Active Tectonic Setting Research Articles

Geochemical characteristics and sedimentary setting of chang 9 shale in the Upper Triassic Yanchang Formation of southeastern Ordos Basin (NW China)

In addition to the organic-rich shale in the Chang 7 Member (Ch7 shale) of the Ordos Basin, the shale hosted in the Chang 9 Member (Ch9 shale) is another source rock and potential unconventional target. However, there is a general lack of consensus on the features and potential of the Ch9 shale. In order to determine the potential of the Ch9 shale, this study analyzes its paleoenvironment and sedimentary setting based on the integrated analysis of geochemistry and sedimentary facies. A comparison of geochemical features between the Ch9 shale and the organic-rich Ch7 shale is made to evaluate the factors controlling organic matter (OM) enrichment of the Ch9 shale. The results show that the Ch9 shale has good hydrocarbon potential, and terrestrial OM is important component of the OM in the Ch9 shale. The Ch9 shale was deposited in climatic transition period from cold and dry to warm and humid, when the water body was suboxic-anoxic and freshwater. The relatively cold and dry climate and suboxic-anoxic water condition in the Ch9 period could not provide the same excellent conditions for OM enrichment as that in the Ch7 period. The Ch9 shale is composed of sediments mixed with felsic rocks that originated from continental island arcs and active continental margins. The active tectonic setting resulted in event deposition (e.g. turbidity flows and hydrothermal fluid activity) during the deposition of Ch9 shale. The input of terrestrial OM carried by turbidity flows also promoted OM enrichment, however, the very high depositional rates caused by turbidity flows diluted the OM, which probably offset the effect of hydrothermal on OM enrichment. Our results are helpful to revealing the source potential of the Ch9 shale, providing a theoretical basis for the shale oil and gas exploration in the Ordos Basin.

Post-rift submarine volcanic complexes and fault activities in the Baiyun Sag, Pearl River Mouth Basin: New insights into the breakup sequence of the northern South China Sea

Post-rift volcanisms and fault activities are significant for understanding the tectonic-sedimentary evolution of rifted continental margins and associated hydrocarbon entrapments. In this study, a series of post-rift fault-associated volcanic activities have been found in the northern continental margin of the South China Sea (SCS). However, their episodes and relationships with the progressive continental breakup are still doubtful. Here, new 2D/3D seismic, borehole, dating, and biostratigraphic data are used to investigate the characteristics of the post-rift volcanism and fault activities in the Baiyun Sag (BYS), Pearl River Mouth Basin. The newly-found fault-associated volcanic complexes include thirteen submarine volcanic mounds and associated lava flows, together with igneous sills that are mainly composed of basalts and tuffs. Seismic stratigraphic analysis combined with KAr dating from cores confirms that post-rift volcanism in the BYS climaxed along with the breakup of the SCS mainly in three episodes: (1) the first episode near Oligocene/Miocene boundary (23.3 Ma); (2) the second at the middle Early Miocene (17.6 Ma); and (3) the new-found third episode at the late Early Miocene (17.1 Ma). Also, fifty-three syn-sedimentary faults are interpreted, which are characterized by a WNW trending normal fault group separated by an ENE trending transcurrent fault group. And there is evidence that these fifty-three faults were long-lived from ca. 32–17.1 Ma and served as important conduits for the upward migration of the magma. The three episodes of volcanism and tectonic fault activities are related to the progressive continental breakup in the northern SCS margin, and they represent the tectonic signature of the early stage of the post-rift subsidence after the lithospheric breakup which extends to the top of a hypothetical breakup sequence (32–17.1 Ma) in the BYS. Furthermore, this study proves that the diachronous continental breakup in the SCS margin can result in corresponding changes in depositional architectures, along with a breakup sequence characterized by the deposition of a ‘sandstone‑carbonate-shale’ regressive-transgressive cycle. The sedimentological changes and associated tectonic features recorded in the Baiyun Sag suggest an exceptionally active tectonic setting in the time of the formation of the breakup sequence (Early Oligocene-Early Miocene) along the northern SCS margin.

