Extensional Activity Research Articles

Lamprophyric rock locations in Greece

Abstract Twenty-four areas with lamprophyric formations have been located through a bibliographic search in Macedonia, Thrace, the islands and Attica. Most lamprophyre types have been identified including rare “alkali minette”. In most localities the dikes/sills appear to be late mantle products associated with deep faulting following extensional activity in granitoids.

Ediacaran extension along the northern margin of the Yangtze Platform, South China: Constraints from the lithofacies and geochemistry of the Doushantuo Formation

Abstract The Ediacaran Doushantuo Formation (Fm) of the Yangtze Platform provides excellent sedimentary records of the Precambrian tectonic evolution of the South China Block related to the final breakup of the Rodinia supercontinent. However, little is known about its sedimentary structural evolution on the northern margin of the Yangtze Platform. This paper presents a detailed lithofacies description and geochemical analysis on the Doushantuo Fm in the vicinity of Chengkou at the northern margin of the Yangtze Platform, in order to understand its tectonic setting and paleogeography. Abrupt variations in stratal thickness and lithofacies over a narrow area, abundant gravity-driven sedimentary structures, and distributions of phosphorite and manganese deposits suggest that both sides of the Chengkou sub-basin were dominated by shallow-water to slope environments, while the center of this sub-basin featured a deep-water environment. Geochemical indicators reveal a clear influence of hydrothermal fluid on the Doushantuo Fm in the center of the Chengkou sub-basin, implying strong synsedimentary extensional activity. These characteristics define a southwest-oriented deep-water subsidence zone (Chengkou sub-basin) that is approximately perpendicular to the strike of the northern margin of the Yangtze Platform. Combined with previous studies, our results indicate that the northern margin of the Yangtze Platform experienced significant extension during the Ediacaran period, which is suggested to be related to the latest breakup of the Rodinia, resulting in the occurrence of alternating uplifts and depressions in a pattern like that of a horst-graben suite. Such widespread extensional activity might have affected the whole Yangtze Platform, with sub-basins like Chengkou extending into its interior, which is of great significance to hydrocarbon exploration.

Chronology, petrology, geochemistry and tectonic environment of the Bailingshan complex in Eastern Tianshan, Xinjiang, NW China

The Bailingshan complex distributed in the Jueluotage tectonic belt of Eastern Tianshan is composed of quartz monzodiorite, granodiorite, monzogranite, and granite. The petrography and petrochemistry of the complex have the features of metaluminous–high K calc‐alkaline series. The ages obtained by zircon U–Pb dating from different rocks indicate that quartz monzodiorite, granodiorite, monzogranite, and granite formed in 314.2 ± 3.5, 302.5 ± 3.8, 298.2 ± 3.6, and 310.9 ± 4.7 Ma, respectively, which is equivalent to the Late Carboniferous–Early Permian. The unconformity between the Lower Carboniferous and Upper Carboniferous in the Jueluotage tectonic belt indicates that the collisional orogen finished in the later period of the Early Carboniferous in this area, and that it began to enter the post‐collisional extension from the Late Carboniferous. The magmatic intrusion is a sign of the extensional activity of the Jueluotage tectonic belt in the Late Carboniferous–Early Permian.

