Revisiting hotspots and continental breakup—Updating the classical three-arm model

ABSTRACT Classic models proposed that continental rifting begins at hotspots—domal uplifts with associated magmatism—from which three rift arms extend. Rift arms from different hotspots link up to form new plate boundaries, along which the continent breaks up, generating a new ocean basin and leaving failed arms, termed aulacogens, within the continent. In subsequent studies, hotspots became increasingly viewed as manifestations of deeper upwellings or plumes, which were the primary cause of continental rifting. We revisited this conceptual model and found that it remains useful, though some aspects require updates based on subsequent results. First, the rift arms are often parts of boundaries of transient microplates accommodating motion between the major plates. The microplates form as continents break up, and they are ultimately incorporated into one of the major plates, leaving identifiable fossil features on land and/or offshore. Second, much of the magmatism associated with rifting is preserved either at depth, in underplated layers, or offshore. Third, many structures formed during rifting survive at the resulting passive continental margins, so study of one can yield insight into the other. Fourth, hotspots play at most a secondary role in continental breakup, because most of the associated volcanism reflects plate divergence, so three-arm junction points may not reflect localized upwelling of a deep mantle plume.

Definition of tectono-sedimentary elements for rifted continental margins of the Norwegian and Greenland seas

The geology of the conjugate continental margins of the Norwegian and Greenland Seas reflects 400 Ma of post-Caledonian continental rifting, continental breakup between early Eocene and Miocene times, and subsequent passive margin conditions accompanying seafloor spreading. During Devonian-Carboniferous time, rifting and continental deposition prevailed, but from the mid-Carboniferous, rifting decreased and marine deposition commenced in the north culminating in a Late Permian open seaway as rifting resumed. The seaway became partly filled by Triassic and Lower Jurassic sediments causing mixed marine/non-marine deposition. A permanent, open seaway established by the end of the Early Jurassic and was followed by the development of an axial line of deep marine Cretaceous basins. The final, strong rift pulse of continental breakup occurred along a line oblique to the axis of these basins. The Jan Mayen Micro-Continent formed by resumed rifting in a part of the East Greenland margin in Eocene to Miocene times. This complex tectonic development is reflected in the sedimentary record in the two conjugate margins, which clearly shows their common pre-breakup geological development. The strong correlation between the two present margins is the basis for defining seven tectono-sedimentary elements (TSE) and establishing eight composite tectono-sedimentary elements (CTSE) in the region.

Moho carbonation at an ocean-continent transition

Carbonation of mantle rocks during mantle exhumation is reported in present-day oceanic settings, both at mid-ocean ridges and ocean-continent transitions (OCTs). However, the hydrothermal conditions of carbonation (i.e., fluid sources, thermal regimes) during mantle exhumation remain poorly constrained. We focus on an exceptionally well-preserved fossil OCT where mantle rocks have been exhumed and carbonated along a detachment fault from underneath the continent to the seafloor along a tectonic Moho. Stable isotope (oxygen and carbon) analyses on calcite indicate that carbonation resulted from the mixing between serpentinization-derived fluids at ~175 °C and seawater. Strontium isotope compositions suggest interactions between seawater and the continental crust prior to carbonation. This shows that carbonation along the tectonic Moho occurs below the continental crust and prior to mantle exhumation at the seafloor during continental breakup.

Catching the Main Ethiopian Rift evolving towards plate divergence

Magmatism accompanies rifting along divergent plate boundaries, although its role before continental breakup remains poorly understood. For example, the magma-assisted Northern Main Ethiopian Rift (NMER) lacks current volcanism and clear tectono-magmatic relationships with its contiguous rift portions. Here we define its magmatic behaviour, identifying the most recent eruptive fissures (EF) whose aphyric basalts have a higher Ti content than those of older monogenetic scoria cones (MSC), which are porphyritic and plagioclase-dominated. Despite these differences, calculations highlight a similar parental melt for EF and MSC products, suggesting only a different evolutionary history after melt generation. While MSC magmas underwent a further step of storage at intermediate crustal levels, EF magmas rose directly from the base of the crust without contamination, even below older polygenetic volcanoes, suggesting rapid propagation of transcrustal dikes across solidified magma chambers. Whether this recent condition in the NMER is stable or transient, it indicates a transition from central polygenetic to linear fissure volcanism, indicative of increased tensile conditions and volcanism directly fed from the base of the crust, suggesting transition towards mature rifting.

Review of the syn-rift to early post-rift depositional systems of the Cretaceous Mauléon rift: sedimentary record of continental crust hyperextension and mantle denudation (Western Pyrenees)

— The Mauléon basin, in the northwestern Pyrenean belt, is related to Early Cretaceous rifting and continental breakup. Here we review the evolution of depositional environments in the hyperextended Mauléon rift basin during Albian and Cenomanian time. This review includes the lithostratigraphy, regional distribution, boundaries, age and facies sedimentology of the basin’s syn-rift formations and their members. We construct paleogeographic maps to elucidate (1) the 3D distribution of sedimentary facies and depositional environments during the Albian and Cenomanian from the Iberian proximal margin to the hyperextended domain and (2) the link between major extensional structures and sedimentation during rifting and continental breakup. The Mauléon rift was supplied during most of the Albian by sediments from the Iberian proximal margin. The southern margin had a steep and abrupt topographic boundary related to a northward crustal rollover along the south-dipping Saint-Palais detachment. This feature controlled the deposition of base-of-slope conglomerates at the base of the margin that abruptly gave way to low-density turbidites, then hemipelagic deposits in the hyperextended domain. During latest Albian to Early Cenomanian time, continental breakup occurred in the eastern Mauléon basin and the vergence of the detachment systems reversed. Minor debris-flow deposits formed at the foot of fault scarps associated with the newly formed north-dipping detachments. Elsewhere, sediment from deltaic systems to the west in the Saint-Jean-de-Luz area deposited low-density turbidites in the hyperextended domain. During the post-rift stage, the flux of coarse sediment from the detachment footwall gradually declined as deformation waned, and low-density turbidites expanded onto the hyperextended domain from the European Late Cretaceous carbonate platform. These paleogeographic reconstructions, in addition to offering a synthetic view of the evolution of sedimentary environments during rifting, offer new insight into the post-rifting exhumation of the lower crust and mantle.

Cenozoic Relative Movements of Greenland and North America by Closure of the North Atlantic-arctic Plate Circuit: the Labrador Sea, Davis Strait and Baffin Bay

The processes that accommodated plate divergence between Greenland and North America are most confidently interpretable from a short-lived (61-42 Ma) sequence of magnetic isochrons in the Labrador Sea. Understanding of the preceding and following periods is impeded by the lack of clear isochrons in the basin’s continent-ocean transition and axial zones. By closing the regional plate circuit, we build and interpret a detailed plate motion model for Greenland and North America that is applicable in, but unaffected by data uncertainty from, the Labrador Sea, Davis Strait, and Baffin Bay. Among our findings, we show the Labrador Sea initially opened during a ~8.3-16.5 Myr-long period of focused extension culminating in continental breakup no earlier than 74-72 Ma, and experienced a ~80° change in spreading direction around 56 Ma. We describe some possible implications for the accommodation of strain prior to continental breakup and during extreme spreading obliquity.