The Turonian-Coniacian stage boundary in an expanded siliciclastic succession: Integrated stratigraphy in deltaic through offshore facies, Bohemian Cretaceous Basin

During the latest Turonian – early Coniacian time, a succession of sand-rich, Gilbert-type deltas was deposited along the faulted margin of the Bohemian Cretaceous Basin (BCB, Central Europe), fining along depositional dip into prodeltaic heterolithic facies and offshore mudstones to marlstones. The active tectonic setting with accelerated subsidence and supply rates contributed to deposition and preservation of an expanded record showing otherwise insufficiently known parts of the Turonian-Coniacian (T–C) boundary interval. In this study a combination of detailed biostratigraphy, genetic sequence stratigraphy, and carbon isotope chemostratigraphy is employed to characterize the T–C boundary in the northern part of the BCB, in both nearshore and offshore facies. The basis of the biostratigraphic framework was the establishment of the same succession of inoceramid bivalves and other molluscan marker taxa and bioevents as that in Salzgitter-Salder (Germany) and Słupia Nadbrzeżna (Poland). All the faunal markers from the Mytiloides scupini through Cremnoceramus crassus crassus inoceramid zones were found both in the nearshore and offshore facies. The linkage of biostratigraphic and carbon isotope-stratigraphic data to a regional stratigraphic picture, as well as to individual outcrop and core sections, provides an important new database for further study of the boundary interval, with a direct link to the transgressive-regressive history of the nearshore depositional systems. Therefore it is proposed here that the T–C interval in the BCB complements the Salzgitter-Salder and Słupia Nadbrzeżna sections and together with them constitutes a broader type region for definition of the T–C boundary.

Syndepositional tectonics and mass-transport deposits control channelized, bathymetrically complex deep-water systems (Aínsa depocenter, Spain)

ABSTRACTThe inception and evolution of channels in deep-water systems is controlled by the axial gradient and lateral confinement experienced by their formative flows. These parameters are often shaped by the action of tectonic structures and/or the emplacement of mass-transport deposits (MTDs). The Arro turbidite system (Aínsa depocenter, Spanish Pyrenees) is an ancient example of a deep-water channelized system from a bathymetrically complex basin, deposited in an active tectonic setting. Sedimentologic fieldwork and geologic mapping of the Arro system has been undertaken to provide context for a detailed study of three of the best-exposed outcrops: Sierra de Soto Gully, Barranco de la Caxigosa, and Muro de Bellos. These locations exemplify the role of confinement in controlling the facies and architecture in the system. Sedimentologic characterization of the deposits has allowed the identification of fifteen facies and eight facies associations; these form a continuum and are non-unique to any depositional environment. However, architectural characterization allowed the grouping of facies associations into four depositional elements: i) weakly confined, increasing-to-decreasing energy deposits; ii) progradational, weakly confined to overbank deposits; iii) alternations of MTDs and turbidites; iv) channel fills. Different styles of channel architecture are observed. In Barranco de la Caxigosa, a master surface which was cut and subsequently filled hosts three channel stories with erosional bases; channelization was enhanced by quasi-instantaneous imposition of lateral confinement by the emplacement of MTDs. In Muro de Bellos, the inception of partially levee-confined channel stories was enhanced by progressive narrowing of the depositional fairway by tectonic structures, which also controlled their migration. Results of this study suggest that deep-water channelization in active tectonic settings may be enhanced or hindered due to: 1) flow interaction with MTD-margin topography or; 2) MTD-top topography; 3) differential compaction of MTDs and/or sediment being loaded into MTDs; 4) formation of megascours by erosive MTDs; 5) basin-floor topography being reset by MTDs. Therefore, the Arro system can be used as an analog for ancient subsurface or outcrop of channelized deposits in bathymetrically complex basins, or as an ancient record of deposits left by flow types observed in modern confined systems.

