Lateral Strike-slip Fault Research Articles

Impact of active faulting on the post LGM infill of Le Bourget Lake (western Alps, France)

We have used high resolution seismic imaging to detect and characterize the recent deformation recorded by the Quaternary sediments of Le Bourget Lake. The last glacial episodes (MIS 6a and 2, i.e., Riss and Würm) scoured out an elongated over-deepened basin to more than 300m below the present lake level and the basin accumulated 150m of post-LGM to Holocene sediments. The well-stratified character of the infill is locally disturbed by tectonic deformations and gravity reworking. A northern fault zone, in continuation with the left-lateral strike–slip Culoz Fault, is imaged within the Holocene and Late Glacial accumulations. A southern fault zone is also detected, which can be related to the sub-lacustrine continuation of a much smaller fault affecting the Jura alpine foreland: the Col du Chat left lateral strike–slip fault. Different generations of fractures have been identified in the lake, allowing correlation and mapping. In pre-Quaternary substratum, the Culoz Fault has a N 160° orientation. Within the post-LGM sediments, fractures related to the Culoz Fault have an orientation between N135° and 95°. A Cloos model (1932) is thus proposed to explain the observed pattern of lacustrine deformations. The calculated horizontal slip rate for Culoz Fault during Holocene is about 1.3mm·yr−1, and for the Col du Chat Fault is around 0.6mm·yr−1.

The Cenozoic rotational extrusion of the Chuan Dian Fragment: New paleomagnetic results from Paleogene red-beds on the southeastern edge of the Tibetan Plateau

Paleomagnetic studies were conducted on the Eocene and Oligocene strata at the western part of the Chuan Dian Fragment in order to describe the crustal deformation induced by continuous penetration of the Indian plate into Eurasia during the late Cenozoic. High-temperature magnetic components with unblocking temperatures of ~680°C were isolated, and positive fold and/or reversal tests reveal the primary nature of the magnetization. The tilt-corrected site-mean directions obtained from the Oligocene and middle-early Eocene strata are, respectively, Ds=200.9°, Is=−31.3°, k=52.8, α95=7.7° and Ds=29.7°, Is=32.0°, k=44.9, α95=5.6°. Comparison of these results with previous paleomagnetic data from the Chuan Dian Fragment shows that the western and central parts of the Chuan Dian Fragment experienced ~20° integral clockwise rotation relative to East Asia since the middle Miocene. However, the eastern part of the Chuan Dian Fragment has experienced different rotational deformation relative to East Asia since the Pliocene, because of the intense regional crustal deformation and activity on fault systems. The eastern boundary of the Chuan Dian Fragment was bounded by the Yuanmou-Luezhijiang left lateral strike-slip fault prior to the Pliocene, and then substituted by the Xiaojiang left lateral strike-slip fault since the Pliocene, due to the eastwards spreading of the clockwise rotational movement of the Chuan Dian Fragment. The evolutionary characteristics of the Ailaoshan-Red River and Xianshuihe-Xiaojiang strike-slip faults were controlled by the difference between the clockwise rotational extrusion velocities of the Chuan Dian Fragment and the Indochina Block.

No surface breaking on the Ecemiş Fault, central Turkey, since Late Pleistocene (~ 64.5 ka); new geomorphic and geochronologic data from cosmogenic dating of offset alluvial fans

The Ecemiş Fault Zone (EF) has been recognized as a major left lateral strike-slip fault in the Central Anatolian Fault Zone (CAFZ) of Turkey. However, its Quaternary slip-rate has been challenging to determine due to the difficulty of dating offset markers. Using high-precision offset measurements and 36Cl cosmogenic nuclide dating, we present the first geochronologically determined Late Quaternary slip-rate for the EF. Our study focuses on the excellent exposures of offset alluvial fan surfaces, originating from the Aladağlar, a Late Quaternary glaciated mountain. Analysis of airborne orthophotogrametry and GNSS (Global Navigation Satellite System) surveys indicates 168±2m left lateral and 31±1m vertical displacements. In-situ terrestrial cosmogenic 36Cl geochronology obtained from eleven surface boulders provides a minimum abandonment/incision age of 104.2±16.5ka for the oldest offset alluvial fan surface. Our geomorphic observations together with Self-potential geophysical surveys revealed the presence of an unfaulted alluvial fan terrace, which allows us to constrain the timing of deformation. The abandonment/incision age of this fan is 64.5±5.6ka based on thirteen 36Cl depth profile samples. Accordingly, we obtained a geologic fault slip-rate of 4.2±1.9mma−1 horizontally and 0.8±0.3mma−1 vertically for the time frame between 104.2±16.5ka and 64.5±5.6ka. Our analysis indicates that the EF has not been producing a major surface breaking earthquake on the main strand at least since 64.5±5.6ka (mid-Late Pleistocene). This could be the result of abandonment of the main strand and accommodation of deformation by other faults within the EF. Nevertheless, a recently occurred (30 September 2011) low magnitude (ML: 4.3) left lateral strike-slip earthquake indicates recent seismic activity of the EF. Comparison of the recent GPS velocity field with the longer slip history along the CAFZ indicates a constant but low strain release without surface breaking and very long large earthquake recurrence intervals.

