Structural, stratigraphic and sedimentological characterisation of a wide rift system: The Triassic rift system of the Central Atlantic Domain

The extension-driven break-up of the Pangean Supercontinent in
The extension-driven break-up of the Pangean Supercontinent in the present-day Central and North Atlantic domains commenced during Permo-Triassic times. Permian rift processes in this region are generally attributed to crustal collapse during the latter part of the Hercynian/Variscan orogeny, but the Triassic rifting style is more complex and the presence of a single over-riding first-order control is equivocal. Basement terrane structures and boundaries provide pre-existing zones of weakness that had significant influences on the location, timing, orientation and extent of Triassic rifting. The 201 Ma Central Atlantic Magmatic Province (CAMP), a Large Igneous Province (LIP), increased lithospheric heat flow at the end of rifting, produced uplift and doming, and also contributed to deformation and subsequent continental breakup.

Analysis of the Triassic basin structures in the Central Atlantic and North Atlantic domains (respectively Georgia to Nova Scotia and conjugate margin, and Newfoundland-Iberia) shows that four main basin types are geographically distributed: 1) southern basins (Georgia to Virginia), small to medium size with limited growth-structures, 2) basins in the Central-East domain (New Jersey-Nova Scotia), wide basins with growth-structures along long-lived inherited bounding faults, 3) basins in the Central-West domain (offshore NS and Morocco) that comprise numerous half-graben sub-basins in a lower unit and an upper very wide tabular unit, and 4) northern basins (Newfoundland-Portugal), that are tabular and extremely wide. Basin architectures suggest that subsidence was regional in extent and not necessarily controlled by main faults. Variations in crustal characteristics are considered to have led to the development of the four main basin types. Stratigraphic data show that the onset of deposition in the Central and southern North Atlantic domain was restricted to a Central Segment between two main structural lineaments. Overall, crustal rheology and inheritance exhibited first-order controls on subsidence and basin development.

Triassic basins in the Central and North Atlantic domain were all filled by continental deposits in various palaeo-environments such as alluvial-fan, mega-fluvial systems, terminal fluvial systems, fan-delta fluvial systems, permanent oxic and anoxic lakes, playa lakes, and saline lakes, some of them interbedded with aeolian and volcanic deposits. Most basins were far from any marine domain but marine ingressions from the Tethys domain are inferred from dolomite and marine fauna in Portugal and from thick salt deposits in Morocco and offshore Canada during the late Rhaetian. A series of palaeogeographic maps are presented utilising plate reconstructions based on Sahabi et al (2004), in order to summarize continental-scale palaeo-environment distribution.

Our tectono-stratigraphic model suggests that initial strain was accommodated in a Central Segment by inherited faults during the Early and Middle Triassic. During the Carnian, the rift domain was increasingly extended with the development of half-graben basins in the Central Segment and some in the Northern Segment. In both segments, the initial strain re-activated inherited faults and later the rift zones evolved towards significantly wider basins marked by the presence of a major erosional truncation surface across the basins and followed by lacustrine sedimentation that eventually overlapped paleo-topographic highs. Subsidence of such regional extent is likely driven by thermal processes following a phase of extreme lithospheric thinning and lower crustal removal associated with mantle uplift. The architecture of the Triassic rifting has the characteristics of the wide rift mode (very large basins with some active faults) for which subsidence is driven by lower crustal flow within a high heat-flow region. Nevertheless some faults were active during Late Triassic sedimentation but accounted for a relatively minor part of the overall subsidence. Pulses of active subsidence and topographic rejuvenation led to fluvial dominated deposition within basins. Non-rejuvenation of hinterland led to more lacustrine dominated sedimentation and eventual connection of basins across topographic highs due to regional (thermal) subsidence. During the latest Rhaetian, basins with the lowest elevations were possibly “flooded” by marine waters resulting in precipitation of thick salt deposits.