Mechanical Mixing of Garnet Peridotite and Pyroxenite in the Orogenic Peridotite Lenses of the Tvaerdal Complex, Liverpool Land, Greenland Caledonides

The Tvaerdal Complex is an eclogite-bearing metamorphic terrane in Liverpool Land at the southern tip of the Greenland Caledonides. It is a Baltic terrane that was transferred to Laurentia during the Scandian orogeny. It exposes a few small garnet dunite and harzburgite lenses, some containing parallel layers of garnet pyroxenite and peridotite (including lherzolite). Sm–Nd mineral ages from the pyroxenites indicate recrystallization occurred at the same time (≈405 Ma) as eclogite recrystallization in the enclosing gneiss. Geothermobarometry indicates these eclogites and pyroxenites shared a similar pressure-temperature history. This congruent evolution suggests pyroxenite-bearing peridotite lenses were introduced from a mantle wedge into subducted Baltic continental crust and subsequently shared a common history with this crust and its eclogites during the Scandian orogeny. Some garnet peridotite samples contain two garnet populations: one Cr-rich (3·5–6·2 wt % Cr2O3) and the other Cr-poor (0·2–1·4 wt %). Sm–Nd analyses of two such garnet peridotites define two sets of apparent ages: one older (>800 Ma) for Cr-rich garnets and the other younger (<650 Ma) for Cr-poor garnets. We propose that the younger Cr-poor garnets were derived from fractured and disaggregated garnet pyroxenite layers (i.e. are M2) and were mixed mechanically with older (i.e. M1) garnets of the host peridotite during intense Scandian shearing. Mechanical mixing may be an important mantle process

Interrelation between rifting, faulting, sedimentation, and mantle serpentinization during continental margin formation-including examples from the Norwegian Sea

The conditions permitting mantle serpentinization during continental rifting are explored within 2-D thermotectonostratigraphic basin models, which track the rheological evolution of the continental crust, account for sediment blanketing effects, and allow for kinetically controlled mantle serpentinization processes. The basic idea is that the entire extending continental crust has to be brittle for crustal scale faulting and mantle serpentinization to occur. The isostatic and latent heat effects of the reaction are fully coupled to the structural and thermal solutions. A systematic parameter study shows that a critical stretching factor exists for which complete crustal embrittlement and serpentinization occurs. Increased sedimentation rates shift this critical stretching factor to higher values as sediment blanketing effects result in higher crustal temperatures. Sediment supply has therefore, through the temperature-dependence of the viscous flow laws, strong control on crustal strength and mantle serpentinization reactions are only likely when sedimentation rates are low and stretching factors high. In a case study for the Norwegian margin, we test whether the inner lower crustal bodies (LCB) imaged beneath the Møre and Vøring margin could be serpentinized mantle. Multiple 2-D transects have been reconstructed through the 3-D data set by Scheck-Wenderoth and Maystrenko (2011). We find that serpentinization reactions are possible and likely during the Jurassic rift phase. Predicted thicknesses and locations of partially serpentinized mantle rocks fit to information on LCBs from seismic and gravity data. We conclude that some of the inner LCBs beneath the Norwegian margin may be partially serpentinized mantle

U–Pb Zircon Geochronology and Tectonostratigraphy of Southern Liverpool Land, East Greenland: Implications for Deformation in the Overriding Plates of Continental Collisions

The East Greenland Caledonides formed in the overriding plate as Baltica was subducted westward beneath Laurentia from 460 to 360 Ma, and offer a unique opportunity to investigate lower crustal deformation in the overriding plates of continental collisions. Field work and new zircon geochronology from gneisses in southern Liverpool Land, exposed in the hinterland ~100 km east of the nearest Caledonian gneisses, define three tectonostratigraphic units that are, from the bottom up, the eclogite+peridotite-bearing Tværdal complex and the granulite-facies Jættedal complex in the footwall of the top-N Gubbedalen shear zone, and the Hurry Inlet granite and associated paragneiss screens in its immediate hangingwall. Zircons from Tværdal complex gneisses yield metamorphic rims that cluster in age from 409 to 401 Ma and overgrow magmatic cores of 1674 and 1665 Ma in two samples, and range from ~1800–1000 Ma in a third sample. In contrast, zircons from three samples in the Jættedal complex and two samples in the paragneiss screens of the Hurry Inlet granite yield metamorphic rims that cluster in age from 438 to 417 Ma with Archean–Early Neoproterozoic detrital cores. A cross-cutting granitic dike in the Jættedal complex yields an age of 394 Ma. Archean–Early Neoproterozoic detrital zircons associated with ~440–420 Ma metamorphism in the Liverpool Land paragneisses suggests correlation with the Krummedal sequence and the Hagar Bjerg thrust sheet of Laurentian affinity. 1670 Ma cores in the Tværdal complex, and ~400 Ma eclogite-facies metamorphism, allow correlation of the Tværdal complex with the Western Gneiss Region in Norway, and it may therefore be of Baltican affinity. Furthermore, the contact between the older Jættedal complex with the younger Tværdal complex requires the existence of a structure, named the Ittoqqortoormiit shear zone herein, which juxtaposed these rocks prior to the initiation of normal-sense slip along the Gubbedalen shear zone. This work provides geochronologic evidence for continental underplating of the overriding plate by the subducting plate during orogenesis, and supports models for high-pressure exhumation in continental collisional settings that identify separate structures associated with initial emplacement in the lower–middle crust and subsequent upper-crustal exhumation

Sedimentation-driven cyclic rebuilding of gas hydrates

Highlights • Sedimentation-driven gas hydrate recycling is cyclic in nature with time scales set by reactive multi-phase transport. • Each cycle can be divided into three distinct phases: 1) gas accumulation phase, 2) gas breakthrough phase and 3) uninhibited hydrate build-up phase. • In the presence of sufficient accumulated gas, convex deposition of hydrate acts like a mechanical nozzle for the ascending gas flow. Gas hydrate recycling is an important process in natural hydrate systems worldwide and frequently leads to the high gas hydrate saturations found close to the base of the gas hydrate stability zone (GHSZ). However, to date it remains enigmatic how, and under which conditions, free gas invades back into the GHSZ. Here we use a 1D compositional multi-phase flow model that accounts for sedimentation to investigate the dominant mechanisms that control free gas flow into the GHSZ using a wide-range of parameters i.e. hydrate formation kinetics, sediment permeability, and capillary pressure. In the first part of this study, we investigate free gas invasion into the GHSZ without any sedimentation, and analyse the dynamics of hydrate formation in the vicinity of the base of GHSZ. This helps establish plausible initial conditions for the main part of the study, namely, hydrate recycling due to rapid and continuous sedimentation. For the case study, we apply our numerical model to the Green Canyon Site 955 in the Gulf of Mexico, where the reported high hydrate saturations are likely a result of hydrate recycling driven by rapid sedimentation. In the model, an initial hydrate layer forms due to the invasion of a specified volume of rising free gas. This hydrate layer is consistent with the local pressure, temperature and salinity state. This hydrate layer is then thermally de-stabilised by sedimentation resulting in free gas formation and hydrate recycling. A key finding of our study is that gas hydrate recycling is a cyclic process which can be divided into three phases of 1) gas hydrate melting and free gas nozzling through the hydrate layer, 2) formation of a new gas hydrate layer as the old layer vanishes, and 3) fast uninhibited grow of a new hydrate layer. High hydrate saturations of about 80% can be attained purely through physical, burial-driven recycling of gas hydrates, without any additional gas input from other sources. Hydrate recycling is, therefore, highly dynamic with its own inherent cyclicity rather than a gradual process paced by the rate of sediment deposition