Structure of the Sibumasu–Indochina collision, central Thailand: A section through the Khao Khwang Fold and thrust belt

Mainland SE Asia is composed of a number of continental fragments and volcanic arcs, separated by oceanic suture zones, which were accreted to the growing Asian continent during the Triassic Indosinian orogeny. The evolution of this orogeny has always been quite controversial. Indeed, the effects of this orogeny in Thailand have often been interpreted without considering the detailed tectonic evolution of the portion of the Indochina Block’s margin formed by Khao Khwang Platform area of the Saraburi Group, in central Thailand. This area is unusual because: (1) an extensive area representing a thin-skinned fold and thrust belt is well-exposed due to quarrying; and, (2) the fold and thrust belt displays a series of E–W and WNW–ESE striking thrusts and associated folds that are not easily explained in the context of the traditional interpretation where the terranes have been accreted broadly along N–S striking collisional zones. Detailed structural observations in numerous quarries around Highway 21 in a 13km long dip-direction traverse have revealed that overall the thrust belt is composed of several large thrusts with an approximately northwards transport direction. In the southern part of the area, south-verging structures are present. Although the dominant structural trend is northwards-verging, interference structures, and late strike-slip faults indicate there is more than one phase of structural development present.

New constraints from seismology and geodesy on the Mw = 6.4 2008 Movri (Greece) earthquake: evidence for a growing strike-slip fault system

The 2008 Mw = 6.4 Movri earthquake ruptured a NNE right lateral strike-slip fault about 30 km south of the city of Patras. Although some strike-slip activity on minor faults was known, there was no tectonic evidence of large scale NS striking fault and such a large event was not anticipated. Following the event, a network of six stations was installed for 4 months in the epicentral area in order to monitor aftershocks and in particular the northern part of the rupture area closest to the city of Patras. We combine these new aftershock observations with GPS measurements of an already existing geodetic network in the area performed just after the earthquake, as well as with SAR interferograms, together with already published source studies, in order to refine already proposed models of this event. The combined data set allows defining much more accurately the lateral and vertical limits of the rupture. Its length inferred from geodesy is ∼15 km and its modelled upper edge ∼17 km. The seismic moment then constrains the lower edge to coincide, within a few kilometres, with the Moho interface. The absence of seismicity in the shallow crust above the co-seismic fault is interpreted as a result of the decoupling effect of possible presence of salt layers above the rupture area, near 14 to 16 km in depth, which favours our interpretation of an immature strike-slip fault system, compatible with the absence of surface ruptures. The immature character of this large crustal fault is further suggested by the high variability of focal mechanisms and of fault geometries deduced from aftershock clusters, in the strike direction. Its geometry and mechanism is consistent with the crustal shear, striking NNE, revealed by GPS in this region. This shear and faulting activity might be generated by the differential slip rate on the subduction interface, 50 km to the south, leading to a north-northeastward propagating strike-slip fault zone. The wide extension of the aftershock distribution forming a NNE alignment, beyond the rupture area towards the north, suggests a localization process of the shear strain, which could be the preliminary stage of fault propagation further to the NNE. An alternative speculative model for this regional stress could be the existence of a well-developed NNE striking shear zone within the uppermost mantle, marking at depth the southward propagation of the northern branch of the North Anatolian fault. Both models may not be exclusive of each other, and in fact their sources may be mechanically interdependent.

