The connection between faults at surface, their subsurface geometry and earthquakes is a long-debated issue. The attempt of making such correlation is even more difficult when earthquakes are not strong enough to reach and break the topographic surface. Even in the latter case, the subsurface geometry of earthquake-causative-faults is not a trivial issue because of the difficulty of imaging the subsurface setting at seismogenic depths.
We take as an example the area of southern Tuscany in Central Italy where several M≈4 strike-slip earthquakes were registered recently, the latest of which occurred in May 2022.
The seismogenic role of transversal SW-NE striking faults in this area is debated as they do not show clear surface evidence even when releasing earthquakes and their recent and/or Quaternary evidence often a matter of discussion. For these reasons they can be extremely dangerous as they receive relatively little attention and are difficult to identify.
We integrate seismic reflection profiles, surface kinematic data and the relocation of seismological data in order to identify and characterize strike-slip active faults geometry at depth in the Valdelsa basin of southern Tuscany. We show that the Montespertoli NE-trending fault, part of a wider (15–20 km) crustal-scale shear zone, is possibly responsible for the 2016 M=3.9 Castelfiorentino earthquake and discuss the role of transversal SW-NE striking faults in controlling the inner Northern Apennines seismicity.

How to cite: Mirabella, F., Braun, T., Brogi, A., and Capezzuoli, E.: Unraveling the subsurface fault geometry of small to moderate strike-slip earthquakes: an example from the Valdelsa basin in Southern Tuscany (Italy), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7915, https://doi.org/10.5194/egusphere-egu23-7915, 2023.

The Altyn Tagh fault (ATF) is a large lithospheric left-lateral strike-slip fault, marking the northwestern boundary of the Tibetan Plateau. Understanding the tectonic history of the ATF provides insights into the growth pattern of the Tibetan plateau, as well as the deformation mechanism of complex fault systems.

However, despite numerous research efforts, the deformation of the ATF is still a subject of discussion, especially its interaction with the other two major faults in the northeast Tibetan Plateau: the strike-slip Eastern Kunlun fault and the Qilian Shan fold-thrust belt. The triple junction analysis has proven successful in explaining the spatial-temporal variations of fault kinematics. Therefore, here we use the principles of triple junctions to discuss the transformation of the ATF in its intersections with the Qilian Shan and Eastern Kunlun Shan, with the assistance of geological evidence from fieldwork and satellite images. We propose that the initiation of the left-lateral motion of the Eastern Kunlun fault led to an FFF triple junction in the former western end of the ATF. Meanwhile, the deformation on the southern Qilian Shan forms an TFF triple junction with the splays of the ATF. The unstable triple junctions will trigger the growth of the ATF and complicate the deformation the Qilian Shan and the Eastern Kunlun Shan. Our research firstly applies triple junction principles to both ends of the ATF, and presents a new model of the evolution of the ATF and its surrounding orogens, shedding lights on the history of Tibetan Plateau.

How to cite: Yi, K. and Guo, Z.: The transformation of the Altyn Tagh fault in its intersections with the Qilian Shan and Eastern Kunlun Shan explained by triple junction analysis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10602, https://doi.org/10.5194/egusphere-egu23-10602, 2023.

In 1978, Ōta and Yoshikawa published a pioneering study describing four distinct zones of marine terrace patterns in Japan and linked them to the large geodynamic processes controlling deformation across the arc. We repeat the exercise of Ōta and Yoshikawa (1978) with a large dataset of 5352 marine terraces of presumed last interglacial high stand age (~120 ka). The data is a subset from the Atlas of Marine Terraces by Koike and Machida (2001) later digitized by Nomura et al. (2016).

Consistent with Ōta and Yoshikawa (1978), we find that, along the subductions, terraces show a near systematic increase in elevation toward the trench reflecting non-recoverable deformation linked to the earthquake cycle. The Pacific Coast has over 1000 terraces that show remarkable regularity in elevation (between 25 and 50 m above sea level, masl). Meanwhile, on the back arc side, terrace elevation can vary over short distances (<20 km) between ~0 and 150 masl. We can identify the signature of the Niigata-Kobe Tectonic Zone responsible for the small block tilting noted by Ōta and Yoshikawa (1978) along the coast of the back arc.

