Decollement Tectonics Research Articles

Extensional tectonic setting of the Triassic metallogenic event in the Xiaoqinling goldfield: Evidence from <sup>40</sup>Ar-<sup>39</sup>Ar chronology of the tectonite

小秦岭金矿田位于华北克拉通南缘，同时也是秦岭复合型造山带的北缘组成部分。这里是我国第二大黄金产地，大规模金的成矿作用形成于早白垩世岩石圈大规模减薄的区域伸展构造背景。此外，越来越多的同位素年代学数据显示区内还存在三叠纪的成矿事件，发育钼、铅、铀、铌等与岩浆热液活动密切相关的多金属矿床，主要分布在小秦岭的南北边缘。目前，对于三叠纪多金属成矿作用的构造背景研究尚属薄弱，还没有直接对有关的构造变形开展过专题研究。为此，我们对小秦岭金矿田东端边缘的构造带进行构造观察和测量，并采用⁴⁰Ar-³⁹Ar年代学方法厘定变形时限，揭示三叠纪成矿事件的构造背景，为深入认识区内三叠纪多金属成矿作用特征和规律提供构造证据。研究表明，小秦岭金矿田三叠纪成矿事件发生于晚三叠世的伸展构造背景，可以划分为两个阶段，早期（221~216Ma）是在碰撞向后碰撞转折阶段的重力滑脱构造环境中，发育火成碳酸岩型钼铅、铀铌铅多金属矿床；晚期（214~203Ma）是在后碰撞阶段由俯冲板片断离或岩石圈拆沉作用等深部过程导致的区域伸展构造环境中，发育石英脉型和斑岩型钼矿床。小秦岭中生代晚三叠世和早白垩世两期成矿事件都是在伸展构造背景中的构造-岩浆-流体活动的产物。;The Xiaoqinling goldfield is located in the southern margin of the North China craton, which is also a marginal part of the Qinling complex orogeny. This goldfield is the second largest gold producer in China, where the large-scale gold mineralization developed in the extensional tectonic setting due to regional lithospheric thinning in Early Cretaceous. In addition, a growing number of isotope chronological data indicates that there also developed metallogenic events in Triassic here, which resulted in the formation of polymetallic deposits, such as molybdenum, lead, uranium, and niobium, etc. The Triassic deposits are closely related to magmatic hydrothermal activities, which are mainly distributed in northern and southern margins of the Xiaoqinling goldfield. However, there is little study on the tectonic setting of the Triassic polymetallic mineralization so far, especially the work on tectonic deformation is almost absent. Therefore, we have carried out the observation and measurement on the structure developed within the tectonic belt occurred in the eastern end of Xiaoqinling goldfield, and then employed the ⁴⁰Ar-³⁹Ar chronology method to determine the time limit of deformation, to reveal the tectonic setting of Triassic metallogenic event, providing tectonic evidence for further understanding characteristics and patterns of Triassic polymetallic mineralization in the Xiaoqinling goldfield. Our study shows that the metallogenic event occurred in the extensional tectonic setting in Late Triassic, which can be divided further into two stages. The early stage (221~216Ma) is of the transition stage from collision to post-collision, developing gravitational decollement tectonics, and carbonatite-type molybdenum, lead, uranium, and niobium polymetallic deposits are well developed in this extensional tectonic setting. The late stage (214~203Ma) is of post-collision, developing regional extensional tectonics caused by the break off of subducting slab or lithospheric delamination, and quartz vein-type and porphyry molybdenum deposits are well developed. The two metallogenic events in Late Triassic and Early Cretaceous in the Xiaoqinling goldfield are the products of the tectonic deformation-magmatic activity-fluid flow in extensional tectonic setting.

Fabric development in a weathering profile at a basement–cover interface, the sub-Cambrian peneplain, Israel: Implications for decollement tectonics

The present study examines a fossil saprock–saprolite–laterite-profile beneath the sub-Cambrian peneplain in the Pan-African Roded Granite, Israel, with regard to structure and magnetic fabrics (anisotropy of magnetic susceptibility, AMS), and image analysis of compaction. The deformed granite shows two pre-weathering foliations, S1m (magmatic) and S2g (gneissic). Pre-Early Cambrian weathering comprised weathering-brecciation in saprock and saprolite, and chemical weathering with clay-formation in saprolite and laterite. During subsequent Phanerozoic burial the laterite was vertically compacted to 73% of its original thickness. In the laterite, compaction produced an unconformity-parallel cleavage (S3d) with increasing intensity towards the unconformity. Bulk susceptibility (κbulk) and anisotropy (P′) decrease from the unweathered granite into the saprolite, as a result of progressive magnetite breakdown, martitization and weathering-brecciation. In the laterite, an enrichment of haematite and relic Fe–Mg–mica lead to increased κbulk. Here, magnetic fabrics trace the compaction fabrics. The subhorizontal, compactional clay–/mica-fabric S3d defines a structurally weak and impermeable layer. The mechanical weakness of a clay-enriched weathering horizon with an unconformity-parallel, planar shape-preferred orientation, combined with the potentially overpressured state due to the sealing character of such a zone provides a viable explanation for the abundant localization of decollement horizons at or beneath basement-cover interfaces.

