The Strzegom – Sobótka Massif has been subject of brittle tectonics studies for more than a century. Due to an ongoing extensive mining activity, numerous good exposures occur in a relatively small area, especially in the western part of the massif. A pioneering tectonic model of jointing in granite was established by Cloos (1922) for the study area, in which the NW-SE striking joint set is the dominant one (Q) and the perpendicular set (S), striking NE-SW, is longitudinal to mineral fabric. Also, there are two sets of the so-called diagonal joints, which are supposedly younger and strike N-S and W-E.

The effects of field work conducted in 20 quarries in the Strzegom – Sobótka Massiff are presented in the form of a tectonic map. In addition to direct measurements in the field, photogrammetric models were produced using aerial photographs to allow structural analysis within hardly accessible walls. For inaccessible quarries joint orientations were extracted using orthophoto maps. Several examples of fault related structures were identified and documented during field work in the studied granite quarries. Faults with slickensides and kinematic indicators were scarce but paleostress analyses were conducted whenever possible.

We discuss our field measurements of joint and fault orientations in relation to different petrographic types of granites and their lateral extent to address the effects of petrographic differentiation on the evolution of brittle tectonic structures in granites. We compare our new measurements to the results of previous tectonic studies of the Strzegom – Sobótka Massiff and paleostress analyses conducted for several other parts of the Sudetes. We also discuss our new results in terms of the Alpine reactivation of the Sudetes Mountains.

How to cite: Fiałkiewicz, M., Grochmal, B., Olkowicz, M., Bulcewicz, K., and Dąbrowski, M.: Brittle tectonics and paleostress analysis of the Strzegom – Sobótka granite massif, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2295, https://doi.org/10.5194/egusphere-egu23-2295, 2023.

Integration of geophysical and geological data is essential to illuminate the configuration and geometry of surface and subsurface structures as well as their long-term evolution history. The NNW–SSE-striking incised valley and parallel mountain range in southeastern margin of the Korean peninsula, extended from Gyeongju to Ulsan cities (~50-km-long on land), have been regarded as the most active geographical feature in Korea, which was named as the Ulsan Fault zone (or system). This study presents a new insight of the structural architecture and its deformation history during the Cenozoic based on a combined data of gravity and electronic survey results with previous field observations. Our major results based on integrated data are as follows. First, the incised fault valley is divided into (1) the northern part of several distributed, buried and exposed fault strands and (2) the southern part of a concentrated deformation zone. Different deformation features between the two parts are controlled by the distribution pattern of the pre-existing Miocene structures (i.e., Yeonil Tectonic Line, YTL). Second, the Ulsan Fault is only constrained as a NNW–SSE-striking Quaternary fault zone within the incised valley-mountain range. The fault zone is composed of several interconnected and disconnected strands forming an imbricate thrust zone located along the western front of the mountain range (or eastern margin of the >2-km-wide incised valley). The constituent fault strands mainly exhibit an east-side-up geometry with moderate to low dip angles and reverse-dominant kinematics in near-surface. These strands are interpreted as reactivated ones of the pre-existing subvertical structures, such as the YTL. In here, we newly designate ‘the Ulsan–Yeonil Fault system’, composed of all NNW–SSE to N–S-striking buried and exposed faults on the incised valley-mountain range, regardless of tectonically controlled sequence of movement stages. Third, movements of the NNW–SSE-striking fault system during the Miocene to Quaternary were arrested by the NNE–SSW-striking Yangsan Fault, which is pre-formed prominent mature structure. Our results highlight the spatiotemporal structural characteristics in SE Korea, emphasizing that the configuration of pre-formed structures have strongly controlled the distribution and characteristics (i.e., geometry and kinematics) of the subsequent deformation during the Cenozoic crustal deformation.

How to cite: Cheon, Y., Shin, Y. H., Park, S., Choi, J.-H., Kim, D.-E., Ko, K., Ryoo, C.-R., and Son, M.: Structure and evolution of the active Ulsan Fault Zone, SE Korea: New insights from geophysical studies, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4713, https://doi.org/10.5194/egusphere-egu23-4713, 2023.

The goal of the ERC funded FOCUS project is to study an active fault offshore Catania using fiber optics, sea-floor geodesy, seismological stations (onshore and OBS on the seafloor) and detailed in-situ observations using an ROV and an AUV. Here, we report on the latter. In October 2020 using the ROV Victor6000 and in January 2022 using the AUV IdefX, we performed micro-bathymetric mapping (at an altitude of 50 m above the seafloor) of a 15-km-long segment of the North Alfeo Fault, covering water depths of about 1600 m to 2300 m. A prominent lozenge-shaped transpressive ridge or “pop-up” type structure is one of the primary features of this portion of the fault zone. It forms a flat-topped plateau culminating at about 1700 m water depth. It is cross-cut by a network of N-S striking faults resembling domino blocks or books in a book-shelf.

