We use the detrital zircon U–Pb geochronology to examine four of these lithotectonic units, now exposed in the Balkans in Bulgaria and Serbia. The obtained age spectra suggest that the Diabase–Phyllitoid Complex (the maximum depositional age, MDA, estimated at 540 + 5/-9 Ma) was presumably an accretionary wedge or a forearc basin sourced from a nearby volcanic arc, however, its palaeo-position remains uncertain. The Vlasina Complex (MDA of 577 + 5/-6 Ma) was the most ʽwesterly’ terrane, adjacent to the Trans-Saharan belt, whereas the Sredna Gora and Stara Planina complexes (MDAs of 546 ± 7 Ma and 579 + 4/-5, respectively) were positioned next to the Saharan Metacraton and Arabian–Nubian shield. To put the Balkan terranes into a broad context, we statistically compare the detrital zircon ages in other terranes from the Eastern Alps to Iran with igneous and metamorphic U–Pb zircon ages from North African source areas. The statistical comparison is done through multi-dimensional scaling (MDS), a more rigorous method than a visual comparison of age spectra, to examine the degree of inter-sample similarity. This information is then transferred to a tentative palaeogeographic map showing position of each terrane with respect to its most likely source region. As a result, we define a ʽwesterly’ terrane assemblage, characterized by Mesoproterozoic ages and sourced from the West African craton and the Trans-Saharan belt and an ʽeasterly’ assemblage formed next to the Saharan Metacraton and the Arabian–Nubian shield. The present-day position of some of these terranes implies significant dextral strike-slip displacement, perhaps due to movement on the Pangea megashear during the Carboniferous and Permian.

FAULT SYSTEMS IN THE UPPER CRUST OF THE FENNOSCANDIAN SHIELD, THE EAST EUROPEAN PLATFORM

Directions of 683 faults located in the southeastern part of the Fennoscandian (Baltic) shield were statistically analyzed, and three orthogonal associations of fault systems were identified in the study area. According to the dynamic analysis of the fault systems and their associations, the main NW-striking faults belong to the fault network originating mainly from the early Paleoproterozoic. These faults functioned in the Paleoproterozoic during four main deformation stages: D1 – sinistral shear transtension and asymmetric rift genesis (2.1–1.9 Ga); D2 – sinistral shear transpression under oblique accretion and convergence (1.9 Ga); D3 – sinistral shear transpression under oblique collision (1.89–1.80 Ga); D4 – dextral strike-slip displacements at the background of complex escape tectonics of the late collision stage (1.80–1.78 Ga). The regional stress field changed as follows: D1 – northeast- or east-trending extension; D2 – northeast compression; D3 – sub-latitudinal compression; D4 – sub-meridian compression. Changes in dynamic loading conditions led to multiple kinematic inversions of the fault networks. Widespread transtension and transpression settings in the southeastern parts of the Baltic Shield give evidence of asymmetric rifting, oblique accretion and collision in the Paleoproterozoic, which must be taken in to account in geodynamic reconstructions.

Structural–Kinematic Parageneses and Dynamic Evolution Model of the Baltic–Mezen Shear Zone in the Phanerozoic, the Northwestern Part of the East European Platform

The tectonic features of the Baltic–Mezen shear zone that developed along the border of the Fennoscandian Shield and the Russian Plate in the north of the East European Platform are considered. It is established that tectonic evolution of the considered zone was associated with strike-slip displacements in the basement rocks, which occurred and were dispersed in cover deposits in the form of low-amplitude faults and diagonal fold-and-thrust systems associated with sublayer detachments. The dynamic relationship of the geological–structural and geomorphic anomalies of the Baltic–Mezen zone with segments of bending of its mainline and virgation, as well as with the nodes of intersection of the main shear zone with transverse faults, is noted. Based on a study of structural and kinematic parageneses, it has been established that the Baltic–Mezen shear zone in the Phanerozoic developed mainly under dextral strike-slip displacement and transpression conditions during eight activation phases of the Caledonian, Hercynian, and recent deformation stages. Periodically there was a change in deformation regimes and kinematic inversions, and a shear zone evolved in conditions of transtension and sinistral strike-slip displacements. The kinematics of the Baltic–Mezen shear zone is consistent with the general dynamic patterns of the evolution of the East European Platform, in particular, with the formation of collision structures of the Scandinavian Caledonides, with extensive Late Devonian tectonic–magmatic events and features of the recent development of the Euro–Arctic region.

