Foreland Fold Research Articles

Ancient synsedimentary structural control on thrust ramp development: an example from the Northern Apennines, Italy

ABSTRACTThe Umbria‐Marche‐Sabina foreland fold and thrust belt (Northern Apennines, Italy) provides excellent test‐cases for the hypothesis of ancient syndepositional structural features controlling thrust ramp development. The sedimentary cover, Late Triassic to Miocene in age, is made of platform and pelagic carbonates, whose deposition was controlled by significant synsedimentary extension. Normal faulting, mainly during the Jurassic and the Late Cretaceous‐Palaeogene, determined sensible lateral thickness variations within the relative sequences. By late Miocene the sedimentary cover was detached from its basement along a mainly evaporitic horizon, and was deformed by means of eastward‐verging folds and thrusts.In order to locate the points where thrust ramps branch‐off the basal detachment, both line‐length and equal‐area techniques were used in the construction of a balanced cross‐section through three major fault‐related folds in southeastern Umbria. The nucleation of thrust ramps was controlled by the occurrence of Jurassic and Cretaceous‐Palaeogene synsedimentary normal faults. These interrupted the lateral continuity of the evaporitic unit (the Late Triassic Anidriti di Burano Fm.) at the base of the sedimentary cover, and acted as obstacles to the eastward propagation of the thrust system, giving rise to major folds which originated from tip‐line folding processes.Therefore, the inferred relationships between ancient normal faults and late thrusts indicate that synsedimentary tectonic structures and the related lateral stratigraphic variations can be envisaged as mechanically important perturbations, which effectively control the nucleation and development of thrust ramps.

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.

The Role of Spaced Cleavage on the Porosity and Permeability within a Reservoir Unit: ABSTRACT

Tectonic deformation plays a significant role in both the enhancement and degradation of the porosity and permeability within a reservoir unit. Enhancing mechanisms generally include fracturing and brecciation, whereas reducing mechanisms include cataclasis and pressure solution. Spaced cleavage is common within argillaceous limestone in foreland fold and thrust belts, including the Sawtooth Range, Montana. The presence of spaced cleavage will have a significant impact on both the permeability and porosity within a reservoir. Spaced cleavage is formed by pressure solution, where calcite is dissolved as a tectonic stress is applied. As the calcite is being removed, relatively insoluble minerals, predominately clay and quartz, are concentrated along the dissolution sites and form thin domains. The minerals within these domains are compacted and aligned perpendicular to the direction of maximum stress. Due to the nature of the aligned minerals, fluid flow will be restricted across these domains, creating severe reservoir anisotropy within the unit. Once the calcite has been dissolved, it must re-precipitate out. This will generally occur in the pores and open fractures present in the cleaved unit, significantly reducing the porosity present in the reservoir rock. However, if the cleavage has been folded during a later deformational event, these domains,more » which represent weaknesses in the rock, could open up around the crests of folds and provide a pathway for increased fluid migration parallel to the cleavage trend.« less

Geochronologic Constraints for the Serrania del Interior Foreland Fold and Thrust Belt, North-Central Venezuela: ABSTRACT

Geochronologic Constraints for the Serrania del Interior Foreland Fold and Thrust Belt, North-Central Venezuela: ABSTRACT

Vertical And Lateral Variations In Fracture Spacing In Folded Carbonate Sections And Its Relation to Locating Horizontal Wells

Abstract Optimum drilling direction and attitude in fractured reservoirs are a function of the width and orientation of the natural fractures present, and the 3-D variation in their fracture intensity or spacing. To make these drilling determinations in fractured carbonate reservoirs, we are faced with determining the relative effect of lithology and structural position on subsurface fracture intensity. Work on several North American folded carbonate sections indicates that weakly deformed or lower curvature portions of the folds display an overall greater stratigraphic variation in fracture intensity than hinge zones or areas of higher curvature. In addition, lithologies exhibiting low fracture intensity off-hinge display larger increases when entering the hinge than those with higher initial off-hinge intensity. The data further indicate that while average fracture intensity is better in hinge zones, flank positions contain layers of optimal properties that have fracture intensities as good if not greater than average intensities in the forelimb or hinge zone. The Conclusion is made that proper deviated or horizontal completions in optimum lithologic layers in flank positions (backlimb or forelimb) could give flow rates as high or higher than average hinge zone completions. Also indicated is a structural style or mode of structural development control on fracture intensity with leading-edge folds containing nearly an order of magnitude more fractures than foreland folds in the same stratigraphic package. In terms of drilling directions, results indicate that backlimb wells should follow optimum stratigraphic horizons, possibly a strike direction; while hinge wells should cross-cut multiple horizons, possibly in a general dip direction. Introduction The porosity and permeability of natural subsurface fracture systems are a function of fracture spacing or intensity (how many fractures) and fracture aperture available for fluid flow (how wide they are). Horizontal wells can be used to optimize the contribution of both parameters in fractured reservoirs (Figure 1). Since we can do little in early exploration to actively high-grade fracture aperture, much of our exploration activity in these reservoirs involves high-grading fracture intensity. Fracture intensity can be defined and predicted by a combination of material property variations (a function of mineral composition, porosity, grain size, and mechanical bed thickness), in situ conditions (depth, pore pressure, temperature, and rate of deformation). and strain distribution within the section (structural position) UI. Because we are mostly interested in determining fracture intensity distributions in individual structures, the environmental parameters at fracturing are usually assumed to have been constant over the vertical and horizontal limits of the field, thus having little effect on relative fracture intensity variations. This leaves us with lithology and structural position as the prime factors to work with in picking optimum well locations, borehole trajectories and completion zones. Past experiences with fractured carbonate reservoirs have led us to conclude that in many cases lithologic variations have a somewhere larger effect on fracture intensity than does structural position. This conclusion will be detailed with use of four carbonate rock sections of similar age and composition in the remainder of this manuscript.