The Lackawanna synclinorium, Pennsylvania: A salt-collapse structure, partially modified by thin-skinned folding

The Lackawanna synclinorium is a 110-km-long structural trough in the Appalachian foreland of northeastern Pennsylvania. In map view, its hinge displays a concave-to-the-foreland (i.e., concave toward the west) curve. Historically, the entire synclinorium was thought to be an Alleghanian thin-skinned contractional structure that formed similarly to the fold trains of the central Appalachian Valley and Ridge province. Interpretation of seismic reflection data across the structure, however, suggests that the synclinorium formed primarily by the removal of salt. The trend of the central and northern synclinorium reflects the location of Upper Silurian Salina salt. In the southern synclinorium, the salt-collapse structure translated northwestward over a thrust ramp joining detachments in the Cambrian Waynesboro Formation and the Salina Group. Interpretation of seismic reflection data also suggests that the basement-cover contact deepens westward and that basement-penetrating reverse faults and a Neoproterozoic rift basin are present beneath the synclinorium. We speculate that (1) partial barriers due to sedimentation and/or reef formation, and (2) localized subsidence over a Neoproterozoic rift zone may have formed a Late Silurian restricted basin that allowed thick accumulations of Salina salt. Regional-scale salt/shale flow and/or foreland-directed (i.e., westward) fluid migration during the Alleghanian orogeny might have removed as much as 700 m of salt. The removal of this salt caused the overlying strata to collapse downward and form the synclinorium. Only in the southern synclinorium was the collapse structure modified significantly by decollement tectonics. Thus, the map-view curvature of the structure reflects greater tectonic shortening and westward translation (toward the foreland) in the southern synclinorium compared to the northern synclinorium.

Frontal belt curvature and oblique ramp development at an obliquely collided irregular margin: Geometry and kinematics of the NW Taiwan fold‐thrust belt

Combined structural and tectonic analyses demonstrate that the NW Foothills of the Taiwan collision belt constitute mainly an asymmetric “primary arc” type fold‐thrust belt. The arcuate belt developed as a basin‐controlled salient in the portion of the foreland basin that was initially thicker, due to the presence of a precollisional depocenter (the Taihsi basin). Additional but limited buttress effects at end points related to interaction with foreland basement highs (Kuanyin and Peikang highs) may have also slightly enhanced curvature. The complex structural pattern results from the interaction between low‐angle thrusting related to shallow decollement tectonics and oblique inversion of extensional structures of the margin on the southern edge of the Kuanyin basement high. The tectonic regimes and mechanisms revealed by the pattern of paleostress indicators such as striated outcrop‐scale faults are combined with the orientation and geometry of offshore and onshore regional faults in order to accurately define the Quaternary kinematics of the propagating units. The kinematics of this curved range is mainly controlled by distributed transpressional wrenching along the southern edge of the Kuanyin high, leading to the development of a regional‐scale oblique ramp, the Kuanyin transfer fault zone, which is conjugate of the NW trending Pakua transfer fault zone north of the Peikang basement high. The divergence between the N120° regional transport direction and the maximum compressive trend that evolved from N120° to N150° (and even to N–S) in the northern part of the arc effectively supports distributed wrench deformation along its northern limb during the Pleistocene. The geometry and kinematics of the western Taiwan Foothills therefore appear to be highly influenced by both the preorogenic structural pattern of the irregularly shaped Chinese passive margin and the obliquity of its Plio‐Quaternary collision with the Philippine Sea plate.

The growth of orogenic belts and the role of crustal heterogeneities in decollement tectonics

