ALBERT

Description: The nature of active deformation in the Gulf of Cadiz is important for developing a better understanding of the interplate tectonics and for revealing the source of the 1755 Great Lisbon earthquake. New, high-resolution 3-D seismic data reveal a classic pull-apart basin that has formed on an east striking fault in the Southern Lobe of the Gulf of Cadiz accretionary wedge. Geometrical relationships between an array of faults and associated basins show evidence for both dextral and sinistral shear sense in the Southern Lobe. Strike-slip faulting within the lobe may provide a link between frontal accretion at the deformation front and extension and gravitational sliding processes occurring further upslope. Inception of the strike-slip faults appears to accommodate deformation driven by spatially variant accretion or gravitational spreading rates, or both. This implies that active deformation on strike-slip faults in the Southern Lobe is unrelated to the proposed modern inception of a transform plate boundary through the Gulf of Cadiz and underscores the importance of detailed bathymetric analysis in understanding tectonic processes.

Description: We present a three-dimensional gas hydrate systems model of the southern Hikurangi subduction margin in eastern New Zealand. The model integrates thermal and microbial gas generation, migration, and hydrate formation. Modeling these processes has improved the understanding of factors controlling hydrate distribution. Three spatial trends of concentrated hydrate occurrence are predicted. The first trend (I) is aligned with the principal deformation front in the overriding Australian plate. Concentrated hydrate deposits are predicted at or near the apexes of anticlines and to be mainly sourced from focused migration and recycling of microbial gas generated beneath the hydrate stability zone. A second predicted trend (II) is related to deformation in the subducting Pacific plate associated with former Mesozoic subduction beneath Gondwana and the modern Pacific-Australian plate boundary. This trend is enhanced by increased advection of thermogenic gas through permeable layers in the subducting plate and focused migration into the Neogene basin fill above Cretaceous-Paleogene structures. The third trend (III) follows the northern margin of the Hikurangi Channel and is related to the presence of buried strata of the Hikurangi Channel system. The predicted trends are consistent with pronounced seismic reflection anomalies related to free gas in the pore space and strength of the bottom-simulating reflection. However, only trend I is also associated with clear and widespread seismic indications of concentrated gas hydrate. Total predicted hydrate masses at the southern Hikurangi Margin are between 52,800 and 69,800 Mt. This equates to 3.4–4.5 Mt hydrate/km2, containing 6.33 × 108–8.38 × 108 m3/km2 of methane.

Description: Submarine groundwater discharge (SGD) into coastal areas is a common global phenomenon and is rapidly gaining scientific interest due to its influence on marine ecology, the coastal sedimentary environment and its potential as a future freshwater resource. We conducted an integrated study of hydroacoustic surveys combined with geochemical porewater and water column investigations at a well‐known groundwater seep site in Eckernförde Bay (Germany). We aim to better constrain the effects of shallow gas and SGD on high frequency multibeam backscatter data and to present acoustic indications for submarine groundwater discharge. Our high‐quality hydroacoustic data reveal hitherto unknown internal structures within the pockmarks in Eckernförde Bay. Using precisely positioned sediment core samples, our hydroacoustic‐geochemical approach can differentiate intra‐pockmark regimes that were formerly assigned to pockmarks of a different nature. We demonstrate that high‐frequency multibeam data, in particular the backscatter signals, can be used to detect shallow free gas in areas of enhanced groundwater advection in muddy sediments. Intriguingly, our data reveal relatively small (typically 〈15 m across) pockmarks within the much larger, previously mapped, pockmarks. The small pockmarks, which we refer to as “intra‐pockmarks”, have formed due to the localized ascent of gas and groundwater; they manifest themselves as a new type of ‘eyed’ pockmarks, revealed by their acoustic backscatter pattern. Our data suggest that, in organic‐rich muddy sediments, morphological lows combined with a strong multibeam backscatter signal can be indicative of free shallow gas and subsequent advective groundwater flow.

