Deformation and kinematic evolution of the subsurface structures: Zagros foreland fold-and-thrust belt, northern Dezful Embayment, Iran
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Anticline
Décollement
Accretionary wedge
In the North Ucayali basin, HC reserves have been already found but the structural style is rather complex due to the variability of the sedimentary covers and the presence of inherited structures. The risk of undercharge of the prospects due to a timing problem exists. A calibration of the erosion at the top of the various structures published by Bertolotti and Moretti (2009) indicated that the tertiary structures have different ages. Although the importance of the Paleozoic depot centres and faults in localizing has been largely published, a major role of the early Mesozoic evaporitic pillows was only recently proposed although seismic data evidences these pillows that influence the thrust geometry. Such behaviour of the evaporites is similar with the one observed in other compressive fronts. However, in the classical example of compressive front above salt layer, the evaporite forms either the main decollement level (as in the Zagros). In the north Ucayali basin, the salt pillows are isolated rather flat bodies, there are also deeper decollement levels. We have designed analogue models to discuss the influence of the depth and continuity of the salt pillows on the structure style.
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Salt tectonics
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Prism
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Recent analyses of multichannel seismic data for the Cascadia margin off central Oregon show a systematic, landward increase in velocity (increase in sediment dewatering) within a protothrust zone about 10 km wide seaward of the toe of the accretionary wedge (Cochrane et al., 1994). Here we analyze the mechanical conditions for the transmission of compression from the rear of the accretionary wedge to the area seaward of the frontal thrust to create the protothrust zone and to cause sediment dewatering. The analysis assumes that sediments are elastoplastic (Coulomb) material; i.e., they behave elastically up to a yield limit at which failure occurs. We show that the necessary conditions for the transmission of compression from the accretionary wedge to the seaward area are (1) the presence of incipient decollements seaward of the frontal thrust and (2) the presence of high pore pressures within the incipient decollement. We also show that the magnitude of pore pressure within the incipient decollement may control the width of the protothrust zone. We suggest that the latter determines the location of the next frontal thrust and hence the ramp spacing between the imbricated thrusts within accretionary wedges. Thus pore pressures within incipient decollements may significantly influence the evolution of the internal structures of accretionary wedges.
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Thrust fault
Continental Margin
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Borehole logs from the northern Barbados accretionary prism show that the plateboundary decollement initiates in a low-density radiolarian claystone. With continued thrusting, the decollement zone consolidates, but in a patchy manner. The logs calibrate a threedimensional seismic reflection image of the decollement zone and indicate which portions are of low density and enriched in fluid, and which portions have consolidated. The seismic image demonstrates that an underconsolidated patch of the decollement zone connects to a fluid-rich conduit extending down the decollement surface. Fluid migration up this conduit probably supports the open pore structure in the underconsolidated patch. on January 19, 2013 geology.gsapubs.org Downloaded from
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Anisotropy of magnetic susceptibility (AMS) results from sediments spanning the basal decollement of the Barbados accretionary prism show a striking progression across this structure that strongly supports the hypothesis that it is strongly overp ressured. In the accretionary prism above the decollement, the minimum AMS axes are subhorizontal and nearly east‐west trending, whereas the maximum AMS axes are nearly north‐south trending, and shallowly inclined. At the top of the decollement, the AMS minimum axes orientations abruptly change to nearly vertical; this orientation is maintained throughout the decollement and in the underthrust sediments below. The AMS orientations in the prism sediments above the decollement are consistent with lateral shortening caused by regional tectonic stress, as the minimum axes generally parallel the convergence vector of the subducting South American Plate, and the maximum axes are trench-parallel. This abrupt change in AMS orientations at the top of the decollement at Site 948 is a direct manifestation of mechanical decoupling of the off-scraped prism sed iments from the underthrust sediments. The decoupling horizon occurs at the top of the decollement zone, coinciding with the location of flowing, high-pressure fluids. Comparison with magnetic fabrics and susceptibilities of the seaward reference site (Site 672) indicates that the AMS fabrics at Sites 948 and 949 record the orientations of neocrystallized (Ti)magnetite and or (Ti)maghemite, and so reflect decoupling of differential stresses (and perhaps also strains) at the top of the decollement. Fur ther comparisons of susceptibility stratigraphy between sediments just above the lithostratigraphic Unit III/Unit II boundary at Sit es 672 and 948 suggest that the lower portion of the structurally defined decollement at Site 948 may in fact be largely intact. T his suggests that (1) there may be little displacement accommodated by sediments below about 498 mbsf; (2) the deformation structures observed in most of the decollement may have formed via low total strains (but perhaps under high strain rates?); an d (3) the basal decollement of the Barbados prism is a narrow plane (490 -492 mbsf), along which stresses are very effectively decoupled, rather than a thick zone of distributed deformation.
