Movement of gravity-driven systems on passive margins is fuelled by the loss of gravitational potential energy. Two end-member modes (gravity spreading and gravity gliding) are defined by whether the ...potential energy loss is due to deformation and movement towards the base of the system (spreading), or by movement parallel to the base of the system (gliding); most natural systems consist of a mixture of the two processes.
Hitherto, use of these concepts has been limited or equivocal due to lack of a quantitative measure. In some cases, characterisation of gliding vs. spreading systems based on secondary attributes has resulted in controversy, because there is a lack of consensus as to which of these are truly diagnostic. This paper presents a new, simple quantitative method based on vector analysis, providing a numerical measure of the relative contribution of spreading vs. gliding. The method is applied to synthetic examples, where deformation can be tracked, and to natural examples where a valid palinspastic reconstruction is available. The results confirm that most natural examples exhibit mixed-mode behaviour, and that some have been mischaracterized; much of the Angola margin is dominated by spreading. The method can also provide an estimate of the absolute amount of gravitational potential energy released in the movement, and the energy contribution made by gliding vs. spreading. Determining the dominant process has implications for predicting the development of seafloor topography and stratal architecture.
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•Gravity gliding and gravity spreading on passive margins.•A simple analysis based on particle displacements reveals the energy source.•Passive margin systems are easily placed on the spreading/gliding spectrum.
Existing models for the initiation of salt withdrawal minibasins focus on the role of triggers that exist within the minibasin, either stratigraphic (e.g. differential deposition) or tectonic ...(extension, translation or contraction). Existing studies tend to focus on complex settings, such as continental margins, which contain many different potential triggering mechanisms. It can be difficult in these settings to identify which process is responsible for minibasin initiation, or the influence of individual factors on their subsequent development.
Salt withdrawal minibasins also exist in simpler settings, without any obvious intrinsic trigger; the region of the North German Basin used by Trusheim (1960) in the classic definition of salt withdrawal geometries was of this nature. There is no overall basal or surface slope, no major lateral movement, and there is no depositional heterogeneity. Previously recognized trigger processes for minibasin initiation do not apply in this benign setting, suggesting that other, potentially more fundamental, influences may be at work.
A simple forward-modelling approach shows how, in the absence of any other mechanism, a new minibasin can develop as the consequence of salt movement driven by its neighbour, and families of withdrawal minibasins can propagate across a region from a single seed point.
This new mechanism may explain how some minibasins appear to initiate before the sediment density has exceeded that of the underlying salt. The forward modelling also indicates that some minibasins begin to invert to form turtle anticlines before the underlying salt has been evacuated, so that the timing of turtle formation may not be diagnostic of weld formation. This mechanism may also give rise to salt-cored turtles that have a lens of salt trapped beneath their cores. These new findings have implications for hydrocarbon migration and trapping.
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•Issue: what triggers the growth of salt withdrawal minibasins and how do they evolve?•Problem: existing studies have complex settings and too many variables.•Approach. Stripped-down analysis, simple scenarios, reduced variables•Results: parent minibasins spawn daughter minibasins, no intrinsic trigger is needed.•.•Turtle anticlines can develop before welding, or even without welding
We present the first sequential structural restoration with flexural backstripping of the Gulf of Mexico US‐Mexico conjugate margin salt basin. We construct four large‐scale (100s of km) balanced, ...sequential structural restorations to investigate spatio‐temporal patterns of subsidence, geometry of the original salt basin, feedbacks between post‐salt structural and stratigraphic evolution, paleo‐bathymetry, and crustal configurations. The restorations are based on interpretations of 2D and 3D seismic data, and include sequential sedimentary decompaction, flexural isostatic backstripping, and thermal isostatic corrections. The spatially variable crustal thinning factor is directly measured from seismic data, and lithologic parameters are determined by well penetrations. We present a model for the original salt basin and discuss evidence for and implications of a deep water salt basin setting for the GoM. Our analysis suggests a salt basin that contained ∼1–2 km thick salt in a basin 175–390 km across with ∼1 km of bathymetry after salt deposition. The base of salt is mostly smooth with <1 km of local relief in the form of normal faults that disrupt a pre‐salt sedimentary section. We find that supra‐salt extension and shortening are not balanced, with measurable extension exceeding shortening by 18–30 km on each cross‐section. Our subsidence analysis reveals anomalous subsidence totaling 1–2 km during Late Jurassic and Early Cretaceous times that may reflect dynamic topography or depth‐dependent thinning. We offer an interpretation of crustal breakup invoking pre‐salt clastic sedimentation, salt deposition in a deep water syn‐thinning basin, and post‐salt lower‐crustal exhumation.
