TS6.2

Conjugate margins along mature oceans indicate two end-member types of rifted margins, distinguished by their crustal architecture, e.g. the Iberia-Newfoundland and Central South Atlantic. Numerical simulations and analogue models show that these types could be explained by rheology, state of stress (depth depended) and role of magmatism (magma assisted). The Red Sea is a young rift, offering the opportunity to study early break-up conditions and to relate them to the architecture and the type of passive margins. The morphology of the Red Sea indicates variability and dissimilarities between its southern and northern regions, nevertheless, the lithospheric structure of the rift remains elusive, mainly due to lack of high-resolution direct geophysical measurements, e.g. seismic profiles.

In this study, we explore the deep architecture of the Red Sea rift using geophysical data, in particular gravity and magnetic data, and constraints from seismic interpretations, receiver functions and tomographic models. We present a 3D structural and density model for the Red Sea, including the African and Arabian shoulders down to 120 km depth. The model includes four main sections: sediments, crystalline crust (continental and oceanic), lithospheric mantle (including a thermal gradient) and a uniform asthenosphere. In order to test different scenarios, we evaluate combinations of (1) exhumed continental mantle lithosphere (Type I margin) versus wide/ultrawide continental crust (Type II margin), and, (2) limited versus extended distribution of oceanic crust. The 3D gravity model favors Type II architecture and limited oceanic crust in the southern-central parts of the Red Sea rift. In the northern parts, the model cannot distinguish between the pre and post break-up stages.

How to cite: Issachar, R., Ebbing, J., Dilixiati, Y., and Gómez-García, Á. M.: Rifting in the Red Sea – insights into the rift architecture from geophysical data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9494, https://doi.org/10.5194/egusphere-egu22-9494, 2022.

The separation and characterisation of different deformation events in superimposed basins can be challenging due to the effects of overprinting and/or fault reactivation, combined with a lack of detailed geological or geophysical data. This study shows how an onshore study can be enhanced using a targeted interpretation of contiguous structures offshore imaged by seismic reflection data.

Two deformation events, including unambiguous evidence of fault reactivation, are recognised in the onshore part of the Lossiemouth Fault Zone (LFZ), southern-central Inner Moray Firth Basin. The basin is thought to record a history of (possibly) Permian to Cenozoic deformation, but it is commonly difficult to conclusively define the age of faulting and fault reactivation. However, structures in Permo-Triassic strata onshore outcrops show no evidence of growth geometries and new interpretation of seismic reflection profiles offshore reveals that Permo-Triassic fills are widely characterised by subsidence and passive infill of post-Variscan palaeotopography. We propose that sequences of reactivated faulting observed onshore and offshore can be correlated and can be shown in the latter domain to be early Jurassic-late Cretaceous, followed by localised Cenozoic reactivation. The workflow used here can be adapted to characterise deformation events in other superimposed rift basins with contiguous onshore surface-offshore subsurface expressions.

How to cite: Tamas, A., Holdsworth, R. E., Underhill, J. R., Tamas, D. M., Dempsey, E. D., McCarthy, D., McCaffrey, K. J. W., and Selby, D.: Correlating deformation events onshore and offshore in superimposed rift basins: the Lossiemouth Fault Zone, Inner Moray Firth Basin, Scotland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5019, https://doi.org/10.5194/egusphere-egu22-5019, 2022.

As the most important hydrocarbon-rich area of Wenchang A sag of Pearl River Mouth Basin, the Wenchang 9/8 area has attracted more scientific attention in fault-related hydrocarbon reservoirs. Here, we employ 2D and 3D seismic data and syn-rifting fault maps to analyze the fault characteristics and evolution of Wenchang 9/8 area, and its response to clockwise rotation of regional extension stress in northern margin of South China Sea. The results demonstrate that three NE- and NW-striking fault belts developed in Wenchang 9/8 area during Cenozoic, respectively. The pre-existing NE- and NW-striking basement faults and clockwise rotation of regional extension stress influenced the evolution of fault system in this area. During Paleocene to Eocene, the NE-striking sag-controlling faults activate intensively, under the control of NW-SE-directed extension. The extension was derived from the NW-direction subduction retreat of Pacific Plate, and the extension direction is perpendicular to pre-existing NE-striking faults, which resulted in the reactivation of the pre-existing NE-striking faults and the formation of the new NE-striking secondary normal faults. During Oligocene to early Miocene, the fault activity of the NE-striking sag-controlling faults weaken rapidly, resulting in segment activate and generate three NE-striking fault belts. These NE-striking fault belts were consisted of a series of new E-W-striking secondary transtensional faults. And the three NW-striking fault belts were started to reactivate in the form of abundant E-W-striking secondary transtensional faults, which were influenced by oblique extension. The characteristics of fault system indicated that N-S-directed extension worked on the study area, and the extension stress shifted clockwise from NW-SE to N-S during this stage. The clockwise rotation of the extension was believed related with the India-Eurasian collision and southern ward subduction of the Proto-South China Sea block. During middle Miocene to present, the NE-striking fault belts stopped. While the NW-striking fault belts activate continually, and each fault belts were consisted of a series of newly-formed NWW-striking secondary transtensional faults distributed in en-echelon. The NWW-striking secondary transtensional faults were formed under the control of NNE-SSW-directed extension, which influenced by regional extension stress further clockwise rotate to NNE-SSW direction. This extension was derived from the Philippine Sea Plate NWW-direction obduction, which leading to arc-continent collision at Taiwan Island, while giving rise to the NNE-SSW-directed extension at Pearl River Mouth Basin. Cenozoic evolution of fault system in Wenchang 9/8 area, Pearl River Mouth Basin revealed by this study not only provides guidance for petroleum exploration, but also affords implication for the research on tectonic stress field in northern margin of South China Sea.

