Strain analysis of a seismically imaged mass‐transport complex, offshore Uruguay

Strain style, magnitude and distribution within mass‐transport complexes (MTCs) are important for understanding the process evolution of submarine mass flows and for estimating their runout distances. Structural restoration and quantification of strain in gravitationally driven passive margins have been shown to approximately balance between updip extensional and downdip contractional domains; such an exercise has not yet been attempted for MTCs. We here interpret and structurally restore a shallowly buried (c. 1,500 mbsf) and well‐imaged MTC, offshore Uruguay using a high‐resolution (12.5 m vertical and 15 × 12.5 m horizontal resolution) three‐dimensional seismic‐reflection survey. This allows us to characterise and quantify vertical and lateral strain distribution within the deposit. Detailed seismic mapping and attribute analysis shows that the MTC is characterised by a complicated array of kinematic indicators, which vary spatially in style and concentration. Seismic‐attribute extractions reveal several previously undocumented fabrics preserved in the MTC, including internal shearing in the form of sub‐orthogonal shear zones, and fold‐thrust systems within the basal shear zone beneath rafted‐blocks. These features suggest multiple transport directions and phases of flow during emplacement. The MTC is characterised by a broadly tripartite strain distribution, with extensional (e.g. normal faults), translational and contractional (e.g. folds and thrusts) domains, along with a radial frontally emergent zone. We also show how strain is preferentially concentrated around intra‐MTC rafted‐blocks due to their kinematic interactions with the underlying basal shear zone. Overall, and even when volume loss within the frontally emergent zone is included, a strain deficit between the extensional and contractional domains (c. 3%–14%) is calculated. We attribute this to a combination of distributed, sub‐seismic, ‘cryptic’ strain, likely related to de‐watering, grain‐scale deformation and related changes in bulk sediment volume. This work has implications for assessing MTCs strain distribution and provides a practical approach for evaluating structural interpretations within such deposits

Sequence stratigraphy: methodology and nomenclature

The recurrence of the same types of sequence stratigraphic surface through geologic time defines cycles of change in accommodation or sediment supply, which correspond to sequences in the rock record. These cycles may be symmetrical or asymmetrical, and may or may not include all types of systems tracts that may be expected within a fully developed sequence. Depending on the scale of observation, sequences and their bounding surfaces may be ascribed to different hierarchical orders.Stratal stacking patterns combine to define trends in geometric character that include upstepping, forestepping, backstepping and downstepping, expressing three types of shoreline shift: forced regression (forestepping and downstepping at the shoreline), normal regression (forestepping and upstepping at the shoreline) and transgression (backstepping at the shoreline). Stacking patterns that are independent of shoreline trajectories may also be defined on the basis of changes in depositional style that can be correlated regionally. All stratal stacking patterns reflect the interplay of the same two fundamental variables, namely accommodation (the space available for potential sediment accumulation) and sediment supply. Deposits defined by specific stratal stacking patterns form the basic constituents of any sequence stratigraphic unit, from sequence to systems tract and parasequence. Changes in stratal stacking patterns define the position and timing of key sequence stratigraphic surfaces.Precisely which surfaces are selected as sequence boundaries varies as a function of which surfaces are best expressed within the context of the depositional setting and the preservation of facies relationships and stratal stacking patterns in that succession. The high degree of variability in the expression of sequence stratigraphic units and bounding surfaces in the rock record means ideally that the methodology used to analyze their depositional setting should be flexible from one sequence stratigraphic approach to another. Construction of this framework ensures the success of the method in terms of its objectives to provide a process-based understanding of the stratigraphic architecture. The purpose of this paper is to emphasize a standard but flexible methodology that remains objective

Arctic Ocean Mega Project: Paper 3 - Mesozoic to Cenozoic geological evolution

We present an atlas of paleogeographic and paleotectonic maps which documents major events in the Arctic for 0–157 Ma. We demonstrate that the Mendeleev Ridge has a continental basement. The following chronology of events in the history of the Arctic Ocean is proposed: (1) Jurassic: continental rifting in the area of the Sverdrup-Banks basins and in the area of the present-day Canada Basin; a system of continental-margin volcanic belts formed in the region of Chukotka and the Verkhoyansk-Omolon; (2) Berriasian-Barremian: formation of the continental-margin Verkhoyansk-Chukotka Orogen; fast opening of Canada Basin (~133–125 Ma); (3) Aptian-Albian: formation of continental igneous provinces, rifting and magmatism in the area of the Alpha-Mendeleev ridges; rifting in the Ust’-Lena, Anisin, North-Chukchi, Podvodnikov and Toll basins; (4) Cenomanian-Campanian: intraplate magmatism in the area of the Alpha-Mendeleev ridges; (5) Campanian-Maastrichtian: a likely start of compressional deformations in the area of the Chukchi Sea; (6) Paleocene: formation of the continental-margin orogen; continental rifting along the present-day Eurasia Basin and the Ust’-Lena Basin; (7) Early-Middle Eocene: onset of opening of the Eurasia Basin started; (8) Middle-Late Eocene: a major restructuring of paleogeography of the Arctic took place at ca. 45 Ma with subaerial emergence of the Barents and Kara Sea shelves and onset of ultra-slow spreading of the Gakkel Ridge, and start of the epoch of formation of normal and strike-slip faults on the Lomonosov and Alpha-Mendeleev ridges and on the shelves of the Chukchi and East Siberian seas. Paleoclimate is discussed in connection with changes in the paleogeography

Arctic Ocean Mega Project: Paper 3 - Mesozoic to Cenozoic geological evolution

We present an atlas of paleogeographic and paleotectonic maps which documents major events in the Arctic for 0–157 Ma. We demonstrate that the Mendeleev Ridge has a continental basement. The following chronology of events in the history of the Arctic Ocean is proposed: (1) Jurassic: continental rifting in the area of the Sverdrup-Banks basins and in the area of the present-day Canada Basin; a system of continental-margin volcanic belts formed in the region of Chukotka and the Verkhoyansk-Omolon; (2) Berriasian-Barremian: formation of the continental-margin Verkhoyansk-Chukotka Orogen; fast opening of Canada Basin (~133–125 Ma); (3) Aptian-Albian: formation of continental igneous provinces, rifting and magmatism in the area of the Alpha-Mendeleev ridges; rifting in the Ust’-Lena, Anisin, North-Chukchi, Podvodnikov and Toll basins; (4) Cenomanian-Campanian: intraplate magmatism in the area of the Alpha-Mendeleev ridges; (5) Campanian-Maastrichtian: a likely start of compressional deformations in the area of the Chukchi Sea; (6) Paleocene: formation of the continental-margin orogen; continental rifting along the present-day Eurasia Basin and the Ust’-Lena Basin; (7) Early-Middle Eocene: onset of opening of the Eurasia Basin started; (8) Middle-Late Eocene: a major restructuring of paleogeography of the Arctic took place at ca. 45 Ma with subaerial emergence of the Barents and Kara Sea shelves and onset of ultra-slow spreading of the Gakkel Ridge, and start of the epoch of formation of normal and strike-slip faults on the Lomonosov and Alpha-Mendeleev ridges and on the shelves of the Chukchi and East Siberian seas. Paleoclimate is discussed in connection with changes in the paleogeography