Luca Clemenzi

Anatomy and paleofluid evolution of laterally restricted extensional fault zones in the Jabal Qusaybah anticline, Salakh arch, Oman

The E-W−trending Jabal Qusaybah anticline, at the western termination of the Salakh arch, Oman Mountains, is characterized by a complex fault network that developed in layered Cretaceous carbonates. This network includes NE-SW left-lateral, N-S extensional, and subordinate E-W extensional fault zones. The N-S−striking extensional faults zones are roughly perpendicular to the fold axis and are best developed in the longitudinally bulged central sector of the anticlinal crest. They are likely due to along-strike outer-arc extension associated with positive fault inversion and salt migration. These extensional fault zones are confined within, and locally abut, major NE-SW left-lateral strike-slip fault zones. Extensional fault displacements range between a few decimeters and ∼60 m, whereas the maximum exposed trace lengths range between a few meters and ∼800 m. Narrow (∼1−15-cm-thick) cataclastic fault cores are surrounded by vein-dominated damage zones as thick as tens of meters. Moreover, fault zones show widespread evidence for substantial dilation in the form of (1) dilation breccias, (2) infilling by large columnar calcite crystals and aggregates, and (3) centimeter- to meter-thick veins. Dilation breccias and calcite infillings are primarily localized at fault tips, fault overlaps, and interaction zones between strike-slip and extensional fault segments. Displacement profiles along the N-S−striking extensional fault zones indicate that they are one order of magnitude shorter than values predicted by most published displacement-length scaling laws. By analyzing fault abutting geometries, detailed vein relative chronology, δ13C and δ18O signatures, and fluid inclusion data from calcite veins and calcite fault infillings, we propose a model whereby a deep-seated, regionally sized, left-lateral strike-slip fault system that was active during anticline growth inhibited the lateral propagation of late-stage transversal extensional fault zones. Our findings show that, in this geological setting, the structural position, rather than fault displacement, is the parameter controlling the location of the more dilatants (and permeable) fault segments. Results of the present work suggest that fault intersections may be more useful than fault throw for predicting zones of enhanced vertical fluid flow in structurally complex carbonate reservoirs.

Impact of erosion and décollements on large-scale faulting and folding in orogenic wedges: analogue models and case studies

Deformation mechanisms, long-term kinematics and evolution of fold and thrust belts subjected to erosion are studied through 2D analogue experiments involving large convergence. First-order parameters tested include (1) décollements and/or plastic layers interbedded at different locations within analogue materials and (2) synconvergence surface erosion. Weak layers, depending on their location in the model, favour deformation partitioning characterized by the simultaneous development of underplating domains in the inner part of the wedge (basal accretion) and frontal accretion where the wedge grows forward. Interaction between tectonics and surface processes influences this behaviour. Development of antiformal thrust stacks controlled by underplating shows small- and large-scale cyclicity. Thin plastic layers induce folding processes, which are studied at wedge scale. Recumbent and overturned folds, with large inverted limbs, develop in a shear-induced asymmetric deformation regime via progressive unrolling of synclinal hinges. Surface erosion and underplating at depth induce further rotation (passive tilting) and horizontalization of fold limbs. Model results give insights to discuss the mechanisms responsible for the large-scale structures (i.e. antiformal nappe stacks, klippen and kilometre-scale recumbent fold–nappes) encountered in several mountain belts such as the Montagne Noire (French Massif Central), the Galicia Variscan belt (Spain) and the northern Apennines (Italy). Supplementary material: Raw data of the experiments are available at www.geolsoc.org.uk/SUP18658.

Complex fault-fold interactions during the growth of the Jabal Qusaybah anticline at the western tip of the Salakh Arch, Oman

The Jabal Qusaybah anticline is located at the western end of the Salakh Arch, a major salient in the foothills of the Oman Mountains. We performed a structural and petrographical-geochemical study of vein sets and fault zones associated with the development of this anticline. Our data illustrate a complex deformation pattern both in space and time, characterized by the unusual presence of widespread NE-SW left-lateral strike-slip fault zones trending oblique to the E-W fold axial strike, and of abundant and well-developed N-S fold-perpendicular extensional fault zones associated with axial bulging and dilation, well developed in the central region of the anticlinal crest. We propose a three-stage evolution for the Jabal Qusaybah anticline, starting with prefolding jointing in the foreland of the late Cretaceous Oman Mountains, and followed by development of extensional faulting in Campanian times. Positive inversion of the Qusaybah Fault, possibly in Miocene times, caused development of a layer-parallel shortening fabric and amplification the Jabal Qusaybah Anticline, in concomitance with the activity of NE-SW left-lateral strike-slip fault zones that triggered N-S, fold-perpendicular extensional faulting, particularly in the axial bump of the anticline. The final evolutionary stage was characterized by further amplification of the axial bump and related N-S extensional fracturing and by uplift and exhumation. To explain the complex noncylindrical fault-fold interactions in the study anticline, we tentatively propose that they were triggered by near foredeep-parallel tapering of the sedimentary/tectonic overburden of the Ara evaporites.

Fluid pressure cycles, variations in permeability, and weakening mechanisms along low-angle normal faults: the Tellaro detachment, Italy

Classical frictional fault reactivation models indicate that slip along misoriented fault planes is not possible under most conditions. Nevertheless, active or exhumed low-angle normal faults have been described in many settings worldwide. This discrepancy is addressed by contrasting models: (1) those proposing that low-angle normal faults result from postkinematic passive rotation of former high-angle extensional faults; and (2) those proposing that specific conditions can promote slip along misoriented fault planes. This paper describes the Tellaro detachment, a mid–late Miocene low-angle normal fault that was responsible for ∼500 m of tectonic vertical thinning in the carbonate-dominated Triassic to Lower Miocene succession of the Northern Apennines, Italy. By integrating structural, petrographic, isotopic, and fluid inclusion data, we show that: (1) the main kinematic activity of the Tellaro detachment occurred between ∼8 and 4 km depths and peak temperature ∼190 °C; (2) dilational breccias, tens of cubic meters in volume, are frequently associated with major low-angle fault segments; (3) slip along misoriented planes was favored by elevated fluid pressures and low differential stress; and (4) the fault system was characterized by transient permeability pulses and overpressure buildups, associated with multiple fracturing and cementation events that caused the downward migration of master slip surfaces. Results presented in this study show that: (1) in a fluid-active regime, continental crustal thinning can occur for shallow values of fault dip; (2) low-angle normal faults have a great influence on fluid circulation within the upper crust; and (3) episodic permeability enhancement and destruction in detachment faults can promote overpressure buildups, triggering deformation episodes.