The large terrace dataset allows us to probe controls on the generation and preservation of marine terraces. Because terrace elevation does not necessarily reflect the elevation of a marine high stand, without absolute dates and depth indicators we avoid using the terraces to calculate rock uplift rates. Instead we use their elevations as an indicator of relative patterns in rock uplift. We identify three main boundary envelopes to the distribution of presumed MIS 5e terraces when the entire dataset is displayed as a function of their mean elevation and surface area, and attribute it to potential controls:

• There are no large terraces preserved at low elevation because waves can more easily erode platforms that reside in or near the swash zone.
• Terrace surface area reaches a maximum around 30 masl before declining again with higher elevation because faster rock uplift rates reduce the time that waves have to erode any given bedrock elevation.
• The minimum area of terraces increases with elevation because under faster rock uplift, subaerial erosion processes tend to be more efficient and destroy small platforms.

Further study of the dataset —in particular accounting for local variations in wave power and rock type — will provide valuable insights to universal controls on marine terrace creation and preservation.

Koike, K., & Machida, H. (2001). Atlas of Quaternary Marine Terraces in the Japanese Islands. Tokyo: University of Tokyo Press.

Nomura K., Tanikawa S.-I. et al. (2016). Compilation of Information on Uplift of the Last Hundred Thousand Years in the Japanese Islands. JAEA reports, (JAEA-Data/Code 2016-015). https://doi.org/10.11484/jaea-data-code-2016-015

Ota, Y., & Yoshikawa, T. (1978). Regional characteristics and their geodynamic implications of late quaternary tectonic movement deduced from deformed former shorelines in japan. Journal of Physics of the Earth, 26(Supplement), S379–S389. https://doi.org/10.4294/jpe1952.26.Supplement_S379

How to cite: Malatesta, L. C., Huppert, K. L., and Finnegan, N. J.: Over 5000+ marine terraces record tectonics of the Japan arc and hint at essential controls on their creation and preservation., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10899, https://doi.org/10.5194/egusphere-egu23-10899, 2023.

Quaternary landscape evolution in the Vienna Basin and the adjacent area west of its subsiding area is controlled by sediment redeposition, aggradation and erosion of the Danube, local normal faulting, and overall regional uplift. Glacial - interglacial climate dynamics highly influence the hydrodynamics and amount of sediment transport. Over the last 9 years sediments exposed during construction and drilling as well as from surface outcrops were sampled for cosmogenic nuclide age determination and uplift/incision rate calculation.

The Vienna Gate marks the transition of the Danube alluvial plain in the west (Tullnerfeld) into the extensional structure of the Vienna Basin. At this border, the Danube flows on top of an approximately 2 km wide segment of Penninic Flysch units before it enters the Vienna Basin to the east. Within the transtensional structure of the Vienna Basin, several fault blocks record local uplift and subsidence. Outside of the Vienna Basin, regional uplift is documented by fluvial terrace deposits at elevated positions located at different heights above the recent Danube riverbed.

The current status and tectonic context of numerical ages ranging between 250 kyr and 3 Ma will be presented in detail at the conference. Few locations appear to be sedimentologically unsuitable for cosmogenic nuclide burial age dating, those scenarios will be explored and discussed.

Funding: HJS 318325/2018; OMAA 90ou17; OMAA 98ou17; NKFIH FK124807

How to cite: Neuhuber, S., Ruszkiczay-Rüdiger, Z., Braucher, R., Salcher, B., Hintersberger, E., Thöny, W., Strauss, P., Grupe, S., Payer, T., Braumann, S., Lüthgens, C., and Fiebig, M.: Terrestrial cosmogenic nuclides in Danube sediments record vertical movement in a transect from the Eastern Alpine Foreland into the Vienna Basin (Austria), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11536, https://doi.org/10.5194/egusphere-egu23-11536, 2023.

The Taiwan mountain belt results from the rapid convergence of the Luzon volcanic arc and Chinese continental margin. While GPS observations showed the progressive decrease in westward shortening across most of the island, they also revealed the tectonic escape of the southwestern part of the island, that is moving towards the southwest at a rate of 4-5 cm/yr. In the past decade, InSAR studies suggested the existence of a southwest striking right-lateral fault in the Holocene Coastal Plain that could play a significant role in this extrusion mechanism.

This study investigates the structure and the Holocene kinematics of this inferred fault based on near-surface geological and geophysical data mainly acquired during a geotechnical consulting project. The study site locates in the Coastal Plain, where the InSAR deformation gradient is highlighted by a topographic scarp and the presence of a mud volcano. The mud volcano displays a dome-shaped topography, 1-km in diameter, cut and offset by the inferred fault. We investigate the deformation of buried Holocene strata using 19 shallow boreholes, radiocarbon (¹⁴C) dating, U-Th dating and Resistivity Image Profiling for stratigraphic correlation across and along the inferred fault.

