Fold-thrust Structures Research Articles

Paleocene–Eocene syncontractional sedimentation in narrow, lacustrine-dominated basins of east-central Tibet

Sedimentologic, stratigraphic, compositional, and structural data from four elongate basins ( 30 km long) in the Nangqian-Yushu region of east-central Tibet (near the headwaters of the Mekong and Yangtze Rivers) indicate nonmarine sedimentation synchronous with Paleocene–Eocene northeast-southwest shortening. Sedimentation in the Nangqian, Niuguoda, Xialaxiu, and Shanglaxiu basins involved (1) mud and carbonate accumulation in offshore to nearshore lacustrine environments and (2) gravel and sand deposition in fan-delta to alluvial-fan environments localized near basin margins. Growth strata in both fine- and coarse-grained deposits, primarily in upper levels of basin fill, provide evidence for sedimentation contemporaneous with motion on fold-thrust structures. Provenance data collected from 25 measured stratigraphic sections, including >1500 paleocurrent measurements and >150 conglomerate compositional analyses, show that each basin was fed sediment from several directions by proximal source areas composed of Carboniferous–Triassic rocks. The source-area proximity and a stratigraphic variability among the basins suggest that each basin evolved independently and was filled by sediment derived from relatively small drainage networks (<103 km2). Age control for basin fill is based on Paleogene fossils, 38–37 Ma 40Ar/39Ar ages from volcanic rocks interbedded with uppermost strata of the Nangqian basin, and 51–49 Ma 40Ar/39Ar ages from igneous rocks that intrude and unconformably overlie strata of the Shanglaxiu basin. Strata containing middle Cretaceous palynomorph and ostracod assemblages are present only locally in the lowermost part of the Nangqian basin. Although the tectonic setting for Cretaceous sedimentation is unclear, early Tertiary basin development was controlled by thin-skinned fold-thrust deformation. We interpret the narrow widths of Paleocene–Eocene basins to be a result of thrust spacing, which in turn may have been controlled by the depth to the decollement (∼5 km deep according to our balanced cross section) from which imbricate thrusts ramped up through the Carboniferous–Triassic section. Sedimentologic and provenance evidence for internal drainage, limited unroofing, and relatively low average sediment-accumulation rates in these syncontractional, plateau-interior basins indicates generally small drainage systems, short main-stem rivers, shallow regional slopes, and limited denudation in east-central Tibet during early Tertiary time. Such geomorphic conditions, which are similar to the modern low-relief interior of the Tibetan plateau, suggest that the deeply incised Mekong and Yangtze Rivers of eastern Tibet were not established until after the termination of Paleogene basin development in the region.

Multi-Stage Deformation in the Nallamalai Fold Belt, Cuddapah Basin, South India - Implications for Mesoproterozoic Tectonism Along Southeastern Margin of India

The common occurrence of small-scale refolded folds, deformed lineations and overprinting of foliation in the northern Nallamalai Fold Belt (NFB) within the intracratonic Cuddapah Basin, South India, suggest three phases of deformation, D 1, D 2 and D 3 during the late Paleoproterozoic to Neoproterozoic. D 1 structures are represented by tight to isoclinal folds, a slaty cleavage and local development of mylonites. D 2 structures include NE trending tight to open folds (F 2) with variable plunge indicating control of large domal structures and a steep crenulation cleavage developed in phyllites and other schistose rocks. The above structures are overprinted by E-W trending D 3 folds and cleavage, which affect Mesoproterozoic rocks of the Srisailam Formation (uppermost Nallamalai Group) in the E-W trending Vami Konda range and metasediments of the Palnad area, thought to be equivalent of the Neoproterozoic Kurnool Group. Taking into account the ages of intrusive Vellaturu Granite (∼1575 Ma) which is post-D 2 deformation in the NFB and the Chelima Lamproite (∼1400 Ma) which intrudes the folded Nallamalai rocks, we suggest that the earlier episodes of multiple deformation in the NFB represent either late Paleoproterozoic or early Mesoproterozoic NW-SE compression in this part of the East Gondwana. The E-W fold-thrust structures affecting the Srisailam Quartzite and adjacent metasediments in the Palnad area represent late Mesoproterozoic and/or Neoproterozoic event. The granite gneisses and schists of the Nellore Schist Belt, bordering the NFB are affected by both early and later events and thrust over rocks of the Nallamalai Group. We propose that early deformation in the NFB lying southwest of the Eastern Ghats and roughly parallel to the present southeastern coast of India, was unrelated to, and occurred prior to the main deformation in the Eastern Ghats, which is generally correlated with the ∼1000 Ma old Grenville event. Alternatively, in view of the complex, protracted evolution of the Eastern Ghats and published geochronological data supporting pre-Grenvillian tectonism particularly in the Western Charnockite Zone of the Eastern Ghats, D 1 and D 2 deformation in the NFB may be linked to this older (1600-1400 Ma) tectonic disturbance in the Eastern Ghats.

