Structure Of Thrust Belt Research Articles

The role of flat slab subduction, ridge subduction, and tectonic inheritance in Andean deformation

AbstractConvergent plate boundaries show sharp variations in orogenic width and extent of intraplate deformation. Analysis of late Cenozoic contractile deformation along the Andean mountain front and adjacent foreland highlights the contrasting degrees of deformation advance toward the plate interior. The retroarc positions of the Andean topographic front (marked by frontal thrust-belt structures) and foreland deformation front (defined by isolated basement block uplifts) range from 300 to 900 km inboard of the trench axis. Over the ~8000 km arcuate length of the Andes (10°N to 55°S), four discrete maxima of inboard deformation advance are spatially co-located with the Peruvian (5°S–14°S) and Pampean (27°S–33°S) zones of flat slab subduction, the subducted Chile Ridge (45°S–48°S), and the anomalously thick Paleozoic stratigraphic wedge of Bolivia (17°S –23°S). The spatial correspondence of retroarc shortening with specific geodynamic configurations demonstrates the mechanical role of flat slab subduction, slab window development, and combined structural and stratigraphic geometries in shaping the orogenic architecture of Cordilleran margins, largely through lithospheric strengthening, weakening, and/or tectonic inheritance.

The seismogenic structure of the southern segment of the Longmen Shan thrust belt, eastern Tibetan Plateau, SW China: A comprehensive analysis of surface geology and deep structure

Since the Ms 7.0 earthquake occurred in Lushan on April 20, 2013, five years after the Wenchuan Ms 8.0 earthquake, the seismogenic potential of southern Longmen Shan has been a considerable concern. Comparative studies of active faults and deep structures of the Lushan and Wenchuan earthquakes are important to evaluate Longmen Shan potential. Terrace measurements, geophysical surveys and borehole drilling reveal that late Quaternary fault activity in the southern segment of the Longmen Shan thrust belt is weaker than that along the central segment. Magnetotelluric data across the southern segment of the Longmen Shan thrust belt show that the central fault and the front-range fault are the electrical boundaries of the crust and that the Longmen Shan thrust belt is a high-resistivity body. The Lushan earthquake occurred at the boundary zone between the high- and low-resistivity bodies, and the crustal structure of the southern Longmen Shan thrust belt matches the multiunit combination seismogenic model, indicating a potential large earthquake in Longmen Shan. Compared to conditions along the central segment, the low-resistivity layer of the upper and lower crust on the western side of the Songpan-Ganzi block develops along the Xianshuihe fault, which may reduce the strain accumulated from the Songpan-Ganzi block to the southern Longmen Shan thrust belt. This interpretation suggests that the main crustal shortening is accommodated by foreland thin fold structures, whereas the structural deformation accommodated by the thrust belt accounts for only a small proportion; thus, the seismogenic potential of southern Longmen Shan is weaker than that of the central segment.

The interaction between strike-slip dominated fault zones and thrust belt structures: Insights from 4D analogue models

Strike-slip motion is a fundamental tectonic process around the globe often resulting in prominent surface expressions. The full reconstruction and comprehension of strike-slip dominated structures can be a major challenge due to the complexity of the associated fault patterns. Further complexities occur when strike-slip dominated structures in the foreland interact with thrust belt fronts. Starting from a well-studied natural example (the Sciacca Fault Zone in the Sicilian Channel, Italy) we ran a set of analogue models simulating the interaction between strike-slip dominated fault zones and thrust structures using quartz sand as the analogue material and X-Ray Computed Tomography as a technique to carry out a 4D analysis of model internal structures. During a first phase, a thrust belt was created in our models, which was subsequently affected by a perpendicular fault system that underwent either pure strike-slip, 10°-, 20°-, 30°-transtensional, or 10°-, 20°-, 30°-transpressional motion. In general, transpressional models form pop-up structures, while the pure strike-slip model develops one sub-vertical fault bounded by two converging reverse faults. The 10˚-transtensional model generates a set of Riedel shear faults, which merge during the later stages of deformation. This model also shows a positive elevation change along the strike-slip dominated structures near the intersection with the thrust belt. The 20˚-transtensional model contains some Riedel shear faulting as well, but is dominated by two normal faults with steep dip angles and some minor sub-vertical strike-slip faults in between. This fault architecture is also observed in our 30˚-transtensional model. Three different tectonic regimes interacting with pre-existing and newly formed thrust front led to very different structural settings. These results were of help in better defining the tectonic regime that controlled the strike-slip dominated part of the Sciacca Fault Zone, but they show a fair match with other natural examples as well.