How to cite: Xu, B., Wu, Z., Cheng, Y., Xu, L., and Sun, W.: Cenozoic faults evolution of Wenchang 9/8 area in Pearl River Mouth Basin, and the response to clockwise rotation of regional extension stress in northern margin of South China Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3305, https://doi.org/10.5194/egusphere-egu22-3305, 2022.

The Luconia-Balingan Provinces are sedimentary basins in Sarawak, Malaysia that presently extends from offshore to onshore, along major NW-SE faults on both sides of the basins. It was formed by several rifting episodes of the Southern China Block, followed by spreading since Eocene. The rifting coeval with the closure of an ancient oceanic crust that induced compression and major uplift in the southern part of the basins. The basins are filled by up to more than 10 km of Cenozoic sediments overlying the Cathaysian derived crystalline basement. Since the Upper Eocene-Oligocene to Upper Miocene, warm-water to tropical carbonate sedimentation has been dominating the stratigraphy mainly in Luconia. To understand the sediment and tectonic subsidence evolution of the Luconia-Balingian Provinces this study analysed data from exploration wells using a Matlab-based open-source tool, the BasinVis 2.0. Updated compaction trend from three different wells to represent the southern-central-north regions of the provinces are adopted. The subsidence history for Luconia-Balingian provinces can be divided into five stages; (i) 37 to 23 Ma: Steadily increase in subsidence is recorded with higher tectonic subsidence rate in the central and west of Luconia-Balingian and moderate tectonic subsidence in the north and south. (ii) 23-18 Ma: Increase in tectonic subsidence for most parts of Luconia-Balingian with slight decrease in total tectonic subsidence recorded in some of the wells. (iii) 18 to 15.5 Ma: Delayed subsidence within the central and northern parts of Luconia-Balingian, coincide with the diachronous timing of Middle Miocene Unconformity. There was minor uplift in the northern section. However, the southern part experienced increased in the total and tectonic subsidence rates. (iv) 15.5 to 11.8 Ma: Overall decrease in tectonic subsidence rate, coinciding with the prolific growth of Middle to Upper Miocene carbonate build-ups. (v) 11.8 to 0 Ma: Increase in tectonic subsidence rate particularly in the wells within the southern part of Luconia-Balingian. Stretching factors ranges between 1.3 to 1.95 are recorded, indicative for foreland basin setting with a very strong influence from the syn- and post-rift phases. It directly related to the effect of extensional tectonics during the South China Sea opening and compressional tectonics during the closure of the proto-South China Sea during the Cretaceous-Eocene, until Middle Miocene. Through this study, accurate subsidence rates are deduced and allows specific characterization of tectonic influences in different parts of Luconia-Balingian at different stages of basin development.

How to cite: Jamaludin, S. N. F., Pubellier, M., and Madon, M.: Variation in Cenozoic tectonic subsidence in Luconia-Balingian provinces, Sarawak Basin, Malaysia: influence of extensional and compressional tectonics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-783, https://doi.org/10.5194/egusphere-egu22-783, 2022.

The Qinling Orogenic Belt (QOB) has been well documented that it was formed by multiple steps of convergence and subsequent collision between the North China Block (NCB) and South China Block (SCB) during the Paleozoic and Late Triassic. Following the collision in the Late Triassic, the whole QOB evolved into an intracontinental orogeny. Large-scale N-S compressional and thrusting deformation tectonics along the previous major boundary faults were developed. Meanwhile, several extension-related tectonics were also developed in the QOB during or slightly later than the compression. However, the expiry date of the Late Mesozoic intracontinental compression orogeny and the tectonic transformation of the QOB remains unclear.