Chapter 7 Småland lithotectonic unit dominated by Paleoproterozoic (1.8 Ga) syn-orogenic magmatism, Svecokarelian orogen

AbstractThe Småland lithotectonic unit in the 2.0−1.8 Ga Svecokarelian orogen, southeastern Sweden, is dominated by a c. 1.81−1.77 Ga alkali–calcic magmatic suite (the Transscandinavian Igneous Belt or TIB-1). At least in its central part, the TIB-1 suite was deposited on, or emplaced into, c. 1.83–1.82 Ga calc-alkaline magmatic rocks with base metal sulphide mineralization and siliciclastic sedimentary rocks (the Oskarshamn–Jönköping Belt). Ductile deformation and metamorphism under low- to medium-grade conditions affected the Oskarshamn–Jönköping Belt prior to c. 1.81 Ga. Both suites were subsequently affected by low-grade ductile deformation, mainly along steeply dipping, east–west to NW–SE shear zones with dip-slip and dextral strike-slip displacement. Sinistral strike-slip NE–SW zones are also present. In the northern part of the lithotectonic unit, 1.9 Ga magmatic rocks, c. 1.87–1.81 Ga siliciclastic sedimentary rocks and basalt, and c. 1.86–1.85 Ga granite show fabric development, folding along steep NW–SE axial surfaces and medium- or high-grade metamorphism prior to c. 1.81 Ga and, at least partly, at c. 1.86–1.85 Ga; base metal sulphide, Fe oxide and U or U–REE mineralizations also occur. Magmatism and siliciclastic sedimentation along an active continental margin associated with subduction-related, accretionary tectonic processes is inferred over about 100 million years.

Basement controls on cover deformation in eastern Chapada Diamantina, northern São Francisco Craton, Brazil: Insights from potential field data

The Proterozoic sedimentary cover in eastern Chapada Diamantina recorded positive inversion tectonics at the Paramirim aulacogen during Neoproterozoic times. Additionally, geological evidence suggests that although the basement was not involved in the contractional tectonics, it exerted control on cover deformation. The Euler deconvolution, based on aeromagnetic and gravimetric databases, integrated with geological framework, was used to delineate the structures and deep architecture of the crystalline basement and to understand their relationships with cover deformation. The 3D view of Euler deconvolution profiles show a graben and horst geometry in the basement architecture. Horsts and grabens are limited by NNW-SSE, N-S and E-W normal faults, which nucleated during two extensional events and favored for deep Archean-Paleoproterozoic discontinuities. The first event was related to the development of a half-graben that had a NNE-SSE-oriented eastern boundary and that was filled with rocks of the Chapada Diamantina Group. The second event gave rise to an asymmetrical graben that evolved from dextral strike-slip displacement and was filled by rocks of the Una Group in the Irecê domain. In the late Neoproterozoic, two contractional, thin-skinned cover deformations with ENE-WSW and N-S shortening directions affected the Chapada Diamantina and Irecê domains. Our geophysical models reveal a parallelism between the thin-skinned structures and N-S and E-W normal faults, interpreted in the crystalline basement, suggesting a control on strain and stress distribution during cover deformation. During the second contractional tectonic event, the NNW-SSE and N-S lineaments at the boundaries of the Irecê domain were reactivated, producing Ubiaí and Serra da Babilônia brittle shear zones in the sedimentary cover, respectively. In summary, the superposed intracratonic basin formation was controlled by older structures in the crystalline basement, while the inversion tectonics was controlled by the geometry of these basins.

Evolution of the transtensional structure in Huimin sag of Bohai Bay basin, eastern China

Transtensional basins containing petroleum reserves are common in eastern China. To better understand the evolution of the oil reservoirs, we investigated the generation mechanism of the Huimin sag transtensional structure, eastern China. We analyzed three-dimensional seismic surveys and drill-core data and concluded that most faults in the Huimin sag are planar or listric with negative flower-structured vertical profiles. The main faults are ENE- and E-striking: the Linshang fault belt to the north and the Xiakou fault belt to the south. Seismic profiles indicate three sets of basement faults in the Huimin sag, striking NNE, ENE, and NNW. The NNW-striking basement faults originated in the middle Triassic; the ENE and NNE faults formed during the late Jurassic. During the Paleogene, the Huimin sag was subjected to N–S extensional forces that reactivated the ENE faults, leading to dextral strike-slip displacement in the Linshang and Xiakou faults, which were oblique to the extensional direction. This generated the conjugate brushed-shaped Linshang and Xiakou fault system, producing a typical oblique extensional basin with characteristic transtensional structure. To reconstruct the evolution of the transtensional structure in Huimin sag, we used Lagrangian analysis software to model the main faults in the western Huimin sag. The simulation verified the re-activation of the NE-, ENE-, and NNW-striking faults under the N–S extension. The NNW fault faded, while the other two strengthened, producing brush-shaped fault systems and E–W- and ENE-striking normal faults. The simulation results show good agreement with the field-data analysis.