Cleavage development within a foreland fold and thrust belt, southern Pyrenees, Spain

In the southern Pyrenees lithologically distinct cleavage fronts are each parallel to bedding and dip ∼20° towards the foreland. Pressure solution was the dominant mechanism of cleavage development. The mudstone cleavage front is coincident with the ∼195°C paleoisotherm and is associated with a pressure solution strain of ∼5%, a mechanical twin strain of ∼4%, and a deviatoric stress magnitude of ∼65 MPa. Illite crystallinity measurements define a geothermal gradient of 15°C km −1 and indicate that the paleoisotherms are bedding-parallel. Deviatoric stress magnitudes, from calcite twins, were regionally constant at ∼65 MPa and principal stress axes were perpendicular to cleavage. Temperature was the primary control on deformation micromechanisms and the position and orientation of the cleavage front within the foreland thrust wedge. Deformation below the cleavage front occurs predominantly by pressure solution, which in conjunction with mechanical twinning and microfracturing produces a quasi-plastic rheology. Stress magnitudes determined from mechanical twinning of carbonate grains and long-term (10 6–10 76 y) strain rates determined for regional folds and faults suggest an apparent macroscopic viscosity of 9.8 × 10 18 to 7.2 × 10 19 Pa s for the lower thrust wedge. Above the cleavage front temperature, pressure solution strain, total strain, and mesoscale deformation diminish. The region of the thrust wedge above the ∼100°C paleoisotherm is characterized by large brittle faults with cataclastic fault zones and negligible grain-scale deformation indicating an elastico-frictional rheology.

40Ar/39Ar evidence that formation of blueschists in continental crust was synchronous with foreland fold and thrust belt deformation, western Brooks Range, Alaska

Abstract 40Ar/39Ar ages from white mica in rocks of the internal zone of the Brooks Range contractional orogen indicate that the Nanielik antiformal duplex developed at about 120 Ma and was remobilized on its southern boundary at c. 108 Ma. Blueschist facies metamorphism accompanied development of the antiform. The timing of the blueschist facies event and creation of the antiform overlap the period of shallow‐seated deformation in the foreland fold and thrust belt and sedimentation in the foreland basin of the Brooks Range. Blueschist facies P‐T conditions may therefore characterize the thicker parts of orogenic wedges in some orogenic systems; ancient blueschists need not necessarily be interpreted as indicators of active subduction or continent‐continent collision.Microprobe analysis using quantitative wavelength‐dispersive and electron backscattered electron imaging methods was used to characterize the composition of white micas in the dated samples. None of the samples was compositionally homogeneous; many contained 2‐3 populations of white mica, including both potassic and sodic varieties. Samples which had undergone (in sequence) amphibolite, albite‐epidote amphibolite and blueschist facies metamorphic events retained muscovites relict of the amphibolite facies event. Samples that had undergone only the blueschist facies event also contained multiple populations of mica, some probably from detrital sources.

Structural transect of the southern Chihuahua Fold Belt between Ojinaga and Aldama, Chihuahua, Mexico