Crustal-Scale Cross-Section of the U.S. Cordillera, California and Beyond, Its Tectonic Significance, and Speculations on the Andean Orogeny

A cross-section across northern California from the San Andreas fault to central Nevada exhibits both major east- and west-vergent structures. East-vergent structures include crustal wedging and fault-propagation folds in the Coast Ranges, emplacement of the Great Valley ophiolitic basement over Sierran basement rocks, early east-vergent structures in the latter, displacement along the eastern margin of the Sierra Nevada batholith, and thrust faults in western Nevada. West-vergent structures include faults within the Franciscan complex and "retrocharriage" structures in the Sierra Nevada A model of evolution of the U.S. Pacific margin emphasizes the role of ophiolites, island arc-continental margin collisions, and subduction of a large oceanic plateau. Early Mesozoic subduction along the Pacific margin of North America was modified by a 165-176 Ma collision of a major intra-oceanic arc/ophiolite complex. A complex SW Pacific-like set of small plates and their boundaries at various times may have been present in southern California between 115 and 40 Ma. Subduction of an oceanic plateau about 85-65 Ma (remnants in the Franciscan) produced east-vergent tectonic wedging in the Coast Ranges, possible thrusting along the eastern Sierra Nevada batholith margin, and development of Rocky Mountain Laramide structures. The "Laramide orogeny" is herein redefined to include all late Cretaceous-Early Tertiary (75-45 Ma) fold-thrust structures from the Pacific Coast to the Rocky Mountains. A speculative model for collisional involvement in the Andean orogeny is also presented, based upon timing of the onset of the Andean orogeny, the presence of oceanic terranes along the western margin of the Andes, and the presence along part of the length of the chain of a remnant marginal basin.

Fold-accommodation faults

Fold-accommodation faults are secondary faults that accommodate strain variations related to structural and stratigraphic position during fold evolution. Four main types of fold-accommodation faults are commonly found. Out-of-syncline and into-anticline thrusts form primarily because of an increase in bed curvature within fold cores, although differential layer-parallel strain at different scales also contributes to fault slip. Depending on the kinematic evolution of the major fold, the thrusts may propagate along the steep or gentle limb of an asymmetric fold or along the hinge of symmetric folds. Wedge thrusts are primarily formed in competent units because of variations in penetrative layer-parallel strain between adjacent units. Limb wedges occur as hanging-wall and/or footwall fault-bend and fault-tip folds, whereas hinge wedges occur as multiple nested faults that tend to thicken the more competent units. Forelimb and backlimb thrusts form by a variety of mechanisms. Forelimb space-accommodation thrusts are low-displacement thrusts that resolve strain discontinuities resulting from increased curvature in fold cores. Forelimb shear thrusts form in the late stages of folding because of rotation and layer-parallel extension on the steep forelimbs of folds. Most backlimb thrusts originate as out-of-syncline thrusts. They may eventually link with forelimb thrusts to form forelimb-backlimb thrusts. Back thrusts accommodate hanging-wall strain during the formation of fault-related folds. They either form selectively in competent units or propagate through the section at the same rate as the main thrust. Although fold-accommodation faults are secondary features, they are important elements that define the geometry and size of structural traps in fold-thrust structures. Accurate mapping of these structures is therefore critical in interpreting the structural geometry of fold and thrust belts. Shankar Mitra holds the Monnett Chair and Professorship of Energy Resources at the University of Oklahoma. He received his Ph.D. in structural geology from Johns Hopkins University in 1977. He spent 19 years at ARCO Research and Exploration as senior and principal research geologist, director of Structural Geology, manager of Geological Research, and senior exploration research advisor. His primary research interests are in the development of structural concepts and methods and their application to exploration and production problems. He is a recipient of AAPG's Wallace Pratt and Cam Sproule awards.