Tectonic strain changes affecting the development of deep seated gravitational slope deformations in the Bohemian Massif and Outer Western Carpathians

This paper describes a pair of intensively studied deep seated gravitational slope deformations from the Czech Republic. Structural geological mapping and the interrogation of high resolution LiDAR DEMs have been used to reassess the extent of each slope deformation and to obtain new information about their surface morphologies. Electrical resistivity tomography has been used to image the subsurface structures while rod dilatometers and permanently installed mechanical extensometers have been used to define their kinematic behaviour. Dilatometric measurements from across a fissure the edge of the Mužský Hill DSGSD show that the progressive trend has been interspersed by five periods of movement acceleration since monitoring began in 1990 while extensometric measurements from across a second fissure about 50m from the edge of the plateau show that this discontinuity was affected by an significant period of extension from 2004 to 2006 and a significant period of compression from 2006 to 2008. These data demonstrate that the Mužský Hill DSGSD is characterised by the lateral spreading of rigid sandstone blocks across a plastic marl formation at the edge of Příhrazy Plateau. Extensometric measurements from the Kněhyně Mountain DSGSD show that a fault located at a depth of about 25m below the surface is characterised by progressive dilation whereas a fault located at a depth of about 57.5m below the surface is characterised by episodic vertical dip-slip displacements. Both faults are affected by horizontal dextral strike slip displacements. These data demonstrate that the Kněhyně Mountain DSGSD is characterised by translational rocksliding associated with the neotectonic uplift of the Outer Western Carpathians. The presented kinematic data have been compared to records of precipitation, groundwater level, earthquakes, and strain changes measured by the fault displacement monitoring network EU-TecNet. Strain changes induced by the widespread redistribution of stress through the crustal appear to represent an important and hitherto underappreciated geomorphological process affecting the development of the slope deformations at Mužský Hill and Kněhyně Mountain. Future research will focus discovering whether this finding holds true for other deep seated gravitational slope deformations in different geomorphological and geological settings.

Strike-slip neotectonic regime and related structures in the Cappadocia region: a case study in the Salanda basin, Central Anatolia, Turkey

The study area is a strike-slip basin of approximately 1-9 km wide, 66 km long and N65°W trending, located between the historical Kesikköprü in the west and the Sarıhıdır settlement in the east along the northern side of the Central Anatolian Volcanic Province. It was evolved on a regional erosional surface of a pre-Quaternary volcanosedimentary sequence during Quaternary. This is evidenced by the stratigraphical, structural, and seismic data. The total amounts of throw and dextral strike-slip displacement accumulated on the basin-boundary faults during the evolutionary history of the basin are 178 m and 5 km, respectively. The average slip rate on the Salanda master fault is approximately 4 mm/year since the late early Pleistocene based on the total dextral strike-slip offset accumulated on it. The throw amount is small compared with the dextral strike-slip offset, which implies a strike-slip regime rather than a tensional tectonic regime in the basin. This is also supported by the combination of both the contractional and extensional structures such as reverse faults, fissure-ridge travertines, and a series of stepped terraces of late Quaternary age. Finally, it would be useful to take this paper into account in new works to be carried out in other sections of the Cappadocia region, because a new neotectonic regime (strike-slip tectonic regime) is first introduced here for this region.

Paleoseismology of the Mount Narryer fault zone, Western Australia: A multistrand intraplate fault system

