Molloy Ridge Research Articles

U-Pb calcite ages date oblique rifting of the Arctic–North Atlantic gateway

Abstract Miocene breakup of Svalbard from Greenland formed a deep oceanic gateway that enabled circulation between the Arctic and Atlantic Oceans, significantly changing the global climate. However, the timing of events remains unclear. An excellent opportunity to constrain this timing is found onshore western Svalbard, where the Sarsbukta fault forms the eastern margin of the Eocene–Oligocene Forlandsundet basin. Here, we present new results from U-Pb dating of calcite precipitated in fault-related veins to constrain the timing of Sarsbukta fault deformation and the evolution of the basin. Our oldest calcite age is Permo-Triassic, suggesting long-lived deformation along the fault. A cluster of ages between 41 and 33 Ma overlaps with fossil-based depositional ages from parts of the Forlandsundet basin. These data indicate that onshore transtension partly pre-dated the well-established Chron 13 (magnetic polarity time scale; 35.5–33.7 Ma) reorganization of spreading ridges in the North Atlantic. Our youngest age of 13 Ma indicates that faulting persisted long after the preserved basin fill was deposited. If seafloor spreading marked the end of extension of continental crust, Molloy Ridge spreading during Chron 5 (19.6–9.8 Ma) may have initiated after 13 Ma.

Acoustic evidence of hydrocarbon release associated with the Spitsbergen Transform Fault, north of the Molloy Ridge, Fram Strait

Hydrocarbon gases formed from biotic and abiotic processes are released through the seafloor at different locations around the world. They have been widely observed directly in video and photo data, and indirectly on echosounder data. Even though biotic gas generation is a very common process, abiotic gas generation is limited to regions where serpentinization of ultramafic rocks occur. Indications of abiotic gas occurrences are therefore sparse and much speculated upon. Here, we investigated the Spitsbergen Transform Fault, the Molloy Ridge, the Molloy Deep, and the Molloy Transform Fault/Fracture Zone, (a transform fault-bounded pull-a-part region offshore western Svalbard) where both processes may be active. Multiple acoustic gas flares, ∼1,770 and ∼3355 m high above the seafloor (tallest ever recorded), were observed indicating active migration and seepage of hydrocarbons. The proximity to the mid oceanic ridge and the documented high heat flow suggests the influence of high temperatures on organic-rich sedimentary deposits. Deep seismic data and other geological information available indicate that the main source of gas could be from thermal cracking of either pre- or syn-rift source rock organic material, potentially mixed with methane from serpentinization of mantle rocks (peridotites). Correlation with seismic stratigraphy from Ocean Drilling Program (ODP) Sites 910 and 912 on the adjacent Yermak Plateau suggests that the sedimentary source rocks may be present at the northern flank of the Molloy Ridge and within the deep graben along the Spitsbergen Transform Fault. The ∼3 km thick sedimentary succession in high heat flow zones within the transform fault and the active bounding faults allow generation and migration of hydrocarbons to the seafloor and sustains present day seepage.

Local Seismicity and Sediment Deformation in the West Svalbard Margin: Implications of Neotectonics for Seafloor Seepage

AbstractIn the Fram Strait, mid‐ocean ridge spreading is represented by the ultra‐slow system of the Molloy Ridge, the Molloy Transform Fault and the Knipovich Ridge. Sediments on oceanic and continental crust are gas charged and there are several locations with documented seafloor seepage. Sedimentary faulting shows recent stress release in the sub‐surface, but the drivers of stress change and its influence on fluid flow are not entirely understood. We present here the results of an 11‐month‐long ocean bottom seismometer survey conducted over the highly faulted sediment drift northwards from the Knipovich Ridge to monitor seismicity and infer the regional state of stress. We obtain a detailed earthquake catalog that improves the spatial resolution of mid‐ocean ridge seismicity compared with published data. Seismicity at the Molloy Transform Fault is occurring southwards from the bathymetric imprint of the fault, as supported by a seismic profile. Earthquakes in the northern termination of the Knipovich Ridge extend eastwards from the ridge valley, which together with syn‐rift faulting identified in seismic reflection data, suggests that a portion of the currently active spreading center is buried under sediments away from the bathymetric expression of the rift valley. This hints at the direct link between crustal rifting processes and faulting in shallow sediments. Two earthquakes occur close to the seepage system of the Vestnesa Ridge further north from the network. We suggest that deeper rift structures, reactivated by gravity and/or post‐glacial subsidence, may lead to accommodation of stress through shallow extensional faults, therefore impacting seepage dynamics.