Late-kinematic timing of orogenic gold deposits and significance for computer-based exploration techniques with emphasis on the Yilgarn Block, Western Australia

Orogenic gold deposits are a widespread coherent group of epigenetic ore deposits that are sited in accretionary or collisional orogens. They formed over a large crustal-depth range from deep-seated low-salinity H 2O–CO 2±CH 4±N 2 ore fluids and with Au transported as thio-complexes. Regional structures provide the main control on deposit distribution. In many terranes, first-order faults or shear zones appear to have controlled regional fluid flow, with greatest ore-fluid fluxes in, and adjacent to, lower-order faults, shear zones and/or large folds. Highly competent and/or chemically reactive rocks are the most common hosts to the larger deposits. Focusing of supralithostatic ore fluids into dilatant zones appears to occur late during the evolutionary history of the host terranes, normally within D 3 or D 4 in a D 1–D 4 deformation sequence. Reactivation of suitably oriented pre-existing structures during a change in far-field stress orientation is a factor common to many deposits, and repeated reactivation may account for multiple mineralization episodes in some larger deposits. Absolute robust ages of mineralization support their late-kinematic timing, and, in general, suggest that deposits formed diachronously towards the end of the 100 to 200 m.y. long evolutionary history of hosting orogens. For example, in the Yilgarn Block, a region specifically emphasised in this study, orogenic gold deposits formed in the time interval between 40 and 90 m.y., with most about 60 to 70 m.y., after the youngest widespread basic-ultrabasic volcanism and towards the end of felsic magmatism. The late timing of orogenic gold deposits is pivotal to geologically-based exploration methodologies. This is because the present structural geometries of: (i) the deposits, (ii) the hosting goldfields, and (iii) the enclosing terranes are all essentially similar to those during gold mineralization, at least in their relative position to each other. Thus, interpretation of geological maps and cross-sections and three-dimensional models can be used to accurately simulate the physical conditions that existed at the time of ore deposition. It is particularly significant that the deposits are commonly related to repetitive and predictable geometries, such as structural heterogeneities within or adjacent to first-order structures, around rigid granitoid bodies, or in specific “locked-up” fold-thrust structures. Importantly, the two giant greenstone-hosted goldfields, Kalgoorlie and Timmins, show a remarkably similar geometry at the regional scale. Computer-based stress mapping and GIS-based prospectivity mapping are two computer-based quantitative methodologies that can utilize and take advantage of the late timing aspect of this deposit type to provide important geological aids in exploration, both in broad regions and more localized goldfields. Both require an accurate and consistent solid geology map, stress mapping requires knowledge of the far-field stresses during mineralization, and the empirical prospectivity mapping requires data from a significant number of known deposits in the terrane. The Kalgoorlie Terrane, in the Yilgarn Block, meets these criteria, and illustrates the potential of these methodologies in the exploration for orogenic gold deposits. Low minimum stress anomalies, interpreted to represent dilational zones during gold-related deformation, coincide well with the positions of known goldfields rather than individual gold deposits in the terrane, and there are additional as-yet unexplained anomalies. The prospectivity analysis confirms that predictable and repetitive factors controlling the siting of deposits are: (i) proximity to, and orientation and curvature of, granitoid-greenstone contacts, (ii) proximity to segments of crustal faults which strike in a preferred direction, (iii) proximity to specific lithological contacts which have similar preferred strike, (iv) proximity to anticlinal structures, and (v) the presence of preferred reactive host rocks (e.g., dolerite). The prospectivity map defines a series of anomalous areas, which broadly conform to those of the stress map (>78% correspondence). The most prospective category on this map covers less than 0.3% of the greenstone belts and yet hosts 16% of the known deposits, which have produced>80% of known gold. Thus, it discriminates in favour of the larger economically more-attractive deposits in the terrane. The successful application of stress mapping and prospectivity mapping to geology-based exploration for orogenic gold deposits indicates that more quantitative analysis of geological map data is a profitable line of research. The computer-based nature of these methodologies is ideal for the production of an ultimate, integrated, deposit target map, which can be compared to other, more conventional, targeting parameters such as geophysical and geochemical anomalies. Such an integrated strategy appears the way forward in the increasingly difficult task of cost-effective global exploration for orogenic gold deposits in poorly exposed terranes.

Kinematics of an arcuate fold–thrust belt: the southern Eastern Carpathians (Romania)