Neotectonics of the Zanjan–Kazvin area, Central Iran: Left lateral strike‐slip induced restraining stepovers

The neotectonics of the Zanjan-Kazvin area are dominated by the region-wide E-W trending left lateral strike-slip faults. These faults create restraining and releasing stepovers indicating strain partitioning. Instead of the NW-SE trending long range-parallel continuous thrust faulting suggested in previous studies, this paper demonstrates that there are E-W trending range-crossing left lateral strike-slip faults creating short range-parallel thrust faults in the restraining stepovers. Establishing this strike-slip-induced structural style might contribute to an understanding of the nature of the Elburz (Alborz) Mountains located between the Central Iran and Caspian blocks.

The Moreira Gomes deposit of the Cuiú-Cuiú goldfield: Fluid inclusions and stable isotope constraints and implications for the genesis of granite-hosted gold mineralization in the Tapajós Gold Province, Brazil

Moreira Gomes is a recently discovered deposit (21.7 t Au) of the Cuiú-Cuiú goldfield, Tapajós Gold Province, Amazonian Craton. The mineralized zone is about 1200 m long, 30–50 m wide, and at least 400 m in depth. The zone is controlled by a subvertical, east–west-trending structure that is related to a left lateral strike-slip fault system. The host rocks are predominantly tonalites of the Creporizão Intrusive Suite (1997 ± 2 Ma) of uncertain tectonic setting (magmatic arc or post-collision). Hydrothermal alteration and mineralization are predominantly of the fissure-filling type and locally pervasive. Sericitization, chloritization, sulfidation, silicification, carbonatization and epidotization are the observed alteration types. Pyrite is the predominant sulfide mineral and bears inclusions of chalcopyrite, galena, sphalerite and minor hessite and bismuthinite. Gold occurs predominantly as inclusions in pyrite and subordinately in the free-milling state in quartz veins. Ag, Pb and Bi have been detected by semi-quantitative EDS analysis.Three types of fluid inclusions, hosted in quartz veins and veinlets, have been identified. (1) one- and two-phase CO2 inclusions; (2) two- and three-phase H2O–CO2-salt inclusions, and (3) two-phase H2O-salt inclusions. The CO2-bearing types are interpreted as the product of phase separation of an immiscible fluid. This fluid presents low to moderate density, low to moderate salinity (1.6–11.8 wt.% NaCl equivalent) and was trapped at 280° to 350 °C. The chemical system of the aqueous inclusions may contain CaCl2 and/or MgCl2, salinity varies from zero to 10.1 wt.% NaCl equivalent. Only locally salinities up to 25% have been recorded. This fluid was trapped between 120° and 220 °C and is interpreted as resulting from mixing of a hotter and more saline aqueous fluid (in part derived from phase separation of the H2O–CO2 fluid) with a cooler and dilute aqueous fluid.The δ34S values of pyrite (−0.3‰ to 3.9‰) are probably related to magmatic sulfur. The isotopic composition of inclusion fluids and of the fluid in equilibrium with hydrothermal minerals (quartz, chlorite, and calcite) show δ18O and δD values that range from +0.5 to +9.8‰, and from −49 to −8‰, respectively. Mineral pairs show equilibrium isotopic temperatures that are compatible with the fluid inclusion homogenization temperatures and with textural relationships of the hydrothermal minerals.Isotopic results combined with mineralogical and fluid inclusion data are interpreted to reflect a magmatic-hydrothermal system that evolved in at least three stages. (1) Exsolution of a CO2-bearing magmatic fluid between 400 °C and 320–350 °C and up to 2.1 kbar (6 km in depth) followed by phase separation and main precipitation of the hydrothermal assemblage composed of chlorite–sericite–pyrite–quartz-gold. (2) Cooling and continuous exsolution of CO2 produced a CO2-depleted and slightly more saline aqueous fluid that was trapped mainly at 250°–280 °C. The predominant hydrothermal assemblage of stage 1 continued to form, but epidote is the main phase at this stage. (3) Mixing of the stage 2 aqueous fluid with a cooler and dilute aqueous fluid of meteoric origin, which was responsible for the main carbonatization phase. The mineralizing fluid was neutral to slightly alkaline and relatively reduced. H2S (and/or HS-) might have been the main sulfur species in the fluid and Au(HS)2- was probably the gold transporting complex. Gold deposition occurred as a consequence of a combination of mechanisms, such as phase separation, mixing and fluid-rock interaction.The Moreira Gomes is a granite-hosted gold deposit that is interpreted to be a product of a magmatic-hydrothermal gold system. The age of ore formation (∼1.86 Ga) is consistent with the final stages of evolution of the widespread high-K, calc-alkaline Parauari Intrusive Suite, although the transitional to predominantly alkaline Maloquinha Intrusive Suite cannot be ruled out. Notwithstanding, the deposit does not show the classic features of (oxidized or reduced) intrusion-related gold deposits of Phanerozoic magmatic arcs.