Bathymetry Refinement over Seamount Regions from SAR Altimetric Gravity Data through a Kalman Fusion Method

Seafloor topography over seamount areas is crucial for studying plate motions, seafloor seismicity, and seamount ecosystems. However, seamount bathymetry modeling is difficult due to the complex hydrodynamic environment, biodiversity, and scarcity of shipborne echo sounding data. The use of satellite altimeter-derived gravity data is a complementary way of bathymetry computation; in particular, the incorporation of synthetic aperture radar (SAR) altimeter data may be useful for seamount bathymetry modeling. Moreover, the widely used filtering method may have difficulty in combing different bathymetry data sets and may affect the quality of the computed bathymetry. To mitigate this issue, we introduce a Kalman fusion method for weighting and combining gravity-derived bathymetry data and the reference bathymetry model. Numerical experiments in the seamount regions over the Molloy Ridge show that the use of SAR-based altimetric gravity data improves the local bathymetry model, by a magnitude of 14.27 m, compared to the result without SAR data. In addition, the developed Kalman fusion method outperforms the traditionally used filtering method, and the bathymetry computed from the Kalman fusion method is improved by a magnitude of 9.34 m. Further comparison shows that our solution has improved quality compared to a recently released global bathymetry model, namely, GEBCO 2022 (GEBCO: General Bathymetric Chart of the Oceans), by a magnitude of 34.34 m. The bathymetry model in this study may be substituted for existing global bathymetry models for geophysical investigations over the target area.

Cruise CAGE-18-4

Cruise CAGE18-4 with UiT’s research vessel R/V Helmer Hanssen was carried out to collect cross-disciplinary data for addressing the objectives of the Norwegian Centre of Excellence for Arctic Gas Hydrate, Environment and Climate, CAGE. The cruise was split into two legs with a port call in Longyearbyen. Leg 1 targeted the Vestnesa Ridge on the western Svalbard margin and acquired 2D and 3D high-resolution P-Cable seismic data in close proximity to the Molloy Ridge in order to better understand tectonic impacts on fluid flow and gas hydrate accumulations. Leg 2 continued our investigations of the newly discovered gas seepage field in the Leierdjupet Fault complex (see CAGE17-3 cruise report) focusing on a better understanding of its complex leakage and source-to-sink plumbing system using seafloor photo and video imagery, water and sediment sampling, multibeam mapping and hydroacoustics. The cruise may be known as: CAGE18_4

Oblique rifting and the Late Eocene–Oligocene demise of Laurasia with inception of Molloy Ridge: Deformation of Forlandsundet Basin, Svalbard