Structural imaging of the East Beni Sueif Basin, north eastern Desert, Egypt

The East Beni Sueif Basin is the only tested hydrocarbon-bearing basin on the eastern side of the Nile in Egypt. The basin is located around 150 km to the south of Cairo. This work introduces the first attempt of seismic interpretation and structural patterns of this basin, for which subsurface published works are lacking. Structural imaging of the area is achieved through interpretation of pre-stack time migration (PSTM) seismic cube and data sets of seven wells. The penetrated sedimentary section is represented by Albian-Middle Eocene sediments.The East Beni Sueif Basin is a type of the whole graben-system and is bounded by two NW-SE bounding faults. These faults had continued activity in an extensional regime associated with fault-propagating folds. The basin is traversed by a N75°E-trending fault system at basement level. This fault system separates the basin into two structural provinces. The Northwestern Province is deeper and shows more subsidence with a predominance of NW-trending longitudinal faults and N60·W oblique faults to the basin trend. The Southeastern Province is shallow and crossed by N14·W-trending faults which are slightly oblique to the basin axis.Albian time had witnessed the main extensional tectonic phase and resulted in major subsidence along basin-bounding faults associated with growth thickening of basal deposits. During Senonian time, the basin experienced a mild phase of transtensional tectonics, which formed negative-flower structures entrapping different folds along the N75°E and N60·W faults. The timing and style of these structures are similar to the Syrian-Arc structures in several Western Desert oil fields. The basin emerged during the Paleocene with scoured and eroded top Cretaceous sediments. Subsidence was resumed during the Early Eocene and resulted in 1500 m-thick carbonate sediments. Lastly, a mild extensional activity possibly occurred during the Oligocene-Miocene time.Despite the possible restricted potentiality of the source rock, the main hydrocarbon accumulation risk is attributed to retention in traps of long-span tectonic history. Reaching of main faults to surface through brittle carbonate cap rocks and limited thickness of the shale in the reservoir section risk hydrocarbon sealing. Buried structures of passive setting during the Tertiary show a minor trapping risk.

Fractures system within Qusaiba shale outcrop and its relationship to the lithological properties, Qasim area, Central Saudi Arabia

The basal Qusaiba hot shale member of Qalibah Formation is considered to be an important source rock in the Paleozoic petroleum system of Saudi Arabia and an exploration target for tight shale as one of the Unconventional resources of petroleum. This work has been carried out to understand the fractures network of Qusaiba shale member in outcrops located to the west of Qusayba' village in Al-Qasim area, Central Saudi Arabia. The main objective of this study is to understand the distribution of natural fractures over different lithological units. Description data sheets were used for the detailed lithological description of Qusaiba shale member on two outcrops. Spot-7 and Landsat ETM+ satellite images were used for lineament mapping and analyses on a regional scale in a GIS environment. Fractures characterization in outcrop-scale was conducted by using linear scanline method. Qusaiba shale member in the study area consists of 5 main lithofacies, divided based on their sedimentary structures and petrographical properties, from base to top in the outcrops, the lithofacies are; fissile shale, very fine-grained micaceous siltstone, bioturbated mudstone, very fine to fine-grained hummocky cross-stratified sandstone, and fine to medium-grained low/high angle cross-stratified sandstone lithofacies. Lineaments interpretation of the Spot-7 and Landsat ETM+ satellite images showed two major directions in the study area; 320° that could be related to Najd fault system and 20° that could be related to the extensional activities which took place after Amar collision. Fractures are much denser in the fissile shale and mudstone lithofacies than sandstones lithofacies, and average spacing is smaller in the fissile shale and mudstone lithofacies than sandstones lithofacies. Lineaments and large-scale fractures are Non-Stratabound fractures and they deal with the area as one big mechanical unit, but small-scale fractures are Stratabound fractures that propose different mechanical units within Qusaiba shale member in the study area. The fractures network in the study area has a wide range of properties related to fractures density, length, spacing, height, and termination degree. The conceptual multi-scale model divides the fractures in the study area into 4 orders depending on the available data that have been observed from satellite images and field. The multi-scale fractures model that was generated in this study could help to understand the distribution of stratigraphically controlled fractures when integrated with flow simulation models. Overall, this work could have a significant contribution to tight shale exploration plans in the subsurface by providing some knowledge about the fractures mechanical behavior of the lower part of Qusaiba shale member of Qalibah Formation.