The Zagreb (Croatia) M5.5 Earthquake on 22 March 2020

On 22 March 2020, Zagreb was struck by an M5.5 earthquake that had been expected for more than 100 years and revealed all the failures in the construction of residential buildings in the Croatian capital, especially those built in the first half of the 20th century. Because of that, extensive seismological, geological, geodetic and structural engineering surveys were conducted immediately after the main shock. This study provides descriptions of damage, specifying the building performances and their correlation with the local soil characteristics, i.e., seismic motion amplification. Co-seismic vertical ground displacement was estimated, and the most affected area is identified according to Sentinel-1 interferometric wide-swath data. Finally, preliminary 3D structural modeling of the earthquake sequence was performed, and two major faults were modeled using inverse distance weight (IDW) interpolation of the grouped hypocenters. The first-order assessment of seismic amplification (due to site conditions) in the Zagreb area for the M5.5 earthquake shows that ground motions of approximately 0.16–0.19 g were amplified at least twice. The observed co-seismic deformation (based on Sentinel-1A IW SLC images) implies an approximately 3 cm uplift of the epicentral area that covers approximately 20 km2. Based on the preliminary spatial and temporal analyses of the Zagreb 2020 earthquake sequence, the main shock and the first aftershocks evidently occurred in the subsurface of the Medvednica Mountains along a deep-seated southeast-dipping thrust fault, recognized as the primary (master) fault. The co-seismic rupture propagated along the thrust towards northwest during the first half-hour of the earthquake sequence, which can be clearly seen from the time-lapse visualization. The preliminary results strongly support one of the debated models of the active tectonic setting of the Medvednica Mountains and will contribute to a better assessment of the seismic hazard for the wider Zagreb area.

Assessing bank dynamics of the Lower Meghna River in Bangladesh: an integrated GIS-DSAS approach

The major conduit of water and sediment discharges from the mighty Ganges-Brahmaputra-Meghna river system to the Bay of Bengal, the Lower Meghna River (LMR), experiences rapid morphological changes with significant environmental and socioeconomic consequences to the surrounding areas. This study measures the rate of bankline shifting, river widening, and change in the braiding index (BI) in the LMR over a period of 30 years (1988–2017) in order to understand the channel dynamics. To extract these parameters, we used the Digital Shoreline Analysis System (DSAS) together with other geospatial techniques on multiple satellite images taken over this period and subsequently validated the spatial outcomes through an extensive field investigation. Our findings revealed that rapid changes occurred in both banks, although at different rates, mostly due to erosion; the right bank has retreated at an average rate of 81 m/yr and the left bank at 28 m/yr, resulting in widening as well as river migration to the west. The channel widening, an overall average increase of about 49%, has been accompanied by a substantial increase in the BI, from 2.2 in 1988 to 6.4 in 2017. This study suggests that the higher erosion rate in the right bank and the consequent westward migration of the river are due to the active tectonic setting of the adjoining areas. Enormous volumes of water and sediment discharges and their periodic variations, heterogeneous bank material properties, climate change, and anthropogenic activities are also possible contributors to this bank instability. This research shows the value of implementing an integrated GIS-DSAS tool for spatiotemporal channel change investigation and could be useful in land-use planning and mitigating bank erosion in the LMR locality.

Continental degassing of helium in an active tectonic setting (northern Italy): the role of seismicity

In order to investigate the variability of helium degassing in continental regions, its release from rocks and emission into the atmosphere, here we studied the degassing of volatiles in a seismically active region of northern Italy (MwMAX = 6) at the Nirano-Regnano mud volcanic system. The emitted gases in the study area are CH4–dominated and it is the carrier for helium (He) transfer through the crust. Carbon and He isotopes unequivocally indicate that crustal-derived fluids dominate these systems. An high-resolution 3-dimensional reconstruction of the gas reservoirs feeding the observed gas emissions at the surface permits to estimate the amount of He stored in the natural reservoirs. Our study demonstrated that the in-situ production of 4He in the crust and a long-lasting diffusion through the crust are not the main processes that rule the He degassing in the region. Furthermore, we demonstrated that micro-fracturation due to the field of stress that generates the local seismicity increases the release of He from the rocks and can sustain the excess of He in the natural reservoirs respect to the steady-state diffusive degassing. These results prove that (1) the transport of volatiles through the crust can be episodic as function of rock deformation and seismicity and (2) He can be used to highlight changes in the stress field and related earthquakes.

Provenance and tectonic setting of the Palaeozoic sediments in the northern Alxa area: Implications for the Central Asian Orogenic Belt