Final breakup of the Laurasian supercontinent occurred within a residual continental bridge that spanned Svalbard, Greenland and Arctic Canada from Late Cretaceous to Oligocene time, but the timing and precise kinematics of that breakup have remained obscure. Dextral oblique motion between Svalbard and northeastern Greenland initially generated the West Spitsbergen Fold-Thrust Belt (a transpressional orogen) but evolved into oblique rifting until the ultimate demise of the continental bridge with the Early Miocene formation of the Molloy Ridge. The Forlandsundet basin stands as a critical entity within the Svalbard continental margin whose fill captured the progressive transition between oblique convergence and divergence. The basin originated as a Late Eocene piggyback basin within the fold-thrust belt during deposition of the oldest unit, the marine fluvio-deltaic Sarsbukta conglomerate. Structural mapping highlights early deformation of the Sarsbukta strata expressed as (1) fractured cobbles in which fractures formed by consistent NNW–SSE stretching were injected with sandy matrix (when poorly lithified), (2) stylolites recording WSW–ENE shortening, and (3) conjugate dextral- and sinistral-normal mesoscale faults that indicate WSW–ENE shortening (P axes) and NNW–SSE extension (T axes). Between 38 and 31Ma, this episode of orogen–normal shortening with orogen–parallel extension evolved progressively into oblique divergence (NW–SE extension) recorded in the Sarsbukta strata as basin-wide strike-slip/normal faults, transtensional folds, and sub-horizontal stylolite surfaces (subvertical P axes). Thus NW–SE extension, consistent with present spreading on the Molloy Ridge, was established at least 9m.y. before the first emergence of recognized oceanic lithosphere. Deformation occurred at upper-crustal temperatures of ≤104°C based on new vitrinite reflectance data. Transtension resulted in intrabasinal exposures of the Caledonide metamorphic basement during deposition of the second unit, the ≥31Ma marine fan-deltaic Sarstangen conglomerate, in the Forlandsundet graben sensu stricto. Such oblique stretching severed the continental bridge heralding ultimate oceanic spreading at the Molloy Ridge.

Seismic view on the svalbard passive continental margin

Deep seismic sounding measurements were performed in the continent-ocean transition zone of the western Svalbard and Barents Sea margin, during the expeditions in 1985–2008. Seismic energy (airgun and TNT shots) was recorded along several profiles by onshore seismic stations and ocean bottom seismometers, and hydrophone systems. Good quality reflected and refracted P waves provided an excellent data base for a seismic modelling along the profiles. TNT sources were recorded even up to 300 km distances. A minimal depth of about 6 km of the Moho interface was found east of the Molloy Deep. The Moho discontinuity dips down to 28 km beneath the continental part of the northernmost profile and down to maximum 32 km beneath other profiles. The evolution of the region is considered to be within a shearrift tectonic setting. The continent-ocean transition zone along the northernmost profile is mostly dominated by extension; therefore, the last stage of the development of the margin can be classified as rifting. The uplifted Moho interface close to the Molloy Deep can be interpreted as a south-western end of the Molloy Ridge. The margin of the southern Spitsbergen is rather of sheared character while the western Barents Sea margin is of slow to ultraslow spreading type.

Active gas venting through hydrate-bearing sediments on the Vestnesa Ridge, offshore W-Svalbard

Abstract Gas hydrate systems offshore western Svalbard are extensive and include the whole Vestnesa Ridge, an elongated sediment drift north of the Molloy Transform and just east of the Molloy Ridge, one of the shortest segments of the slow spreading North Atlantic Ridge system. The crest of the Vestnesa Ridge at water depth between 1200–1300 m is pierced with fluid flow features. Seafloor pockmarks vary in size up to 700 m in diameter. Pockmarks are generally larger at the eastern start of the Vestnesa Ridge than at its western termination. A recent cruise with R/V Jan Mayen discovered methane flares in the water column above the pockmark field at the onset of the Vestnesa Ridge. Over a period of two days at least 3 pockmarks were continuously active and methane flares in the water column reached a height of approximately 800 m. It is still unclear what has triggered the increase in gas expulsion from seafloor sediments. High-resolution 3D seismic data acquired in 2007 and 2010 show significant differences of the subseafloor expression of these fluid leakage systems. At the western end of the Vestnesa Ridge, sub-seafloor fluid flow features resemble well-described chimney structures. However, the seismic expression of the active fluid flow features is much broader, much more chaotic, dome-shaped and is not very similar to a typical chimney structure. The Vestnesa Ridge gas-hydrate and free-gas system occurs within few kilometers of a mid-oceanic ridge and transform fault, which makes this gas-hydrate system unique on Earth. The close proximity to the spreading center and its hydrothermal circulation system affects the dynamics of the gas-hydrate and free-gas system. The high heat flow together with the high tectonic activity of this region, a thick sedimentary cover, a shallow maturation window and an accelerated rate of biogenic and thermogenic gas production cause substantial disturbance to the free-gas system leading to high variability in gas supply, gas migration and gas hydrate build up and dissociation.