The role of tectonic flow of crustal material in the formation of the Sea of Japan and the Sea of Okhotsk

Based on the concept of tectonic delamination of the lithosphere, we revealed that the Sea of Japan and the Sea of Okhotsk were formed as a result of the tectonic flow of crustal material. The intermittent southward movement of southwestern Japan (Late Cretaceous–Cenozoic) along the eastern Japanese leftlateral strike-slip fault zone resulted in the formation of paired structures: back-arc extensional (Central Japan rift) and frontal compressional (South Japan imbricate–thrust belt) structures. The Sea of Okhotsk was formed in a similar tectonic setting: South Okhotsk rift (back-arc extensional structure) and Kamuikotan–Susunai compressional belt (frontal imbricate-thrust structure). Synchronous extension, compression, and strike-slip movements suggest that the tectonic flow of crustal material played a critical role in the formation of the Sea of Japan and the Sea of Okhotsk.

Geologic and Geotechnical Factors Controlling Incipient Slope Instability at a Gravel Quarry, Livermore Basin, California

Mine pit slopes at Arroyo del Valle Quarry in the Livermore Basin of northern California expose late Quaternary sandy gravel. In the deep subsurface, the gravel unconformably overlies gently folded lacustrine sediments. Within the lacustrine sediments, a bed of sheared, unoxidized clay overlies a marl bed, forming a distinctive marker bed couplet. Structure contours on this marker bed show an anticline and a syncline in the vicinity of the quarry. These northwest-striking folds are aligned parallel to regional Quaternary fold and thrust belt structures. Where the marker bed dips toward the pit, slope inclinometers consistently deflected toward the pit at the depth of the unoxidized clay. Where the marker bed dips away from the quarry pit, no slope inclinometer deflections were recorded. Thus, slope instability was controlled by the site stratigraphy, geologic structure, and the location of quarry slopes relative to that geologic structure. High pore-water pressures within the unoxidized clay also contributed to slope instability. Shearing of the unoxidized clay occurred prior to excavation of the quarry pit, and the resulting low residual strength of this high-plasticity clay made it particularly vulnerable to incipient landsliding when lateral confinement was removed during excavation of the quarry pit. Analysis of the critical region between the quarry pit and the anticline axis showed that the static factor of safety remained below 1.5. Seismic displacement analyses indicated that moderate to large displacements would be anticipated. Thus, depressurization wells and an earth-fill buttress were designed and implemented to mitigate deep-seated slope instability.

The Archean–Paleoproterozoic evolution of the Quadrilátero Ferrífero (Brasil): Current models and open questions

The Quadrilátero Ferrífero is a metallogenic district (Au, Fe, Mn) located at the southernmost end of the São Francisco craton in eastern Brazil. In this region, a supracrustal assemblage composed of Archean greenstone and overlying Neoarchean–Paleoproterozoic sedimentary rocks occur in elongated keels bordering domal bodies of Archean gneisses and granites. The tectonomagmatic evolution of the Quadrilàtero Ferrífero began in the Paleoarchean with the formation of continental crust between 3500 and 3200 Ma. Although this crust is today poorly preserved, its existence is attested to by the occurrence of detrital zircon crystals with Paleoarchean age in the supracrustal rocks. Most of the crystalline basement, which is composed of banded gneisses intruded by leucogranitic dikes and weakly foliated granites, formed during three major magmatic events: Rio das Velhas I (2920–2850 Ma), Rio das Velhas II (2800–2760 Ma) and Mamona (2760–2680 Ma). The Rio das Velhas II and Mamona events represent a subduction-collision cycle, probably marking the appearance of a modern-style plate tectonic regime in the Quadrilátero Ferrífero. Granitic rocks emplaced during the Rio das Velhas I and II events formed by mixing between a magma generated by partial melting of metamafic rocks with an end member derived by recycling gneissic rocks of older continental crust. After deformation and regional metamorphism at ca. 2770 Ma, a change in the composition of the granitic magmas occurred and large volumes of high-K granitoids were generated.The ca. 6000 m-thick Minas Supergroup tracks the opening and closure of a basin during the Neoarchean–Paleoproterozoic, between 2600 and 2000 Ma. The basal sequence involves continental to marine sediments deposited in a passive margin basin and contain as a marker bed the Lake Superior-type Cauê Banded Iron Formation. The overlying sediments of the Sabará Group mark the inversion of the basin during the Rhyacian Minas accretionary orogeny. This orogeny results from the collision between the nuclei of the present-day São Francisco and Congo cratons, generated the fold-and thrust belt structure of the Quadrilátero Ferrífero. Afterwards, the post- orogenic collapse resulted in the deposition of the Itacolomi Group and in the genesis of the dome-and-keel structure.In this paper, we review current knowledge about the 1500 Ma long-lasting tectonomagmatic and structural evolution of the Quadrilátero Ferrífero identifying the most compelling open questions and future challenges.