Flysch and molasse nappes of the Eastern Carpathians strike mainly N–S and were folded and imbricated since the late-Early Cretaceous. Boundaries of stratigraphic and structural units and calculated fold-trend axes from the southern part of the Eastern Carpathians indicate a curvature of the orogen by 80°. Kinematic data from fault-slip analysis of all nappe systems exposed in the arc document an overall NW–SE contraction. Nappes of the external Moldavides nappe complex underwent deformation since the Late Cretaceous, peaking in the Miocene, and exhibit fanning subhorizontal contraction axes from E–W in the N-striking part of the Eastern Carpathians through WNW–ESE and NW–SE in the northern part of the arc to NNW–SSE in its southwestern part. Geometric relations between faults and folds indicate that this fan pattern reflects an extended interval and was contemporaneous to nappe stacking and subsequent continental collision. Statistical analysis reveals correlation between orientation of the fanning kinematic axes and the structural trend, where fanning of subhorizontal contraction axes is systematically smaller than change of strike of fold–thrust structures. Orogen-parallel extension in the Tarcau nappe of the Moldavides nappe complex is estimated to be ≤20%. This low value of orogen-parallel extension indicates a primary arc in contrast to bending of a previously straight fold–thrust belt. Low orogen-parallel extension and the observed pattern of structural and kinematic axes can be simulated in sand-wedge indentation models where the indenter moves oblique to its margins. In these models, thrust traces and fold axes are curved around the arc but are parallel to the margins of the indenter away from the corner area. The movement directions of the thrust faults in the models are deflected from normal to fault-strike towards the movement direction of the indenter, leading to fanning contraction directions similar to those observed in the Eastern Carpathian arc. Thus, the main controlling factors on arc formation were the plate margin geometry (i.e. the shape of the upper plate) and the movement direction of the upper plate relative to the orientation of its margins. The influence of the geometry of the continental margin in the lower plate was mainly by imposing constraints on the movement direction of the upper plate.

Sediment accumulation on top of the Andean orogenic wedge: Oligocene to late Miocene basins of the Eastern Cordillera, southern Bolivia

Research Article| September 01, 1998 Sediment accumulation on top of the Andean orogenic wedge: Oligocene to late Miocene basins of the Eastern Cordillera, southern Bolivia Brian K. Horton Brian K. Horton 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1998) 110 (9): 1174–1192. https://doi.org/10.1130/0016-7606(1998)110<1174:SAOTOT>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 Brian K. Horton; Sediment accumulation on top of the Andean orogenic wedge: Oligocene to late Miocene basins of the Eastern Cordillera, southern Bolivia. GSA Bulletin 1998;; 110 (9): 1174–1192. doi: https://doi.org/10.1130/0016-7606(1998)110<1174:SAOTOT>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 large volume of Tertiary synorogenic sediment accumulated on top of the Eastern Cordillera of southern Bolivia as the Andean orogenic wedge was shortened, thickened, and uplifted. Oligocene to upper Miocene strata were deposited in five basins that were separated by active, north-trending, fold-thrust structures of the then-frontal part of the orogenic wedge. These coarse-grained deposits recorded accumulation in the most proximal sector of the Andean foreland basin system, the wedge-top depozone.Analyses of depositional systems, sediment dispersal patterns, and clast provenance of 0.6–2.3-km-thick, Oligocene to upper Miocene wedge-top successions demonstrate that fault-propagation and fault-bend folds commonly isolated individual basins while serving as primary sediment sources. Growth strata which formed by progressive fold-limb rotation indicate thrust-fault displacement and related folding concurrent with deposition. Alluvial fans defined most basin margins, whereas braided streams or small lakes occupied basin axes. Diagnostic stratigraphic units confined to individual basins suggest that streams were rarely able to cut across growing folds to connect adjacent basins.Growth strata and crosscutting and onlapping relationships between contractional structures and wedge-top strata delineate the chronology of fold-thrust deformation in the Eastern Cordillera. Five new 40Ar/39Ar dates and previously published K-Ar dates, ranging from 30 to 8 Ma, define an Oligocene phase of west-vergent backthrusting followed by primarily east-vergent thrusting during Miocene time. Timing of displacement on two east-vergent thrusts is determined by 40Ar/39Ar ages of tuffs within adjacent growth strata sequences of the Tupiza Formation conglomerate (16.14 ± 0.06 Ma) and Oploca Formation (13.33 ± 0.15 Ma, 15.7 ± 2.4 Ma). These ages, combined with basin depositional histories, demonstrate synchronous and out-of-sequence thrust displacement during Miocene shortening.Upper-crustal contractional deformation and wedge-top deposition terminated in the Eastern Cordillera during late Miocene time as the thrust front propagated eastward into the Subandean Zone. Continued thrust-front migration produced the present-day configuration in which Eastern Cordillera wedge-top basins, originally developed above the toe of the orogenic wedge at relatively low elevations, are now >250 km west of the active thrust front and at ∼3 km elevation. The modern wedge-top depozone overlies the active frontal part of the orogenic wedge and consists of strata in thrust-bounded, Subandean Zone basins and sediment overlying blind structures beneath the westernmost Chaco Plain.In general, wedge-top deposits become highly susceptible to erosional recycling as the orogenic wedge propagates forward and the wedge surface is uplifted. Nevertheless, Eastern Cordillera wedge-top deposits have been preserved for ∼10–30 m.y. in southern Bolivia. Such long-term preservation may reflect the inability of drainage systems to remove sediment mass from this low-precipitation region of the central Andes. Retention of sediment mass within the orogenic belt may promote critical thrust-wedge conditions in which propagation of the thrust front is favored. 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.