Landscape evolution of the graben of Puerto Vallarta (west-central Mexico) using the analysis of landforms and stream long profiles

The analysis of landforms and stream long profiles are useful tools that can provide information about the tectonic activity in the landscape. The west-central portion of Mexico is a region that is mostly dominated by an extensional tectonics initiated in the Miocene and continuing in the Quaternary, where there has also been a vast emission of volcanic products. Of particular importance is the graben of Puerto Vallarta which was formed during the extensional activity that opened the Gulf of California in the Miocene. Nevertheless, the geomorphology and landscape evolution of this structure has not been studied in detail. Here I present a study of the rivers, landforms and topography of Puerto Vallarta graben in order to assess its evolution, focusing on the evidences of its initial phase of formation. The geomorphological map elaborated in this study reveals the presence of lava flows and volcanic structures in the eastern sector of the graben. Here it is proposed that this volcanism occurred during the formation of the graben in the Miocene, nevertheless, radiometric dating of lavas is still required to determine the precise timing of this event. Analysis of stream long profiles confirms that knickpoints are propagating in the fluvial network at a rate of ∼0.07 to ∼0.7 mm yr−1 and these were probably triggered by a drop in the base-level of rivers during the graben formation. The rates of knickpoint retreat in the graben of Puerto Vallarta are slightly lower than those observed in other recent (Quaternary) tectonic areas, however, the migration rates of the graben of Puerto Vallarta were probably higher at the onset of the faulting. Here the analysis of landforms and stream long profiles were useful to elucidate one part of the evolution of the graben of Puerto Vallarta. This study emphasizes the importance of incorporating the tectonic geomorphology in future studies of the extensional zones of west-central Mexico.

Major unconformities/termination of extension events and associated surfaces in the South China Seas: Review and implications for tectonic development

The distribution of unconformities and end of Cenozoic rifting events in the South China Seas (SCS) reflects both the modes of rift development, and the effects of driving mechanisms. Continental rifting began in the eastern basins during the Paleocene, and propagated westwards to the Vietnam basin margin in the Late Eocene. Continental breakup around 32–28Ma caused a regional reduction or cessation in extensional activity, particularly affecting basins furthest from the spreading centre. Basins in the slope and deepwater area north of the spreading centre exhibit reduced fault activity until 21–20Ma. Propagation of oceanic crust westwards between ∼25 and 23Ma, and termination of seafloor spreading sometime between 20.5 and 16Ma affected fault activity in the Qiongdongnan, and Nam Con Song basins. In the Phu Khanh Basin and South, in the Dangerous Grounds area, extension continued until about 16Ma, ending at the Red Unconformity. The end of seafloor spreading around 20.5Ma reflects loss of extensional driving force as thinned continental crust entered the NW Borneo subduction zone. Controversially, a key component of the driving force maybe attributed to slab-pull. A transitional period of about 5–7my between the onset of subduction of continental crust, and final jamming of the subduction zone (Deep Regional Unconformity, DRU) is inferred. The last pulse of extension was focussed in the western SCS, and terminated around 10.5Ma. Detailed understanding of proto South China Seas development remains uncertain and controversial.

Characteristics and Numerical Simulation of Extensional Structure Since Late Mesozoic in Western Shandong, China

The extensional structure in western Shandong is composed of steep normal fault and decollement fault, which two together constitute a unique “extensional structure framework”. With the regional geological survey of West Shandong Rise and seismic data from Jiyang Depression, we make comprehensive researches on extensional structural features in western Shandong. A number of NW-trending and NWW-trending normal faults, together with several NE-trending and nearly EW-trending faults, constitute a typical “fault-block style” all over the research area. In the steep normal fault zone, there are dynamic breccia, fault clay and scraping trace making up signs of faults activity. Low-angle decollement faults are mainly found at unconformity / lithologically abrupt interfaces or disconformity surfaces, of which the most outstanding in western Shandong occurred along unconformity surface between the Archean and Cambrian (Ar/∈), and the disconformity surface between the Ordovician and Carboniferous (O/C). Based on the finite element method, we use the software of Ansys12.0 to carry out tectonic stress field numerical simulation of the evolution in research area since late Mesozoic. The result indicates that the strength of the tectonic stress field has experienced a process from strong to weak since late Mesozoic. The evolution of extensional structures of western Shandong commenced in late Jurassic, with the direction of NE-SW. And the research area entered its first massive extended fault-depression phase in Late Jurassic-Early Cretaceous (140 - 65 Ma). Then the extension continued in Paleocene-Early Eocene (65 - 53 Ma) which was a transforming transitional period with its extending orientation transformed from NE-SW to SN. In Eocene-Oligocene (53 - 23.3 Ma) the extension met its large-scale extensional activity with the direction of NW-SE. As can be seen in the study, the essential dynamic origin is mainly influenced by the structural changes since late Mesozoic, and the chief factors leading to the tectonic stress field change are the subduction of the Pacific Plate and the strike slip motion of the boundary faults. Key words: Numerical simulation; Extensional structures; Since late Mesozoic; Western Shandong