The extensional shear zone developed in the West QOB provides a crucial clue for revealing the orogenic belt's tectonic transformation and uplift and exhumation process. This paper focuses on kinematics, geochronology, and deformation temperature of the Taibai shear zone in the west QOB (TBSZ) to understand the intracontinental orogenic evolutionary process of the QOB.

The Taibai shear zone was developed at the northwestern margin of the Taibai pluton. It cuts across the QOB and separates the Taibai pluton in the east from the Baoji pluton in the west. The NE–NEE-striking and NW–NNW-dipping TBSZ is an extensional shear zone showing a top-to-the-NW sinistral shear sense. Mineral deformation characteristics and two-feldspar geothermometers constrain that the TBSZ was formed under the high greenschist–low amphibolite facies conditions (300-550 ℃) in the middle-upper crust (8-15 km). Zircon U-Pb dating and biotites and muscovites ⁴⁰Ar-³⁹Ar dating analysis suggest that the TBSZ was formed and uplifted rapidly during 120-113 Ma.

Combined with the regional geological data, the TBSZ records the extensional collapse of the Late Mesozoic intracontinental orogenic belt during the 120–113 Ma. The TBSZ also led to the rapid uplift and exhumation of the Taibai pluton and the North Qinling Belt in the east.

How to cite: Zhang, L. and Li, W.: Structural and geochronological constraints on a Late Mesozoic extension event in the West Qinling Orogenic Belt, China, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11808, https://doi.org/10.5194/egusphere-egu22-11808, 2022.

Abstract: To clarify the relationship between the complexity of the fault network systems and hydrocarbon migration in petroleum basin, 3D seismic data and hydrocarbon migration data from T50, T60 and T70 horizon in Wenchang 9/8 area were analyzed. The complexity and connectivity of the fault network system at different horizons in Wenchang 9/8 area are quantitatively characterized based on the topology and fractal theory. The relationship between the distribution characteristics of the complexity of the fault network system and hydrocarbon migration at different horizons in Wenchang 9/8 area was revealed combined with hydrocarbon migration data in typical zones. It indicates that the fractal dimension high value areas, and the topological high value areas and hydrocarbon migration area shave a good coupling relationship in the fault network system. The results showed that: there has a good coupling relationship among the fault interaction zone (fracture tip, structural transition zone, etc.), the fractal dimension high value areas and the topological high value areas. The peak values of the number of nodes (Nc) in T50, T60 and T70 horizons are in the range of 1-2, 6-8 and 5-7, respectively. The peak values of the fractal dimension (D) in T50, T60 and T70 horizon are among 1.4-1.6, 1.6-1.8 and 1.5-1.8, respectively. There is a good coupling relationship between fault interaction zone and hydrocarbon migration zone. Comparing with other structural area, the fault interaction zone has higher topological value and fractal dimension value. The topological high value area has good connectivity, and the fractal high value area has more faults, which is conducive to the formation of fault traps. Therefore, the overlapping zone with high topological value and high fractal dimension value are the dominant channels for oil and gas migration, which are favorable for the formation of favorable oil and gas reservoirs.

How to cite: Ma, S., Wu, Z., and Wang, D.: Topological structure, fractal characteristic of fault network system and their relationship with hydrocarbon migration：Taking Wenchang 9/8 area in the Pearl River Mouth Basin as an example, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4708, https://doi.org/10.5194/egusphere-egu22-4708, 2022.

Introduction. The Nepluyevka pluton is the Early Carboniferous polyphase batholith situated in the East Ural zone. The batholith is subdivided into 4 phases ranging from basic to felsic in composition. The pluton formed during the Early Sudetian orogenic phase of the East Ural zone, which was characterized by a complex alternation of pure and sub-simple shear kinematic settings over its’ duration. Evidently, this alternation was connected with changes in the kinematics of the subduction zones preceding the Late Visean collision of Laurussia and Kazakhstania. These transformations had defined the characteristic features of the tectono-magmatic evolution of the southern part of the East Ural zone. Thus, their investigation is crucial for improving our understanding of the geological history of the Southern Urals.

Methods and materials. We have investigated anisotropy of magnetic susceptibility (AMS) and magnetic mineralogy of the rocks of the Nepluyevka batholith to gain insights into the circumstances of its’ formation and its’ deformation history. Totally 186 oriented specimens from 16 sites spread over all the phases of the pluton were collected. MFK-1A kappa-bridge was used to measure MS and AMS, temperature dependencies of induced magnetization were studied with Curie balance, magnetic hysteresis loops were obtained on J_meter coercivity spectrometer.

Results. The specimen appeared to contain PSD high-Ti magnetite (magmatic), MD low-Ti magnetite (hydrothermal), as well as the minerals of goethite and maghemite-hematite series.