Using high-resolution stratigraphy to date fold and thrust activity: examples from the Neogene of south-central Sicily

The integration of structural and stratigraphic data is fundamental for determining rates of deformation in the uppermost continental crust. The high temporal resolution provided by Neogene marine sediments is used here to examine deformation rates in part of a thrust belt chosen from the Maghrebian orogen of Sicily. Conventional biozonal stratigraphy, calibrated against the geomagnetic polarity time scale, shows that individual thrust-fold structures grew steadily over many millions of years. Structures across the thrust belt were active at the same time and accommodated bulk shortening rates of c. 0.5 mm a -1 . In contrast the basal detachment operated about ten times faster. These results are in broad agreement with some theoretical models for orogenic wedge kinematics. Shore-line carbonate successions, calibrated with precession cycles of sea-level change, provide a very high-resolution, temporal scale with which to chart tilt rates on fold limbs (1°/27.6 ka = 0.036° ka -1 ). These show that fold amplification was continuous although the rates may have varied with time. Incremental tilting of limbs during fold amplification is not predicted by popular models of fault-bend and 'fault-propagation' folding. Geometric modelling suggests that folding occurred by limb rotation, with minor hinge migration during buckling above buried thrusts. Thus stratigraphic data may be used to examine the kinematic evolution of thrust-fold systems along regional cross-sections, and of local structures. However, the types of structural models that can be tested using estimates of deformation rates depend upon the chrono-stratigraphic resolution available for the syn-tectonic sediments.

End-Carboniferous fold-thrust structures, Oxfordshire, UK: implications for the structural evolution of the late Variscan foreland of south-central England

Interpretation of commercial seismic reflection data from the west of Banbury, south-central England, has provided information on the late Carboniferous structural evolution of the Oxfordshire Syncline. End-Carboniferous structures are preserved in the footwall of the main basin-bounding faults of the Permo-Triassic Worcester Graben. Restoration of major pre-Triassic normal faults has allowed reconstruction of earlier compressional structures. In contrast to the WNW-ESE trend of structures to the south, in the Variscan foreland fold-thrust belt, major faults and folds in Oxfordshire are approximately N-S trending. The structures seen to deform Westphalian D-Stephanian-aged sequences on the seismic profiles are a combination of steep (30-40° dip) fault-bounded uplifts and less dominant thin-skinned tectonics, apparently showing transport to the east. In the south, some thin-skinned structures are present, and in the north cover sequences are uplifted in a narrow zone of deformation on reactivated basement faults. The amount of shortening also changes along strike, from c. 1600 m in the south to c. 950 m in the north. Reactivation of major basement faults with differential basement shortening accounts for the main structures. Localized detachment in the young and weak sediments close to the palaeo-land surface explains the thin-skinned structures seen on some of the seismic lines. These end-Carboniferous structures in Oxfordshire and structures in the Malvern-Abberley Hills represent the structural limits of a large N-S-striking intra-Stephanian uplift in the late Variscan foreland. It is suggested that the structures formed during a discrete phase of intra-Stephanian E-W-directed regional shortening prior to the final phase of movement in the Variscan belt.

Late Paleogene extensional collapse of the Cordilleran foreland fold and thrust belt

