Progressive Thrusting Research Articles

San Albino, Nicaragua: A Low-Angle, Thrust-Controlled Orogenic Gold Deposit

Abstract The San Albino deposit is an orogenic gold occurrence hosted by a low-angle thrust that is the site of a new open-pit mine in northern Nicaragua. The deposit is hosted in greenschist facies rocks of the Jurassic metasedimentary Neuvo Segovia Formation. The schist was uplifted and exposed during arc accretion and Cretaceous thin-skin deformation, forming the NE-striking Colon fold-and-thrust belt. Deformation included emplacement of the 119 to 113 Ma NE-trending Dipilto batholith into the regionally metamorphosed clastic rocks about 5 km northwest of the San Albino deposit. Mineralization is dominated by three laminated quartz vein systems (i.e., San Albino, Naranjo, Arras) that broadly follow shallowly dipping (approx. 30°) carbonaceous shears roughly concordant to schistosity along the limbs of a doubly plunging antiform. The three main parallel shears are each separated by about 90 m and individually reach a maximum thickness of about 8 m. Maximum thickness of ore zones is where post-ore local folding and reverse motion along the shallow shears has duplicated the laminated low-angle gold-bearing veins (D2 and early D3). Additional gold was added to the veins, with abundant sulfides, during a subsequent brecciation event of the early formed quartz veins that accompanied progressive thrusting (late D3). This predated boudinage of the veins during continued compression and thrust loading (D4); high gold grades are particularly notable along pyrite- and arsenopyrite-bearing stylolites formed during D4 pressure solution. The D2 to D3 gold event is likely coeval with Albian uplift of the Dipilto batholith and with back thrusting in the schist aided by the stress inhomogeneities provided by the igneous complex. Low-angle thrust-controlled orogenic gold deposits may represent world-class exploration targets because of their large linear footprints, although they are traditionally looked at as less favorable exploration targets relative to gold systems developed more commonly along high-angle reverse faults. Our case study of the San Albino deposit shows that although low-angle deposits are not inherently misoriented for failure like the more common subvertical reverse fault-related deposits, they may be sites of significant pressure buildup due to hydrothermal mineral precipitation during initial water-rock interaction or slight temperature decreases along the low-angle flow path. Resulting fluid cycling may lead to thick laminated vein development, such as seen at San Albino, where especially high-grade zones may be associated with local steepening and/or dilational zones within the broader, low-angle vein-hosting shear system.

New insight on the structural architecture of northern Tunisia with a multidisciplinary approach: Association of Miocene piggy-back basin and active Plio-quaternary out-of-sequence thrusts

In order to better understand the intricate structural architecture of northern Tunisia, we use a multidisciplinary approach that includes field observations, geomorphic, sedimentological, and gravity data across this area. We offer a model of progressive thrusting in which four deformation phases occurred in the context of plate convergence (i) Uplift of the Jebel Ichkeul Triassic succession via a NE-trending thrust fault during the Eocene; (ii) Advance of Numidian thrust sheets during the late Oligocene-Middle Miocene, causing the formation of the Lake Ichkeul/Jalta as being piggy-back basin; (iii) Initial out-of-sequence thrusting during the Late Miocene, resulting in the formation of the Sejnane and El Mejel basins with syntectonic Pliocene infill; and (iv) Subsequent out-of-sequence thrusting during the Early Pleistocene, leading to the formation of the Oued (River) Zyatine basin with a syntectonic Quaternary deposits. We used the gravity data analysis to mark the thicknesses of syntectonic deposits and their bounding thrust faults. The kinematic analysis indicates the occurrence of progressive multiphase out-of-sequence thrusting events with thrusts verging to the southeast. The combination of climate and mechanical fractures induced gradually long-term damage and changed the original landscape of the evidenced out-of-sequence thrusts. Furthermore, the existence of local normal faults in the backlimbs and reverse faults in the forelimbs, together with the drainage network type, earthquakes, the slope erosion, and the aggraditional terrace system, implies a gradual landscape evolution of thrust topography. This physical evolution occurred following an active tectonics across the exhibited out-of-sequence thrusting since the Pliocene.

Deformation mechanisms accommodating progressive simple shear thrusting of quartzite and metacarbonate in the southwestern Espinhaço Range, Brazil

The accommodation of low-temperature ductile deformation in foreland fold-and-thrust belts is often described in terms of outcrop-based geometric analysis, but microtextural observations are also important, as they relate stress, strain, fluids and temperature and thus the rheology in the peripheral part of the orogen. Such microtextural observations are lacking from the foreland part of the Araçuaí orogen in Brazil, and are here investigated through EBSD-based textural analysis of quartzites and metacarbonate samples from the southwestern Espinhaço Range. The quartzites overthrust the metacarbonates, and deform solely by the activation of dissolution-precipitation creep, whereas the metacarbonates show a much greater variety of deformation mechanisms that closely relate to grain-size, composition and strain, in the long (low-angle) and short (steep) limbs of shear-related folds. The metacarbonates show a centimeter-scale alternation of coarse-grained domains of calcite and quartz, and fine-grained domains of a mix of phases (calcite, dolomite, phyllosilicates, quartz, and apatite). In the strongly sheared long limbs, coarse calcite was deformed by dislocation creep with slip on (c)<a>, whereas fine calcite displays evidence of oriented dissolution-precipitation creep, with both cases displaying <c> axis orientations consistent with the top-to-the-west thrusting. Coarse calcite shows evidence of recovery by subgrain rotation and grain boundary migration. Coarse-grained calcite in the short limbs was deformed by dislocation creep, assisted by twin boundary migration recrystallization with twinning on {e}, indicating simple shear-dominated deformation. In contrast, fine-grained calcite was deformed by fluid-assisted grain boundary sliding. Dissolution-precipitation in both short and long limbs produced random textures, with the exception of coarse quartz in the short limbs, where dissolution-precipitation creep led to <c> axis orientation parallel to X. This study demonstrates that the foreland thrust zone of the Araçuaí orogen underwent consistent W-directed progressive thrusting, and that this deformation produced a wide variety of texture types in the metacarbonates while a simpler structural and textural pattern developed in adjacent quartzitic metasediments.

