Hemin Koyi

Progressive unconformities within an evolving foreland fold–thrust belt, Zagros Mountains

A major angular unconformity between the Bakhtyari conglomerates and the underlying Agha Jari Formation has long been interpreted as indicating that orogeny in the Zagros Simply Folded Zone took place in Plio-Pleistocene times. This study uses field evidence of unconformities between older units in conjunction with geological maps and cross sections to argue that the front of the Zagros Simply Folded Zone has propagated in time and space. These unconformities indicate that deformation started as early as end Eocene in the northeast of the Simply Folded Zone and propagated progressively to the southwest, where unconformable contacts are only seen between younger units. As shortening continued, the southwest migration of the deformation front drove the foreland basin in front of it to its present position along the Persian Gulf and Mesopotamia. The climax of orogeny took place at end Pliocene time when the most extensive unconformity in the Zagros Simply Folded Zone developed between the (upper) Bakhtyari Formation and older units. Active seismicity and documented present uplift imply that the Simply Folded Zone is still propagating southwestward.

Modeling of thrust fronts above ductile and frictional detachments: Application to structures in the Salt Range and Potwar Plateau, Pakistan

Series of scaled sandbox models are used to simulate the development of thin-skinned simultaneous shortening above adjacent ductile and frictional substrates, These models simulate the evolution of the Potwar Plateau and Salt Range in Pakistan, where Pale

Effect of spatial distribution of Hormuz salt on deformation style in the Zagros fold and thrust belt: an analogue modelling approach

Scaled analogue models of thin-skinned simultaneous shortening above adjacent viscous and frictional décollements simulate the effect of Hormuz salt on the shortening in the Zagros fold and thrust belt. The models consisted of sand layers that partly overlay a viscous layer of silicone and were shortened from one end. Spatial distribution of the viscous décollement varied along strike and dip, as occurs in part of the Zagros fold and thrust belt. In this belt, Phanerozoic sedimentary cover was shortened partly above the Hormuz salt lying on the Precambrian crystalline basement, behaving as a basal viscous décollement. Model results display how the nature of the décollement affects the evolution of an orogenic belt. Using model results, we explain the development of deflection zones, and discuss strain partitioning, formation of different topographic wedges and differential sedimentation along the Zagros fold and thrust belt. Model results suggest the formation of a gentle taper, consisting of both foreward and backward thrusts above a viscous décollement and a relatively steeper taper consisting only of forward-vergent imbricates above a frictional décollement. However, in our models, the steepest wedge with the highest topography formed where the viscous substrate had a limited extent with a transitional boundary (pinch-out) perpendicular to the shortening direction. Shortening of this boundary led to development of frontal ramps associated with significant uplift of the area behind the deformation front.

Mode of internal deformation in sand wedges

Sequential sections of a sand box model are used to quantify displacement along imbricate surfaces and their rotation and volume loss history within an accreting sand wedge. Model results show that ...

Three-dimensional geometry and kinematics of experimental piggyback thrusting

The three-dimensional geometry and kinematics of piggyback stacks of imbricate thrust sheets are illustrated and discussed using a single model shortened in a squeeze box. Strike-parallel geometric elements simulated include lateral ramps, eyed sheath folds, splays, and thrust/thrust interference. Fine details of these structures were exposed by eroding a shortened wedge of sand using a newly developed vacuum-eroding technique. A kinematic analysis of the model shows a stepwise increase in imbricate thrust spacing and/or a decrease in rate of nucleation of imbricate thrusts in the direction of thrust transport. Despite the steady forward advance of a rear wall, the piggyback wedge accreted episodically, recording different strain domains in longitudinal cross sections. Strain partitioning in single layers by bed-length balancing showed an increase in layer shortening with volume loss and a corresponding decrease in imbricate thrusting and ramp folding with depth.

Experimental Modeling of Role of Gravity and Lateral Shortening in Zagros Mountain Belt

Dynamically scaled analogs of the geologic structures of the Zagros mountain belt are used to argue that different parts of the Zagros Mountains of Iran record different combinations of the effects of a gravity-driven overturn and a southwest-northeast lateral shortening superimposed on the Zagros overturn. Partially scaled material models have been used to simulate the Zagros geodynamics, which involve layer-parallel compression of a 6 to 7 km-thick Phanerozoic carbonate cover containing a pattern of preshortening diapirs. The folds in the Zagros form rapidly (1.5 mm/yr in a 20 to 30 km-wide zone), reactive some of the preshortening diapirs, and generate new synshortening listric diapirs. A third set of postshortening diapirs rises from the Hormuz decollement behind the fold-thrust front. Model buckle folds superimposed on diapirs or pillows tend to avoid and curve around preshortening diapirs, which flatten in the synclines. Model profiles show that lateral shortening induces residual salt at depth to flow toward and rise through the anticlinal cores as synshortening or postshortening diapirs. I suggest that any salt pillows in currently diapir-free zones of the Zagros fold-thrust belt may surface as diapirs through the anticlines in the future. In the absence of well data, seismic, and field observations, I have used only three map pattern criteria to recognize the timing of diapirs; these are locally obscured by extrusive salt sheets. Preshortening diapirs are generally small and elongate parallel to fold axes and are restricted to synclines. Synshortening diapirs are elongate perpendicular to the fold axes and are restricted to anticlines. Postshortening diapirs can be the same size as synshortening diapirs or larger and can be circular; they are more common in the anticlines.