Geological structures inferred from airborne geophysical surveys around Lützow-Holm Bay, East Antarctica

The area around Syowa Station, the Japanese Antarctic wintering station in Lützow-Holm Bay, is widely considered as a junction of the continents of Africa, India, Madagascar, and Antarctica, according to a reconstruction model of Gondwana that considers the suture between East and West Gondwana. This area is therefore key for investigating the formation of Gondwana. To reveal the tectonic evolution that contributed to Gondwana's formation in this area, joint Japanese-German airborne geophysical surveys were conducted around Syowa Station in January 2006 during the 47th Japanese Antarctic Research Expedition, from 67° S to 73° S latitude and from 35° E to 45° E longitude. Ice radar, magnetic, and gravity data were obtained from onshore areas. Several characteristic features that are possibly related to the tectonic evolution of Gondwana were inferred, primarily from magnetic anomalies, as well as from gravity anomalies and bedrock topography. The boundaries of the Lützow-Holm Complex, the Yamato-Belgica Complex, and the Western Rayner Complex are defined, but the inland extension of the boundary between the Lützow-Holm and the Yamato-Belgica Complexes is unknown south of 71° S. The main geological structural trends of the Lützow-Holm Complex derived from magnetic anomalies are NW–SE and are concordant with the geological results in the coastal region. However, nearly NE–SW-trending magnetic anomalies cut across the NW–SW magnetic anomaly trends, and NE–SW right lateral strike-slip faults were deduced from the magnetic and the gravity anomaly data of the Lützow-Holm Complex. The Lützow-Holm Complex was sub-divided into four blocks based on the estimated strike-slip faults. These strike-slip faults may have been generated during a younger stage of Pan-African orogeny, after the formation of NW–SE-striking geological structures. Cape Hinode, which is considered an allochthonous unit in the Lützow-Holm Complex according to its surface geology, may have originated from the Rayner Complex and been displaced by right lateral strike-slip motions. These results provide new constraints on the tectonic evolution of Gondwana during the Pan-African orogeny.

Geometry of co-seismic surface ruptures and tectonic meaning of the 23 October 2011 Mw 7.1 Van earthquake (East Anatolian Region, Turkey)

The 23 October 2011 Mw 7.1 (USGS, 2011a) Van earthquake was generated by the rupture of a main thrust fault plane striking N60°E and dipping 55°NW with a very small left-lateral offset in the western part of the Bardakçı-Saray thrust fault zone located in the north of the Bitlis-Zagros Suture Zone. The incompatibly between the subsurface seismologic data and surface deformation data indicates that the fault that generated the earthquake could be blind. The main thrust fault plane developed between the northern (Erciş) and southern (Van) continental blocks and indicates that the region is under the influence of a constant NW–SE pure compressional tectonic regime. Along with the primary thrust rupture, a secondary back thrust fault, a left lateral fault with NE strike and a right lateral fault with NW strike occurred in the contraction province. Additionally, the tensional ground cracks that developed on the hanging wall block of the ruptured thrust fault plane due to intra-continental collision are either transtensional that mainly fits to left lateral strike-slip faulting, or extensional deformations on the elevated block.

The Alpar canyon system in the Pannonian Basin, Hungary – its morphology, infill and development