Crustal structure of northern Spitsbergen along the deep seismic transect between the Molloy Deep and Nordaustlandet

A deep seismic sounding experiment was performed during the expedition ARKTIS XV/2 of the RV Polarstern and the Polish ship Eltanin in 1999 in the continent–ocean transition zone of northwestern Svalbard, along the 430-km-long profile AWI-99200. The profile runs from the Molloy Deep in the vicinity of an active spreading axis in the northern Atlantic to Nordaustlandet. Seismic energy (airgun and TNT shots) was recorded by seismic land (onshore) stations, OBSs and hydrophone systems, with airgun shots recorded up to 200 km onshore and 50 km offshore. The data recorded along the entire profile provide an excellent database for a detailed seismic modelling of the crustal P-wave velocity field along the profile track. A minimal depth of about 6 km to the Moho discontinuity was found east of the Molloy Deep. Here, the upper mantle exhibits a P-wave velocity of about 7.9 km s−1, and the crustal thickness does not exceed 4 km. The continent–ocean transition zone to the east is characterized by a complex seismic velocity structure. The Moho interface reaches a maximum depth of 28 km beneath the continental part of the profile, with a P-wave velocity in the upper mantle of 8.15 km s−1. The continental crust consists of three layers with P-wave velocities of about 5.5, 5.9–6.0 and 6.2–6.6 km s−1, respectively. In addition, we have found two reflectors in the mantle lithosphere at depths of 14–42 and 40–50 km dipping NE. The evolution of the region appears to be within a shear-rift tectonic setting. The continent–ocean transition zone is mostly dominated by extension, so the last stage of the development of this margin can be classified as rifting. The uplifted Moho boundary close to the Molloy Deep can be interpreted as the southwestern end of the Molloy Ridge.

A deep seismic transect from Hovgård Ridge to northwestern Svalbard across the continental-ocean transition: A sheared margin study

New seismic refraction data were collected across the western Svalbard continental margin off Kongsfjorden (Ny Ålesund) during the cruise leg ARK15/2 of RV Polarstern. The use of onshore and offshore seismic receivers and a dense air-gun shot pattern provide a detailed view of the velocity structure of Svalbard's continental interior, the continent–ocean transition, and oceanic crust related to the northern Knipovich Ridge and the Molloy Ridge. The proposed Caledonian central and western terranes of Svalbard are not distinguishable on the basis of seismic velocity structure. Below a 7 to 8 km thick Palaeozoic sedimentary cover the crystalline crust reveals a three-layer structure with seismic velocities ranging between 6.1 and 6.9 km s−1. The geological suture between the terranes is imperceptible. The middle and upper crust below the Tertiary Forlandsundet graben shows low velocities. This can be related to faulting during the Early Palaeozoic movements between Svalbard and northern Greenland, followed by the continental break-up. Moreover, a sedimentary Palaeozoic core is may be buried below the Forlandsundet graben. The continent–ocean transition can be classified as an obliquely sheared (transform) continental margin. The Moho dips with an angle of 45° eastwards at the continent–ocean transition that exhibits higher seismic velocities of more than 7.2 km s−1on the continental side. The sheared margin evolution is linked to the Spitsbergen Transform Fault, today located north of the Molloy Ridge spreading segment. During a later evolutionary stage the Molloy Ridge passed the continental margin. The separating boundary between continental and oceanic crust off northwestern Svalbard is today part of the inactive Spitsbergen Fracture Zone. The high seismic velocities at the continent–ocean boundary can be interpreted as minor mantle-derived intrusions, probably induced by interaction of the passing spreading ridge during the sheared margin evolution. The oceanic crust generated at the Knipovich Ridge and the Molloy Ridge is thin (2 to 4 km), compared to the global mean, and is thinner as previously observed. The oceanic crust is characterized by the absence of oceanic layer 3. These observations can be ascribed to conductive cooling of the ascending mantle as a result of the extremely low divergence rate. The underlying mantle is slightly serpentinized below the Knipovich Ridge segment, reflected by low seismic velocities of ∼7.7 km s−1. A thicker sequence of syn- and post-rift sediments and sedimentary rocks are observed on the Molloy Ridge oceanic segment, which most likely results from greater subsidence relative to the Knipovich Ridge segment.