Our paleoseismological study of faults and fault-related folds comprising the Mount Narryer fault zone reveals a mid- to late Quaternary history of repeated morphogenic earthquakes that have influenced the planform and course of the Murchison, Roderick, and Sanford Rivers, Western Australia. The dominant style of deformation involves folding of near-surface sediments overlying discrete basement faults. Carbon-14, optically stimulated luminescence, and in situ–produced 10 Be constrain the timing of the events and late Quaternary slip rates associated with fault propagation folds in tectonically uplifted and deformed alluvial channel deposits. A flight of five inset fluvial terraces is preserved where the Murchison River flows across the Roderick River fault. These terraces, which we infer to be coseismic, are consistent with at least four late Quaternary seismic events on the order of moment magnitude (M w ) 7.1 within the last ~240 k.y. Secondary shears expressed on the folds indicate a component of dextral strike-slip displacement. Quaternary slip rates on the underlying faults range from 0.01 to 0.07 mm yr –1 , with a total slip rate for the zone between 0.04 and 0.11 mm yr –1 . These rates are intermediate to those in the adjacent Mesozoic basin (>0.1 mm yr –1 ) and Precambrian craton ( –1 ) and so provide insight into how tectonic strain is partitioned and transferred across a craton margin.

The use of speleothem damage and in situ fault displacement monitoring to characterise active tectonic structures: an example from Západní Cave, Czech Republic

The EU-TecNet fault displacement monitoring network records three-dimensional displacements across specifically selected tectonic structures within the crystalline basement of central Europe. This paper presents a study of recent and active tectonic fault displacement recorded at Západní Cave in northern Bohemia (Czech Republic). It extends previous geological research by measuring speleothem damage within the cave and monitoring displacements across two fault structures within the Lusatian Thrust Zone. The measured speleothem damage, which are characterised by strike-slip movements, show dextral strike-slip displacement along the WSW-ENE striking fault and sinistral strike-slip displacement along the NNW-SSE striking fault. It is found that the compressive stress σ1 is located in the NW or SE quadrant while the tensile stress σ3 is oriented perpendicular to σ1, i.e. in the NE or SW quadrant. The results of fault displacement monitoring show a long-term dextral strike-slip trend along the WSW-ENE striking fault while the NNW-SSE striking fault presents evidence for uplift of the eastern block. There are notable periods of increased geodynamic activity, referred to as pressure pulses, in 2008, 2010-2011, and 2012. The significant correspondence between the measured speleothem damage and the results of fault displacement monitoring confirm the supposition that, at this site, the compressive stress σ1 persists in the NW or SE quadrant. The presented results offer an insight into the periodicity of pressure pulses, demonstrate the necessity for protracted monitoring periods as a means for better understanding geodynamic processes, and show that it is possible to characterise the displacements that occur across individual faults in a way that cannot be accomplished from geodetic measurements made using Global Navigation Satellite Systems.

Structural evidence for superposition of transtension on transpression in the Zagros collision zone: Main Recent Fault, Piranshahr area, NW Iran

The Main Recent Fault of the Zagros Orogen is an active major dextral strike-slip fault along the Zagros collision zone, generated by oblique continent–continent collision of the Arabian plate with Iranian micro-continent. Two different fault styles are observed along the Piranshahr fault segment of the Main Recent Fault in NW Iran. The first style is a SW-dipping oblique reverse fault with dextral strike-slip displacement and the second style consists of cross-cutting NE-dipping, oblique normal fault dipping to the NE with the same dextral strike-slip displacement. A fault propagation anticline is generated SW of the oblique reverse fault. An active pull-apart basin has been produced to the NE of the Piranshahr oblique normal fault and is associated with other sub-parallel NE-dipping normal faults cutting the reverse oblique fault. Another cross-cutting set of NE–SW trending normal faults are also exist in the pull-apart area. We conclude that the NE verging major dextral oblique reverse fault initiated as a SW verging thrust system due to dextral transpression tectonic of the Zagros collision zone and later it has been overprinted by the NE-dipping oblique normal fault producing dextral strike-slip displacement reflecting progressive change of transpression into transtension in the collision zone. The active Piranshahr pull-apart basin has been generated due to a releasing damage zone along the NW segment of the Main Recent Fault in this area at an overlap of Piranshahr oblique normal fault segment of the Main Recent Fault and the Serow fault, the continuation of the Main Recent Fault to the N.

