Rift kinematics preserved in deep-time erosional landscape below the northern North Sea

Our understanding of continental rifting is largely derived from the stratigraphic record. This archive is, however, incomplete as it does not capture the geomorphic and erosional record of rifting. New 3D seismic reflection data reveals a Late Permian-Early Triassic landscape incised into the pre-rift basement of the northern North Sea. This landscape, which covers at least 542 km2, preserves a drainage system bound by two major tectonic faults. A quantitative geomorphic analysis of the drainage system reveals 68 catchments, with channel steepness and knickpoint analysis of catchment-hosted paleo-rivers showing that the landscape preserved a >2 Myrs long period of transient tectonics. We interpret that this landscape records punctuated uplift of the footwall of a major rift-related normal fault at the onset of rifting. The landscape was preserved by a combination of relatively rapid subsidence in the hangingwall of a younger fault and burial by post-incision sediments. We show how and why erosional landscapes are preserved in the stratigraphic record, and how they can help us understand the tectono-stratigraphic evolution of ancient continental rifts.

Mantle flow and orography: the effect of basal lithospheric shearing and lateral erosion gradients on continental rifting

<p><span>Mantle plume-lithosphere interactions modulated by surface processes across extensional tectonic settings give rise to outstanding topographies and sedimentary basins. However, the nature of these interactions and the mechanisms through which they control the evolution of continental rifts are still elusive. Basal lithospheric shearing due to plume-related mantle flow leads to extensional lithospheric rupturing and associated magmatism, rock exhumation, and topographic uplift away from the plume axis by a distance inversely proportional to the lithospheric elastic thickness. When moisturized air encounters a topographic barrier, it rises, decompresses, and saturates, leading to enhanced erosion on the windward side of the uplifted terrain. Orographic precipitation and asymmetric erosional unloading facilitate strain localization and lithospheric rupturing on the wetter and more eroded side of an extensional system. This simple model is validated against petro-thermo-mechanical numerical experiments where a rheologically stratified lithosphere above a mantle plume is subject to fluvial erosion proportional to stream power during extension. These findings are consistent with Eocene mantle upwelling and flood basalts in Ethiopia synchronous with distal initiation of lithospheric stretching in the Red Sea and Gulf of Aden as well as asymmetric topography and slip along extensional structures where orography sets an erosional gradient in the Main Ethiopian Rift (MER). I conclude that, although inherently related to the lithosphere rheology, the evolution of continental rifts is even more seriously conditioned by the mantle and surface dynamics than previously thoughts.</span></p>

Exploring rift magmatism and evolution through gravity analysis of North America's failed rifts

<p>Comparative study of North America’s failed continental rifts allows investigation of the effects of extension, magmatism, magmatic underplating and rift inversion in the evolution of rifting. We explore this issue by examining the gravity signatures of the Midcontinent Rift (MCR), Reelfoot Rift (RR), and Southern Oklahoman Aulacogen (SOA). The ~1.1 Ga MCR records aspects of the complex assembly of Rodina, while the structures related to the ~560 Ma RR and SOA formed during the later breakup of Rodinia and subsequent assembly of Pangea. Combining average gravity anomalies along each rift with seismic data, we examine whether these data support the existence of high-density residual melt underplates (“rift pillows”), reflect the possible amounts of inversion, and whether these rifts should be considered analogs of one another at different stages in rift evolution. The MCR and SOA have strong gravity highs along much of their length. Furthermore, the west and east arms of the MCR have different gravity signatures. The west arm of the MCR has a positive gravity anomaly of 80-100 mgals, while the east arm and SOA have positive anomalies of only 40-50 mgals. The RR does not exhibit a high positive anomaly along much of its length. The positive anomalies of both arms of the MCR and SOA reflect 10-20 km thick underplates at the base of the crust. These gravity anomalies also reflect greater amounts of inversion, during which the rift-bounding normal faults are reactivated by later compression, bringing the high-density igneous rocks closer to the surface. By averaging gravity data along the length of each failed rift, we can more easily distinguish between the history of individual rifts and general features of rifting that apply to other failed or active rifts around the world.</p>

The influence of laterally varying crustal strength on rift physiography – Combining 3D numerical models and geological observations

<p>Continental rifts form across a mosaic of crustal units, each unit displaying properties that reflect their own unique tectonic evolution and lithology. The physiography of rift systems is largely reflective of this underlying crustal substrate, which may change over short distances along-strike of the rift. Pervasive, well-developed structural heterogeneities represent sites where strain may localise and may thus weaken a crustal volume. In contrast, relatively pristine areas of crust, such as igneous batholiths, contain few heterogeneities and may be considered relatively strong. Characteristic rift physiographies associated with these ‘strong’ and ‘weak’ crustal units, and how rift physiography changes across areas where these units are juxtaposed remain elusive.</p><p>In this study we use the 3D thermo-mechanical numerical code ASPECT to investigate how areas of differing upper crustal strength influence rift physiography. We extend a 500x500x100km volume, within which we define four 125km wide upper crustal domains of either ‘strong’, ‘normal’ or ‘weak’ crust. Crustal strength is determined by varying the initial plastic strain in the model across 5km blocks, producing a static-like pattern. Weak domains contain weakened blocks with large initial plastic strain values, creating large contrasts with adjacent blocks. In contrast, 5 km blocks within the strong domain have relatively low values of initial plastic strain, producing little variation between adjacent blocks.</p><p>Our modelling simulations reveal that strain rapidly localises onto high-displacement structures (equivalent to faults) in the weak domain. Fault spacing and the strain accommodated by each fault decreases in the normal domain, with the strong domain characterised by closely-spaced, low displacement faults approximating uniform strain. When heterogeneities are incorporated into the strong domain, we find that these rapidly localise strain, effectively partitioning the domain into a series of smaller, strong areas separated by faults. Faults are initially inhibited at the boundaries with adjacent stronger domains; as extension progresses, these faults break through the barrier and propagate into the stronger domains.</p><p>Our observations have important implications for rift system development, particularly in areas of highly heterogeneous basement. Studies have shown that the Tanganyika rift developed at high angles to cratonic and mobile belt basement terranes, with localisation inhibted in the stronger cratonic areas. Similarly, extension in the Great South Basin (GSB), New Zealand, initially localised in weak, dominantly sedimentary, terranes, compared to stronger, more homogenous granitic areas. Terrane boundaries in the GSB also inhibit the lateral propagation of faults. Comparing our model results with observations from these and other systems globally, we determine characteristic structural styles and examine how rift physiography varies across ‘strong’ and ‘weak’ crustal volumes.</p>