Description: Key Points High-resolution reflection seismic data reveals that the internal architecture of the Kolumbo Volcanic Chain The Kolumbo Volcanic Chain evolved during two episodes along NE-SW striking normal faults A prominent volcanic ridge connects the Kolumbo Volcanic Chain with Santorini highlighting a former connection between both systems Abstract The Christiana-Santorini-Kolumbo volcanic field in the southern Aegean Sea is one of the most hazardous volcanic regions in the world. Forming the northeastern part of this volcanic field, the Kolumbo Volcanic Chain (KVC) comprises more than submarine volcanic cones. However, due to their inaccessibility, little is known about the spatio-temporal evolution and tectonic control of these submarine volcanoes and their link to the volcanic plumbing system of Santorini. In this study, we use multichannel reflection seismic imaging to study the internal architecture of the KVC and its link to Santorini. We show that the KVC evolved during two episodes, which initiated at ~1 Ma with the formation of mainly effusive volcanic edifices along a NE-SW trending zone. The cones of the second episode were formed mainly by submarine explosive eruptions between 0.7 and 0.3 Ma and partly developed on top of volcanic edifices from the first episode. We identify two prominent normal faults that underlie and continue the two main trends of the KVC, indicating a direct link between tectonics and volcanism. In addition, we reveal several buried volcanic centers and a distinct volcanic ridge connecting the KVC with Santorini, suggesting a connection between the two volcanic centers in the past. This connection was interrupted by a major tectonic event and, as a result, the two volcanic systems now have separate, largely independent plumbing systems despite their proximity

Description: Focused gas migration through the gas hydrate stability zone in vertical gas conduits is a global phenomenon. The process can lead to concentrated gas hydrate formation and seafloor gas seepage, which influences seafloor biodiversity and ocean biogeochemistry. However, much is unknown about how gas and gas hydrate co-exist within and around gas conduits. We present seismic imaging of the gas hydrate system beneath a four-way closure anticlinal ridge at New Zealand's southern Hikurangi subduction margin. Gas has accumulated beneath the base of gas hydrate stability to a thickness of up to ∼240 m, which has ultimately led to hydraulic fracturing and propagation of a vertical gas conduit to the seafloor. Despite the existence of an array of normal faults beneath the ridge, these structures are not exploited as long-range gas flow conduits. Directly beneath the conduit, and extending upward from the regional base of gas hydrate stability, is a broad zone characterized by both negative- and positive-polarity reflections. We interpret this zone as a volume of sediment hosting both gas hydrate and free gas, that developed due to partial gas trapping beneath a mass transport deposit. Similar highly reflective zones have been identified at the bases of other gas conduits, but they are not intrinsic to all gas conduits through gas hydrate systems. We suggest that pronounced intervening sealing units within the gas hydrate stability zone determine whether or not they form.

Description: Although submarine landslides have been studied for decades, a persistent challenge is the integration of diverse geoscientific datasets to characterise failure processes. We present a core-log-seismic integration study of the Tuaheni Landslide Complex to investigate intact sediments beneath the undeformed seafloor as well as post-failure landslide deposits. Beneath the undeformed seafloor are coherent reflections underlain by a weakly-reflective and chaotic seismic unit. This chaotic unit is characterised by variable shear strength that correlates with density fluctuations. The basal shear zone of the Tuaheni landslide likely exploited one (or more) of the low shear strength intervals. Within landslide deposits is a widespread “Intra-debris Reflector”, previously interpreted as the landslide’s basal shear zone. This reflector is a subtle impedance drop around the boundary between upper and lower landslide units. However, there is no pronounced shear strength change across this horizon. Rather, there is a pronounced reduction in shear strength ∼10-15 m above the Intra-debris Reflector that presumably represents an induced weak layer that developed during failure. Free gas accumulates beneath some regions of the landslide and is widespread deeper in the sedimentary sequence, suggesting that free gas may have played a role in pre-conditioning the slope to failure. Additional pre-conditioning or failure triggers could have been seismic shaking and associated transient fluid pressure. Our study underscores the importance of detailed core-log-seismic integration approaches for investigating basal shear zone development in submarine landslides.

Description: The Formosa Ridge cold seep is among the first documented active seeps on the northern South China Sea passive margin slope. Although this system has been the focus of scientific studies for decades, the geological factors controlling gas release are not well understood due to a lack of constraints of the subsurface structure and seepage history. Here, we use high-resolution 3D seismic data to image stratigraphic and structural relationships associated with fluid expulsion, which provide spatio-temporal constraints on the gas hydrate system at depth and methane seepage at modern and paleo seafloors. Gas has accumulated beneath the base of gas hydrate stability to a critical thickness, causing hydraulic fracturing, propagation of a vertical gas conduit, and morphological features (mounds) at paleo-seafloor horizons. These mounds record multiple distinct gas migration episodes between 300,000 and 127,000 years ago, separated by periods of dormancy. Episodic seepage still seems to occur at the present day, as evidenced by two separate fronts of ascending gas imaged within the conduit. We propose that episodic seepage is associated with enhanced seafloor sedimentation. The increasing overburden leads to an increase in effective horizontal stress that exceeds the gas pressure at the top of the gas reservoir. As a result, the conduit closes off until the gas reservoir is replenished to a new (greater) critical thickness to reopen hydraulic fractures. Our results provide intricate detail of long-term methane flux through sub-seabed seep systems, which is important for assessing its impact on seafloor and ocean biogeochemistry. Key Points - Gas has accumulated beneath the base of gas hydrate stability, causing vertical gas conduit formation and seabed mounds - Mounds imaged within the conduit record episodic seepage between 300 and 127 kyrs ago - Quiescence may be associated with enhanced seafloor sedimentation that increases effective stress at the top of the gas reservoir