Methane-generated(?) pockmarks on young, thickly sedimented oceanic crust in the Arctic: Vestnesa ridge, Fram strait

Acoustic backscatter imagery in the Fram strait (between Greenland and Spitzbergen) reveals a 1-3-km-wide, 50-km-long belt of ∼50 pointlike backscatter objects decorating the ∼1300-m-deep crest of Vestnesa Ridge, a 1- >2 km thick sediment drift possibly underlain by a transform-parallel oceanic basement ridge (crustal ages ∼3-14 Ma). A 3.5 kHz seismic-reflection profile indicates that at least some objects are pockmarks ∼100-200 m in diameter and 10-20 m deep. The pockmarks (possibly also mud diapirs) may have been formed by evolution of methane generated by the decomposition of marine organic matter in the Vestnesa ridge sediment drift. The ridge may be underlain by an anticlinal carapace of methane-hydrate calculated to be 200-300 m thick, comparable to the hydrate thickness measured just to the south. The rising methane would collect in the ridge-crest trap, intermittently escaping to the sea floor. This hypothesis is supported by multichannel evidence for bright spots and bottom-simulating reflectors in the area. The pockmark belt may also be located above a transcurrent fault. Sediment slumps on the flanks of Vestnesa ridge and northeast of Molloy ridge may have been triggered by plate-boundary earthquakes and facilitated by methane hydrates.

Rifting in the northern Norwegian‐Greenland Sea: Thermal tests of asymmetric spreading

The analysis of heat flow, seismic, and topographic data collected in the northern Norwegian‐Greenland Sea reveals an asymmetric evolution of the Eurasian and North American plates. These data are compared to predictions from three kinematic models of extension which produce asymmetry about the Knipovich Ridge: (1) uniform asymmetric pure shear, (2) lithospheric simple shear, and (3) rift/ridge jumping. The data are consistent with a range of deformation scenarios, from one ridge jump occurring at about 25 m.y. after the initiation of spreading to continuous asymmetric extension. The simple shear model can match the data only when a detachment fault dips more steeply than 45° under Svalbard. Tectonic and heat flow evidence suggests that asymmetry may have evolved from a combination of all three models. When the Mohns Ridge (propagating to the east) encountered the preexisting northward trending Spitsbergen Shear Zone, the direction of ridge propagation shifted to the north, being influenced by the abrupt change in the regional stress field at the shear zone‐ridge axis intersection. As a result, the nascent Knipovich Ridge entered into and propagated along the shear zone. Consequently, formerly active shear faults became the new detachment surfaces along which new crust is minted and asymmetrically extended. The high level of deviatoric stress about the Mohns and Knipovich Ridge intersection may cause a gradual eastward migration of the Knipovich Ridge, resulting in multiple zones of magma intrusion. “Off‐axial” bands of high heat flow and volcanism located along the Barents Sea and Svalbard margin, along the Yermak Plateau, and on continental Svalbard just southeast of the Yermak Plateau may be evidence for this migration. Propagation from the Nansen Ridge may have entered the same shear zone from the north, explaining the creation of the small Molloy Ridge and off‐axis volcanism on the Yermak Plateau.

