Basin Research - Volume 26, Issue 6, 2014

Interpretation of long‐offset 2D depth‐imaged seismic data suggests that outer continental margins collapse and tilt basinward rapidly as rifting yields to seafloor spreading and thermal subsidence of the margin. This collapse post‐dates rifting and stretching of the crust, but occurs roughly ten times faster than thermal subsidence of young oceanic crust, and thus is tectonic and pre‐dates the ‘drift stage’. We term this middle stage of margin development ‘outer margin collapse’, and it accords with the exhumation stage of other authors. Outer continental margins, already thinned by rifting processes, become hanging walls of crustal‐scale half grabens associated with landward‐dipping shear zones and zones of low‐shear strength magma at the base of the thinned crust. The footwalls of the shear zones comprise serpentinized sub‐continental mantle that commonly becomes exhumed from beneath the embrittled continental margin. At magma‐poor margins, outer continental margins collapse and tilt basinward to depths of about 3 km subsea at the continent–ocean transition, often deeper than the adjacent oceanic crust (accreted later between 2 and 3 km). We use the term ‘collapse’ because of the apparent rapidity of deepening (<3 Myr). Rapid salt deposition, clastic sedimentation (deltaic), or magmatism (magmatic margins) may accompany collapse, with salt thicknesses reaching 5 km and volcanic piles 1525 km. This mechanism of rapid salt deposition allows mega‐salt basins to be deposited on end‐rift unconformities at global sea level, as opposed to deep, air‐filled sub‐sea depressions. Outer marginal collapse is ‘post‐rift’ from the perspective of faulting in the continental crust, but of tectonic, not of thermal, origin. Although this appears to be a global process, the Gulf of Mexico is an excellent example because regional stratigraphic and structural relations indicate that the pre‐salt rift basin was filled to sea level by syn‐rift strata, which helps to calibrate the rate and magnitude of collapse. We examine the role of outer marginal detachments in the formation of East India, southern Brazil and the Gulf of Mexico, and how outer marginal collapse can migrate diachronously along strike, much like the onset of seafloor spreading. We suggest that backstripping estimates of lithospheric thinning (beta factor) at outer continental margins may be excessive because they probably attribute marginal collapse to thermal subsidence.

We present field and seismic evidence for the existence of Coniacian–Campanian syntectonic angular unconformities within basal foreland basin sequences of the Austral or Magallanes Basin, with implications for the understanding of deformation and sedimentation in the southern Patagonian Andes. The studied sequences belong to the mainly turbiditic Upper Cretaceous Cerro Toro Formation that includes a world‐class example of conglomerate‐filled deep‐water channel bodies deposited in an axial foredeep depocentre. We present multiple evidence of syntectonic deposition showing that the present internal domain of the fold‐thrust belt was an active Coniacian–Campanian wedge‐top depozone where deposition of turbidites and conglomerate channels of Cerro Toro took place. Cretaceous synsedimentary deformation was dominated by positive inversion of Jurassic extensional structures that produced elongated axial submarine trenches separated by structural highs controlling the development and distribution of axial channels. The position of Coniacian‐Campanian unconformities indicates a ca. 50–80 km advance of the orogenic front throughout the internal domain, implying that Late Cretaceous deformation was more significant in terms of widening the orogenic wedge than all subsequent Andean deformation stages. This south Patagonian orogenic event can be related to compressional stresses generated by the combination of both the collision of the western margin of Rocas Verdes Basin during its closure, and Atlantic ridge push forces due to its accelerated opening, during a global‐scale plate reorganization event.

However salt has a viscous rheology, overburden rocks adjacent to salt diapirs have a brittle rheology. Evidence of deformation within the overburden has been described from diapirs worldwide. Gravity‐driven deposits are also present along the flanks of several diapirs. The well‐known example from the La Popa Basin in northern Mexico shows that such deposits may be organized into halokinetic sequences. This leads to several questions: (i) How does diapir growth contribute to overburden deformation? (ii) Are halokinetic sequence models valid for other areas beyond the La Popa Basin. The Bakio diapir and its well‐exposed overburden in Basque Country, Spain provides key elements to address these questions. The Bakio diapir consists of Triassic red clays and gypsum and is flanked by synkinematic middle to upper Albian units that thin towards the diapir. The elongate diapir parallels the Gaztelugatxe normal fault to the NE: both strike NE–SW and probably formed together during the middle Albian, as synkinematic units onlap the fault scarp. The diapir is interpreted as a reactive diapir in response to middle Albian motion on the Gaztelugatxe fault. The rate of salt rise is estimated to be about 500 m Myr⁻¹ during this passive stage. During Late Albian, the diapir evolved passively as the Gaztelugatxe fault became inactive. Synkinematic units thinning towards the diapir, major unconformities, slumps and other gravity‐driven deposits demonstrate that most deformation related to diapir growth occurred at the sea floor. Halokinetic sequences composed of alternating breccias and fine‐grained turbidites recorded cyclic episodes of diapir flank destabilization. This work provides insights into drape fold and halokinetic sequence models and offers a new simple method for estimating rates of diapir growth. This method may be useful for outcrop studies where biostratigraphical data are available and for other passive diapirs worldwide.

Fluvio‐deltaic stratigraphy develops under continuous morphodynamic interactions of allogenic and autogenic processes, but the role and relative contribution of these processes to the stratigraphic record are poorly understood. We analysed synthetic fluvio‐deltaic deposits of several accommodation‐to‐supply cycles (sequences) with the aim to relate stratigraphic variability to autogenic and allogenic controls. The synthetic stratigraphy was produced in a series of long time‐scale (10⁵ years) numerical experiments with an aggregated process‐based model using a typical passive‐margin topography with constant rates of liquid and solid river discharge subjected to sinusoidal sea‐level fluctuation. Post‐processing of synthetic stratigraphy allowed us to quantify stratigraphic variability by means of local and regional net sediment accumulation over equally spaced time intervals (1–10 kyr). The regional signal was subjected to different methods of time‐series analysis. In addition, major avulsion sites (>5 km from the coastline) were extracted from the synthetic stratigraphy to confirm the interpretations of our analyses. Regional stratigraphic variability as defined in this study is modulated by a long‐term allogenic signal, which reflects the rate of sea‐level fluctuation, and it preserves two autogenic frequency bands: the intermediate and high‐frequency components. The intermediate autogenic component corresponds to major avulsions with a median inter‐avulsion period of ca. 3 kyr. This component peaks during time intervals in which aggradation occurs on the delta plain, because super‐elevation of channel belts is a prerequisite for large‐scale avulsions. Major avulsions occur occasionally during early stages of relative sea‐level fall, but they are fully absent once the coast line reaches the shelf edge and incision takes place. These results are consistent with a number of field studies of falling‐stage deposition in fluvial systems. The high‐frequency autogenic component (decadal to centennial time scales) represents mouthbar‐induced bifurcations occurring at the terminal parts of the system, and to a lesser extent, partial or small‐scale avulsions (<5 km from the coastline). Bifurcation intensity correlates strongly with the rate of progradation, and thus reaches its maximum during forced regression. However, its contribution to overall stratigraphic variability is much less than that of the large‐scale avulsions, which affect the entire area downstream of avulsion nodes. The results of this study provide guidelines for predicting fluvio‐deltaic stratigraphy in the context of co‐existing autogenic and allogenic processes and underscore the fact that the relative importance and the type of autogenic processes occurring in fluvio‐deltaic systems are governed by allogenic forcing.