The Iapetan rifted margin of southern Laurentia

The Iapetan rifted margin of southern Laurentia includes the northeast-striking Blue Ridge, Ouachita, and Marathon rifts, which are offset by the northwest-striking Alabama-Oklahoma and Texas transform faults, framing the Alabama and Texas promontories and the Ouachita and Marathon embayments of the continental margin. Interpretations of the original trace, structural style, and age of the rifted margin rest on identification of synrift rocks and structures, as well as continental-shelf and off-shelf sedimentary deposits on the passive margin. Both late Paleozoic Ouachita-Appalachian allochthons and post-orogenic Atlantic-Gulf passive-margin deposits cover the Iapetan rift margin, necessitating the use of data from deep wells and geophysical surveys along with geologic maps of the exposed Ouachita-Appalachian thrust belts to characterize the synrift and post-rift rocks and structures. The continental margin and passive-margin shelf strata are primarily in the footwall of the Ouachita allochthon; however, some Ouachita thrust faults displaced shelf-margin basement and cover. Appalachian thrust faults imbricate synrift fill of the intracratonic Birmingham graben and the passive-margin shelf. Palinspastic restoration of thrust-belt structures uses balanced cross sections to locate the original trace of the Iapetan margin. Thickness and subsidence history of the passive-margin successions, as well as a general lack of preserved synrift deposits, indicate an upper-plate structure along the Blue Ridge rift on the Alabama promontory and along the Ouachita rift on the Texas promontory. The upper plate on the Texas promontory is conjugate to a lower-plate rift structure on the Argentine Precordillera. Although data are limited, the evolution of the passive margin along the Marathon rift in the Marathon embayment suggests a lower-plate structure. Geophysical modeling supports a steep continental margin along the Alabama-Oklahoma transform, and a similar structure can be inferred for the Texas transform. The Blue Ridge rift north of the Alabama promontory is dated by synrift volcanic rocks as young as 564 Ma, and passive-margin transgression beginning in earliest Cambrian is documented along the Alabama promontory and farther north. The age of the Ouachita rift is documented by the 530–539 Ma synrift volcanics of the transform-parallel intracratonic Southern Oklahoma fault system, by Early Cambrian synrift sediment along the conjugate rift margin in the Argentine Precordillera, and by late synrift graben-fill of Early to early Late Cambrian age in the rift-parallel intracratonic Mississippi Valley and Birmingham graben systems, as well as by subsidence history of the passive margin on the Texas promontory. The diachroniety of rifting reflects an inboard shift from the Blue Ridge rift to the Ouachita rift along the Alabama-Oklahoma transform and rifting of the Argentine Precordillera from the Ouachita embayment.

The Geological Structure of the North-West Highlands of Scotland – revisited: Peach et al. 100 years on

Abstract Following years of sporadic debate, in the early 1880s consensus was reached that thrust tectonics explained hitherto controversial geological field relationships in NW Scotland. This spawned a major research effort there by the Geological Survey of Great Britain that culminated in a series of highly detailed geological maps, preliminary research papers and, eventually, the publication of a memoir to the region. These works became highly influential to early-20th century geoscience, especially structural geology. Not only did they provide the first major synthesis of thrust belt structure, they also provided the basis for descriptions of fault and shear zone processes and deductive methods for unravelling tectonic histories in metamorphic basement. A common misconception is that the results arose from mapping alone, without regard to extant models and theory and this approach is held up as an ideal for fieldwork. Yet the notebooks and writings of the surveyors show the application of learning not only from other research groups but also between themselves. As with modern mapping, the Survey team created interpretations that built on contemporary knowledge. This work in turn has driven subsequent research for over 100 years, in the NW Highlands and in deformed rocks throughout the world.