Sub-bottom profiling (using a chirp system on the AUV IdefX at an altitude of 70 m above the seafloor) crossed the transpressive ridge and imaged the network of narrowly spaced (typically 100 - 200 m spacing) N-S striking faults, which are steeply W dipping normal faults. This suggests the transpressive ridge is currently collapsing. Indeed, the eastern part of the plateau is marked by a small (600 m long from headscarp to toe) submarine landslide. The overall pattern in the northern portion of the mapped area (west of the plateau) is a series of oblique secondary faults, crossing the primary fault at an angle of about 30°. Using a very simple analog model of a thin layer of cohesive granular material above two rigid plates, with a slightly curved fault track, it was possible to produce a primary strike-slip fault directly above the cut between the two plates, and several distinct transpressional ridges (pop-ups) as well as transtensional fissures or gashes. Secondary faults form obliquely to the primary fault and are oriented at about a 30° angle clockwise from the trend of the primary fault. This pattern reproduces the large-scale features observed in the micro-bathymetry from the NW prolongation of the North Alfeo fault. A series of analog experiments using different rheologies in the sediment layer is planned in the future to test the likely detachment (nucleation) depth for the strike-slip fault in the basement.

How to cite: Giovanni, B., Arnaud, G., Frauke, K., Pauline, D., Edgar, L., Vincent, C., Stèphane, D., and Marc-Andrè, G.: Ultra-high resolution (micro-bathymetric mapping and sub-bottom profiling) imaging of an active strike-slip fault, the North Alfeo Fault, offshore Catania, Eastern Sicily (Ionian Sea, Central Mediterranean), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9272, https://doi.org/10.5194/egusphere-egu23-9272, 2023.

Lithospheric flexure is the primary mechanism for the development of normal faults in foreland basins. While the tectonic regime defines the overall fabric of such flexure-induced faults, mechanical heterogeneity of the sedimentary sequence and pre-existing faults exert a major control on the geometry of the individual faults. Interpretation of 3-D seismic reflection data in the central part of the German Molasse Basin, a northern Alpine foreland basin, reveals a normal fault network that exhibits varying degrees of vertical segmentation. Two major faults oriented parallel to the strike of the Alpine orogen are characterised by geometrically coherent displacement of deeper Mesozoic strata and shallower Cenozoic strata. In contrast, another major fault system, oriented obliquely to the orogenic strike, shows an along-strike variation in geometric coupling between deeper and shallower structural levels. Although a thoroughgoing fault in the northeast, it bifurcates laterally to the southwest, with the deep and shallow segments decoupling across a southeastwardly-thickening, mechanically-weak layer. To establish the geometric evolution of these faults and understand to what extent it was governed by mechanical stratigraphy and structural inheritance, we here analyse throw distribution on the faults and variations in stratal thicknesses across the faults. High-resolution throw mapping indicates a general updip decrease in throw for the orogen-parallel faults, whereas the obliquely-oriented fault, in its coupled portion, has two throw maxima separated by a throw minimum at the mechanically incompetent interval. These results, together with syn-kinematic strata observations, show that the former faults initiated with the onset of the Cenozoic foreland flexure and grew upward by radial propagation, whereas the latter fault formed by an oblique reactivation of precursory Mesozoic faults and developed in the Cenozoic as a segmented structure. We hypothesise that the coupling of its deep and shallow segments to the northeast was established by a dip-linkage mechanism, which was inhibited further to the southeast as the mechanical barrier thickens. The reactivation of the pre-existing structures explains the non-optimal orientation of the younger fault segments at a shallower level, with the former acting as kinematic attractors for the latter faults. This study demonstrates how a detailed fault kinematic analysis can help to decipher the effect of multi-layered mechanical stratigraphy and structural inheritance on the spatial evolution of individual flexure-induced faults.

How to cite: Shipilin, V., Tanner, D., Ziesch, J., and Buness, H.: Influence of mechanical stratigraphy and structural inheritance on the geometrical evolution of normal faults in the German Molasse Basin, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12583, https://doi.org/10.5194/egusphere-egu23-12583, 2023.