Episodic accretion and strain partitioning in a model sand wedge

We model led thrust wedge accretion and deformation partitioning in a passively layered sand, detached and shortened above a smooth rigid decollement. The thrust wedge accretes in piggyback style during forward advance of a rear wall. The passively layered sand accommodates the shortening penetratively, by kinking/ramp folding, and by imbrication. In single layers, the style of compression changes with time and/or stratigraphic position within the sand prism. Initial penetrative layer shortening, above a slipped zone of decollement is succeeded by formation of monoclinal or conjugate shear bands at temporary terminations of decollement. The shear bands subsequently narrow down and lock as thrust ramps. The model sand wedge accrets episodically rather than steadily. Episodicity is controlled by the stick/slip mode of decollement propagation, and by volume loss and compaction of the wedge material in response to convergence. This results in formation of a wedge with a convex-upwards cross-sectional topography

Analogue benchmarks of shortening and extension experiments

Abstract We report a direct comparison of scaled analogue experiments to test the reproducibility of model results among ten different experimental modelling laboratories. We present results for two experiments: a brittle thrust wedge experiment and a brittleviscous extension experiment. The experimental set-up, the model construction technique, the viscous material and the base and wall properties were prescribed. However, each laboratory used its own frictional analogue material and experimental apparatus. Comparison of results for the shortening experiment highlights large differences in model evolution that may have resulted from (1) differences in boundary conditions (indenter or basal-pull models), (2) differences in model widths, (3) location of observation (for example, sidewall versus centre of model), (4) material properties, (5) base and sidewall frictional properties, and (6) differences in set-up technique of individual experimenters. Six laboratories carried out the shortening experiment with a mobile wall. The overall evolution of their models is broadly similar, with the development of a thrust wedge characterized by forward thrust propagation and by back thrusting. However, significant variations are observed in spacing between thrusts, their dip angles, number of forward thrusts and back thrusts, and surface slopes. The structural evolution of the brittle-viscious extension experiments is similar to a high degree. Faulting initiates in the brittle layers above the viscous layer in close vicinity to the basal velocity discontinuity. Measurements of fault dip angles and fault spacing vary among laboratories. Comparison of experimental results indicates an encouraging overall agreement in model evolution, but also highlights important variations in the geometry and evolution of the resulting structures that may be induced by differences in modelling materials, model dimensions, experimental set-ups and observation location.

Epicenter distribution and magnitude of earthquakes in fold‐thrust belts: Insights from Sandbox Models

Scaled analogue models are used to illustratethe eect of basal friction and erosion on fault activity andhence on epicentre distribution and magnitude of earth-quakes in the sedimentary cover of active fold-thrust belts.Model results suggest that in fold-thrust belts shortenedabove low-friction ductile decollements (rock salt or over-pressured mudstone), low- to moderate-magnitude earth-quakes (Mw=5:3−5:6), distributed over a wide area, occuralong several long-lived thrust faults. Conversely, in areasshortened above high-friction decollements large-magnitudeearthquakes (Mw=6:6−6:8), distributed over a narrowzone are likely to occur along few short- lived thrust ramps.Calculated magnitude of earthquakes from models and theirdistribution are in agreement with recorded earthquake pat-tern from the Zagros mountain belt, which is partially short-ened above a ductile decollement of Hormuz salt formation.Model results also showed that erosion reactivates older in-active thrusts and promotes formation of out-of-sequencethrusts.

The shaping of salt diapirs

Abstract Six parameters shape the geometry of passive diapirs associated with stiff overburden: rates of salt supply ( S ′); dissolution ( D ′; sediment accumulation ( A ′); erosion ( Er′ ); extension ( E ′); and shortening ( Sh′ ). These parameters change in space and time, and hence influence the geometry of the structure as it forms. A complex six-parameter plot, representing the evolution history of a salt diapir, can be simplified into three separate graphs. This study recommends adding to plots of S ′ against A ′, a second of rate of salt supply ( S ′), this time with extension rate ( E ′), and a third of rate of sediment accumulation ( A ′) plotted against extension rate ( E ′). Integrating these three plots on a single diagram results in a complete description of the evolution history of a diapir when it is applied to three-dimensional data. However, if applied to a profile, the plot shows only the two-dimensional evolution history of the diapir. Lateral forces (extension or compression) have a significant role in moulding the geometry of a salt diapir by influencing the space which it occupies. By incorporating extension in the moulding plots of salt diapirs, this study introduces the rate of sediment accumulation multiplied by extension (( E · A )′) as a significant factor in moulding salt diapirs. By using this rate against the rate of salt supply, this study redefines the conventionally accepted interpretation of upward-narrowing, upward-widening and columnar diapirs. Upward-narrowing diapirs form when the rate of salt supply is less than the rate of sediment accumulation multiplied by extension. Upward-widening diapirs form when the salt supply is greater than the rate of sediment accumulation multiplied by extension. Columnar diapirs form when the rate of salt supply is equal to the rate of sediment accumulation multiplied by extension. This new relationship explains the absence of columnar and upward-widening diapirs in passive margins where thin-skinned extension dominates, and emphasizes the significance of lateral movement (extension and shortening) in moulding the geometry of salt diapirs.