Influence of Syntectonic Sedimentation and Décollement Rheology on the Geometry and Evolution of Orogenic Wedges: Analog Modeling of the Kuqa Fold‐and‐Thrust Belt (NW China)

Abstract

Contractional deformation in the outer parts of fold‐and‐thrust belts is in part controlled by the presence of syntectonic sediments and multiple décollements (e.g., the Apennines, the Appalachians, the Pyrenees, the Zagros, or the Sub‐Andean and Kuqa fold‐and‐thrust belts). To better understand the influence of these parameters in the kinematic evolution of fold‐and‐thrust systems, we carried out an experimental study including four 3‐D sandbox models inspired by one of the previously mentioned prototypes, the Kuqa fold‐and‐thrust belt. This belt contains two décollements: a weak synorogenic salt layer and a deeper, preorogenic, and frictionless décollement (i.e., organic‐rich shales) showing along strike variations of rheology. The experimental results show that increasing synkinematic sedimentation rate (i) generates a progressive change from distributed to localized deformation and (ii) delays the development of frontal contractional structures detached on the salt, favoring the formation and reactivation of more hinterland thrusts and backthrusts. With respect to the rheology, our study reveals that as the viscosity of the prekinematic décollement increases, (i) the deformation propagates more slowly toward the foreland, and (ii) the underlying thrust stack becomes broader and lower and has a gentler thrust taper angle. The rheology of the prekinematic décollement defines the distribution and geometry of the structures detached on it that in turn influence the development of overlying, salt‐detached structures. Subsalt structures can (i) determine the areal extent of the salt and therefore of any fold‐and‐thrust system detached on it and (ii) hamper or even prevent the progressive foreland propagation of deformation above the salt.

Influence of Overlapping décollements, Syntectonic Sedimentation, and Structural Inheritance in the Evolution of a Contractional System: The Central Kuqa Fold‐and‐Thrust Belt (Tian Shan Mountains, NW China)

“…Shortening in basement was transferred via the salt to suprasalt folding and thrusting localized mostly in the Qiulitage and Tuzimaza structures that were separated from each other by the broad Baicheng syncline. Analogue (Duerto & McClay, ; Pla et al, ) and numerical models (Fillon et al, ) demonstrate that localization of shortening in a few, long‐lived, and widely spaced structures results from the increase in syntectonic sedimentary thicknesses (Figure b). Similar geometries are found in other natural analogues such as the south Central Pyrenees (Muñoz, ) that are characterized by a thick succession of syntectonic strata deformed by long thrusts and a wide wedge‐shaped basin transported over an underlying décollement.…”

“…The sequential restorations (Figures b and c) suggest that the salt‐detached structures were initially located above coal‐detached thrusts, with the hinges of suprasalt and presalt folds being slightly shifted. This structural style is common in (although not exclusive of) contractional scenarios where salt thicknesses are comparable to those of the overlying suprasalt units (such as the Zagros (Fard et al, ) and can be simulated in sand‐silicone analogue models (Pla et al, ; Figure ). The geometries result from a combination of disharmonic folding and salt flow from above the hinges of presalt anticlines toward the cores of suprasalt anticlines, the latter locally evolving to thrusted folds and diapirs (Fard et al, , Figure b).…”

“…Analogue models (from Pla et al, ) displaying key deformation features that are recognized in the studied cross section. Models consist of (1) a precontractional sand package that thins toward the foreland and contains a thin interbedded silicone layer (representing the Jurassic coal in the Kuqa fold‐and‐thrust belt) and (2) a syncontractional sequence involving a basal silicone plate (analogous to the Kumugeliemu salt in the Kuqa fold‐and‐thrust belt).…”

“…Modeling results also show that thrusts initiate at the point where the total work needed to slide on the décollement and to break through the overburden is minimal (Fillon et al, ; Hardy et al, ). This location could be either where the overlying strata have the minimum thickness (Fillon et al, ) or at the frontal termination of the weak décollement (Pla et al, ). In the Kuqa fold‐and‐thrust belt, the foreland salt pinchout controlled the locus of the frontalmost structures (Figures and b) and the earlier development of the southern Qiulitage anticline with regard to the northern one (Figure ).…”

Rotational Kinematics of Basement Antiformal Stacks: Paleomagnetic Study of the Western Nogueras Zone (Central Pyrenees)

“…The first mechanism requires that the décollement separating cover and basement units remained inactive and allowed to fully transfer rotations from the basement to the cover (i.e., rotation values in cover and basement units being equal). Several plausible reasons can be invoked for that mechanism, such as the décollement being too thin in comparison to the overlying cover units (Pla et al, ) or the deformation velocity being too fast to permit its fully ductile behavior (Couzens‐Schultz et al, ). Besides, the early compartmentalization of the décollement during precontractional stages (due to basement faulting or early evaporite flow; López‐Mir et al, ; McClay et al, ) is an additional factor that can considerably diminish décollement effectiveness.…”