A reappraisal of the early slip history of the San Andreas fault, central California, USA

The modern San Andreas fault system (California, United States) is widely considered to have formed in response to the initiation of Pacific–North American plate interaction ca. 27 Ma. Although there is general consensus on the magnitude and timing of Neogene displacement along the San Andreas system, its Paleogene history remains unresolved. In particular, ∼100 km of right-lateral offset between mid-Cretaceous plutonic rocks of the northern Salinian block and the western edge of Sierra Nevada basement remains unaccounted for after restoration of Neogene displacement along strike-slip faults of the San Andreas system. Our detrital zircon data invalidate a key Paleogene piercing point by demonstrating that displaced portions of the hypothesized Middle Eocene Butano–Point of Rocks submarine fan were never contiguous across the San Andreas fault. We instead show that the Eocene provenance characteristics exhibited by northern Salinian strata closely match those of the southern Sierra Nevada and northwestern Mojave Desert. This implies that the northern Salinian block was located at least 75–50 km farther south in Eocene time than previously recognized. Our data require (1) pre–23 Ma dextral slip along the San Andreas fault in central California, and/or (2) slip along a predecessor fault that formed prior to Pacific–North American plate interaction. This previously undocumented slip may indicate that significant Pacific–North American plate interaction propagated from the plate margin into the continental interior much earlier than conventionally believed. Alternatively, late Paleogene slip could predate the development of the modern plate boundary and represent inboard dextral strike-slip displacement along the eastern margin of the Salinian block, similar to the deformation that occurs today along the strike-slip Sumatra fault system.

Quantitative kinematic analysis within the Khlong Marui shear zone, southern Thailand

The NNE trending Khlong Marui shear zone has a strong geomorphic signal with marked fault-strike parallel topographic ridges. The lithologies within the strike-slip zone mainly consist of vertical layers of mylonitic meta-sedimentary rocks associated with orthogneisses, mylonitic granites, and pegmatitic veins. The pegmatitic veins concordantly intrude the mylonitic foliation but were sheared at the rims indicating syn-kinematic emplacement. Microstructures and mineral assemblages suggest that the rocks in the area have been metamorphosed at amphibolite facies and low to medium greenschist facies by the first deformation. The Khlong Marui shear zone was deformed under dextral simple shear flow with a small finite strain. The ductile-to-brittle deformation involves a period of exhumation of lenses of higher grade rocks together with low grade fault rocks probably associated with positive flower structures. The final stage brittle deformation is reflected by normal faulting and formation of proto-cataclasites to cataclasites of the original mylonitic meta-sedimentary host rock. Although clear age-constraints are still missing, we use regional relationships to speculate that earlier dextral strike-slip displacement of the Khlong Marui shear zone was related to the West Burma and Shan-Thai collision and subduction along the Sunda Trench in the Late Cretaceous, while the major exhumation period of the ductile lens was tectonically influenced by the early India-Asia collision. The changing stress field has responded by switching from dextral strike-slip to normal faulting in the Khlong Marui shear zone, and is associated with “escape tectonics” arising from the overall India-Asia collision.

Basement character and basin formation in Gorontalo Bay, Sulawesi, Indonesia: new observations from the Togian Islands

AbstractWe present a new stratigraphy for the Togian Islands, Sulawesi, and interpret the age, character and evolution of Gorontalo Bay. At its western end the bay is underlain by continental crust. The central part is underlain by Eocene to Miocene oceanic and arc rocks, although the area south of the Togian Islands could have continental crust of the Banggai-Sula microcontinent thrust beneath this and the East Arm ophiolite. Gorontalo Bay was not a significant deep bathymetric feature before the Miocene. Field relationships indicate a latest Miocene to Pliocene age for inception of the basin. Medium-K to shoshonitic volcanism in the Togian Islands is not due to subduction but reflects crustal thinning and extension in the Pliocene and Pleistocene, causing the underlying mantle to rise, decompress and melt. Extension is continuing today and is probably the cause of volcanism at Una-Una. Volcanic activity migrated west with time and volcanic products have been offset by dextral strike-slip displacement along the Balantak Fault. Extension and subsidence was driven by rollback of the subduction hinge at the North Sulawesi Trench with a possible contribution due to flow of the lower crust.