Giant incised canyons were recently recognized in Late-Miocene post-rift sediments in the central part of the Pannonian Basin. Though not connected to the world seas, Lake Pannon shows significant signs of relative lake level variations controlled by tectonics and climate changes. The incision surface of the Alpar canyon system is connected to SB Pa-4 (6.8Ma sensu Vakarcs, 1997), earlier reported to represent a significant relative base-level fall in the basin, however, debated recently.Incised several hundred meters in the preexisting substrate, the individual canyon valleys of the Alpar canyon system are enormous in size and display a multi-story nature. They loose topographic expression headwards and basinward. Widths of individual valleys range from 5 to 10km, with smaller tributaries. In the study area several adjacent canyon valleys can be seen on seismic profiles. The valley depth is greatest near their confluence, where a major trunk valley (600–700m deep) was formed by eroding most of the Upper Miocene succession. The canyons are filled with clay marls. They are incised into an extremely thick aggrading deltaic complex and are overlain by fluvial sediments, suggesting a major transgression in between.The Late Miocene Alpár canyon system developed on the southern margin of the Mid-Hungarian Mobile Belt, the latter is characterized by NE-SW oriented fold axis and NE-SW oriented left lateral strike-slip faults. The canyon system coincides with a large releasing bend and/or extensional duplex of the Paks–Szolnok strike-slip system being active as sinistral during the Late Miocene.Presumably, the formation of the deep canyons was generated by the close interaction of several factors and events in space and time, among them tectonic uplift forced relative base-level fall, the reactivation and bending/duplexing of a strike-slip fault system located near the coeval zone of the lake shoreline and shelf edge, and the possible change of sediment supply carried by overfed rivers.

Rupture segmentation and slip partitioning of the mid-eastern part of the Kunlun Fault, north Tibetan Plateau

The Kunlun Fault, an active fault on the border between the Bayan Har and Kunlun-Qaidam blocks, is one of the major left lateral strike-slip faults in the Tibetan Plateau. Previous research has not reached a consensus on agreeable slip rates along much of its length and the slip rate gradient along the eastern part, both of which play critical roles in a range of models for the eastward extrusion and thickened crust of the Tibetan Plateau. New slip rates have been determined at sites along the eastern part of the Kunlun Fault by dating deposits and measuring atop displaced fluvial terrace risers. Field investigations and interpretation of satellite images reveal geometrical features of the fault and the late Quaternary offset, new earthquake ruptures and surface-rupturing segmentation, from which long-term slip rates and earthquake recurrence intervals on the fault are estimated. The tectonic geomorphology method has determined that the long-term horizontal slip rates on the Tuosuohu, Maqin and Maqu segments from west to east are 11.2±1, 9.3±2, and 4.9±1.3 mm/a while their vertical slip rates are 1.2±0.2, 0.7±0.1, and 0.3 mm/a in the late Quaternary. Results indicate that the slip rates regularly decrease along the eastern ∼300 km of the fault from >10 to <5 mm/a. This is consistent with the decrease in the gradient such that at the slip rate break point is at the triple point intersection with the transverse fault, which in turn is transformed to the Awancang Fault. The vector decomposition for this tectonic transformation shows that the western and eastern branches of the Awancang Fault fit the slip-partitioning mode. The slip rate of the southwestern wall is 4.6 mm/a relative to the northeastern wall and the slip direction is 112.1°. The mid-eastern part of the Kunlun Fault can be divided into three independent segments by the A’nyemaqen double restraining bend and the Xigongzhou intersection zone, which compose the surface rupture segmentation indicators for themselves as well as the ending point of the 1937 M7.5 Tuosuohu earthquake. The average recurrence interval of the characteristic earthquakes are estimated to be 500–1000 a, respectively. The latest earthquake ruptures occurred in AD 1937 on the western Tuosuohu segment, as compared to ∼514–534 a BP on the Maqin segment, and ∼1055 to 1524 a BP on the Maqu segment. This may indicate a unidirectional migration for surface rupturing earthquakes along the mid-eastern Kunlun Fault related to stress triggered between these segments. Meanwhile, the long-term slip rate is obtained through the single event offset and the recurrence interval, which turn out to be the same results as those determined by the offset tectonic geomorphology method, i.e., the decreasing gradient corresponds to the geometrical bending and the fault’s intersection with the transverse fault. Therefore, the falling slip rate gradient of the mid-eastern Kunlun Fault is mainly caused by eastward extension of the fault and its intersection with the transverse fault.

The Puerto Armuelles Earthquake (Southwestern Panama) of July 18, 1934

The Puerto Armuelles Earhquake of July 18 1934 and its six main aftershocks nave been relocated using a new version of Dewey's Joint Hypocenter Determination program: JHD89. The main event focal mechanism has been determinated from historical seismograms first arrivals. The results of this study suggest that the active Panama, Balboa and possibly the Mykland fracture zones extent through the southwestern Panama continental shelf up to connect with faults inland as proposed by BERRAGE & THORPE (1988). The Burica Peninsula medial fault zone, stratigraphically infered by CORRIGAN (1986) also seems to be active. This group of parallel northwest trending right lateral strike-slip faults interact, at least for large events. The focal mechanism solution of the 1934 Puerto Armuelles Earthquake indicates a right lateral strike slip solution with a normal component.