Research Article| December 01, 1995 The growth of orogenic belts and the role of crustal heterogeneities in decollement tectonics Dennis L. Harry; Dennis L. Harry 1Department of Geology, University of Alabama, Box 870338, Tuscaloosa, Alabama 35487-0338 Search for other works by this author on: GSW Google Scholar John S. Oldow; John S. Oldow 2Department of Geology and Geophysics, Rice University, P.O. Box 1892, Houston, Texas 77251 Search for other works by this author on: GSW Google Scholar Dale S. Sawyer Dale S. Sawyer 2Department of Geology and Geophysics, Rice University, P.O. Box 1892, Houston, Texas 77251 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1995) 107 (12): 1411–1426. https://doi.org/10.1130/0016-7606(1995)107<1411:TGOOBA>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Dennis L. Harry, John S. Oldow, Dale S. Sawyer; The growth of orogenic belts and the role of crustal heterogeneities in decollement tectonics. GSA Bulletin 1995;; 107 (12): 1411–1426. doi: https://doi.org/10.1130/0016-7606(1995)107<1411:TGOOBA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A series of finite element models of contractional deformation are presented that examine the role preexisting mechanical heterogeneities within the lithosphere play in controlling the partitioning of stress and strain during orogenesis. The models focus particularly on orogens that develop near previously rifted continental margins, although many of the observations are relevant to other fold and thrust belts. The models indicate that orogen evolution is strongly influenced by crustal architecture during the first 150 to 200 km of shortening. Major decollements develop at midcrustal and lower crustal levels, partitioning strain into upper crustal, lower crustal, and mantle strain domains. The magnitude and spatial distribution of strain within each domain are nearly uniform but differ significantly between domains. The decollements extend throughout the entire width of the orogen, providing a successively increasing degree of decoupling between deformation in the shallow crust and deeper levels. The midcrustal decollement terminates beneath the leading edge of the thrust front, but the deep crustal decollement extends more than 500 km beyond the foreland. This allows up to 5%–10% strain to develop well in front of the orogenic belt. In the interior of the orogen, deformation extends through all levels of the crust beginning during the earliest stages of contraction. Deformation propagates continentward with increasing shortening, with the foreland regions undergoing the most intense shortening in the shallow crust. The loci of maximum total shortening within the upper and lower crust are laterally offset, with the thickest crust lying continentward of the greatest elevations and the site of most intense surface deformation. This pattern of crustal thickness variation is a primary feature of orogenesis in the models and is not related to postorogenic collapse. After 150 to 200 km of shortening, sufficient topography develops to cause the balance between rock strength, gravitational forces, and shear stress on the basal detachment to dominate the style of deformation. Preexisting mechanical heterogeneities thereafter exert little influence on orogenic processes. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Neotectonic state of stress in the Jura Mountains

AbstractFor the determination of the tectonic stress field in the region of the Jura Mountains, three different relief techniques have been used in 18 test sites. In six test sites measurements have been carried out in horizontal boreholes using the doorstopper method. In the remaining test sites measurements were made in vertical boreholes by the doorstopper method (8), triaxial strain cell method (3) and borehole slotter (1). Additionally, analyses of borehole breakouts from six deep wells are presented.Orientation of maximum horizontal stress clearly deviates from the uniform NW-SE orientated central and western European stress field. Local stress sources, which are probably related to active decollement tectonics in the Jura Mountains, may explain the deviation in stress orientations in comparison to the foreland. Normal stress magnitudes perpendicular to the trend of compressional structures in the Folded Jura are largest in the southeastern Upper Rhine Graben and its southwards prolongation into the...

Tracing the Sole of a Thrust Through Thick and Thin of Salina Group (Upper Silurian): Decollement Tectonics of Southern Tier, New York: ABSTRACT

The elusive decollement of the Allegheny Plateau is identified by repeat stratigraphic sections on gamma-ray logs, and is mapped along the Southern Tier of New York. The detachment surface is found at three progressively lower stratigraphic levels from east to west within the Salina Group. In the eastern counties, Chemung, Schuyler, Tompkins, and Tioga, Unit F (salt) sections of the Syracuse Formation are vertically repeated two and three times along splays of the main thrust fault residing within Unit F. Fault throws as great as 400 ft (120 m) are shown on gamma-ray logs. Correlations provide evidence for overthickened sections, which are accounted for by imbricate thrusting of the section, thereby stacking the salt beds atop each other over decollement. The three-dimensional dome shape of the Syracuse Formation with planar base supports a thin-skinned tectonic origin and negates the primary depositional genesis. The thrust was upward to the northwest, and subsurface faults are associated with folds at the surface. In Steuben and Allegany Counties, the overthickened Unit E sections of the Syracuse Formation again suggest the presence of stacked repeat sections rather than a depocenter. The detachment surface is near the base of Unit E. Mapping indicates that the rocks from the northwest have slid and overridden those to the southeast in the central region. To the west in County, decollement terminates in a structure with northeast strike, here named the Chautauqua anticline. This prominent subsurface thrust zone is of smaller proportions, but structurally comparable to the Burning Springs anticline of West Virginia. At the northwest edge of the underlying salt beds, the horizontal decollement bends upward into the Upper Silurian and Lower Devonian rocks, eventually dying out in the fissile shales of the Hamilton Group. Thin-skinned slippage has occurred on at least two planes--those of the evaporite beds in the Syracuse and Vernon Formations. The anticline forms the structural trap for the targets of recent plays in the Bass Islands and Akron dolostones of the Rondout Group overlying the Salina. End_of_Article - Last_Page 1452------------