Understanding the tectonic nature of the Alxa area is significant in tectonic reconstruction of the Central Asian Orogenic Belt, but their tectonic correlation, affinity, and implications have not been well defined. The late Palaeozoic sediments in the northern Alxa area can help to understand this question. The sedimentary sequence of the late Palaeozoic strata (Benbatu Formation) indicates that the sedimentary environment of the study area gradually changed from a stable carbonate platform to a bathyal‐abyssal environment with relatively active tectonic setting. The results of the grain size analysis of the sandstone debris show the typical characteristics of a turbidity current. The composition of clastic particles is mainly feldspar, which is characterized by short transport distance and rapid deposition of sediments. Combined with the geochemical characteristics of sandstone, the northern Alxa Block is considered to be an active continental margin in the late Palaeozoic. Through summarizing the zircon data of the Alxa Block, Tarim Craton, and North China Craton, it is concluded that the Palaeozoic clastic zircons in the study area come from the Alxa Block and the surrounding magmatic arc, the Neoproterozoic‐Middle Proterozoic zircons come from the Alxa Block, the Archean zircons come from the North China Craton. According to the age distribution of clastic zircons, the magmatic activity may exist in the early Palaeozoic, which suggests it may not be a stable passive continental margin in the margin of northern Alxa Block in the early Palaeozoic. Based on the provenance analyses, the sources block shows no tectonic affinity to the Tarim Craton, but the Hanwula area in the north of the study area might have been accreted to the Tarim Craton, which means the size of the Paleo‐Asian Ocean between the study area and the Hangwula area is large. The earliest Palaeozoic clastic zircon ages in sedimentary rocks are recorded in the late Cambrian, whereas the earliest Palaeozoic detrital zircons in the Hangwula area are recorded in the Ordovician, which means that the magmatic activity recorded in the study area is earlier than the Hangwula area and indicates that the subduction of the south side of the Paleo‐Asian Ocean began earlier than the north side. The evidence of magmatic rocks discovered so far confirms this view.

Seismic sources for southern Pakistan and seismic hazard assessment of Karachi

Karachi is Pakistan’s largest city with population exceeding 18 million and is amongst the top five most congested cities in the world. Karachi has experienced no earthquake related damage in the recorded history of past ~ 175 years. Yet, Karachi is located in a seismically active tectonic setting often compared to Los Angeles with active plate boundary faults and triple junctions within a radius of ~ 150 km. This discrepancy in earthquake history and seismotectonic setting has led to diverse seismic hazard assessments for Karachi ranging from assignment of seismic hazard zones I (least severe) to IV (most severe). Recent assessment adopted in Building Code of Pakistan (2007) assigns an intermediate seismic hazard value of 0.16–0.24 g (Zone IIB) to Karachi, which is broadly accepted but sometimes criticized to be an underestimation (e.g. Bilham et al. Seismol Res Lett 78(6):601–631, 2007). The present study based on a new active faults compilation and seismic sources definition together with incorporation of maximum possible information on historical earthquakes (up to 893AD) has led to a re-assessment of seismic hazard for Karachi using probabilistic and deterministic seismic hazard assessment approaches. According to this study, Karachi is assessed to be prone to ground motions ~ 0.25 g with metropolitan areas having hazard values between 0.21 and 0.25 g for 10% probability of exceedance in 50 years (475-year return period). The deterministic seismic hazard analysis suggest maximum that peak ground acceleration (PGA) varies from 0.19 to 0.99 g in Karachi and its higher values are concentrated around the Nagar Parker fault that is controlling and hazardous for Karachi.

Constraining mountain front tectonic activity in extensional setting from geomorphology and Quaternary stratigraphy: A case study from the Matese ridge, southern Apennines

Rugged peaks, large intermontane basins and frequent seismicity all characterize the active extensional tectonic setting of the southern Apennines. The Matese ridge typifies the active tectonic setting of the southern Apennines with steep carbonate mountain fronts and large depositional centres. Moderate to high magnitude earthquakes have affected the northern, western and eastern sectors of the Matese ridge in historical times. However, the seismogenic potential of the extensional fault system bounding the southern Matese mountain front has not been fully assessed to date. To unravel the active tectonic setting of the southern Matese mountain front, we have carried out a comprehensive geomorphological and tectonic-geomorphology investigation of the mountain front and its piedmont and have constrained results through chronological (i.e., tephrostratigraphical and 40Ar/39Ar) and structural data.Our study highlights that in the last ∼600 ka, activity along E-W trending normal faults has identified a locus of higher slip rate tectonic activity in the central part of the analysed mountain front. These active E-W-striking normal faults are inherited, reactivated structures, which have interacted with newly formed NW-SE-striking normal faults during NE-SW extension active on the regional scale, causing fault bending and local extension to be oriented N-S. Consequently, lower slip rates have been recorded along the NW-SE-striking normal faults at the north-western and south-eastern tips of the southern Matese front. The long-term displacement rate of the fault system at the boundary of the central part of the southern Matese front is consistent with mean values of displacement of faults that, in the southern Apennines, show evidence of activity during the late Quaternary. Despite strong historical seismicity clustering primarily around the study area, our data highlight that it cannot be ruled out that moderate to high magnitude seismicity could affect the southern Matese mountain front.Our case study represents an example of the possible modes of formation and evolution of mountain front-basin systems in extensional setting, and shows how the combination of different data sets allows unravelling the interaction between tectonic, erosional and sedimentary processes, which lead to landscape evolution of active mountain belts.