The May 29th 2008 earthquake aftershock sequence within the South Iceland Seimic Zone: Fault locations and source parameters of aftershocks

On May 29th 2008, two $M_w$ ∼6 earthquakes struck the western part of the South Iceland Seismic Zone. The first event was followed within seconds by a similar size event on a second fault ∼5 km further west. Earthquakes, detected by a temporary network of 11 seismometers and three permanent SIL-network stations were located using an automated Coalescence Microseismic Mapping technique. The epicenters delineate two major and several smaller N–S faults as well as an E–W zone of activity stretching further west into the Reykjanes Peninsula Rift Zone. Fault plane solutions show right lateral strike slip mechanisms along the two major N–S faults and suggest both smaller N–S right-lateral strike slip faults further west as well as an E–W zone of left lateral strike slip fault. The aftershocks deepen from 3–5 km in the north to 8–9 km in the south, suggesting that the main faults dip southwards. A brief increase in aftershock seismicity is most likely caused by short-term static stress buildup on adjacent faults. The faulting is interpreted to be driven by the local stress due to transform motion between two parallel segments of the divergent plate boundary crossing Iceland.

Time-Domain Moment Tensors for shallow (h≤40km) earthquakes in the broader Aegean Sea for the years 2006 and 2007: The database of the Aristotle University of Thessaloniki

Abstract We present a catalog of moment tensor (MT) solutions and moment magnitudes, Mw, for 119 shallow (h ≤ 40 km) earthquakes in Greece and its surrounding lands (34°N–42°N, 19°E–30°E) for the years 2006 and 2007, computed with the 1D Time-Domain Moment Tensor inversion method (TDMT_INV code of Dreger, 2003 ). Magnitudes range from 3.2 ≤ Mw ≤ 5.7. Green's functions (GF) have been pre-computed to build a library, for a number of velocity profiles applicable to the broader Aegean Sea region, to be used in the inversion of observed broad band waveforms (10–50 s). All MT solutions are the outcome of a long series of tests of different reported source locations and hypocenter depths. Quality factors have been assigned to each MT solution based on the number of stations used in the inversion and the goodness of fit between observed and synthetic waveforms. In general, the focal mechanisms are compatible with previous knowledge on the seismotectonics of the Aegean area. The new data provide evidence for strike-slip faulting along NW–SE trending structures at the lower part of Axios basin, close to the heavily industrialized, and presently subsiding, region of the city of Thessaloniki. Normal faulting along E–W trending planes is observed at the Strimon basin, and in Orfanou Gulf in northern Greece. A sequence of events in the east Aegean Sea close to the coastline with western Anatolia sheds light on an active structure bounding the north coastline of Psara–Chios Islands about 20–25 km in length exhibiting right lateral strike-slip faulting.

Tectonics related to the North Anatolian Fault in the Sea of Marmara: evidence from seismic reflection data

The North Anatolian Fault crosses the Sea of Marmara from east to west. Tectonic features of the Sea of Marmara were studied using multi-channel deep seismic reflection data. The northern branch of the North Anatolian Fault is active as a right lateral strike-slip fault zone and indicates both negative and positive flower structures. The North Anatolian Fault splays into two faults at the Sea of Marmara as a northern branch and north segment of the southern branch. The northern branch named the Main Marmara Fault extends in a complicated manner from the north of the Kapidagi Peninsula to westward in the Sea of Marmara. The north segment of southern branch extends between the Gemlik and Bandirma gulfs in the south of the Sea of Marmara. In addition, uplift areas arose by compression and a push-up style in between the Kapidagi Peninsula and the Main Marmara Fault. The North Anatolian Fault is characterized by a negative flower structure in basins and push-up style in uplift areas in the Sea of Marmara. An uplift area arose between the north segment of the southern branch and the northern branch of the North Anatolian Fault. The north segment of the southern branch of the North Anatolian Fault is a strike-slip fault and displays a pull-apart style in the seismic reflection data.