The AMS data tells the history of the formation and structural evolution of the batholith. Gabbro (1st phase) and granodiorites (2nd phase) in the center of the pluton are characterized by prolate magnetic fabrics. Lineation there is steep to sub-vertical and marks the flow direction near the feeder shared by both phases. Granodiorites (2nd phase) in the north and adamellites (3rd phase) in the north and the south of the pluton are characterized by predominantly oblate, flat-dipping fabrics, corresponding to lateral spreading of the melt. The magnetic fabrics of the adamellites (3rd phase) near the pluton’s southern boundary are oblate and dip steeply in the SW direction, marking the melt flowing parallel to the contact. The magnetic fabrics of the adamellites in the NE part of the batholith are similarly oblate and subparallel to the contact.

Discussion. We propose the model of “magmatic duplex” for the formation of the pluton. The upper-crust transtensional structure associated with a sinistral strike-slip fault was draining the lower-crust magma chamber. Due to the fractionation and assimilation of the chamber’s wall material, it was supplying increasingly felsic melt. Formation of the first two phases was controlled mainly by the central feeder. The 3rd phase adamellites intruded two weakened contact zones of the pluton as the transtensional structure continued to grow sub-longitudinally. The pluton has experienced secondary heating and some metasomatic alteration, but no significant deformations occurred.

Financial support. The research has been funded by RFBR and CNF as a part of the research project № 19-55-26009 with the use of materials of the "Geoportal" Center of the Lomonosov Moscow State University.

How to cite: Koptev, E., Kazansky, A., Tevelev, A., Borisenko, A., Pravikova, N., and Zák, J.: Granitic batholith emplacement mechanism in a transtensional setting: petromagnetic evidence from the Southern Urals, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4514, https://doi.org/10.5194/egusphere-egu22-4514, 2022.

Introduction. Fracture analysis of rock formations allows us to reconstruct formation history and structural development of magmatic blocks. This study investigates the Cheka block of alkaline granitoids (Southern Urals, Chelyabinsk Oblast). The objective of this study was to evaluate the main deformation characteristics of the sincollisional Cheka block (pluton). For this purpose, stress fields were reconstructed.

The Cheka pluton is composed of the Cheka Mountain and has a meridional strike and dimensions of 6.5 km long and 1-2 km wide. The pluton is composed of alkaline rocks of three intrusion phases: first – monzodiorites, second – alkaline syenites, third – alkaline granites and granosyenites. The pluton is Triassic and intrudes Carboniferous volcanics. The western contact of the Cheka pluton is limited by a dextral fault. The pluton is situated in the Magnitogorsk zone.

During the formation of the pluton, extension changed to compression. This led to formation of a right-lateral transpression setting with a system of meridional strike-slip and near-slip extension zones.

Materials and methods. Space images show several fracture systems with approximate strike lengths: -20° and 310°. During the field work more than 180 fracture orientations were measured Samples were taken for petro- and paleomagnetic, geochemical investigations, and isotope dating at five locations.

The Stereonet v.11.3.0 software was used to analyse the fractures. Schematics (Mohr circles) with fracture poles were created for each location. From these, five swarms of poles with Kamb contours were extracted, showing the statistical concentration of the poles. At locations 701 and 702, three swarms of sub-perpendicular poles were most clearly observed and interpreted as a system of tectonic fractures. The S, Q, and L fractures were identified among the prototectonic fractures based on the relation to linearity and pluton contacts. All the poles that fell within these three swarms were treated as prototectonic, while remaining locations outside these zones were treated as a system of fractures of tectonic origin. Numerous fractures, which are not part of the described systems, are most likely random and require more detailed research.

Results and discussion. A series of vertical fractures, arranged in a pattern relative to each other, were considered. Based on these swarms, a deformation model was built, and the directions of tension and compression were determined. Sub-horizontal compression was oriented northeastward, resulting in the formation of sub-meridional right-lateral shear and a general right-lateral transpression setting. The predominant fractures were synthetic P (according to Riedel), and they are also the most pronounced geomorphologically and on satellite images. Less pronounced are synthetic R and antithetic R' fractures.

This study of the Cheka pluton made it has possible to separate two fracture systems. These systems point to right-lateral transpression, which confirms the model of the massif formation as a shear magmatic duplex.

Financial support. The reported study was funded by RFBR and Czech Science Foundation according to the research project № 19-55-26009. Centre of collective usage ‘Geoportal’, Lomonosov Moscow State University (MSU), provided access to remote sensing data.

How to cite: Shestakov, P., Tevelev, A., Pravikova, N., Volodina, E., Borisenko, A., Kazansky, A., and Koptev, E.: Formation history of the Cheka pluton of alkaline granitoids (Southern Urals): fracture analysis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5946, https://doi.org/10.5194/egusphere-egu22-5946, 2022.