The North Anatolian Fault (NAF), located in Turkey, is one of the world's most active faults and accommodates Anatolia's westward motion relative to Eurasia. Over the last century, several earthquakes (M>6.8) have migrated from east to west. It is in the Marmara region, south of Istanbul, that the subsequent rupture is expected. However, this is where the geometry of the fault becomes more complex. It divides into three branches, one of which borders Lake Iznik and the southern Marmara Sea. As there is now very little seismic activity along this portion of the NAF (MNAF), and GPS only detects small displacements (Reilinger et al., 2006), it is thought to be inactive. However, the city of Iznik, the cradle of Christianity, has preserved valuable historical evidence in contrast to its observations. Therefore, to better understand the seismic hazard in this area, it is necessary to catalogue the seismic activity and locate past ruptures.

Two active faults were discovered in Lake Iznik thanks to our geophysical and coring campaigns (Gastineau et al., 2021). The study of short (<4m) sediment cores sampled on both sides of the E-W fault running close to Iznik city reveals that the previous rupture (1065 CE) coincides with a highly destructive historical earthquake recorded in the city's archaeological structures (Benjelloun et al., 2020). In addition to this localised rupture, numerous other event deposits are present in the sediments (laterally and temporally). We demonstrated that the same earthquake in 1065 CE is associated with various deposit types. One type of deposition is only observed for the 1065 CE earthquake, which takes place in the lake, unlike the others, suggesting that this type of deposition may depend on ground motion parameters besides the source-core distance.

A compilation of marine and lacustrine palaeoseismological studies was carried out at the scale of the western part of the NAF. We show that the relationship between sedimentation rate and the presence of earthquake-induced slope destabilisation doesn't work in the marine environment, unlike in the lacustrine environment. We also show that Lake Iznik records earthquakes from the NNAF and the MNAF, whereas the Sea of Marmara records only NNAF earthquakes. These observations open new perspectives and demonstrate the need to consider seismology and site effects in marine and lacustrine paleoseismology.

References:

Benjelloun, Y., De Sigoyer, J., Dessales, H., Baillet, L., Guéguen, P., Şahin, M., 2020. Historical earthquake scenarios for the middle strand of the North Anatolian Fault deduced from archeo-damage inventory and building deformation modeling. Seismol. Res. Lett. https://doi.org/10.1785/0220200278

Gastineau, R., De Sigoyer, J., Sabatier, P., Fabbri, S.C., Anselmetti, F.S., Develle, A.L., Şahin, M., Gündüz, S., Niessen, F., Gebhardt, A.C., 2021. Active Subaquatic Fault Segments in Lake Iznik Along the Middle Strand of the North Anatolian Fault, NW Turkey. Tectonics 40, e2020TC006404. https://doi.org/10.1029/2020tc006404

Reilinger, R., McClusky, S., Vernant, P., Lawrence, S., Ergintav, S., Cakmak, R., Ozener, H., Kadirov, F., Guliev, I., Stepanyan, R., others, 2006. GPS constraints on continental deformation in the Africa-Arabia-Eurasia continental collision zone and implications for the dynamics of plate interactions. J. Geophys. Res. Solid Earth 111. https://doi.org/10.1029/2005JB004051

How to cite: Gastineau, R., Sabatier, P., Fabbri, S. C., Anselmetti, F. S., Roeser, P., Şahin, M., Gündüz, S., Gebhardt, A. C., Franz, S. O., Niessen, F., and De Sigoyer, J.: From Lake Iznik to the Marmara Sea (NW Turkey): new insights in marine and lacustrine paleoseismology., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13935, https://doi.org/10.5194/egusphere-egu23-13935, 2023.

The undersea portion of the Northern Apennines is characterized by blind thrust faults running parallel to the Adriatic Sea coastline in northeastern peninsular Italy. These thrusts are buried below a thick cover of syntectonic Quaternary deposits. Their elusive geological signature at shallow depths and the low seismicity associated with them gave rise to diverging interpretations and views concerning the current activity of these thrusts and their earthquake potential.

On 9 November 2022, a seismic sequence started with an Mw 5.5 earthquake in the Pesaro Offshore. Hypocentral depth, focal mechanism, and aftershocks location all suggest that the earthquake was generated by one of the outermost thrusts of the Northern Apennines front that was already mapped as a potential seismogenic source in the DISS database (https://diss.ingv.it/diss330/sources.php?ITCS106).