Research Article| January 01, 1996 Late Paleogene extensional collapse of the Cordilleran foreland fold and thrust belt Kurt N. Constenius Kurt N. Constenius 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Author and Article Information Kurt N. Constenius 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1996) 108 (1): 20–39. https://doi.org/10.1130/0016-7606(1996)108<0020:LPECOT>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 Kurt N. Constenius; Late Paleogene extensional collapse of the Cordilleran foreland fold and thrust belt. GSA Bulletin 1996;; 108 (1): 20–39. doi: https://doi.org/10.1130/0016-7606(1996)108<0020:LPECOT>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 The Cordilleran foreland fold and thrust belt collapsed and spread to the west during a middle Eocene to early Miocene (ca. 49–20 Ma) episode of crustal extension. The sedimentary and structural record of this event is preserved in a network of half grabens that extends from southern Canada to central Utah. Extensional structures superposed on this allochthonous terrain are rooted to the physical stratigraphy, structural relief, and sole faults of preexisting thrust-fold structures. The sole faults dip 3°–6° west above an undeformed Precambrian crystalline basement and accommodated tectonic transport of a thick (up to 20+ km) eastward-tapering hanging wall during regimes of crustal shortening and extension. The chronology of tectonism for the foreland fold and thrust belt is established here by dating latest thrusting and initial normal faulting and is best defined where thrusts and normal faults are linked by common detachment surfaces. Dated movement on two extensionally reactivated thrusts, the Lewis thrust of northwest Montana and southeast British Columbia and the Medicine Butte thrust of southwest Wyoming and northeast Utah, suggests that the hiatus between the end of crustal shortening in the early or early middle Eocene and the start of extension in the early middle Eocene was brief.Lateral spreading and extensional basin formation in the Cordilleran foreland fold and thrust belt were partly concurrent with formation of metamorphic core complexes and regional magmatism. Conceptually linking extensional processes that were simultaneously deforming both the hinterland and foreland of the late Paleogene Cordilleran orogenic wedge is accomplished by applying the extensional-wedge Coulomb critical-taper model. The rapid drop in North America–Pacific plate convergence rate and/or steepening of the subducted oceanic slab at ca. 50 Ma resulted in a large reduction in east-west horizontal compressive stress in the Cordillera. As a result, the Cordilleran orogenic wedge was left unsupported, and it gravitationally collapsed and horizontally spread west until a new equilibrium was established at ca. 20 Ma. Subsequently, crustal extension and magmatism during the Basin and Range event (ca. 17–0 Ma) overprinted much of this earlier phase of extension. 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.

Petroleum Geology of the Southern Pre-Uralian Foredeep with Reference to the Northeastern Pre-Caspian Basin

The southern Pre-Uralian Foredeep and the northeastern Pre-Caspian Basin of southern Russia and Kazakhstan are at the juncture of two major oil-producing regions, the Volga-Ural Basin and the new fields of the Northern Caspian Basin (e.g., Tengiz). The southern Pre-Uralian Foredeep has produced little oil; nevertheless, the Permian-Carboniferous stratigraphy and the general fold-thrust structure of the Pre- Uralian Foredeep, and adjacent Pre-Caspian Basin, afford the possibility for classic and largely untested sub-salt and sub-thrust plays. Prior to the onset of Uralian orogenic activity, Late Devonian-Early Carboniferous rifting disrupted the East European continent, forming a series of rift basins including the Kama-Kinel troughs and the Pre- Caspian Basin. The Middle Carboniferous to Early-Middle Triassic Uralian Orogenic Belt consists of a complicated series of lower Paleozoic continental margin sequences, basement nappes, and accreted terranes, structurally interleaved via large-scale folding and th...

The displacement-distance method for contractional kink bands and its application to fold-thrust structures

The displacement-distance ( d− x) method can be modified to study the geometry and development of contractional kink bands by dividing displacements into cartesian component vectors. Kink bands are idealised as having constant layer lengths, enabling simple trigonometry to be used to determine the displacement of one wall of the kink band relative to the other wall. In a consideration of several applications of the d− x method for kink bands, it is shown that displacement is transferred between conjugate and overstepping kink bands in a similar way to displacement transfer between conjugate and overstepping faults and extension fractures. The several different models of kink band formation are shown to each have different displacement characteristics. The d− x method can also be used to study the geometry and evolution of folds related to thrust-propagation and ramps, which are often modelled as having kink band geometries. For instance, the d− x method can be used to show how fault-tip and fault-bend folds cause or accommodate thrust displacement variations, and to estimate displacement rates from the amounts of deformation in different syn-thrust sedimentary layers.

Deformation partitioning during polyphase oblique convergence in the Karawanken Mountains, southeastern Alps

The Karawanken Mountains of southeastern Austria straddle the Insubric Line, a major fault zone that corresponds to the Austroalpine-South Alpine border of the eastern Alps. They display polyphase structural patterns that resulted from deformation partitioning at shallow crustal levels. Following a poorly constrained late Cretaceous deformation history, Paleogene contraction produced NW—SE-trending fold—thrust structures and possibly related sinistral (?) cross faults. Miocene deformation produced both NW- and SE-directed thrust faults and NW-striking high-angle dextral cross faults which merged with or displaced the W-striking Insubric Line. Distributed dextral strike-slip displacements of the order of 30–40 km and thrust displacement of 4–5 km along the northern Karawanken front initiated subsidence of a small foreland basin north of the Karawanken Mountains. Recent regional deformation seems to be concentrated along cross-cutting NNW-striking dextral faults within a shifting system of seismogenic transfer faults that probably merge with the convergent South Alpine thrust front to the west. There are conspicuous parallels in the style of deformation along the Karawanken Mountains with that found along other seismogenic convergent strike-slip faults, such as those of southern California, New Zealand and Anatolia.