Deep Resistivity Structure in Longmen Shan and the Wenchuan Earthquake in the East of Qinghai-Tibet Plateau, China

The magnitude 8.0 Ms Wenchuan earthquake of 12 May 2008 occurred along the Longmen Shan fault zone, southwest of China, is a huge hazard. Why the earthquake is happened along the NE-SW tending and in an inactive orogenic area? What is the geodynamics caused the earthquake erupting abruptly? Usage of the results of the processing and interpretation of the magnetotelluric data observed before the Wenchuan earthquake from Luqu-Zhongjiang passed through the Qinling tectonic zone, Songpan-Garze block and Longmen Shan as well as Sichaun basin, this paper reveals crustal architecture of 50 km depth of the Eastern Qinghai-Tibet plateau and Western Sichuan foreland basin and the relationship between them, with the characteristics of eastern pressing of the Qinghai-Tibet plateau forcing Songpan-Garze block thrusting upon the Yangzi continent block, later obstructing the eastern movement of the Qinghai-Tibet Plateau. The Longmen Shan is located at the joint of two block and has been found there is a west inclined low-conductivity layer in the crust of 10-20 km depth of Longmen Shan, it is deduced as the deep conductivity characteristics of thrustbelt structure in Longmen Shan. The huge earthquake has caused about 300 kilometers long rupture zone,it happened mainly in faults of Beichuan-Yingxiu and Pengxian-Guanxian of Longmen Shan faults setup, which is going down with the north-western inclined along the thrustbelt in the Longmen Shan. The two faults of Beichuan-Yingxiu and Pengxian-Guanxian seem going to converge together at the deep but they are probably separated with the Maoxian-Wenchuan fault in the Longmen Shan structure zone. There are a little different characters for the Sichuan basin and the Songpan-Garze foldbelts located at the two side of the Longmen Shan, for Sichuan basin has the characteristics of the upper thick low resistance sedimentary, and the under stable high resistance basement, while Songpan-Garze block has high resisitivity cover of upper crust with continuous low resistance bed of the crust. Resulting from the Songpan-Garze block thrust upper the Sichuan Basin stability block, the Longmen Shan structure zone formed three layer of geoelectrical structure which is a high resistance in the upper and lower part of high resistance basin basement, and mingling with low resistance thrust fault zone in the middle. As explanation above, the conclusion could be focused on (1) there is a western inclined thrusting zone range from surface to depth 20km or more under Longmen Shan and Songpan Block, which caused the earthquake happen as it be activated; (2) the different action of surface breaking of Longmen Shan thrusting faults depend on the deep structure characteristics; (3) the earthquake is caused by the mutually promotive with Eurasian plate, Indian plate and Pacific plate in a broad sense. So the research on the MT profile is important for both continent dynamics in the orogenic zone and deep mechanism of Wenchuan earthquake at Longmeshan fault zone in the east of the Qinghai-Tibet plateau.

Cenozoic tectonic evolution of Qaidam basin and its surrounding regions (Part 1): The southern Qilian Shan-Nan Shan thrust belt and northern Qaidam basin