Early Cretaceous redbeds from the Minle Basin, Hexi Corridor, northwest China: Mineralogy and geochemistry implications for paleoweathering, provenance, and tectonic settings

Redbeds with a large thickness in the lower Cretaceous record abundant geologic information in the Minle Basin. We have conducted the paleoweathering conditions, provenance, and tectonic settings based on mineralogy and geochemistry. Our results indicate that mudstone samples are characterized by abundant illite with negligible amounts of K-feldspars and analcime. The lower part of the lower Cretaceous is rich in quartz, whereas the upper part is dominated by dolomite and analcime. We suggest that this is caused by the decreasing input of the clastic influx during the middle-late early Cretaceous. High index of compositional variation values (average 1.33) indicate first-cycle sediment supply, suggesting an overall compositional immaturity and short-distance transportation. These characteristics are consistent with an active regional extension tectonic setting. The [Formula: see text] system ([Formula: see text];[Formula: see text];[Formula: see text]) and Th/U versus Th consistently reveal that the lower Cretaceous experienced a positive gradient in chemical weathering from young to old formations. Although the patterns of trace elements in three formations of the lower Cretaceous are different, those of the rare earth elements (REEs) tend to be consistent. The significant enrichment of light REEs, heavy REEs fractionation, and distinctive negative Eu anomalies suggest derivation from an old, upper continental crust composed of predominantly felsic sediments. This interpretation is supported by several discrimination diagrams such as titanium dioxide-nickel ([Formula: see text]), which shows the characteristics of immature recycled sediments. A few sensitive elements, ratios, and normalized REE patterns indicate a provenance of an active continental margin and a continental island arc (CIA). The La-Th-Sc, Th-Co-Zr/10, and Th-Sc-Zr/10 discrimination plots further confirm the CIA signature. Thus, we conclude that the early Cretaceous redbeds in the Minle Basin, Hexi Corridor, were deposited in a dustpan-shaped half-graben basin in a CIA setting when northwest China was influenced by intense regional extension.

Evidence for active faulting in the eastern part of the Gulf of Saros, northern Aegean, from high-resolution seismic data

This study focuses on the determination of the near-surface tectonic features in the eastern part of the Gulf of Saros, a highly active seismic region within the North Anatolian Fault Zone. Approximately 350-km-line high-resolution seismic data were acquired to understand the active tectonic setting of the inner side of the gulf. The Ganos Fault, which generated the 9 August 1912 earthquake (Mw = 7.3) in western Turkey, enters into the Gulf of Saros from the east, cuts the southern margin of the gulf forming a trough which becomes broader and deeper westward. The seismic data reveal that the fault is not made up of a single segment or a pair of border faults bounding the trough. According to the seismic images of the fault segments, however, the emanating earthquake energy will possibly be carried by two different faults; namely the Ganos and Saros segments. These fault segments will be presented first in this paper in detail from new collected seismic data at the inner part of the gulf, and will be used to help a new tectonic model that explains how the Ganos segment controls the northern margin of the trough while the Saros segment controls the central part of the main trough. The latter is responsible from the development of an active basin and it forms the sudden deepening observed on the trough.

Weathering of Ophiolite Remnant and Formation of Ni Laterite in a Strong Uplifted Tectonic Region (Yuanjiang, Southwest China)