Deformation characteristics and formation mechanism of the Yunmengshan metamorphic core complex

The Yunmengshan metamorphic core complex in the middle part of the Yanshan Fold and Thrust Belt records crust extension processes of the eastern North China Craton during its peak destruction. Development of the metamorphic core complex was controlled by the generally NNE-striking Dashuiyu Shear Zone. The shear zone dips SE and becomes shallower NE-wards, leading to exposures of a ductile shear zone in the southern and middle parts and brittle faults in the northern part. Exposure structures, microstructures, and quartz C- axis fabrics indicate that the ductile shear zone belongs to an extensional shear zone with a top-to-the-SE shear sense. Deformation temperatures of 300-520 ℃ suggest a midcrustal origin for the ductile shear zone. A ductile deformation belt in the footwall of the shear zone is only as wide as 1-3 km, indicating no widespread mid-crustal ductile flow in the region during the deformation. Zircon U-Pb dating of dykes and plutons as well as hornblende and biotite 40 Ar/ 39 Ar dating demonstrate that the metamorphic core complex originated at 135 Ma and experienced intense shearing of the Dashuiyu Shear Zone, development of the supradetachment basins, and synkinematic intrusion during 135-125 Ma. The metamorphic core complex was subjected to rapid exhumation during 125-114 Ma when the Dashuiyu Shear Zone suffered continuous activity and passive doming. The shear zone and its hanging wall were cut or replaced by a series of brittle faults when they were uplifted to a brittle regime, showing that exhumation took place in continuous extensional activities. The metamorphic core complex turned into slow exhumation in an extensional regime in the following latest Early Cretaceous. The evolution history suggests that the Yunmengshan metamorphic core complex was developed by the rolling-hinge model, a common formation mechanism for intraplate metamorphic core complexes in the North China Craton, under the continuous NW-SE extension during the Early Cretaceous (135-100 Ma).

Organic matter appraisal of the Lower Eocene (Ypresian) Bou Dabbous Formation in Tunisia