Cenozoic Qaidam basin, the largest active intermountain basin inside Tibet, figures importantly in the debates on the history and mechanism of Tibetan plateau formation during the Cenozoic Indo-Asian collision. To determine when and how the basin was developed, we conducted detailed geologic mapping and analyses of a dense network of seismic reflection profiles from the southern Qilian Shan-Nan Shan thrust belt and northern Qaidam basin. Our geologic observations provide new constraints on the timing and magnitude of Cenozoic crustal thickening in northern Tibet. Specifically, our work shows that the southernmost part of the Qilian Shan-Nan Shan thrust belt and contractional structures along the northern margin of Qaidam basin were initiated in the Paleocene-early Eocene (65–50 Ma), during or immediately after the onset of the Indo-Asian collision. This finding implies that stress was transferred rapidly through Tibetan lithosphere to northern Tibet from the Indo-Asian convergent front located >1000 km to the south. The development of the thrust system in northern Qaidam basin was driven by motion on the Altyn Tagh fault, as indicated by its eastward propagation away from the Altyn Tagh fault. The eastward lengthening of the thrust system was spatially and temporally associated with eastward expansion of Qaidam basin, suggesting thrust loading was the main control on the basin formation and evolution. The dominant structure in northern Qaidam basin is a southwest-tapering triangle zone, which started to develop since the Paleocene and early Eocene (65–50 Ma) and was associated with deposition of an overlying southwest-thickening, growth-strata sequence. Recognition of the triangle zone and its longevity in northern Qaidam basin explains a long puzzling observation that Cenozoic depocenters have been located consistently along the central axis of the basin. This basin configuration is opposite to the prediction of classic foreland-basin models that require the thickest part of foreland sediments deposited along basin edges against basin-bounding thrusts. Restoration of balanced cross sections across the southern Qilian Shan-Nan Shan thrust belt and northern Qaidam basin suggests that Cenozoic shortening strain is highly inhomogeneous, varying from ~20% to >60%, both vertically in a single section and from section to section across the thrust belt. The spatially variable strain helps explain the conflicting paleomagnetic results indicating different amounts of Cenozoic rotations in different parts of Qaidam basin. The observed crustal shortening strain also implies that no lower-crustal injection or thermal events in the mantle are needed to explain the current elevation (~3000–3500 m) and crustal thickness (45–50 km) of northern Qaidam basin and the southern Qilian Shan-Nan Shan thrust belt. Instead, thrusting involving continental crystalline basement has been the main mechanism of plateau construction across northern Qaidam basin and the southern Qilian Shan-Nan Shan region.

Geometry, segmentation pattern and displacement variations along a major Apennine thrust zone, central Italy

Hundreds of kilometres-long, arc-shaped, continuous thrust faults commonly imply very large displacements by detachment-dominated thrusting. Therefore, their occurrence has a large impact on fold–thrust belt structure. One of the major thrust faults of the central-northern Apennines, the Umbria–Marche–Sabina Thrust Zone, is traditionally believed to represent one such feature, characterised by displacements of several tens of kilometres. However, detailed studies of this structure revealed that it is actually composed of a series of partially overlapping fault segments, rather than consisting of a single, continuous thrust. Cross-section balancing and restoration, carried out by the integration of surface geological data with available sub-surface information, points out relatively limited amounts of thrust displacement (<10 km). Displacement–distance profiles show moderate displacement gradients well compatible with those reported for coherent thrust sheets. They also suggest that individual thrust segments were originally isolated and then grew by lateral propagation, leading to overlap and variable fault interaction within relay zones. The relationship between maximum displacement and fault trace length tends to follow a power-law distribution, as it commonly occurs for fault populations. However, a better correlation could be obtained by further segmentation of the northernmost (blind) fault in an area of no seismic data, thus confirming that fault scaling relationships may be useful for pointing out possible problems with the structural interpretation of poorly constrained areas and for exploring viable alternative solutions.

Fold and thrust belt structures and strike‐slip faulting at the SE margin of the Salar de Atacama basin, Chilean Andes

A tectonic reinterpretation is reported for the southeastern margin of the Salar de Atacama basin of northern Chile. Detailed structural mapping revealed the presence of an east vergent thin‐skinned fold and thrust belt affecting Oligocene‐Miocene Paciencia Group rocks and the overlying Plio‐Pleistocene volcanic rocks. Along‐strike segmentation of the main fold implies local foreland influence on footwall ramp geometry leading to local thrust sheet rotation. To the east the adjacent western slope of the Western Cordillera displays two different structural domains, probably controlled by preexisting basement structures. The southern domain comprises two N‐S oriented sigmoidal belts of linear arranged pressure ridges, indicating left‐lateral transpression. In contrast, the northern domain is characterized by east vergent fold and thrust belt structures and reactivated NW‐SE striking sinistral strike‐slip faults, governing clockwise block rotations. An indenter‐driven deformation model is proposed to explain sinistral transpression and clockwise block rotations around vertical axes. This variant of a small‐block rotation mechanism is discussed in the context of oroclinal bending of the central Andes, emphasizing the significance of ancient structures in controlling rotations.