Late quaternary seismic stratigraphy and active faults of the Gulf of İzmit (NE Marmara Sea)

The Upper Quaternary seismic stratigraphy and active faults of the Gulf of Izmit were investigated by means of high-resolution shallow seismic profiling data in the source region of 1999 Izmit earthquake. High-resolution seismic reflection data correlated with borehole data indicate that the stratigraphy of Izmit Bay consists of three distinct depositional sequences formed in response to middle Pleistocene-Holocene sea-level changes. Reflector R, separating the pre-Holocene sequences (1 and 2) from the Holocene sequence (3), represents an erosional unconformity produced by the subaerial fluvial erosion of the continental shelves at the time of the last glacial maximum. Occasional, anomalous reflections (acoustic turbidity) observed within the Holocene sequence are interpreted as gas accumulations. The maximum thickness of the Holocene sediments is found to be about 25 m. The isopach map of Holocene sediment implies that the thickness of the Holocene decreases from the east towards the central and western basins of Izmit Bay. Two distinct fault systems are interpreted in Izmit Bay. The main fault system extending roughly in an E-W direction along the Gulf of Izmit is an active right lateral strike slip fault with a normal component. The secondary faults are normal faults striking in different directions and these are identified as being both active and inactive. In addition, prominent compressive features are identified in the seismic cross-sections of some profiles acquired to the east of Hersek Peninsula where the focal mechanisms of the aftershocks of the 1999 Izmit earthquake also reveal predominantly reverse faulting mechanisms, as identified by a local dense seismic network.

The Late Paleozoic structural evolution of the Magnitogorsk Megazone in the southern Urals

Various models of the evolution of the southern Urals at the stage of the Late Paleozoic collision of the East European and Kazakhstan continents have been proposed [3‐5, 7, and others]. The diversity of geodynamic models is caused by poor knowledge of regional tectonics in terms of structural geology. The results of regional structural and tectonophysical studies in the Magnitogorsk Megazone allowed us to recognize and characterize two stages of the Late Paleozoic (postDevonian) tectonic deformations: (1) thrusting in the Middle Carboniferous and (2) strike-slip faulting in the left-lateral transpression regime in the Middle Carboniferous‐Permian. Middle Carboniferous thrusting. The most intense tectonic deformations were confined to the flanks of the megazone within the Main Ural and East Magnitogorsk boundary fault zones, where the high-amplitude counter-dipping listric thrust faults were formed at a depth of 25‐30 km (Figs. 1, 2). The Late Paleozoic thrust structure of the Main Ural Fault Zone is characterized by nappes of higher orders deformed into NNEstriking antiforms and synforms. The westward verging reverse and thrust faults of the second generation are conjugated with the folds. The identification of these structural elements is difficult because of the distortion of the East Magnitogorsk Fault Zone by late strike-slip faults. They are represented by fragments of bivergent imbricate reverse and thrust faults retained in shear slices. The inner sectors of the Magnitogorsk Megazone are characterized by thin submeridional thrust and reverse faults with an amplitude of a few kilometers, which are accompanied by subordinate transfer faults and folds of mainly the open type (Figs. 1‐3a). The transfer faults bound the thrust and reverse fault zones, as well as imbricate packets therein, and serve as

Structural pattern at the northwestern sector of the Tepic-Zacoalco rift and tectonic implications for the Jalisco block, western Mexico

AbstractAnalysis of the aeromagnetic anomalies over the northwestern sector of the Tepic-Zacoalco rift documents a NE-SW pattern of lineaments that are perpendicular to the inferred NW-SE boundary between the Jalisco block and the Sierra Madre Occidental. The boundary lies within the central sector of the Tepic-Zacoalco rift immediately north of the Ceboruco and Tepetiltic stratovolcanoes and extends up to the San Juan stratovolcano, where it intersects the NE-SW magnetic anomaly lineament that runs toward the Pacific coast (which intersects two volcanic centers). This N35°E lineament separates the central rift zone of low amplitude mainly negative anomalies (except those positive anomalies over the stratovolcanoes) from the zone to the north and west characterized by high amplitude positive long wavelength anomalies. The NE-SW lineament is parallel to the western sector of the Ameca graben and the offshore Bahia de Banderas graben and to the structural features of the Punta Mita peninsula at the Pacific coast, and thus seems to form part of a regional NE-SW pattern oblique to the proposed westward or northwestward motion of the Jalisco block. The orientation of this regional structural pattern at the northern end of the Tepic-Zacoalco rift seems consistent with proposed dominant SW-directed extension along the rift during the Pliocene and Quaternary, rather than with NW-SE lateral strike-slip faulting. The orthogonal pattern that characterizes the northernmost boundary of the Tepic-Zacoalco rift is oblique to the pattern observed in the Grande de Santiago river (which conforms the northern limit of the rift) and for the central-eastern sectors of the Ameca graben (south of the rift). This spatial arrangement of major lineaments and structural elements points to a complex tectonic history for the region that includes the rifting of the Gulf of California and margin deformation due to plate convergence and kinematic re-organization events, and which may have resulted in distinct episodes of left-lateral, right-lateral and normal faulting in Neogene times, as proposed in recent studies.