The Yuanjiang Ni deposit in southwestern margin of the Yunnan Plateau is the only economically important lateritic Ni deposit in China. It contains 21.2 Mt ore with an average grade of 1.05 wt % Ni and has been recognized as the second largest Ni producer in China following the Jinchuan super-large magmatic Ni–Cu deposit. This Ni deposit is hosted within the lateritic regolith derived from serpentinite within the regional Paleo-Tethyan Ophiolite remnants. Local landscape controls the distribution of the Ni mineralized regolith, and spatially it is characterized by developing on several stepped planation surfaces. Three types of lateritic Ni ores are identified based on Ni-hosting minerals, namely oxide ore, oxide-silicate mixed ore and silicate ore. In the dominant silicate ore, two phyllosilicate minerals (serpentine and talc) are the Ni-host minerals. Their Ni compositions, however, are remarkably different. Serpentine (0.34–1.2 wt % Ni) has a higher Ni concentration than talc (0.18–0.26 wt % Ni), indicating that the serpentine is more significantly enriched in Ni during weathering process compared to talc. This explains why talc veining reduces Ni grade. The geochemical index (S/SAF value = 0.33–0.81, UMIA values = 17–60) indicates that the serpentinite-derived regolith has experienced, at least, weak to moderate lateritization. Based on several lines of paleoclimate evidence, the history of lateritization at Yuanjiang area probably dates to the Oligocene-Miocene boundary and has extended to the present. With a hydrology-controlled lateritization process ongoing, continuous operation of Ni migration from the serpentinite-forming minerals to weathered minerals (goethite and serpentine) gave rise to the development of three types of Ni ore in the regolith. Notably, the formation and preservation of the Yuanjiang lateritic Ni deposit has been strongly impacted by regional multi-staged tectonic uplift during the development of Yunnan Plateau. This active tectonic setting has promoted weathering of serpentinite and supergene Ni enrichment, but is also responsible for its partial erosion.

Geochemistry and geochronology of the Paleozoic sedimentary rocks in the Shar Khutul area, Central Mongolia

The study area is located in the central part of Tsetserleg terrane in the southwestern margin of the Khangai-Khentey orogenic system. The paper presents new data on geochemistry and geochronology of sedimentary rocks from the Shar Khutul area, where the Tsetserleg terrane consists of Silurian-Devonian oceanic plate stratigraphic unit and Carboniferous shallow water sediment. The Upper Silurian to Middle Devonian Erdenetsogt Formation (S3-D2er), which is an oceanic plate stratigraphic unit, is mainly composed of siliceous siltstone, volcanites, tuffs, quartzite, and cherts. The shallow water sediments are divided into Upper Devonian to Lower Carboniferous Tsetserleg Formation (D3-C1cc) and Lower–Middle Carboniferous Dzargalant Formation (C1-2dz). The Tsetserleg Formation (D3-C1cc) consists of only sedimentary rocks such as bluish-grey sandstones and siltstones, and Lower–Middle Carboniferous Dzargalant Formation (C1-2dz) is principally composed of medium- to coarse-grained, brown-greenish grey sandstones with thin-layers of dark siltstones and gravelites. 
 The SiO2 content of the Shar Khutul area sandstones ranges from 63.85 to 67.95 wt.% and the average content of TiO2 is 0.72 wt.% and Al2O3 content is 14.38 wt.%. The Chemical Index of Alteration (CIA) value ranges from 48.71 to 56.94 and the range of Index of compositional variations (ICV) is from 0.98 to 1.24. Moreover, the samples studied show that most of the sandstones are generally immature and were derived from weakly weathered source rocks.
 The ratios of Eu/Eu* (0.83), La/Sc (3.81), La/Co (5.30), and Cr/Th (13.81) indicate that the derivation of the Shar Khutul area sandstones from felsic rock sources and confirm the signatures of a felsic igneous provenance and suggest an active continental margin tectonic setting of the source area.
 The clastic zircons from the medium grained sandstone (Erdenetsogt formation) yield ages between 2.5 Ga and 236 Ma and the detrital zircons exhibit four peak ages at 1.7-2.5 Ga (n = 13), 455-499 Ma (n = 6), 337-382 Ma (n = 13) and 236–250 Ma (n = 5).