Description: The Christiana‐Santorini‐Kolumbo (CSK) volcanic field has hosted more than 100 explosive eruptions in the past 250,000 years, including the 1650 CE eruption of Kolumbo Volcano. Previous studies have established a link between regional tectonics and volcanism in the CSK volcanic field. While 2D seismic reflection data have given valuable insight into regional faulting, detailed fault zone characterization has been precluded by the sparsely spaced profiles. Using 3D seismic reflection data around Kolumbo Volcano, we provide the first 3D characterization of fault zones in the CSK volcanic field. Beneath the volcano's northwestern flank, and farther to the northwest, normal faults are predominantly NE‐SW trending, with mean fault trends between 044° and 049°. Normal faults beneath the southeastern flank are slightly more north‐oriented, with mean fault trends between 028° and 038°. Our detailed fault zone analysis reveals clear NW‐SE directed extension around the volcano, consistent with published focal mechanisms from microseismicity. The Kolumbo Fault Zone, ∼6 km northwest of Kolumbo Volcano, is characterized by distinct relay ramps between major overstepping normal faults. Regional 2D seismic profiles reveal a previously undocumented volcanic cone directly above the fault zone. Magma ascent to this cone has likely exploited enhanced vertical permeability associated with distributed deformation within a relay ramp. We suggest that fault relay structures may play an important role, over a range of spatial scales, in focusing magma ascent within the CSK volcanic field. Plain Language Summary In the last 250,000 years, more than 100 explosive eruptions have occurred in the “Christiana‐Santorini‐Kolumbo” volcanic field in the Aegean Sea. Eruptions like these represent a serious natural hazard for the region. In this study, we explored how tectonic processes are related to volcanic activity. We did this by studying tectonic deformation around the submarine Kolumbo Volcano, which last erupted violently in 1650 CE. We used three‐dimensional (3D) seismic reflection data, which provide high‐resolution imagery of the seafloor and underlying sediments. The data set shows how the sediments beneath the seafloor have been disrupted by tectonic faults, which have formed as the crust is being slowly pulled apart (extended). The orientations of the faults show that extension in and around the volcano is happening along a northwest to southeast orientation. Based on our new data, we suggest that the movement of magma through the crust might occur preferentially through structural features called “relay ramps.” Relay ramps are regions of complex tectonic deformation that exist between overlapping extensional faults. Our 3D imagery of fault zones in this volcanic field gives a better understanding of how tectonic and volcanic processes interact with each other. Key Points 3D seismic data reveal unprecedented detail of normal faulting around the submarine Kolumbo Volcano, Aegean Sea Long-term extension (NW-SE oriented) around Kolumbo Volcano is consistent with previous studies of seismicity and field mapping on Santorini Relay ramps accommodate strain in step-overs between normal faults and may be exploited as permeable zones for vertical magma ascent

Description: Sub-seabed fluid flow, gas hydrate accumulation and seafloor methane seepage are tightly interwoven processes with implications for marine biodiversity, ocean chemistry and seafloor stability. We combine long-offset seismic reflection data with high-resolution seismic data to investigate shallow structural deformation and its relationship to focused gas migration and hydrate accumulation in the southern Hikurangi subduction wedge. Anticlines, effective traps for focusing free gas, are characterized by both normal faults and vertical zones of hydraulic fracturing within the hydrate stability zone. The normal faults form as a result of sediment layer folding and gravitational collapse of ridges during uplift. We document both longitudinal (ridge-parallel) and transverse (ridge-perpendicular) extensional structures (normal faults and elongated hydraulic fracture zones) in the sub-seafloor of anticlinal ridges. Intriguingly, gas flow through ridges close to the deformation front of the wedge exploits longitudinal structures, while ridges further inboard are characterized by gas flow along transverse structures. This highlights pronounced changes in the shallow deformation of ridges in different parts of the wedge, associated with a switching of the least and intermediate principal stress directions. It is critical to understand these shallow stress fields because they control fluid flow patterns and methane seepage out of the seafloor. Key Points Gas migration through ridges occurs along both longitudinal (ridge-parallel) and transverse (ridge-perpendicular) zones of fracturing Shallow stress fields differ significantly between ridges, reflecting differences in ridge evolution and deformation Seismic reflection images of the base of gas hydrate stability and gas-water contacts are strongly affected by seismic frequency content