Various facies characterize the Lower Eocene (Ypresian) that had been named formations which include the Bou Dabbous Formation. It is one of the four lateral equivalents of the Metlaoui Group and consists of micrite and marls with abundant planktonic foraminifera (Globigerina). In this paper, we report on the organic matter (OM) analyses of the Bou Dabbous Formation from nine outcrop cross sections in central and northern Tunisia. The total organic carbon (TOC) values and Rock-Eval analysis showed that this formation had preserved type II/III marine OM. Locally, such as at Oueds Sidi Mansour and Bou Jabia, TOC values are very high. These values are reliable to the structural framework (subsiding grabens) during the deposition of the Bou Dabbous Formation. Biomarker (steranes and terpanes) analysis showed that this OM is very rich in these compounds. Our results exposed that the Bou Dabbous Formation OM is represented by an open planctonic to estuarine algal facies with bacterial contribution as revealed by the relative abundance of the C27 (37–43 %), C28 (26–31 %), and C29 (30–34 %) steranes and the dominance of the hopane compounds for all the samples. The good preservation of OM in the Bou Dabbous Formation has been evidenced by the high abundance of gammacerane and the preservation of the C34 and C35 homohopanes in central Tunisia (Oueds Souinia and Sidi Mansour) compared to the northern part (Tellian Atlas). We interpreted that the development of anoxic conditions in central Tunisia had been induced by water stratification of super-salt water under arid and warm climate. These results concord with the structural and the paleogeographic setting where the Bou Dabbous Formation in central Tunisia was deposited in subsiding grabens separated from the open sea by high sills (horst). In northern Tunisia, eastern upwelling waters rich nutriments enhanced primary organic productivity and therefore the development of anoxic conditions. Rock-Eval maximum pyrolysis temperature (T max) values fitting with the interval 425–454 °C show that the OM thermal maturity ranges from immature (central Tunisia), marginally mature (Bizerte region) to the peak of oil generation window (NW Tunisia). We showed that these thermal maturity differentiations were related to the geodynamic evolution that had gone separately central and northern Tunisia. Therefore, the Bou Dabbous formation in central Tunisia had gone subsidence inversion and tendencies to emersion since the Late Eocene epoch and consequently not enough to reach the oil generation window. On the other hand, at the Oligocene–Early Miocene, extensional activity favored dissymmetric subsidence where northern Tunisia (Tellian Atlas) had gone active subsidence associated with high thermal flux in relationship to magmatic activities. Consequently, this feature explains the thermal maturation of the Bou Dabbous OM at this province.

Retroarc extension in the last 6 Ma in the South-Central Andes (36°S–40°S) evaluated through a 3-D gravity modelling

Abstract The Andean retroarc between 35° and 40°S is the locus of debate regarding its Pliocene to Quaternary tectonic setting. Retroarc volcanic eruptions since 6 Ma to the Present are, based on some hypotheses, associated with widespread extension. In these works, geological data point to the existence of normal faults affecting previous (Late Cretaceous to Miocene) contractional structures. In order to evaluate such interpretations we have collected data from various geological and geophysical studies and scales. Based on these data, an existing large-scale 3-D gravity model could be improved and used to investigate the lithospheric structure of this region. Moreover, using the gravity model, an attenuated crust could be localized and quantified throughout the retroarc area. Deep seismic data available from this region are limited to the forearc – arc area, while in general the retroarc zone lacks deep seismic constraints. The only deep seismic profile extending to the retroarc is a receiver function profile at 39°S, showing crustal attenuation. This observation correlates with the extensional activity recognized at the surface. When analysing the gravity field, positive residual anomalies are observed. They correlate with crustal attenuation at the areas of extension. Also, computed elastic thickness in the retroarc shows good correlation between the areas of crustal stretching and low flexural rigidity, explained by thermal processes. The present extensional deformation reflected in positive residual gravity anomalies points to the influence of reactivated Triassic rifting inherited from early phases of Pangea break-up. Finally, the present local uplift and consequent fluvial incision at the retroarc zone are explained by crustal stretching and not by crustal shortening, the common mechanism in Andean orogenesis.

Andean evolution of the Aluminé fold and thrust belt, Northern Patagonian Andes (38°30′–40°30′S)

The Aluminé fold and thrust belt between 38°30′ and 40°30′S is the result of two periods of progression of deformation toward the foreland. The chronology of deformation and its relationship with magmatism through time show spatially and temporally separated magmatic events closely linked to distinct deformational stages. Data presented here confirms a Late Cretaceous mountain-building phase that coexisted in space and time with an eastward arc-migration. During this stage, a belt of deformation expanded through the foreland where it produced the Southern Neuquen Precordillera. This eastern independent mountain grew separately from the main Andean axis through a combination of inversion of the old rift systems and interaction with a pre-Andean belt which acted as a foreland obstacle. On the basis of tectonostratigraphic controls we define the last Andean contractional phase between the Late Miocene and the Pliocene. This event induced the reactivation of both sectors of the fold and thrust belt with minor propagation toward the foreland, leading to the uplift of the Patagonian Andes and reshaping the Southern Neuquén Precordillera. Both intervals of shortening are separated by a period of localized extension that resulted in the development of the Collón Cura basin within this Andean segment. Here, large thicknesses of volcanosedimentary sequences accumulated contemporaneously with the extensional activity between the earliest Oligocene and the Early Miocene.