Plain Language Summary
The Gulf of Mexico is a large basin that formed over 200 million years ago due to tectonically driven extension of a supercontinent. Early in its formation it accumulated thick salt deposits. Due to that salt and the later deposition of several kilometers of sedimentary rock that conceal the deep geology, it is difficult to know exactly how extension started and progressed. This study uses new 2D and 3D seismic data that images the deep geology corresponding to that early extension. We sequentially remove each rock layer to reconstruct what the margin looked like in the Mesozoic. By systematically moving back in time we are able to reconstruct the changing geometry, deformation, and bathymetry of the Gulf of Mexico. Our results reveal periods of time when the bathymetry was influenced by unknown factors, which we posit reflects mantle forces.
Key Points
We present sequential structural restorations with flexural backstripping of the post‐rift eastern GoM conjugate US‐MX margin from Mesozoic to present
We interpret the geometry and bathymetry of the restored Mesozoic salt basin
Our analysis indicates significant and widespread Mesozoic anomalous subsidence
In salt‐detached gravity‐gliding/spreading systems the detachment geometry is a key control on the downslope mobility of the supra‐salt sequence. Here, we used regional 3D seismic data to examine a ...salt‐stock canopy in the northern Gulf of Mexico slope, in an area where supra‐canopy minibasins subsided vertically and translated downslope above a complex base‐of‐salt. If thick enough, minibasins can interact with, and weld to, the base‐of‐salt and be obstructed from translating downslope. Based on the regional maps of the base of allochthonous salt and the base of the supra‐canopy sequence, the key controls on minibasin obstruction, we distinguished two structural domains in the study area: a highly obstructed domain and a highly mobile domain. Large‐scale translation of the supra‐canopy sequence is recorded in the mobile domain by a far‐travelled minibasin and a ramp syncline basin. These two structures suggest downslope translation on the order of 40 km from Plio‐Pleistocene to Present. In contrast, translation was impeded in the obstructed domain due to supra‐canopy bucket minibasins subsiding into feeders during the Pleistocene. As a result, we infer that differential translation occurred between the two domains and argue that a deformation area between two differentially translating supra‐canopy minibasin domains is difficult to recognize. However, characterizing domains according to base‐of‐salt geometry and supra‐canopy minibasin configuration can be helpful in identifying domains that may share similar subsidence and downslope translation histories.
The effect of minibasin obstruction processes are investigated in our study area, where two different domains, each containing several minibasins are described. The two domains are characterized by different degrees of mobility (or obstruction) according to the presence (or absence) of highly obstructed minibasins (e.g., bucket minibasins). The obstruction of one domain as opposed to the continuous translation of the other domain, results in differential translation of the sedimentary cover.
Brun and Fort (2011) use mechanical analysis, experimental models, and geologic data to suggest that deformation in passive-margin salt basins is dominantly a result of gravity gliding rather than ...gravity spreading. They claim that only seaward tilt of the salt layer is effective in driving basinward translation of the salt and overburden and that differential loading alone requires extreme conditions that do not occur in nature. In this Discussion, we refute many of their arguments and conclusions. We show that: i) a more thorough mechanical analysis indicates that gravity spreading is effective if the proximal overburden is at least three times thicker than the distal overburden, a common occurrence on passive margins; ii) more realistic analogue models also demonstrate that extreme thickness variations are not necessary for gravity spreading; iii) their analysis of structures or structure associations is sometimes misleading; and iv) there is abundant evidence that gravity spreading is dominant on some margins. In particular, modern data from the northern Gulf of Mexico confirm traditional interpretations that Cenozoic failure was mainly due to downslope movement driven by sedimentary loading, not SW-directed gliding driven by tilt of the deep salt as claimed by Brun and Fort (2011). We conclude that both gravity gliding and gravity spreading are common processes which may vary spatially and temporally in any one salt basin.