A new type of decollement thrusting

One of the fundamental rules of decollement tectonics is that decollement horizons form in mechanically weak layers. Here we document two examples of decollement-style thrust faults that detach within thick platformal dolostones in preference to apparently weaker layers of shale, siltstone and limestone underlying the dolostones. The thrusts are the Keystone–Muddy Mountain–Glendale thrust system (the KMG thrust system), , and the underlying Contact–Red Spring–North Buffiington–Mormon thrust system (the CRM thrust system), both of southern Nevada. They form part of the Sevier orogenic belt, and extend for roughly 250km along strike, together showing at least 65 km tectonic overlap (Fig. 1). Although the thrust systems are closely related spatially, the higher KMG system is younger than the underlying CRM thrust system, and the two developed essentially independently. Three points are important: first, that both the KMG and the CRM thrust systems are decollement style thrusts; second, that the decollement horizon is primarily restricted to a narrow stratigraphical interval within a bedded sequence of essentially homogeneous dolostones of the Middle Cambrian Bonanza King Formation; the third, that the thrust faults formed at a very shallow crustal level (<5 km).

Structural Revelations from Seismic Interpretation, Southern Tier, New York: ABSTRACT

When published structure contour maps of the Devonian outcrop, Tully formation, base Hamilton Group, and Oriskany Sandstone are overlain, the locations and trends of anticlinal and synclinal axes are not coincident, suggesting a lack of vertical continuity. The traces of fold axes within these specific stratigraphic units and their vertical orientation are not well defined by previous work. In order to develop a cohesive, three-dimensional structural model for the Trenton Group (Middle Ordovician) through the Tully formation (Middle Devonian) interval, seismic surveys from six prospect areas in the southern tier were interpreted and well data from over 2,000 wells were used to make computer-drawn geologic structure and isopach maps as well as cross sections. The stratigraphic framework of such an analysis is built upon the strong seismic reflectors, in this case carbonates, specifically the Tully formation, Onondaga Formation, Lockport Group, and Trenton Group. Subsurface facies were identified for the thickest intervals, namely the Hamilton, Salina, Lockport, and Clinton Groups, as well as the Queenston-Oswego-Lorraine sequence. The lithologic facies analysis is then used in conjunction with the seismic sections to define seismic facies. The importance of seismic facies is that they may reveal how certain lithologies deform in the stack of End_Page 1164------------------------------ subsurface sedimentary layers. Seismic mapping indicates much greater variation of near-surface horizons than mapping with well data. Axial planes of Acadian folds appear to be vertical. Fold amplitude diminishes between the Tully formation and the Lockport Group in some areas, while in other areas amplitude remains constant through the Devonian and Silurian section. In Chautauqua County, an anomalously thick Onondaga to Lockport interval parallel to regional fold axes has been interpreted as an anticline with a salt core, suggesting decollement tectonics. End_of_Article - Last_Page 1165------------

Decollement tectonics in the Hercynian Cordillera of Northwest Spain

ANATOMY AND LITHOFACIES OF AN INTACT AND SEISMIC-SCALE CARBONIFEROUS CARBONATE PLATFORM (ASTURIAS, NW SPAIN): ANALOGUES OF HYDROCARBON RESERVOIRS IN THE PRICASPIAN BASIN (KAZAKHSTAN)

Structural Evolution of Santa Lucia Range, California: ABSTRACT

The rocks of the Santa Lucia Range are divisible into a bedrock complex and a superjacent series, separated by a major unconformity which represents early Late Cretaceous deformation (Santa Lucia orogeny). Decollement tectonics, involving gravity sliding of the Franciscan rocks over carbonate rocks of the Gabilan mesa, played an important role in the deformation of the bedrock complex during Late Jurassic and middle Cretaceous times (Hsu, 1965). The superjacent rocks were deformed by wrench faulting, and by folding during several Cenozoic orogenic episodes. Thrust faults have been observed in wrench-fault zones. Local changes from wrench to thrust faulting are related to slight changes in the magnitude of the vertical and horizontal principal stresses. These thrusts sho ld not be confused with the decollement tectonics which affected only the bedrock complex. The Franciscan-Recent succession of the region is illustrated and discussed. The stratigraphy of the superjacent sediments is based on the work of previous investigators. The stratigraphy of the bedrock complex is based mainly on the writer's structural interpretations. Although the Franciscan rocks were deformed during late Mesozoic and denuded during early Tertiary, they furnished very little debris to the Cretaceous and early Tertiary sedimentary formations of California. End_Page 648------------------------------ Nevertheless, large volumes of Franciscan debris must have been removed from the Santa Lucia Range before the Miocene transgression. This debris probably was transported west and deposited as thick early Tertiary clastic sequences in offshore basins. End_of_Article - Last_Page 649------------