An investigation of educational needs of Guilan- Iran's tea-planters

Agriculture has various roles on the development of Iran in different aspects. The most obvious roles are: food preparation, rural's migration prevention to urban areas, preservation of economical and political independence and material preparation for many industries. Tea is one of the crops among agricultural productions and is still confronted with a series of poor natural and managerial barriers and lack of modern technology as well because of its key role including the most popular drink among Iranians, job creation, currency loss prevention and so on. This research was conducted with the goal of investigation of educational needs of Guilanian tea-planters in 2006 in Guilan province, Iran. 80% of domestic tea in Iran is produced in Guilan province. Statistical population of this research is farmers who are known as tea-planter according to the local norms. Among 67681 Guilanian tea-planters, 280 ones were selected randomly using Cochran's cluster test (formula) to represent the whole population. Independent variables of this research are: age, educational level, range of income, under-cultivation level, social participation, sex (gender), participation in educational and extensional activities, and the use of communicative channels and their relationship were also considered and contrasted with educational needs of tea-planters. The major device for data collection was questionnaire, which was determined after its validity and reliability. For data analysis and interactions existed among variables, descriptive statistics (frequency, percentage, mean, standard deviation, variance…) and inferencing statistics were used. The results obtained through the test of theories cleared that there was a meaningful and negative relationship among educational level, economical situation, under-cultivation level, connection of tea-planters with the extension agents of Agricultural Jihad with their educational need. There was also a meaningful and positive relationship between the contact of tea-planters studied and other tea-planters in the area and their educational needs. There was no meaningful relationship among age, use of communicative channels, use of educational and extensional classes, connection of tea-planters with local leaders and factory owners, tea-planter's use of radio and TV and studying of extensional periodicals with the range of educational need. Educational needs of the tea-planters who participated in educational-extensional activities or those who participated in cooperations and unions were less than that of tea-planters who did not participate in such activities, cooperatives, and unions. Key words: Tea, educational needs, tea-planters, educational and extensional activities.

Two types of strike-slip and transtensional intrabasinal structures controlling sandbodies in Yitong Graben

Recently, the researches on structure controls on sandbodies have provided a new method for predicting petroleum reservoirs. The Yitong (伊通) graben is situated in the northern section of the Tan-Lu (郯-庐) fault system in eastern China. It was characterized by dual properties of strike-slip and extension in Cenozoic. Two types of intrabasinal structures were identified as oblique fault and transverse uplift in the graben. The oblique faults arranged en echelon in plain and locally presented negative rosette structures on seismic profile, so they were closely derived from strike-slip movement of the northwestern boundary faults. Moreover, these oblique faults were divided to five zones. The three transverse uplifts, located corresponding to flattened southeast boundary faults, were mainly originated by displacement-gradient folding due to segmental extensional activities of southeast boundary faults. The large-scale sandbodies of subaqueous fan facies and fan delta facies had developed at the two types of intrabasinal structure zone. Based on analyzing the seismic facies, logging facies and seismic attribute extractions, and on discovering many incised valleys at the oblique fault zones, the two types of intrabasinal structures were revealed to have conducted drainage entering basin and further dispersing, and to have consequently controlled the development and distribution of sandbodies.