Neogene tectonic evolution of the Gibraltar Arc: New paleomagnetic constrains from the Betic chain

New paleomagnetic results from Neogene sedimentary sequences from the Betic chain (Spain) are here presented. Sedimentary basins located in different areas were selected in order to obtain paleomagnetic data from structural domains that experienced different tectonic evolution during the Neogene. Whereas no rotations have been evidenced in the Late Tortonian sediments in the Guadalquivir foreland basin, clockwise vertical axis rotations have been measured in sedimentary basins located in the central part of the Betics: the Aquitanian to Messinian sediments in the Alcalà la Real basin and the Tortonian and Messinian sediments in the Granada basin. Moreover, counterclockwise vertical axis rotations, associated to left lateral strike-slip faults have been locally measured from sedimetary basins in the eastern Betics: the Middle Miocene to Lower Pliocene sites from the Lorca and Vera basins and, locally, the Tortonian units of the Huercal-Overa basin. Our results show that, conversely from what was believed up to now, paleomagnetic rotations continued in the Betics after Late Miocene, enhancing the role of vertical axis rotations in the recent tectonic evolution of the Gibraltar Arc.

CO 2 emissions and heat flow through soil, fumaroles, and steam heated mud pools at the Reykjanes geothermal area, SW Iceland

Carbon dioxide emissions and heat flow through soil, steam vents and fractures, and steam heated mud pools were determined in the Reykjanes geothermal area, SW Iceland. Soil diffuse degassing of CO 2 was quantified by soil flux measurements on a 600 m by 375 m rectangular grid using a portable closed chamber soil flux meter and the resulting data were analyzed by both a graphical statistical method and sequential Gaussian simulations. The soil temperature was measured in each node of the grid and used to evaluate the heat flow. The heat flow data were also analyzed by sequential Gaussian simulations. Heat flow from steam vents and fractures was determined by quantifying the amount of steam emitted from the vents by direct measurements of steam flow rate. The heat loss from the steam heated mud pools was determined by quantifying the rate of heat loss from the pools by evaporation, convection, and radiation. The steam flow rate into the pools was calculated from the observed heat loss from the pools, assuming that steam flow was the only mechanism of heat transport into the pool. The CO 2 emissions from the steam vents and mud pools were determined by multiplying the steam flow rate from the respective sources by the representative CO 2 concentration of steam in the Reykjanes area. The observed rates of CO 2 emissions through soil, steam vents, and steam heated mud pools amounted to 13.5 ± 1.7, 0.23 ± 0.05, and 0.13 ± 0.03 tons per day, respectively. The heat flow through soil, steam vents, and mud pools was 16.9 ± 1.4, 2.2 ± 0.4, and 1.2 ± 0.1 MW, respectively. Heat loss from the geothermal reservoir, inferred from the CO 2 emissions through the soil amounts to 130 ± 16 MW of thermal energy. The discrepancy between the observed heat loss and the heat loss inferred from the CO 2 emissions is attributed to steam condensation in the subsurface due to interactions with cold ground water. These results demonstrate that soil diffuse degassing can be a more reliable proxy for heat loss from geothermal systems than soil temperatures. The soil diffuse degassing at Reykjanes appears to be strongly controlled by the local tectonics. The observed diffuse degassing defines 3–5 elongated N–S trending zones (000–020°). The orientation of the diffuse degassing structures at Reykjanes is consistent with reported trends of right lateral strike slip faults in the area. The natural CO 2 emissions from Reykjanes under the current low-production conditions are about 16% of the expected emissions from a 100 MWe power plant, which has recently been commissioned at Reykjanes.