Recent subsalt petroleum discoveries associated with rifted‐margin salt basins have piqued interest in the presalt geology of the Gulf of Mexico margin. Available subsurface data does not uniquely ...constrain the subsalt geometry, so creating an interpretation of the crustal architecture requires the application of geological models for crustal extension and breakup. However, published interpretations of the nature of the transition from continental rifting to seafloor spreading range from magma‐rich to magma‐poor. To address this uncertainty, we present 2D forward kinematic models for crustal configurations generated by diverse models (symmetric extension, depth‐dependent extension, and volcanic extension). Through a series of conceptual balanced cross‐sections grounded in a ~600 km 2D seismic line from the NE Gulf of Mexico, we demonstrate the implications of each model for the limit of oceanic crust, basement morphology, crustal architecture, and hydrocarbon prospectivity. We discuss evidence for the dominant crustal processes, including geodynamic factors and structural and stratigraphic observations. Based on our observations and the geologic history, we favour an asymmetric, magma‐poor to ‐intermediate margin interpretation for the NE Gulf of Mexico, but suggest that the degree of volcanic input and width of the ocean‐continent transition zone may vary along strike. The along‐strike variability highlights the importance of understanding all potential presalt crustal configurations, their key features, and their implications. With increased data availability on the presalt geology in the Gulf of Mexico the relevance of these scenarios can be assessed, allowing development of comprehensive geodynamic and tectonic models of the margin and consideration of petroleum system elements in the presalt sequence.
Salt‐detached gravity gliding/spreading systems having a rugose base‐of‐salt display complex strain patterns. However, little was previously known about how welding of supra‐salt minibasins to the ...sub‐salt may influence both the downslope translation of minibasins on salt‐detached slopes and the regional pattern of supra‐salt strain. Using a regional 3D seismic reflection data set, we examine a large salt‐stock canopy system with a rugose base on the northern Gulf of Mexico slope, on which minibasins both subside and translate downslope. Some minibasins are welded at their bases and others are not. We suggest that basal welds obstruct downslope translation of minibasins and control regional patterns of supra‐canopy strain. The distribution of strain above the canopy is complex and variable. Each minibasin that becomes obstructed modifies the local strain field, typically developing a zone of shortening immediately updip and an extensional breakaway zone immediately downdip of the obstructed minibasin. This finding is corroborated by observations from a physical sandbox model of minibasin obstruction. We also find in our natural example that minibasins can be obstructed to different degrees, ranging from severe (e.g., caught in a feeder) to mild (e.g., welded to a flat or gently dipping base‐of‐salt). By mapping both the presence of obstructed minibasins and the relative degree of minibasin obstruction, we provide an explanation for the origin of complex 3‐D strain fields on a salt‐detached slope and, potentially, a mechanism that explains differential downslope translation of minibasins. In minibasin‐rich salt‐detached slope settings, our results may aid: i) structural restorations and regional strain analyses; ii) prediction of subsalt relief in areas of poor seismic imaging; and iii) prediction of stress fields and borehole stability. Our example is detached on allochthonous salt and where the base‐of‐salt is rugose, with the findings applicable to other such systems worldwide (e.g., Gulf of Mexico; Scotian Margin, offshore eastern Canada). However, our findings are also applicable to systems where the salt is autochthonous but has significant local basal relief (e.g., Santos Basin, Brazil; Kwanza Basin, Angola).
In the early stages of margin development when minibasins are relatively thin and do not interact with the base‐of‐salt relief, the strain pattern on the salt‐detached slope is expected to be relatively simple. In contrast, later in margin development, when minibasins are thick enough such that their bases weld to the rugose base‐of‐salt, they become obstructed. During the latter stage a more complex strain pattern develops, with shortening strains typically developed immediately upslope of each obstructed minibasin and an extensional breakaway developed immediately downslope. In this paper, we propose that minibasin obstruction exerts a first‐order control on the mobility of minibasins and the pattern of supra‐salt strain.
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