We present a 3D reconstruction of the thrust system that caused the Pesaro Offshore seismic sequence obtained through the reinterpretation of publicly available seismic reflection profiles and well logs. The 3D geometry and size of the thrust activated during the seismic sequence suggest that it can also host larger earthquakes. We also present the application of a well-established workflow for calculating the slip rates of this buried thrust already tested in nearby structures. The outcomes of this study represent a step forward for earthquake and tsunami hazard models, the study of the seismic source, the enhancement of earthquake location by mix and match of seismological and geological independent data, and the expected kinematics of future potential earthquake ruptures.

These results are particularly relevant in offshore areas, where neither surface co-seismic ruptures nor GPS/InSAR deformation data are available in the aftermath of a significant earthquake. In these cases, multichannel seismic reflection profiles represent the only tool to appraise the subsurface structural setting. 

How to cite: Maesano, F. E., Buttinelli, M., Maffucci, R., Toscani, G., Basili, R., Bonini, L., Burrato, P., Fedorik, J., Fracassi, U., Panara, Y., Tarabusi, G., Tiberti, M. M., Valensise, G., Vallone, R., and Vannoli, P.: 3D geological modeling of the blind thrust system activated during the November 2022 Pesaro offshore seismic sequence (Adriatic sea, Italy)., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6973, https://doi.org/10.5194/egusphere-egu23-6973, 2023.

The central Mediterranean is a geodynamically very complex area included in the convergence zone between the European plate and the African plate. We investigated the western sector of the Sicily Channel , which shows, according to literature data, different deep and shallow tectonic structures than the eastern sector.  Structural data show the presence of a crustal-scale discontinuity that has generated major seismic events such as the Belice earthquake of 1968. This structure has been identified as a wideband roughly oriented N-S from the San Vito Lo Capo to the Sciacca area (SVCS band, San Vito Lo Capo - Sciacca band) (Di Stefano et al., 2015) and continuing offshore to the Pantelleria area. In this work, through multidisciplinary data analysis, we aim to investigate the correlation between the surface structures highlighted onshore and the offshore continuation. For this purpose, we considered offshore data from the Sicily Channel including: gravimetric data, which show negative anomalies in the Pantelleria graben (Palano et al., 2020) and in the Sciacca offshore and velocity models showing the lateral variation of the Moho with values ranging from 30 to 33 km depth and values ranging from 20 to 23 km depth respectively west and est of the Pantelleria graben (Finetti, 2005). These data were compared with our interpretation of crustal reflection seismic profiles and seismic events (since 2005 with M≥2). The results show an alignment of seismic events with roughly N-S direction from offshore Sciacca to Lampedusa. Moreover, the seismic profiles show a lateral variation of the Moho depth deepening estward. From the joint analysis of these data we obtained a geological model of the investigated sector defining the offshore prosecution of the SVCS band present onshore. The present work may be useful for understanding the geodynamic evolution and for studying the seismic hazard of this area.

References

Di Stefano P., Favara R., Luzio D., Renda P., Cacciatore M. S., Calò M., Napoli G., Parisi L., Todaro S., Zarcone G. (2015). A regional-scale discontinuity in western Sicily revealed by a multidisciplinary approach: A new piece for understanding the geodynamic puzzle of the southern Mediterranean, Tectonics, 34, 2067–2085, doi:10.1002/2014TC003759.

Finetti I. R. (Ed.). (2005). CROP project: deep seismic exploration of the central Mediterranean and Italy. Elsevier.

Palano M., Ursino A., Spampinato S., Sparacino F., Polonia A., Gasperini L. (2020). Crustal deformation, active tectonics and seismic potential in the Sicily Channel (Central Mediterranean), along the Nubia–Eurasia plate boundary. Scientific reports, 10(1), 1-14.

How to cite: Bongiovanni, S., Maiorana, M., D'Alessandro, A., Martorana, R., and Sulli, A.: Multidisciplinary approach to the study of a crustal tectonic discontinuity: an example from the Central Mediterranean offshore (Sicily channel), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11843, https://doi.org/10.5194/egusphere-egu23-11843, 2023.

Seismic reflection datasets can help unpick the long term (i.e., 100 kyr to Ma) slip history of active normal faults (e.g., Nicol et al., 2005). To constrain slip-rate from seismically imaged normal faults you measure the across-fault offset of stratigraphic markers of known age.  Ideally, this is undertaken parallel to the slip-vector, i.e., orthogonal to fault-strike. In many active systems this is not possible with only non-optimally orientated 2D surveys available. Here we assess how obliquity effects continuous and discontinuous fault properties (throw, heave, dip, displacement) extracted from normal faults imaged in a 3D seismic cube. We targeted ‘straight’ faults and extracted cut off data from sequentially oblique sample lines ranging from ±50˚, comparing oblique data to that extracted from optimally orientated lines. Additionally, repeat picks were undertaken on two horizons to investigate the relative importance of measurement obliquity and human error.