The fault-perpendicular cross-sections show that the bedrock and Holocene strata on the southeast block have been uplifting along a fault dipping 70^o to the southeast. The boreholes allow to identify a characteristic sandy layer, interpreted as a shoreface environment and dated at 4.7 ka. Along fault-parallel sections, this layer lies sub-horizontally, in contrast to the dome-shaped topography. Near the mud volcano mouths, the cores show mud dikes within this 4.7-ka layer and several mud flows within the overlying layer, which base was dated 4.1 ka. This suggests that the dome-shaped topography is the result of accumulated mud flows at the surface with mud-fluid transported through fractures induced by fault activity and/or fluid overpressure. The formation of the dome-shaped topography coincides with the transition from a shallow marine to a coastal and then continental environment at 4.1 ka. In parallel, using a high-resolution topographic dataset, we use the morphology of the mud volcano to estimate the right-lateral offset accumulated since 4.1 ka or later. We estimate an average horizontal offset of 54.4 ± 6.7 m and a minimum horizontal fault slip rate of 13.2 ± 1.6 mm/yr since 4.1 ka. Using the vertical offset of distinct layers across the fault leads to a vertical fault slip rate of 4.2 ± 1.8 mm/yr since 10 ka. The horizontal slip rate in our study is compatible with the horizontal deformation gradient of 15 mm/yr observed from GPS during 2015-2018. While GPS observations suggest that the fault may be at least partly creeping, the presence of Holocene growth strata at our study site suggest the possible occurrence of earthquakes during the Holocene. 

Keywords: Active tectonics, fault slip rates, mud volcanoes, Gutingkeng formation, Holocene

How to cite: Nguyen, N.-T., Le Beon, M., Ching, K.-E., and Pathier, E.: Near-surface structure and morphology of an offset mud volcano constrain the structure and Holocene kinematics of a reverse strike-slip fault in the Coastal Plain of southwestern Taiwan , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15198, https://doi.org/10.5194/egusphere-egu23-15198, 2023.

The Cenozoic Karvandar Basin is situated at the intersection of the Sistan Suture Zone and the Makran accretionary wedge, in SE Iran. This intersection represents the junction of the continental Lut and Afghan/Helmand blocks in the west and east, respectively, and the northward subducting oceanic lithosphere of the Arabian plate in the south, hereafter called Makran-Sistan triple junction. The plate tectonic framework in Late Cretaceous is comparable to the present situation in the Mediterranean, with several microcontinents divided by smaller branches of the Neo-Tethys (Nain-Baft, Fannuj, Sistan, and Sabsevar oceans) surrounding the Central Iranian Blocks and the main Neo-Tethys Ocean to the south.

The Karvandar Basin hosts a series of elongated, doubly-plunging growth synclines connected by variably thick shale walls while anticlinal structures are mostly absent. In this study, we unravel the tectonostratigraphic development of these synclines by geologic field investigations and precise magnetostratigraphic dating, pinpointed by U-Pb zircon ages of interlayered tuffs. Detailed information on the timing of sediment accumulation, limb rotation, and the geometry of unconformities allow identifying the character of their formation, i.e. gravitational downbuilding vs. tectonic forcing, and help understanding the tectonic context of the Karvandar Basin, specifically, how it relates to adjacent plate boundaries such as the Makran subduction zone and the Sistan Suture Zone, which is still under debate.

The stratigraphic record of the Karvandar Basin is dominated by a 6-kilometer-thick sequence, showing a gentle deepening towards the west. The basin records a relatively rapid shallowing upwards trend at the base. After this first phase, the record is dominated by shallow marine to non-marine alluvial Molasse-like sediments. During this phase, the sedimentary environment remained steady for thousands of meters, suggesting a balance between accommodation and sedimentation. This reveals a fast and steady subsiding system, and points to high sedimentation rates and an expanded stratigraphy.

Magnetostratigraphic dating of a approx. 4km sedimentary sequence suggests that the basin formed between ~23–17 Ma, resulting in an accumulation rate of ~1 m/kyr. Angular blocks of volcanic heritage and corrals in the underlaying shale potentially suggest an olistostrome nature with a respective age >24 Ma. We propose that the closure of the South Sistan Basin and the related orogeny led to tectonic subsidence, where a Molasse-type continental sequence was deposited onto a kilometer-thick, mechanically weak olistostrome.