Fault‐related fold styles and progressions in fold‐thrust belts: Insights from sandbox modeling

AbstractFault‐related folds of variable structural styles and assemblages commonly coexist in orogenic belts with competent‐incompetent interlayered sequences. Despite their commonality, the kinematic evolution of these structural styles and assemblages are often loosely constrained because multiple solutions exist in their structural progression during tectonic restoration. We use a sandbox modeling instrument with a particle image velocimetry monitor to test four designed sandbox models with multilayer competent‐incompetent materials. Test results reveal that decollement folds initiate along selected incompetent layers with decreasing velocity difference and constant vorticity difference between the hanging wall and footwall of the initial fault tips. The decollement folds are progressively converted to fault‐propagation folds and fault‐bend folds through development of fault ramps breaking across competent layers and are followed by propagation into fault flats within an upper incompetent layer. Thick‐skinned thrust is produced by initiating a decollement fault within the metamorphic basement. Progressive thrusting and uplifting of the thick‐skinned thrust trigger initiation of the uppermost incompetent decollement with formation of a decollement fold and subsequent converting to fault‐propagation and fault‐bend folds, which combine together to form imbricate thrust. Breakouts at the base of the early formed fault ramps along the lowest incompetent layers, which may correspond to basement‐cover contacts, domes the upmost decollement and imbricate thrusts to form passive roof duplexes and constitute the thin‐skinned thrust belt. Structural styles and assemblages in each of tectonic stages are similar to that in the representative orogenic belts in the South China, Southern Appalachians, and Alpine orogenic belts.

Strain localization, granulite formation and geodynamic setting of ‘hot orogens’: a case study from the Eastern Ghats Province, India

The Eastern Ghats Province underwent major orogenic events in the Neoproterozoic–Cambrian period: 980–930, 900–650 and 550–500 Ma. At each time interval, deformation occurred synchronous with high‐grade metamorphism. The first event was characterized by distributed strain throughout the entire province, while strain and thermal anomalies during the later orogenies were confined to the province boundaries. In the first case, pre‐orogenic rifting caused ultrahigh‐temperature metamorphism at the base of the crust, and was closely followed by crustal shortening related to the collision of the granulite belt with the Indian craton. The thermal anomaly persisting from the rifting event, followed by the thermal relaxation associated with crustal shortening led to prolific melt production and transfer of heat producing elements (HPE) into the middle and upper parts of the thick post‐collisional crust. Heating and associated melting caused rheological weakening of the crustal section, and explains why strain was distributed across the entire thermally perturbed zone. Erosion removed a substantial portion of the HPE‐rich upper crust, and deposited the detritus in cratonic sedimentary basins to the west. Subsequently, the rheologically stronger Eastern Ghats Province crust could effectively transmit stresses and concentrate them along pre‐existing zones of weakness near the province boundaries. Progressive thrusting in these domains led to consistent loading of the footwall, even as it underwent thermal relaxation. Since both strain and heating were associated with the footwall of pre‐existing discontinuities, the ‘hot orogen’ so formed was also spatially restricted. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Structural style of the Marathon thrust belt, West Texas

The Marathon portion of the Ouachita thrust belt consists of a highly deformed allochthonous wedge of Cambrian-Pennsylvanian slope strata (Marathon facies) that was transported to the northwest and emplaced over Pennsylvanian foredeep sediments. The foredeep strata in turn overlie early-middle Paleozoic shelfal sediments which are deformed by late Paleozoic basement-involved reverse faults. The Dugout Creek thrust is the basal thrust of the allochthon. Shortening in this sheet and overlying sheets is ∼80%. Steep imbricate faults link the Dugout Creek thrust to upper level detachments forming complex duplex zones. Progressive thrusting and shortening within the allochthon folded the upper level detachments and associated thrust sheets. The Caballos Novaculite is the most competent unit within the Marathon facies and controlled development of prominent detachment folds. Deeper imbricate sheets composed of the Late Pennsylvanian foredeep strata, and possibly early-middle Paleozoic shelfal sediments developed concurrently with emplacement of the Marathon allochthon and folded the overlying allochthon. Following termination of thrusting in the earliest Permian, subsidence and deposition shifted northward to the Delaware, Midland and Val Verde foreland basins.

Generation and modification of finite strain patterns by progressive thrust faulting in the Laksefjord Nappe, Finnmark

Using deformed conglomerates as finite strain markers, a strain map of the base of the Laksefjord Nappe has been produced and is presented. Finite strains are predominantly of flattening type and are explained in terms of a variable simple shear superimposed on a constant pure shear. Strain patterns are examined in terms of their three dimensional geometry and are shown to be clearly related to the presence of the basal Laksefjord thrust. Late, brittle thrust faulting has modified early finite strain patterns and a progressive thrusting model is presented to explain present-day strain patterns. Finite strains developed in a mid-crustal, sub-horizontal shear zone at lower greenschist facies account for only a small proportion of total nappe translation.