ABSTRACT The North American Cordillera is generally interpreted as a result of the long-lived, east-dipping subduction at the western margin of the North American plate. However, the east-dipping subduction seems problematic for explaining some of the geological features in the Cordillera such as large volume back-arc magmatism. Recent studies suggested that westward subduction of a now-consumed oceanic plate during the Cretaceous could explain these debated geological features. The evidence includes petrological and geochemical variations in magmatism, the presence of ophiolite that indicates tectonic sutures between the Cordillera and Craton, and seismic tomography images showing high-velocity bodies within the underlying convecting mantle that are interpreted as slab remnants from the westward subduction. Here we use 2-D upper mantle-scale numerical models to investigate the dynamics associated with westward subduction and Cordillera-Craton collision. The models demonstrate the controls on slab breakoff (remnant) following collision including: (1) oceanic and continental mantle lithosphere strength, (2) variations in density (eclogitization of continental lower crust and cratonic mantle lithosphere density), and (3) convergence rate. Our preferred model has a relatively weak mantle lithosphere, eclogitization of the lower continental crust, cratonic mantle lithosphere density of 3250 kg/m 3 , and a convergence rate of 5 cm/yr. It shows that collision and slab breakoff result in an ~2 km increase in surface elevation of the Cordilleran region west of the suture as the dense oceanic plate detaches. The surface also shows a foreland geometry that extends >1000 km east of the suture with ~4 km of subsidence relative to the adjacent Cordillera.

Abstract The fluid budget of a composite crustal column is a critical parameter that influences many lithospheric processes. The amount of water introduced into the middle and lower crust can be quantified using phase equilibrium modelling. The Dharwar Craton, India, displays a now-exposed continuous crustal section from near-surface conditions to c. 30 km depth. This section records the different steps of a c. 15 myr-long high-temperature metamorphic event (60°C kbar −1 ) responsible for the formation of syn- to post-tectonic anatectic intrusions. The global water budget is assessed using thermodynamic modelling on bulk-rock compositions of an average early Proterozoic supracrustal unit and c. 3.0 Ga felsic basement, the Peninsular gneisses. Results show the fast burial of a water-saturated supracrustal package (1.6 wt%) will release c. 50% of its mineral-bound water, triggering water-fluxed partial melting of the basement. Modelled anatectic magma compositions match the observed granitoid chemistries, and distinction can be made between water-fluxed melting and water-absent melting in the origin of syn- to post-tectonic anatectic granites. Findings from this study show the importance of crustal pile heterogeneity in controlling the nature of partial melting reactions, the composition of the magmas and the rheology of the crust.

ABSTRACT The structural evolution of the T-Block (U.K. 16/17) Brae Formation fields in the southern part of the South Viking Graben reflects a history of Late Jurassic rifting and Early Cretaceous inversion. Triassic rifting follows an inherited Caledonian trend, with Permian and Triassic depocenters to the northwest and southeast of a ridge trending north–northeast through the South Viking Graben from the area of the Thelma field. In the northern part of the area, in Trees Block (U.K. 16/12), halokinesis has created accommodation space for Middle Jurassic deposition. Further south, in T-Block, Middle Jurassic deposition does not appear to have been influenced by Caledonide structures. Rifting commenced in Trees Block in the early part of the Late Jurassic, with development of a north–south striking northern fault segment. The faulting propagated southward from the northern segment and northward from a segment to the south of T-Block, to create a relay zone opposite Thelma and Toni. At the segment centers, the fault throws are large, and the Middle Jurassic sequence dips to the west, toward the footwall. In comparison, at the Thelma relay zone, the fault displacements are much smaller, and the Middle Jurassic dips to the east. Flexural uplift and back-tilting have affected the footwall sediments and normal faults. The fault segment evolution is likely to have been a significant control on Brae sedimentation, the back-tilting of the footwalls at the segment centers funneling sediment supply into the Thelma relay zone, and footwall uplift providing emergent source areas adjacent to the developing graben. The basin morphology has been modified by postrifting thermal subsidence, increasing the eastward dip of the fault terraces. Inversion in the Early Cretaceous caused uplift of the hanging wall, creating a bulge over Thelma and Toni, and uplift on the fault adjacent to Trees Block. This inversion event is likely to be the result of oblique northwesterly compression, causing shortening and left-lateral strike-slip on the marginal faults. This event can be related to an unconformity between the Valhall and the Carrack formations, which constrains timing to the late Barremian–Aptian.

ABSTRACT The South Viking Graben (SVG) hosts many large oil and gas condensate reservoirs, some within Middle Jurassic and Cenozoic rocks, but most within thick submarine fan sandstone and conglomerate sequences of the Upper Jurassic Brae Formation and their correlative equivalents, collectively termed here the Brae Play. Regional studies carried out over the last few years (based on the extensive well database and a variety of 3-D seismic data) and the recent acquisition of extensive, high-quality, broadband 3-D seismic data across the SVG have led to better definition of the half-graben geometry and the extents of the Upper Jurassic submarine fans that host these hydrocarbon accumulations. A summary structure map, seismic sections that extend across the graben, and a 3-D image of the “Base Cretaceous” are used to illustrate the main structural features. On its western side, the top of an eroded scarp, which grades downdip into the major fault plane, can be used as the lateral limit of the postrift graben fill. The uppermost Kimmeridge Clay Formation (KCF; termed Draupne Formation in Norway), which is the top seal and dominant source rock for Brae Play fields, onlaps this eroded slope and limits the western extent of the synrift section. At depth, the top of the prerift Bathonian Sleipner Formation can be mapped along this fault margin abutting the uneroded footwall fault; this boundary defines the edge of the thickest Upper Jurassic synrift section within the graben. The top of the prerift section becomes progressively shallower to the east, where an approximate minimum limit of the graben can be defined along much of its length by the eastern limit of seismically mappable KCF (Draupne) Formation. Thick sequences of Upper Jurassic conglomerates and sandstones within the KCF (i.e., the Brae Formation) were deposited as submarine fans within the graben. Most sediment was derived from the west (i.e., the Fladen Ground Spur), but some important fan systems were fed from the east (i.e., the Utsira High). The maximum limits of these fan systems are delineated, aided by the use of lithofacies correlation, reservoir pressure, and biostratigraphic data; changes in fan distributions through the Late Jurassic are also shown. An updated palynological zonation scheme that has been widely used throughout the area is also presented. Although the area is in a mature stage of exploitation, further mapping using the most recent high-quality 3-D seismic, available extensive well datasets, and the mapped extents of the fan systems might lead to additional hydrocarbon accumulations being identified.