How to cite: Ruh, J. B., Vouga, J., Valero, L., Najafi, M., Landtwing, F., and Guillong, M.: Tectonic evolution of the Makran-Sistan triple junction: Field study and magnetostratigraphy from the Molasse-type Karvandar Basin, SE Iran, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16382, https://doi.org/10.5194/egusphere-egu23-16382, 2023.

Based on the detailed description of the fault system and the regional dynamic background of the study area, the Cenozoic structural development and evolution characteristics of the southwest Bohai sea and the migration law of the sedimentary-subsidence center were studied by using 3D seismic data and drilling data.The results show that the NW, NNE, NE and EW trending faults were mainly developed in the study area. The NW-trending faults were Cenozoic revived faults, which control the development of the NW-trending structural belt. The NNE-trending faults control the formation of the uplift, including Kendong fault, Gudong fault and Changdi fault, which all belong to co-direction shear faults of the Tan-lu fault zone, and have obvious strike-slip characteristics. The NE-trending faults and EW-trending faults were extensional faults, which further complicate the tectonic pattern. Under the control of the NNE-trending faults and near EW-trending faults, the sedimentary thickness of the Paleogene strata in the study area changed from thick in the south and thin in the north in the early stage to thin in the south and thick in the north in the late stage. In the sedimentary period of Es₃, the uplift was highly segmented. The mountains were high and the surrounded lakes was deep, and the water bodies were connected between the depressions. During the sedimentary period from Es₂ to Es₁, the regional structure subsided and the lake area expanded. In the sedimentary period of Ed, it was high in the south and low in the north, and basically distributed regionally. After the Neogene, it finally became a unified whole to accept deposition. Generally, the overall evolution can be divided into four stages: ① Confined fault-depression stage of Ek to Es₄；② Strong fault-depression stage of  Es₃ to Es₂；③Weak fault depression stage of Es₁ to Ed；④ Weak extended depression stage of the Ng-Nm.

How to cite: Hao, R., Wang, Y., and Wu, Z.: Cenozoic Tectonic Characteristics and Evolution of the Southwest Bohai Sea, China, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11, https://doi.org/10.5194/egusphere-egu23-11, 2023.

The area of the High Agri Valley, located in the central part of the Southern Apennines, has been extensively studied in the past, due to the presence of important economic resources and active faults. In particular, attention was focused on the large-scale faults, affecting the allochthonous tectonic units the area, with a direction nearly parallel to the chain axis. Based on that, the previous authors identified two different fault systems located on the opposite sides of the valley. Less attention, however, has been paid to the transversely oriented faults that make up Transverse Tectonic Lines (TTL). The Agri valley is one of the NW-SE elongated basin formed during the extensional phase that, starting from lower Pliocene, affected the Southern Apennines. In the area important structures recorded the brittle and ductile deformation that involves all the tectonic units that make up the Southern Apennines thrust and fold belt. This latter results from the tectonic collision between the African and European plates in the present-day Mediterranean area. These allowed the allochthonous wedge to migrate with NE vergence on the autochthonous Apulian carbonates.

The TTL in the eastern part of the High Agri Valley appear to have similar lengths (segments rarely reach 8 km in length) and are characterized by a NE-SW orientation, nearly parallel to the main thrust vergence direction. There are few transverse fault planes directly visible in the field and most of the faults have been deduced from the displacement of stratigraphic contacts as well as from the observation of satellite images. The maximum vertical displacements in the central part of the major fault segments exceed 1500 m, thus allowing us to consider these structures of considerable importance on the scale of the Southern Apennines.

The throw profiles derive from the analysis of cut-off lines of formational tops displaced from selected faults obtained from a static 3D model. This allows us to hypothesize its growth pattern and kinematics. Most of the throw profiles of LLTs have a characteristic bell-shaped geometry with greater displacement in the central part that gradually decreases at the tips. Moreover, the observation of the hanging-wall and footwall curves of the cut-off lines of the formational tops allow to hypothesize the kinematics of the studied faults.

The analysis of LTTs and throw profiles in fault developed within highly deformed allochthonous Units can be considered as a new approach that can be proposed for further studies in fold and thrust belts. The transverse faults could be interpreted as linkage structures between segments of faults parallel to the chain axis or be confined by the latter which inhibit their lateral propagation. This could also be important in relation to the seismicity of the Southern Apennines as well as in the compartmentalization of aquifers hosting important water resources in the study area.

How to cite: Olita, F. and Prosser, G.: Study of the fault propagation process in the High Agri Valley area (Southern Apennines), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11872, https://doi.org/10.5194/egusphere-egu23-11872, 2023.