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Echelon formation
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The tectonic and deformation processes active during transfer of material in accretionary prisms are the results of a complex interplay of different factors. Among those, as demonstrated by studies on modern accretionary prisms, the mutual dependence between fluid circulation and sediment budget seems to play a crucial role in influencing the morphology and dynamics of both prisms and decollement zone.
In this work we report a detailed structural study of two examples of fossil prisms characterized by different sediment budgets, and representative of different accretionary processes: the Franciscan Complex (FC, N-California) and the Internal Ligurian Units (ULI, N-Apennines). Two fluid circulation models are proposed and their comparison suggests that sediment budget controls prism hydrogeology and tectonics.
The analyses of the two analogues have shown that, despite a different structural style, in both examples deformation is strongly controlled by repeated injection of overpressured fluids during underthrusting and accretion. Hydrofracturing occurs through dilatant fractures sub-parallel to the decollement zone, favored also by the wedge regional stress field. Fluid injection is transient, is associated with variation of local stress field at the decollement, and cyclical variation of permeability and cohesion states of rocks and sediments, allowing cyclical variation of deformation mechanisms and crosscutting veining episodes.
The sediment budget at trenches controls the fluid pathways, which are located along the sedimentary layers of the turbiditic deposits in the ULI. Moreover, the presence of thick sequences of sediments, which undergo modification while moving to depth, allows the deformation to be strongly influenced by diagenetic processes. While deformation mechanisms, hydrofracturing and veining are essentially “shear zone (i.e. tectonically)-controlled” in the FC, in the ULI they are syn-diagenetic, and change when mechanical and diagenetic conditions in the turbiditic sequence, such as lithification state and competency contrasts, change.
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Echelon formation
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The dynamics of accretionary convergent margins are severely influenced by intense deformation and fluid expulsion. To quantify the fluid pressure and fluid flow velocities in the Hellenic subduction system, we set up 2‐D hydrogeological numerical models following two seismic reflection lines across the Mediterranean Ridge. These profiles bracket the along‐strike variation in wedge geometry: moderate compression and a >4 km thick underthrust sequence in the west versus enhanced compression and <1 km of downgoing sediment in the center. Input parameters were obtained from preexisting geophysical data, drill cores, and new geotechnical laboratory experiments. A permeability‐porosity relationship was determined by a sensitivity analysis, indicating that porosity and intrinsic permeability are small. This hampers the expulsion of fluids and leads to the build up of fluid overpressure in the deeper portion of the wedge and in the underthrust sediment. The loci of maximum fluid pressure are mainly controlled by the compactional fluid source, which generally decreases toward the backstop. However, pore pressure is still high at the decollement level at distances <100 km from the deformation front, either by the incorporation of low permeability evaporites or additional compaction of the wedge sediments in the two profiles. In the west, however, formation of a wide accretionary complex is facilitated by high pore pressure zones. When compared to other large accretionary complexes such as Nankai or Barbados, our results not only show broad similarities but also that near‐lithostatic pore pressures may be easier to maintain in the Hellenic Arc because of accentuated collision, some underthrust evaporates, and a thicker underthrust sequence.
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