Abstract: Normal faults grow via a sympathetic increase in their displacement and length (‘isolated model’) or by rapid establishment of their near-final length prior to significant displacement accumulation (‘constant-length model’). The isolated model has dominated the structural geology literature for >30 years, although some 3D seismic data-based studies support the constant-length model. Because they make different predictions regarding rift development, and earthquake size and recurrence intervals in areas of continental extension, it is critical to test these models with data from natural examples. Here we outline a range of techniques that constrain the kinematics of synsedimentary normal faults and thus test competing fault growth models. We then apply these techniques to three seismically imaged faults, showing that, in general, they grew in accordance with the constant-length model, although periods of relatively minor tip propagation and coeval displacement accumulation, characteristics more consistent with the isolated model, also occurred. We argue that analysis of growth strata represents the best way to test competing fault growth models; most studies utilizing this approach support the constant-length fault model, suggesting it may be more widely applicable than is currently assumed. It is plausible that the very early development of large faults is, however, characterized by the development of faults that, pre-linkage, grow in accordance with the isolated model; we may simply lack the data resolution, especially in the subsurface, to resolve this very early stage of fault growth.

Abstract Black shales are integral parts of most foreland-basin deposits and, because they typically reflect maximum basin subsidence, their distributions serve as proxies for the extent of foreland-basin development. In the United States Appalachian area, the distribution of Middle–Upper Ordovician black shales suggests that the Taconian Orogeny proceeded from south to north along the eastern Laurentian margin and that Taconian tectophases were mediated by convergence at continental promontories. In the Late Ordovician Taconic tectophase, changes in the distribution of the Martinsburg and Utica black shales support a reversal of subduction polarity that effected the reactivation of basement structures and basin migration sufficient to yoke the Appalachian foreland basin with adjacent intracratonic basins. Shale distribution suggests that early Chatfieldian (late Sandbian–early Katian), east-verging subduction early in the tectophase generated a cratonic extensional regime with a narrow foreland basin that developed along reactivated Iapetan basement structures. Abruptly, in late Chatfieldian–early Edenian (early Katian) time, westwards migration of basinal Utica black shales and an underlying unconformity suggests change to a compressional regime and westwards subduction vergence. The coincidence of changes in basin shape and migration with the shifts in subduction polarity suggests a causal relationship.

Abstract We use the Bay of Biscay and Western Pyrenees as a natural laboratory to develop and apply an approach to characterize and identify distinctive rifted margin domains in offshore and onshore settings. The Bay of Biscay and Western Pyrenees offer access to seismically imaged, drilled and exposed parts of one and the same hyperextended rift system. Offshore, we use gravity inversion and flexural backstripping techniques combined with seismic interpretation to provide estimates of accommodation space, crustal thickness and lithosphere thinning. Onshore, we focus on key outcrops of the former rift domain to describe the nature of sediment and basement rocks, and of their interface. This qualitative and quantitative characterization provides diagnostic elements for the identification of five distinct structural domains at magma-poor rifted margins and their fossil analogues (proximal, necking, hyperthinned, exhumed mantle and oceanic domains). This new approach can be used to reconcile offshore and onshore observations, and to aid interpretation when only local observations are available. Onshore remnants can be placed in an offshore rifted-margin context, enabling the prediction of first-order crustal architecture. For the interpretation of offshore seismic reflection sections, geological insights into rift structures and basement nature can be suggested based on onshore analogies. Supplementary material: Sensitivity of backstripping results to flexural rigidity is available at http://www.geolsoc.org.uk/SUP18778 .