Pressure, temperature and time constraints on Himalayan metamorphism from eastern Kashmir and western Zanskar

Himalayan metamorphism in the Kashmir and Zanskar sector of the High Himalaya resulted from thrust- and fold-related crustal thickening within the Indian plate following the collision of India and Asia at c. 50 Ma. Interbanded metapelites, marbles, calcareous schists, amphibolites and quartzites represent metamorphosed equivalents of the Palaeozoic-Lower Mesozoic continental margin rocks. Granitic rocks include pre-collision K-feldspar megacrystic and biotite granites and post-collision two-mica ± garnet ± tourmaline granites. Average pressure-temperature conditions of equilibration using the self-consistent thermodynamic data-set of Holland &amp; Powell (1990) are presented across a 45 km traverse of the eastern Kashmir-western Zanskar High Himalaya. Kyanite is the stable aluminosilicate phase across 35 km outcrop width in the middle of the slab. Complex microstructures indicate that prograde metamorphism up to kyanite grade and fabric development in the upper structural nappes is early and unrelated to the Main Central thrust. Diachronous metamorphism propagated southwards with the overall structural evolution. Early isograds and thrust-fold structures were carried passively in the hanging-wall of the Main Central thrust. Peak metamorphism, based on 40 Ar/ 39 Ar hornblende ages, occurred pre–30.7 ± 2.0 Ma at the top of the slab, and in the middle of the slab was pre-22 ± 1.0 Ma, probably 25-28 Ma. The lower structural levels in Zanskar record peak metamorphic conditions around 700–750 °C and 8 kbar, reflecting depths of burial of 28–30 km. A new younger schistosity, which is not present in the higher structural levels, was developed under kyanite grade conditions. The regional distribution of high temperatures recorded by thermo-barometry does not support the concept of additional heat being supplied by frictional heating along the Main Central thrust or magmatic heat resulting from anatexis. Exhumation of the Himalayan metamorphic rocks was achieved by three major processes: erosion as a result of crustal thickening, uplift of the rocks along the hangingwall of the Main Central thrust above a major frontal ramp in the Kishtwar Window area, and extensional unroofing along the footwall of a large-scale, NE-dipping normal fault at upper crustal levels, probably synchronous with compression at depth.

The deep structure of the ardennes variscan thrust belt from structural and ECORS seismic data

A new crustal tectonic model is proposed for the Ardennes external Variscides of northern France-Belgium on the basis of original microtectonic data, further complemented by revised ECORS deep seismic information. This model is illustrated by both restored and balanced crustal sections that clearly emphasize the direct control exerted by pre-existing extensional structures on the collision-related processes of Variscan deformation. Indeed, the deep structure of the Ardennes belt appears as a continental accretionary prism that thickens south towards the hinterland above a staircase-type basal thrust. Innermost crustal shortening is here assigned to a deeply rooted foreland-directed ramp thrust system soled at depth by a thick ductile decoupling shear zone, actually imaged at about 20 km depth on the ECORS section; this thrust system is bounded upwards by a shallow infra-Devonian roof thrust. To account for the present-day high structural position of the Rocroi Cambrian inlier, as well as for its internal superimposed ductile fabrics, major uplift of a previously sheared crust is assumed to have occurred along a frontal ramp which nucleated on an early S-dipping extensional fault (the Rocroi Fault). According to the ECORS data, the hanging wall of the Rocroi Fault has been translated about 40 km northwards along the Dinant infra-Devonian shallow décollement. This estimate is quite consistent with the 40% stratal shortening undergone at its front by the Devonian detached cover whose thin-skinned fold-thrust structures also reflect the direct influence of early Devonian normal faulting. Basin inversion tectonics are clearly expressed along the frontal thrust zone where the thin Condroz basement sheet is now interpreted as resulting from slicing of the Dinant Décollement through the footwall of a major Lower Devonian extensional fault scarp that separated the Namur shallow shelf from the Dinant downthrowing basin. The present thrust model also implies that the Dinant Décollement extends as a blind thrust beneath the slightly detached Namur Carboniferous coalfield, and that the Midi frontal ramp only represents a minor out-of-sequence thrust reactivating a rotated basement-cover contact.