Structural Style Analysis as a Hydrocarbon Exploration Basis for the Carpathian Thrust Belt of Romania

As a result of a Cretaceous-Tertiary sedimentary wedge movement over a basal detachment surface, the present structure of the Carpathian Thrust Belt was successively completed by the Miocene compressional stages of the Alpine Orogenesis. The major basal detachment overpassed the platform, ending at different stratigraphic levels as a [open quotes]blind[close quotes] thrust. The inner thrust sheets were formed by a fault bend folding mechanism, while the outer sheets were formed, primarily, by a propagation fold. The vertical play of the platform blocks created a complex ramp geometry which represents the main control for the deep structural style. The platform thresholds retained the displacement of the thrust sheets, generating duplex systems. In order to illustrate the main structural features, several regional cross sections have been analyzed and balanced. Selected examples point out how the depositional and structural history of parts of the Thrust Belt and their associated hydrocarbon accumulations can be interpreted and evaluated to reduce the exploration risk. Considering the intense exploration of the shallow and medium depths of the Carpathian Thrust Belt petroliferous basins, indentification of new targets relies upon understanding the original distribution of the source and reservoir rocks and the tectonic transport mechanisms which placed them inmore » an allochthonous position over the platform. Therefore, some restored sections have been used to map the initial location of the hydrocarbon prospective facies. Local structural styles of deep levels have also been examined in an attempt to discover new duplex systems.« less

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.

The Cordilleran Foreland Thrust Belt in Northwestern Montana and Northern Idaho From COCORP and Industry Seismic Reflection Data (1)

COCORP and petroleum industry seismic reflection profiles in northwestern Montana reveal the structure of the Cordilleran foreland thrust belt. The Front Ranges consist of thick thrust sheets containing Precambrian Belt Supergroup and Paleozoic miogeoclinal shelf rocks above a thin remnant of Paleozoic rocks and gently westward-dipping North American basement. Interpretation of the seismic data and results from a recent petroleum exploration well suggest that 15-22 km of Precambrian Belt Supergroup sedimentary rocks are present in several thrust plates beneath the eastern Purcell anticlinorium. Previous hypotheses of a large mass of Paleozoic miogeoclinal sedimentary rocks or slices of crystalline basement located beneath the eastern Purcell anticlinorium do not appear to be supported by the data. The easternmost occurrence of allochthonous basement is interpreted to be in the western part of the anticlinorium near the Montana-Idaho border. Comparison of the Cordilleran foreland thrust belt in northwestern Montana and southern Canada suggests that a change in the deep structure of the Purcell anticlinorium occurs along strike. The anticlinorium in southern Canada has been interpreted as a hanging-wall anticline that was thrust over the western edge of thick Proterozoic North American basement, whereas in northwestern Montana the anticlinorium appears to consist of a complex series of thrust sheets above highly attenuated North American basement.

Dillon cutoff-Basement-involved tectonic link between the disturbed belt of west-central Montana and the overthrust belt of extreme southwestern Montana

The front of the Cordilleran fold and thrust belt in western Montana follows the disturbed belt in the north, merges with the southwest Montana transverse zone in the west-central part of the region, and in southwestern Montana is marked by a broad zone characterized by complex interaction between thrust belt structures and basement uplifts. The front margin of the thrust belt in Montana reflects mainly thin-skinned tectonic features in the north, an east-trending lateral ramp that curves southwest in the central part into the Dillon cutoff, an oblique-slip, thick-skinned displacement transfer zone that cuts through basement rocks of the Lima recess, and a zone of overlap between thin- and thick-skinned thrusts in extreme southwestern Montana. The transverse ramp and basement-involved thrust faults are controlled by Proterozoic structures.

SCAT Dipmeter Rules for Interpretation of Thrust Belt Kink-Fold and Detachment Structures: ABSTRACT