10.1007/s11479-008-1004-1

The geological and geophysical data primarily on the structure of the upper sedimentary sequence of the northern Knipovich Ridge (Norwegian-Greenland Basin) that were obtained during Cruise 24 of the R/V Akademik Nikolai Strakhov are considered. These data indicate that the recent kinematics of the northern Knipovich Ridge is determined by dextral strike-slip displacements along the Molloy Fracture Zone (315° NW). This stress field is superimposed by a system related to rifting and latitudinal opening of rifts belonging to the ridge proper. Thus, the structural elements formed under the effect of two stress fields are combined in this district. Several stages of tectonic movements are definable. The first stage (prior to 500 ka ago) is marked by the dominant normal faults, which are overlain by the lower and upper sedimentary sequences. The second stage (prior to 120–100 ka ago) is characterized by development of normal and reverse faults, which displace the lower sequence and are overlain by the upper sequence. Both younger and older structural features reveal peaks of tectonic activity separated by intermediate quiet periods 50–60 ka long. The stress field of the regional strike-slip faulting is realized in numerous oblique NE-trending normal and normal-strike-slip faults that divide the rift valley and its walls into the segments of different sizes. Their strike (20°–30° NE) is consistent with a system of secondary antithetic sinistral strike-slip faults. The system of depressions located 40 km west of the rift valley axis may be considered a paleorift zone that is conjugated at 78°07′ N and 5°20′ W with the NW-trending fault marked by the main dextral offset. The stress field that existed at this stage was identical to the recent one. The rift valley axis migrated eastward to its present-day position approximately 2 Ma ago (if the spreading rate of ∼0.7 cm/yr is accepted). The obtained data substantially refine the understanding of the initial breakup of continents with the formation of oceanic structural elements. The neotectonic stage is characterized by combination of different stress fields that resulted in the formation of a complex system of tectonic structural units, including those located beyond the recent extension zone along the rift axis of the Knipovich Ridge. The tectonic deformations occurred throughout the neotectonic stage as discrete recurrent events.

The continuation of the Kazerun fault system across the Sanandaj–Sirjan zone (Iran)

The Kazerun (or Kazerun-Qatar) fault system is a north-trending dextral strike-slip fault zone in the Zagros mountain belt of Iran. It probably originated as a structure in the Panafrican basement. This fault system played an important role in the sedimentation and deformation of the Phanerozoic cover sequence and is still seismically active. No previous studies have reported the continuation of this important and ancient fault system northward across the Sanandaj–Sirjan zone. The Isfahan fault system is a north-trending dextral strike-slip fault across the Sanandaj–Sirjan zone that passes west of Isfahan city and is here recognized for the first time. This important fault system is about 220 km long and is seismically active in the basement as well as the sedimentary cover sequence. This fault system terminates to the south near the Main Zagros Thrust and to the north at the southern boundary of the Urumieh–Dokhtar zone. The Isfahan fault system is the boundary between the northern and southern parts of Sanandaj–Sirjan zone, which have fundamentally different stratigraphy, petrology, geomorphology, and geodynamic histories. Similarities in the orientations, kinematics, and geologic histories of the Isfahan and Kazerun faults and the way they affect the magnetic basement suggest that they are related. In fact, the Isfahan fault is a continuation of the Kazerun fault across the Sanandaj–Sirjan zone that has been offset by about 50 km of dextral strike-slip displacement along the Main Zagros Thrust.

Perkembangan Tektonik Daerah Busur Muka Selat Sunda dan Hubungannya dengan Zona Sesar Sumatera

The subduction of the Indian-Australian Plate beneath the Eurasian Plate is oblique (~45o) along Sumatra. A major zone of dextral strike slip displacement along Sumatra Island, called the Sumatra Fault, partially accommodates the oblique convergence off Sumatra. On the contrary, the subduction is normal along Java; therefore, no major fault zone can be found along the Java Island. The transition zone of the Sunda Strait fore arc, which is located between Java and Sumatra fore arc, is subject to northwest-southeast extension related to the motion of the Sumatra Fault and north-south compression because of subduction. Geophysical studies show that continuous extension dominated the Sunda Strait fore arc region. The results are interpreted as showing ongoing separation of the Sunda Strait fore arc region as the Sumatra fore arc plate has moved northwest, bounded by the Sumatra Fault. Therefore, the Sumatra Fault can also be interpreted to extend across the fore arc to the trench in the form of several graben systems. Key words: Sunda Strait, Sumatra Fault Zone, fore arc tectonic

Complicated phenomena of the Beichuan-Yingxiu surface fracture zone during Wenchuan Ms8.0 earthquake in Sichuan Province, China

After Wenchuan Ms8.0 earthquake, the surface fractures have been investigated in Beichuan City and Yingxiu Town. It was found that the Beichuan-Yingxiu earthquake fracture zone trends northeastward, dipping to northwest. The earthquake fracture zone is mainly characterized by thrust faulting with small amounts of strike-slip movement, and demonstrating various complexities for different areas. The phenomena related to dextral strike-slip displacement are found in Beichuan City and those related to left-lateral strike-slip movement in Yingxiu Town. The compressive shortening of the two surface fracture zones in Beichuan City is 2.8–3.9 m and the left-lateral strike-slip displacement of the surface fractures in Yingxiu Town is 0.52 m.