New findings on the tectono-metamorphic history of the western Rhenish Massif (Germany) by K–Ar dating of metasedimentary illite

Metapelites, clay-rich sandstones and volcanics from Cambrian, Ordovician and Lower Devonian strata of the western Rhenish Massif underwent a complex regional Variscan tectono-thermal evolution, as shown by mineralogical and K–Ar isotopic analyses of the illite to mica components from three NW-SE transects. The metamorphic degree extended from an anchimetamorphic to an epimetamorphic intensity during two major episodes of illite crystallization at 328 ± 6 and 282 ± 12 Ma. A further late orogenic or post-orogenic extensional activity could also be detected, but not precisely, around 270 Ma, probably recorded by the precipitation of illite in new or reactivated extensional faults with upward moving heat flows. Three studied cross-sections through the massif display a distribution of the illite crystallinity index that scatters within the diagenesis-to-anchizone range with increased crystallization grades along the thrust zones. The determined K–Ar ages probably frame the Variscan convergence in the western Rhenish Massif, which allows calculation of a “shortening velocity” of 140 km in 40 Ma, or of about 0.35 cm/a, that can be suggested on the basis of a total shortening of about 50% for the entire massif.

Plate-driven extension and convergence along the East Gondwana active margin: Late Silurian–Middle Devonian tectonics of the Lachlan Fold Belt, southeastern Australia

The Lachlan Fold Belt of southeastern Australia developed along the Panthalassan margin of East Gondwana. Major silicic igneous activity and active tectonics with extensional, strike-slip and contractional deformation have been related to a continental backarc setting with a convergent margin to the east. In the Early Silurian (Benambran Orogeny), tectonic development was controlled by one or more subduction zones involved in collision and accretion of the Ordovician Macquarie Arc. Thermal instability in the Late Silurian to Middle Devonian interval was promoted by the presence of one or more shallow subducted slabs in the upper mantle and resulted in widespread silicic igneous activity. Extension dominated the Late Silurian in New South Wales and parts of eastern Victoria and led to formation of several sedimentary basins. Alternating episodes of contraction and extension, along with dispersed strike-slip faulting particularly in eastern Victoria, occurred in the Early Devonian culminating in the Middle Devonian contractional Tabberabberan Orogeny. Contractional deformation in modern systems, such as the central Andes, is driven by advance of the overriding plate, with highest strain developed at locations distant from plate edges. In the Ordovician to Early Devonian, it is inferred that East Gondwana was advancing towards Panthalassa. Extensional activity in the Lachlan backarc, although minor in comparison with backarc basins in the western Pacific Ocean, was driven by limited but continuous rollback of the subduction hinge. Alternation of contraction and extension reflects the delicate balance between plate motions with rollback being overtaken by advance of the upper plate intermittently in the Early to Middle Devonian resulting in contractional deformation in an otherwise dominantly extensional regime. A modern system that shows comparable behaviour is East Asia where rollback is considered responsible for widespread sedimentary basin development and basin inversion reflects advance of blocks driven by compression related to the Indian collision.

Mud volcanoes along the Chishan fault in Southwestern Taiwan: A release bend model

Both land and offshore mud volcanoes have been investigated thoroughly in southwestern Taiwan in terms of morphological expression, spatial distribution, composition, and evasion flux of extruded material. The interactive relationship between mud volcanoes and their associated structures, however, has rarely been examined. This study proposes a release bend model that explains the spatial distribution and relative activity of mud volcanoes along the Chishan Fault, southwestern Taiwan. The surface movement of the Chishan Fault and mud volcano activity were monitored by GPS stations and gas flux measurements, respectively. The resistivity sub-structure of strata across the fault was investigated by the magnetotelluric (MT) method and resistivity image profiling (RIP). Analytical results support the contention that extensional activity at the release bend provides openings to fissures associated with the Chishan Fault, thereby increasing the potential for a mud volcano. The proposed model may facilitate future exploration of mud volcanoes.