1.39 Ga mafic dyke swarm in southwestern Yilgarn Craton marks Nuna to Rodinia transition in the West Australian Craton

The Archean Yilgarn Craton in Western Australia hosts at least five generations of mafic dykes ranging from Archean to Neoproterozoic in age, including the craton-wide ca. 2408 Ma Widgiemooltha and the 1210 Ma Marnda Moorn Large Igneous Provinces (LIP), the 1888 Ma Boonadgin dykes in the southwest and the 1075 Ma Warakurna LIP in the northern part of the craton. We report here a newly identified NNW-trending mafic dyke swarm, here named the Biberkine dyke swarm, in the southwestern Yilgarn Craton dated at 1390 ± 3 Ma by ID-TIMS U-Pb geochronology of baddeleyite. The regional extent of the dyke swarm is uncertain but aeromagnetic data suggest that the dykes are part of a linear swarm several hundred kilometers long, truncated by the Mesoproterozoic Albany-Fraser Orogen to the south. Geochemical data indicate that the dykes have tholeiitic compositions with a significant contribution from metasomatically enriched subcontinental lithospheric mantle and/or lower continental crust. Paleogeographic reconstructions suggest that a prolonged tectonic quiescence in the Yilgarn Craton from ca. 1600 Ma was interrupted by renewed subduction along the southern and southeastern margin at ca. 1400 Ma, reflecting a transition from Nuna to Rodinia configuration. The 1390 Ma Biberkine dykes may be a direct consequence of this transition and mark the change from a passive to active tectonic setting, which culminated in the Albany-Fraser Orogeny at ca. 1330 Ma. The Biberkine dykes are coeval with a number of other mafic dyke swarms worldwide and provide an important target for paleomagnetic studies.

Novel discrete element modelling of Gilbert‐type delta formation in an active tectonic setting—first results

AbstractGilbert deltas are now recognised as an important stratigraphic component of many extensional basins. They are remarkable due to their coarse‐grained nature, large size and steep foresets (up to 30–35°) and may exhibit a variety of slope instability features (faulting, slump scars, avalanching, etc.). They are also often closely related to major, basin‐margin normal faults. There has been considerable research interest in Gilbert deltas, partly due to their economic significance as stratigraphic traps for hydrocarbons but also due to their sensitivity to relative base level changes, giving them an important role in basin analysis. In addition to field studies, numerical modelling has also been used to simulate such deltas, with some success. However, until now, such studies have typically employed continuum numerical techniques where the basic data elements created by simulations are stratigraphic volumes or timelines and the sediments themselves have no internal properties per se and merely represent areas/volumes of introduced coarse‐grained, clastic and sedimentary material. Faulting or folding (if present) are imposed externally and do not develop (naturally) within the modelled delta body itself. Here, I present first results from a novel 2D numerical model which simulates coarse‐grained (Gilbert‐type) deltaic sedimentation in an active extensional tectonic setting undergoing a relative base level rise. Sediment is introduced as packages of discrete elements which are deposited beneath sea level, from the shoreline, upon a pre‐existing basin or delta. These elements are placed carefully and then allowed to settle onto the system. The elements representing the coarse‐grained, deltaic sediments can have an intrinsic coefficient of friction, cohesion or other material properties appropriate to the system being considered. The spatial resolution of the modelling is of the order of 15 m and topsets, foresets, bottomsets, faults, slumps and collapse structures all form naturally in the modelled system. Examples of deltas developing as a result of sediment supply from both the footwall and hanging‐wall of a normal fault, and subject to changes in fault slip rate are presented. Implications of the modelling approach, and its application and utility in basin research, are discussed.

A Conceptual Model For the Preservation of Thick, Transgressive Shoreline Successions: Examples from the Forearc Nanaimo Basin, British Columbia, Canada

Abstract Sequence stratigraphic models derived from passive margins and foreland basins characterize transgressive shorelines (excluding incised valleys) as recording periods of sediment starvation, and are commonly represented as thin ( 10 m) are more likely to be developed in active tectonic settings (e.g., forearc and rift basins), which generally experience high sedimentation rates and relatively rapid changes in base level. Settings with high sedimentation rates and moderate base-level rise lead to progradational trends despite overall transgression of the shoreline. In settings where changes in eustatic sea level mainly control the resulting stratigraphic architecture (e.g., passive margins), thin transgressive successions (1–10 m) are more likely to be produced. Finally, in settings defined by relatively slow base-level change (e.g., foreland basins), the resultant transgressive succession typically comprises a lag (< 1 m) overlain by offshore mudstones. This conceptual model highlights the importance of the interplay between sedimentation rates and base-level rise, and is intended as a guide for the predictive modeling of sedimentary basins.