Dipmeter interpretation in thrust belts can be difficult because thrust belt structure is often complicated. However, interpreters can construct reliable transverse sections by combining statistical curvature analysis techniques (SCAT) with the geometric techniques for constructing thrust belt sections. By itself, the simplistic tadpole plot furnished by logging companies is inadequate for structural interpretation even in simple settings. In contrast, the six dipmeter displays of SCAT establish the correct direction for transverse (T) sections and the correct method for projecting both the well bore and the location of stratigraphic boundaries, faults, and apparent dips onto such sections. Trend line shift on plots of transverse dip component vs. measured depth (T-plots) identify either kink planes or hanging wall or footwall cutoffs. Kink planes are entered on the T-section and expanded upward to the surface, to a nonbedding parallel fault (where they bend), or to a singular point where multiple link planes meet and terminate; and downward to the causative thrust or to a singular point. Charts relating axial angles of fault-propagation folds or fault-bend folds to ramp angles, together with the location of fault from well data, aid interpretation of duplex structures, antiformal stacks, backlimb thrusts, and imbricate fault-propagation folds. Curved trendmore » lines on T-plots identify curved bedding related to listric thrusts and disharmonic or detachment folds. Transverse steps produce telltale patterns on longitudinal dip-component plots and dip vs. azimuth and polar tangent crossplots.« less

Balanced Section in Thrust Belts Part 1: Construction

Balanced geological cross sections are an important aid to understanding thrust-belt structures and to estimating their hydrocarbon-bearing potential. Two approaches to section balancing have been taken in the past: construction of retrodeformable sections from raw data, and modification of previous interpretations after an evaluation procedure. These approaches may be unified by use of a Langrangian grid. Projection of datum locations, dips, and stratigraphic thicknesses onto the section plane is performed directly by calculation, or graphically, using plunge lines. Spline interpolation and parallel-fold modeling are the simplest ways of filling gaps in the sectional data, but isogon interpolation yields rheologically more realistic results. Faults are interpolated using cutoff geometries as constraints. Fault tips are located from distance-displacement plots, whereas the depth to detachment is obtained by a modified Chamberlin construction. Interpolations made on the section are directed back to the geologic map via plunge lines. Although these techniques of section construction do not guarantee a balance, especially when complexities such as growth structures, diapirs, or strike-slip faults are present, they eliminate many potential errors.

Paleo-Reconstruction of Shelf-Slope Margin Along San Emigdio Mountains, Southern San Joaquin Valley, California: ABSTRACT

Deformation along the San Emigdio Mountains, California, is characterized by large overthrust sheets that carried Eocene through Miocene nonmarine to shallow marine strata over their deeper marine equivalents. The Pleito thrust has at least 20,000 ft of throw and is the major structural feature of this overthrust belt. The upper plate of the Pleito thrust carries an extensively exposed block of three prograding sequences and, along strike, partially exposes the shelf-slope boundaries of these units. Equivalent changes are observed in the subsurface beneath the overthrust. Total crustal shortening along this region ranges from 25 to 50%, with most of the shortening taken up by the Pleito thrust. The thrust has a low-angle, ramp-and-glide configuration, but on the south, the strata eventually extend downward into basement. The northern boundary to the deformation belt is the White Wolf and Pioneer fold and thrust structures. These features form a transfer zone, where one structure ends and the other feature begins. The amplitude of these folds can be up to 10,000 ft from crest to trough. Because of the large overthrust of the Pleito thrust, reservoir rocks are found up to 7 mi south of the fault's surface trace in the lower plate. Themore » buried strata lie south and west of producing fields along the White Wolf fault and the Tejon embayment. The complicated stratigraphic changes combined with the thrust-belt structures require retrodeformable crustal profiles to take into account the facies distributions to model the hydrocarbon potential of this lightly explored province.« less

The NW external Alpine Thrust Belt and its implications for the geometry of the Western Alpine Orogen

Summary The NW external Alpine Thrust Belt contains an imbricated basement—cover sequence which is broadly continuous with the French cratonic foreland. This thrust belt developed in the late stages of Alpine orogenesis and carried the previously deformed and metamorphosed internal zones to the WNW, piggy-back fashion, onto the French Craton. Balanced cross-sections restore the thrust-belt structures to lie for over 100 km to the ESE of their present, innermost outcrop trace. The regional sole thrust commonly lies in the upper crust. To acheive a balance between different crustal levels requires a layer of lower crust, on which the external zones originally lay, to underlie the entire present outcrop width of the Alps. Therefore models of Alpine orogenesis, which require the internal zones to be underlain by their deep crustal root, are not valid and new models are required to explain the sub-surface geometry of the NW Alps. This paper stresses the importance of resolving the external parts of orogens, particularly using balanced cross-sections, before attempting to either interpret the internal parts of orogenic belts or make palaeogeographical reconstructions.