Bathymetry of Molloy Deep: Fram Strait between Svalbard and Greenland

The sea floor of Fram Strait, the over 2500 m deep passage between the Arctic Ocean and the Norwegian-Greenland Sea, is part of a complex transform zone between the Knipovich mid-oceanic ridge of the Norwegian-Greenland Sea and the Nansen-Gakkel Ridge of the Arctic Ocean. Because linear magnetic anomalies formed by sea-floor spreading have not been found, the precise location of the boundary between the Eurasian and the North American plate is unknown in this region. Systematic surveying of Fram Strait with SEABEAM and high resolution seismic profiling began in 1984 and continued in 1985 and 1987, providing detailed morphology of the Fram Strait sea floor and permitting better definition of its morphotectonics. The 1984 survey presented in this paper provided a complete set of bathymetric data from the southernmost section of the Svalbard Transform, including the Molloy Fracture Zone, connecting the Knipovich Ridge to the Molloy Ridge; and the Molloy Deep, a nodal basin formed at the intersection of the Molloy Transform Fault and the Molloy Ridge. This nodal basin has a revised maximum depth of 5607 m water depth at 79°8.5′N and 2°47′E.

Thermal evolution of the western Svalbard margin

The northern Norwegian-Greenland Sea opened up as the Knipovich Ridge propagated from the south into the ancient continental Spitsbergen Shear Zone. Heat flow data suggest that magma was first intruded at a latitude of ≈75° N around 60 m.y.b.p. By 40–50 m.y.b.p. oceanic crust was forming at a latitude of 78° N. At ≈12 m.y.b.p. the Hovgard Transform Fault was deactivated during a northwards propagation of the Knipovich Ridge. Spreading is now in its nascent stages along the Molloy Ridge within the trough of the Spitsbergen Fracture Zone. Spreading rates are slower in the north than the south. For the Knipovich Ridge at 78° N they range from 1.5–2.3 mm yr-1 on the eastern flank to 1.9–3.1 mm yr-1 on the western flank. At a latitude of 75° N spreading rates increase to 4.3–4.9 mm yr-1. Thermal profiles reveal regions of off-axial high heat flow. They are located at ages of 14 m.y. west and 13 m.y. east of the northern Knipovich Ridge, and at 36 m.y. on the eastern flank of the southern Knipovich Ridge. These may correspond to episodes of increased magmatic activity; which may be related to times of rapid north-wards rise axis propagation. The fact that the Norwegian-Greenland Sea is almost void of magnetic anomalies may be caused by the chaotic extrusion of basalts from a spreading center trapped within the confines of an ancient continental shear zone. The oblique impact of the propagating rift with the ancient shear zone may have created an unstable state of stress in the region. If so, extension took place preferentially to the northwest, while compression occurred to the southeast between the opening, leaking shear zone and the Svalbard margin. This caused faster spreading rates to the northwest than to the southeast.

Thermal implications for the evolution of the spitsbergen transform fault

Heat flow taken between Svalbard and Greenland reveal three thermal provinces: 1. (1) the Molloy Ridge within the Spitsbergen Transform, 2. (2) the Yermak Plateau 3. (3) the northeastern margin of Svalbard (Nordaustlandet). The Molloy Ridge is a short spreading segment and the average heat flow is much above the Sclater et al. (1971), cooling curve but agrees with values from the Norwegian-Greenland Sea. An additional zone of intrusion identified by heat flow lies to the northwest of the Molloy Ridge. It straddles both the visible fracture zone and part of the Yermak Plateau. A thermal boundary lies between the warm western segment of the Yermak Plateau and the shelf off Nordaustlandet. If the thermal subsidence of the western Yermak Plateau can be traced to the latest heating episode then it is likely that the crust is similar to oceanic in composition and not older than 13 m.y. (approximately 20 m.y. younger than the northeastern segment of the plateau). Plate rotation shows that there was no room for the western segment of the plateau prior to anomaly 7. We postulate that the original transform is associated with the Hornsund Fault zone. In response to deviatoric stress across the oblique ridge-transform system, the Nansen Ridge propagated southwestward aborting the old transform trace, and shifted to its present position. It is suggested that this propagation and migration of the ridge-transform system across a zone of extensional deviatoric stress allowed the massive intrusion of basalt forming the Western Yermak Plateau. The propagation phenomenon coincides with large-scale Tertiary volcanic activity on Svalbard. Readjustment and migration of the oblique transform is still taking place. As the transform-ridge system is liberated from continental constraints, the migration rate will diminish as orthogonality is approached.