In Situ Permeability and Scale Dependence of an Active Accretionary Prism Determined From Cross‐Borehole Experiments

AbstractPermeability controls fluid flow in the Earth's crust and affects a wide range of processes including advective transport and pore pressure generation. However, in situ measurements of permeability are few, especially in active tectonic settings or at scales relevant to regional flow. We analyze formation fluid pressure records from oceanic boreholes in the Nankai accretionary prism offshore southwest Japan, focusing on unexpected responses to drilling operations conducted at boreholes ~100 m to the northeast. We develop a 2‐D numerical model of transient fluid flow and conduct a parametric grid search to define hydraulic diffusivity. A value of 0.19–0.46 m2/s (corresponding to a permeability of 9.8 × 10−13 to 2.4 × 10−12 m2) yields the best fit to observed pressure responses. Together with laboratory measurements on core samples and drillstrem tests reported in previous studies, our analysis indicates a strong scale dependence of permeability, likely reflecting the presence of permeable faults and fractures.

Jurassic sediments in the Irkut basin and southwestern Transbaikalia: correlations based on paleobotanical and geochronological data

The Jurassic growth of mountain ranges along the southern edge of the Siberian platform occurred in an active tectonic setting related to the closure of the Mongol-Okhotsk Ocean. The oceanic subduction and subsequent continent collision events induced compressive deformations at the platform boundary. Understanding the paleogeography related to the Mesozoic closure of the Mongol-Okhotsk Ocean requires dating and correlation of the Jurassic Prisayan Formation in the Irkut basin and Tugnuyskaya Formation in southwestern Transbaikalia. This work presents structural and paleobotanic results within both formations. 40Ar/39Ar dating of underlying volcanics from the upper member of the Ichetuyskaya Formation is used to refine the age of the sediment series and provide probable correlation. The results show that the Tugnuyskaya Formation initiated at the end of the Middle Jurassic-beginning of the Late Jurassic and was not coeval with the Prisayan Formation, whose upper fine-grained members were deposited in the early Middle Jurassic. 40Ar/39Ar dating of volcanics from the upper member of the Ichetuyskaya Formation yielded a Middle Jurassic age of 167.7 ± 1.2 Ma (Bajocian to Bathonian). The paleogeographic data analysis based on facies and mineralogical composition of sediments and on a study of source areas from Sm-Nd data and the U-Pb ages of detrital zircons from the deposits in the southern Irkut basin indicates that the deposition of the Prisayan Formation was followed by the intensification of relief building along the southern edge of the Siberian Platform. Our geochronological data show that active tectonic deformations in southwestern Transbaikalia evidenced in the volcanoclastic Ichetuyskaya Formation in the Tugnuy basin also occurred during the Middle Jurassic. The uppermost sediments of the Tugnuy basin were deposited at the end of the Middle Jurassic-Late Jurassic in a quiet tectonic setting with low relief and lacustrine-boggy depositional environments.

Controls on Neogene carbonate facies and stratigraphic architecture of an isolated carbonate platform – the Caribbean island of Bonaire

Abstract The Neogene carbonate succession on the island of Bonaire (Netherland Antilles) shows complex geometries associated with a sequence of depositional and erosional events which reflects the history of this isolated platform and the interaction between eustasy and tectonics. Three major episodes of carbonate platform deposition are defined which show contrasting depositional styles: 1) aggradational platform (Lower-Middle Miocene) with sediments showing a fining-upward trend from mixed coral rudstone to medium-grained coralgal grain/packstone, partly dolomitized and tilted by tectonic deformation; 2) progradational platform (Upper Miocene-Pliocene) which is formed of several shallowing-upward prograding units mainly composed of reworked red algal grain/packstone, with significant dolomitization, passing upward to shoreline and aeolian deposits formed of cor-algal grain/packstone and large benthic foraminifera grainstone, and 3) flat-topped platform (Pleistocene) with a reefal framework composed of a rich variety of corals in a bioclastic pack/wackestone matrix. These platform episodes exhibit contrasting stacking patterns and are separated by erosional unconformities. Overprinting this depositional succession is a series of Quaternary near-horizontal shoreline erosional terraces and vertical cliffs which have been cut into the island stratigraphy and complicate the stratal field relationships. However, this terrace morphology clearly does not represent depositional episodes, as has been suggested before. The internal architecture of each of the three carbonate platform reflects of the dynamics of sedimentation with allogenic controls. The latter relate to major oceanographic and tectonic events in the region, including changing ocean circulation as a result of the closure of the Panama isthmus, and Caribbean plate dynamics that affected sea-floor and island topography. The Bonaire succession provides a model for understanding and predicting isolated carbonate platform development, as well as architecture, facies and potential diagenetic changes, in an active tectonic setting.