Magnetic fabric as a structural indicator of the deformation path within a fold-thrust structure: a test case from the Corbières (NE Pyrenees, France)

In order to define the deformation path and the distribution of internal deformation in a foreland foldthrust structure, the anisotropy of magnetic susceptibility was measured in 150 samples taken from 11 sites throughout the Vitrollian sequence of the Lagrasse structure (most external Pyrenean Zone). Whereas surrounding rocks seem completely undeformed, the Vitrollian Formation exhibits a complete set of microstructural markers thereby allowing fault striation measurements at sites also sampled for magnetic fabric analysis. Structural analysis shows that two mechanisms acted successively during thrust motion. The first one resulted from a NW-SE layer parallel shortening (LPS). It was responsible for a well defined vertical magnetic foliation parallel to the regional cleavage, in which a vertical magnetic lineation developed in conjunction with an increase in the shortening intensity towards the zone which would subsequently fail, leading to the development of an imbricate thrust system. During this second event, internal deformation was concentrated around the hinge and in the forelimb of the frontal fold. Around the hinge, the magnetic lineation rotated to an horizontal NE-SW trend parallel to the fold axis, indicating a significant component of longitudinal stretching during the development of the frontal ramp-anticline. Subsequently, the magnetic lineation swung again and became parallel to the NW-SE tectonic transport direction beneath out-of-sequence forelimb thrusts. This complex deformation history is not accompanied by a linear increase in the degree of anisotropy. On the contrary, we observe a cycle of construction and destruction of the magnetic fabric during LPS and imbricate thrusting.

Computerized tomographic analysis of displacement trajectories and three-dimensional fold geometry above oblique thrust ramps

Computerized tomographic analysis of physical models of fold-thrust structures permits nondestructive, systematic analysis of variations in displacement trajectories and variations in three-dimensional fold geometry as a function of thrust-fault geometry. The authors studied models in which sand layers with embedded transport-parallel marker lines were shortened incrementally up to a maximum of 24% over rigid, footwall fault templates. These templates simulated a right-stepping oblique ramp connecting two ramps perpendicular to the shortening direction. In response to movement over the oblique-ramp templates, overturned folds formed and marker-line deflections indicative of out-of-plane strains developed. Marker lines within layers originally truncated by fault templates were deflected leftward away from the right-stepping oblique ramp during deformation, whereas marker lines within sand layers that originally extended across the upper flat of the fault template were deflected in a more radial pattern. Differences in marker-line deflections imply that displacement trajectories within natural thrust sheets may vary as a function of overburden and structural position. Furthermore, their tomographic studies graphically demonstrate that out-of-plane strain and differential rotation of layers within thrust sheets occur in the vicinity of oblique ramps.

New Excess-Area/Depth-to-Detachment Relationship for Fold-Thrust Structures

New Excess-Area/Depth-to-Detachment Relationship for Fold-Thrust Structures

Centrifuge modelling of fold-thrust structures in a tripartite stratigrahic succession: Dixon, J M; Tirrul, R J Struct GeolV13, N1, 1991, P3–20

Centrifuge modelling of fold-thrust structures in a tripartite stratigrahic succession: Dixon, J M; Tirrul, R J Struct GeolV13, N1, 1991, P3–20

Influence of layering and boundary conditions on fault-bend and fault-propagation folding

The influence of heterogeneous layering and boundary conditions on the structural development of fault-bend and fault-propagation folds has been investigated through petrographic study of nonscaled rock models. The models are deformed in a triaxial rock-deformation apparatus at room temperature and a 50-MPa confining pressure. The models consist of a single layer of sandstone containing a saw-cut ramp that is inclined 20° to the layering, and an overlying, intact, thinly layered unit that is composed of limestone interlayered with lead or mica. Analysis of the fold-thrust structures generated in sequentially shortened models with different loading conditions and layer types suggests that the mode of fold-thrust interaction activated upon shortening will depend on fault zone drag, bending and shearing resistance of the hanging wall, shear strength of layer interfaces, and loading conditions. For the models, these parameters may be expressed as a strength ratio describing the resistance to foreland translation relative to the resistance to internal deformation of the thrust system. Low strength ratios favor fault-bend folding. High strength ratios favor internal shortening of the sheet; isotropic and thick (relative to ramp height) units above a propagating thrust tip will shorten primarily by faulting, whereas thinly layered, anisotropic units will shorten by fault-propagation folding. During both modes of fold-thrust interaction, the dips of the fold limbs increase, interlimb angles decrease, and imbricate faults form in the hanging wall or footwall with shortening. In one model suite, the imbrication is associated with a transition from fault-bend folding to fault-propagation folding and produces a highly asymmetric ramp anticline similar to a second-mode fault-bend fold or to a transported fault-propagation fold. The model data suggest that fault-propagation folding in heterogeneously layered rock occurs by the discontinuous formation, growth, and linkage of faults below the growing fold. Amplification of the fault-propagation fold is affected by the amount of slip transferred out of the deforming region, imbrication, and buckling. The changes in the mode of fold-thrust interaction and modifications in the local geometry and strain distribution that occur during shortening result from slip hardening on faults, or from rotation- or strain-induced variations in the strength of the layers.