Oblique measurements showed different along-fault profiles compared to orientated sample lines. This causes some datasets to be statistically different, with >100 % difference occasionally observed. Continuous deformation is more prone to obliquity errors, with the measurement of an apparent dip causing heave, and therefore displacement and dip, to display large differences at high obliquity. The dip of horizons close to the fault and localised complexity at the sample location (e.g., nearby faults) are also important factors. Differences regularly exceed 20% at high obliquity and we therefore suggest obliquity should not exceed 15˚ and were this is not possible measurements are corrected using fault cut offs and local fault strike.

For repeats picks, the shape of along-fault profiles is similar; however, subtle differences exist. Variability depends on the fault and fault parameter, with greater differences observed for faults with low displacement. Several locations along the fault exceeded the population difference. This was locally associated with a particular dataset; however, trends rarely persisted along the whole fault. Factors influencing this include a) shallow folding close to the fault, b) localised complexity at the sample location, and/or c) poor imaging near the fault plane. Unexpectedly, no correlation between variability and obliquity was found. Overall, we suggest errors due to human factors could be ~10-15% for throw and 20-25% for heave, with higher errors possible.

If we consider the fault data extracted using 2D seismic lines across the Cape Egmont Fault by Nicol et al., 2005, >50% of the measurement points exceed our recommended maximum obliquity. Nicol et al. (2005) report that the maximum throw between the 3.2 and 3.7 Ma horizons as 1364 m, giving a throw rate of 0.0028 mm/yr. However, the 2D survey at this location has a measurement obliquity of 21˚. Considering our findings throw rate could range from 0.0022 to 0.0033 mm/yr due to obliquity and human factors. It is therefore important users are aware that spatio-temporal variations in slip-rate may be caused by geological controls, human errors, and measurement obliquity.

Nicol et al., 2005. Growth of a normal fault by the accumulation of slip over millions of years. J. Struct. Geol. 27 (2), 327-342.

How to cite: Andrews, B., Mildon, Z., and Jackson, C.: The impact of human factors and measurement obliquity when extracting geological slip-rate from seismically imaged normal faults., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14121, https://doi.org/10.5194/egusphere-egu23-14121, 2023.

The Coefficient of Complexity (CoCo) is a metric that quantifies the relative structural complexity of the fault system surrounding a specific study site on a primary fault extending through the system. Specifically, the CoCo metric has been used successfully to correlate the relative constancy or non-constancy of incremental slip rates on major strike-slip faults with the proximity, number, and slip rates of other neighboring active faults within a given radius of observation. Interestingly, our analysis shows that faults that extend through more structurally complex plate-boundary fault systems are characterized by more temporally variable slip behavior than faults that are embedded within simpler settings. Complex stress interactions within structurally complicated tectonic networks, as well as possible temporal changes in fault strength and kinematic interactions amongst mechanically complementary faults, likely explain these different behaviors. The CoCo metric thus not only provides a potential means for better evaluating the future behavior of large plate-boundary faults in the absence of well-documented incremental slip-rate behavior, but also can be used to differentiate faults that typically slip at a constant rate from the ones which do not. Using these results, we explore the relationship between incremental fault slip rates averaged over both short and large displacements on major strike-slip faults and geodetic estimates of strain accumulation rate on faults with different CoCo values. As might be anticipated, our analysis shows that the relatively constant slip rates on faults embedded within structurally simple strike-slip tectonic networks (i.e., low-CoCo faults) generally match rates of elastic strain accumulation of the faults’ shear zones, as measured by geodetic slip-deficit rates. In marked contrast, geodetic slip-deficit rates for faults embedded within structurally complex fault systems (high-CoCo faults) are less consistent with geological rates, whether averaged over short or large displacement scales, indicating significant variations in strain-accumulation rate on high-CoCo faults. We use these data to suggest possible patterns of geodetic-to-geologic rate ratios that may be indicative of the likely near-future behavior of the fault in question.

How to cite: Gauriau, J. and Dolan, J.: Using the CoCo metric of relative structural complexity of major plate-boundary fault networks to explore potentially time-variable fault loading rates on major strike-slip faults, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8544, https://doi.org/10.5194/egusphere-egu23-8544, 2023.