Obstructed minibasins on a salt‐detached slope: An example from above the Sigsbee canopy, northern Gulf of Mexico

Abstract

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). K E Y W O R D Sbase-of-salt relief, extension, salt tectonics, salt-detached slopes, shortening, supra-salt strain, welding 506 | EAGE DUFFY et al. | 523 EAGE DUFFY et al.

How fast can minibasins translate down a slope? Observations from 2D numerical models

“…When minibasins translate downslope over a rugose base-of-salt, if thick enough, the minibasin can weld at its base, or buttress against a high-relief base-salt feature, obstructing the minibasin from further downslope translation (e.g. Krueger, 2010;Wagner and Jackson, 2011;Duffy et al, 2020). The complex deformation patterns that result from different degrees of minibasin obstruction at both the minibasinscale and the sub-regional scale have been recently described in detail in an area where the base-of-salt has very high relief (i.e.…”

“…The complex deformation patterns that result from different degrees of minibasin obstruction at both the minibasinscale and the sub-regional scale have been recently described in detail in an area where the base-of-salt has very high relief (i.e. the northern Gulf of Mexico canopy; Duffy et al, 2020.…”

“…The complex threedimensional strains due to differential translation of the sedimentary cover have been previously described using seismic reflection data imaging natural systems (e.g. Krueger, 2010;Duffy et al, 2020;Fernandez et al, 2020), and have also been described from physical models (Dooley et al, 2019;Duffy et al, submitted). In those previous works, strike-slip patterns around minibasins are discussed within the context of minibasins obstructed or stopped due to welding.…”

“…Furthermore, the downslope transation of minibasins can be obstructed by base-salt relief or friction associated with primary welding, processes that result in locally complex strain patterns on the slope (e.g. Duffy et al, 2020) (Figure 1c).…”

“…Furthermore, if minibasins translating at different velocities can coexist on a slope, this can result in differential translation between minibasins and complex strain patterns around them (e.g. Krueger, 2010;Duffy et al, 2020).…”