Decollement

Key Takeaways

• Thin-skinned vs. thick-skinned tectonics is defined by whether the decollement level is within the sedimentary section (thin-skinned, with basement remaining undeformed) or whether deformation penetrates the crystalline basement (thick-skinned, with no decollement in the sedimentary section): thin-skinned fold-and-thrust belts with salt or shale decollements produce the gentle, detached anticlines and thrust sheets that are the most prolific structural traps in compressional petroleum provinces, because the sedimentary section above the decollement can be folded and thrust without the mechanical complications of basement involvement; thick-skinned provinces (such as the Laramide foreland of Wyoming and the Sierras Pampeanas of Argentina) produce broad, structurally complex basement uplifts that are more difficult to image with seismic and more geologically complex to evaluate as petroleum traps; the recognition of a basal decollement in a fold-and-thrust belt seismic section is therefore a key observation that determines which tectonic model (thin-skin or thick-skin) applies to the province and guides the choice of structural interpretation templates for trap mapping.
• Salt as a decollement horizon is uniquely effective because salt (halite and anhydrite) has almost zero internal friction angle and very low yield strength, flows plastically at geological strain rates, and provides a nearly frictionless surface along which the overlying sedimentary section can translate horizontally under the driving force of gravity spreading (in passive margin settings) or horizontal tectonic compression (in fold-and-thrust belts): in the northern Gulf of Mexico, Cenozoic sedimentary wedges have detached from the underlying Jurassic Louann Salt and translated southward for distances of 30 to 80 kilometers, with the resulting salt diapirs, canopy systems, and salt-detached thrust belts containing some of the Gulf of Mexico's most important deepwater oil and gas fields; in the Jura Mountains of France and Switzerland, Triassic evaporites serve as the decollement beneath a fold-and-thrust belt that has translated northwestward from the Alps for more than 30 kilometers, creating the symmetrical Jura anticlines that are the textbook examples of decollement-controlled fold geometry; salt decollement fold geometry (with anticlinal crests over thinner salt and synclinal troughs over thicker salt accumulations) is the opposite of the gravity-driven salt dome structural pattern, reflecting the fundamentally different kinematics of compressional versus extensional salt tectonics.
• Overpressured shale decollements are the most common decollement type in compressional fold-and-thrust belts worldwide because many sedimentary basins contain overpressured shale horizons where elevated pore pressure reduces the effective normal stress on the bedding plane and lowers the shear strength of the layer to the point where it can accommodate large-magnitude horizontal displacement: Cretaceous shales in the Canadian Foothills of Alberta and British Columbia have served as decollements for the eastward transport of the Rocky Mountain fold-and-thrust belt, with the decollement horizon visible on seismic profiles as a relatively continuous reflection beneath the complex thrust sheet structures above; in the Appalachian Valley and Ridge province, Cambrian carbonate shales and Silurian salt beds have served as multiple decollement levels at different depths, producing a structurally complex stack of thrust sheets (duplexes and triangle zones) that has been extensively mapped with both surface geology and deep seismic reflection profiles; the depth, dip, and lateral continuity of the shale decollement directly controls the geometry of the anticlines above it, making accurate decollement mapping the foundation of trap geometry evaluation in these provinces.
• Structural interpretation above decollements uses the geometric relationship between decollement depth and anticline geometry encoded in the fault-bend fold and fault-propagation fold models to predict the geometry of the subsurface structure from the surface or seismic expression: in a simple fault-bend fold model (where the thrust ramp climbs from the decollement at angle theta to reach the surface or another flat above), the relationship between ramp angle, flat length, and anticline width and height is geometrically determined, allowing the decollement depth to be predicted from the surface anticline geometry (or vice versa); this geometric predictability of decollement-related folds is what makes fold-and-thrust belt structural traps particularly amenable to petroleum exploration based on surface mapping or regional seismic grids, because the structural geometry of the trap can be predicted from incomplete seismic data using the fault-bend fold framework as a geometric constraint; seismic reflection profiling directly images the decollement in many fold-and-thrust belts where the velocity contrast between the decollement shale or salt and the overlying carbonate or sandstone section is large enough to produce a strong reflector at the decollement surface.
• Petroleum system implications of decollements include both the trapping of hydrocarbons in decollement-related anticlines and the potential for decollement shales or salts to serve as effective regional seals that separate different fluid systems above and below the detachment: salt decollements are particularly effective seals because salt is essentially impermeable to all fluids, creating a barrier between the salt-detached structural traps above the decollement and any deep pre-salt petroleum system below; the pre-salt petroleum systems of the South Atlantic (offshore Brazil and West Africa), which have produced some of the most significant oil discoveries of the past two decades (Lula, Tupi, Jubilee), exploit carbonate reservoirs below the Aptian salt decollement that are isolated from the post-salt system above; the Zagros fold belt of Iran and Iraq, where Hormuz salt and lower Paleozoic shales serve as multiple decollement levels, hosts the world's most productive oil fields (Ghawar, Burgan, Kirkuk) in anticlines formed above decollement-controlled thrusts, with the Hormuz salt providing both the decollement mechanism for anticlinal development and an effective regional seal over the deep Paleozoic petroleum system.