K-Ar dating of extensional fault gouge from the Yi-Shu segment of the Tan-Lu fault zone

Two grabens were developed in the Yi-Shu segment of the Tan-Lu fault zone (TLFZ) during its extensional activities, and are now confined by four major NNE-trending normal faults and filled with Cretaceous sediments. These faults were developed due to their reactivities, containing gouge and cutting the graben sediments. Detailed fieldwork demonstrates that the faults experienced sinistral transtensional moment related to regional NE-SW extension during the reactivity. X-ray diffraction (XRD) analysis of the finest gouge samples gives illite crystallinity values higher than 0.42°Δ2θ, indicating temperatures experienced by the gouge samples were less than 150°C. From the relation between K-Ar data and proportions of detrital illite in different size fractions of the gouge samples, we conclude that refaulting for the western boundary fault of the TLFZ, abbreviated to F4, took place at ca. 90 Ma and for the eastern boundary fault, abbreviated to F1, happened from 70 to 60 Ma. During the two phases of reactivity imposed by the same NE-SW extension, the TLFZ experienced uplifting and no sediments were deposited in the two grabens. It is suggested that the TLFZ experienced extension during the Late Cretaceous, which supports the inference that lithospheric thinning was still undergoing in the east of the North China Craton during the Late Cretaceous magmatic hiatus.

Vertical movements in the Ionian margin of the Sila Massif (Calabria, Italy)

Detailed geological-geomorphological investigations, carried out in the Ionian coastal belt between the Trionto River and the Colognati River, allowed us to set up a tectonic-sedimentary model of the northern flank of the Sila Massif during Pliocene-Pleistocene times. The study area is characterised by a distinct step-like topography displaying a well-preserved flight of coastal plain (alluvial/marine) terraces, arranged in five altimetric orders (T1 to T5). The deposition of sandy-clayey marine sediments of Middle Pliocene age ( CTSL Unit ) and beach sandstones grading upward to gray silty clays, referred to the Emilian-Sicilian ( Bisciglia Unit ) on the basis of micropaleontological analysis, has been related to a phase of subsidence, likely induced by the activity of E-W trending extensional faults. The Bisciglia Unit grades westward into conglomerates, sandstones and silty-clay sediments forming part of juxtaposed and superimposed deltaic (and subordinate alluvial/beach) deposits. A phase of subaerial landscape modelling, starting from the Middle Pleistocene, marks the end of subsidence and the onset of a rapid uplift which, interacting with eustatic sea-level changes, gave rise to the five orders of terraces. The uplift rate of the whole area, inferred by correlating the terrace surfaces with the paleoclimatic curve proposed by Bintanja et alii (2005), is 0.65 ± 0.1 mm yr −1 . However, the contemporaneous occurrence of extensional fault activity in the area induced variations in the uplift rates, which actually range between 0.52 and 0.88 mm yr _1 in relation to the distance from the fault trace. These data highlight the competing role of fault activity and regional uplift in controlling vertical movements and surface topography at the local scale.

The relationship between rifting and magmatism in the northeastern Arabian Sea

The rifting of the Seychelles microcontinent from India involved two phases of extensional activity. The initial separation of the Laxmi Ridge from India was accompanied by extensive magmatism but the later separation of the Seychelles from the Laxmi Ridge was only weakly magmatic. The causal mechanisms linking continental flood basalts, lithospheric extension and mantle plumes, as well as the relative timing of extension and volcanism, are controversial1,2,3,4. The eruption of the Deccan flood basalts was approximately contemporaneous with the separation of the Seychelles microcontinent from India. However, between these continental blocks lies the enigmatic Laxmi Ridge, and the sequence of extensional events that formed these various tectonic elements is poorly understood. Here we present wide-angle seismic data along a profile across Laxmi Ridge that permit delineation of offshore igneous bodies associated with the Deccan magmatism; these bodies are similar to those associated with flood-basalt volcanism and rifting in the Atlantic region and elsewhere1. From the geometry of these bodies, we infer that there were two periods of extension. The first phase, which involved extension between Laxmi Ridge and the Indian subcontinent, was accompanied by significant Deccan-related magmatism. Full development of a continental margin was achieved during the second phase of weakly magmatic extension between Laxmi Ridge and the Seychelles. We suggest that between these rifting events the region passed beyond the reach of lateral flow from the source region of the Deccan flood basalts.