A restorable structural transect has been constructed across the southern end of the Chihuahua Fold Belt segment of the Cordilleran foreland fold and thrust belt of northeastern Chihuahua, Mexico, and west Texas. The 160‐km transect begins near Ojinaga and ends near Aldama, Chihuahua. It has been found that the Mesozoic Chihuahua Trough was inverted by Laramide tectonism and regionally shortened approximately 20 km (about 9%) along the transect to form the Chihuahua Fold Belt. Along the transect the belt consists of two allochthons of opposing vergence directions. The décollement of the eastern, northeast vergent allochthon shallows from within the Jurassic evaporites, along the center of the belt, and surfaces in Upper Cretaceous clastic rocks at the eastern thrust front south of Ojinaga, Chihuahua. The eastern allochthon was shortened approximately 6 km and may be divided, from west to east, into main ranges and an eastern frontal zone. Structures of the main ranges are detached in Jurassic evaporites and were greatly influenced by syntectonic flowage, resulting in salt‐cored anticlines and salt withdrawal synclines. To the east the décollement shallows over the Tascotal basement block which formed a buttress to northeastward transport forming a frontal zone containing ramp‐related anticlines and emergent thrusts. The western allochthon is southwest vergent. The décollement cuts up section from the center of the belt, where Precambrian basement is detached in the Plomosas Uplift, to Lower Cretaceous carbonates of the western frontal zone. The western allochthon was shortened approximately 20 km at the Paleozoic level and approximately 11 km at the Mesozoic level. Local basement involvement in the Plomosas Uplift may result from strike‐slip deformation along a northwest trending basement fault.

Wrench faulting in the northern Pakistan foreland region

This paper represents an introduction to an extensive geologic and geophysical data set recently acquired by Amoco Pakistan Exploration Company (Apec) in the Kohat Plateau and surrounding regions of northern Pakistan. This data set has given us a comprehensive understanding of the surface and subsurface geology of what has formerly been described as a classic foreland fold and thrust belt, and has lead us to the conclusion that northern Pakistan has been deformed through wrench tectonics, rather than purely compressional tectonics.

Age and regional tectonic implications of Late Cretaceous thrusting and Eocene extension, Cabinet Mountains, northwest Montana and northern Idaho

Research Article| August 01, 1994 Age and regional tectonic implications of Late Cretaceous thrusting and Eocene extension, Cabinet Mountains, northwest Montana and northern Idaho JEFFREY A. FILLIPONE; JEFFREY A. FILLIPONE 1Department of Earth and Space Sciences, University of California, Los Angeles, California 90024-1567 Search for other works by this author on: GSW Google Scholar AN YIN AN YIN 1Department of Earth and Space Sciences, University of California, Los Angeles, California 90024-1567 Search for other works by this author on: GSW Google Scholar Author and Article Information JEFFREY A. FILLIPONE 1Department of Earth and Space Sciences, University of California, Los Angeles, California 90024-1567 AN YIN 1Department of Earth and Space Sciences, University of California, Los Angeles, California 90024-1567 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1994) 106 (8): 1017–1032. https://doi.org/10.1130/0016-7606(1994)106<1017:AARTIO>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 JEFFREY A. FILLIPONE, AN YIN; Age and regional tectonic implications of Late Cretaceous thrusting and Eocene extension, Cabinet Mountains, northwest Montana and northern Idaho. GSA Bulletin 1994;; 106 (8): 1017–1032. doi: https://doi.org/10.1130/0016-7606(1994)106<1017:AARTIO>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 Understanding the kinematic and temporal relationships between the Cordilleran magmatic belt and the foreland fold and thrust belt can provide insights into the development of thrust systems in retro-arc settings. Combining data from field mapping and 40Ar/39Ar geochronometry and thermochronometry, we show that Late Cretaceous thrusting in the Cabinet Mountains of the northwest Montana thrust belt coincided both spatially and temporally with hinterland magmatism. Thrusting and magmatism in the hinterland of the northwest and southwest Montana thrust belt were coeval with development of the foreland fold and thrust belt during the Late Cretaceous. Movement on the Moyie thrust, a principal thrust in the western part of the Purcell anticlinorium, occurred between ca. 71 and 69 Ma. In the Cabinet Mountains, the anticlinorium developed by systems of east- and west- directed trusts, with a southward decrease in slip on the Moyie thrust that the heloped contribute to less overall shortening in the anticlinorium in Montana than in British Columbia. The lithosphere underlying the anticlinorium may have been thermally weakened by magmatic heating, promoting the development of both the Late Cretaceous thrusts and early to middle Eocene normal faults. 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.

Early Palaeozoic foreland thrusting and basin reactivation at the Palaeo-Pacific margin of the southeastern Australian Precambrian Craton: a reappraisal of the structural evolution of the Southern Adelaide Fold-Thrust Belt

Regional and detailed structural mapping, kinematic analysis and balancing and restoration of cross sections has lead to a re-interpretation of the Adelaide Fold Belt in South Australia. In this belt sedimentary rocks of the Late Proterozoic Adelaidean and Early Cambrian Normanville and Kanmantoo sequences were deposited during at least two episodes of subsidence related to major crustal attenuation. Contraction and crustal thickening accompanied by granitoid intrusions are related to the Cambro-Ordovician Delamerian orogeny. During this event both basins were reactivated (“inverted”) and the sedimentary rocks are now incorporated in a WNW-verging foreland fold and thrust belt at the margin of the Proterozoic southeast Australian craton. Maximal shortening of the orogen was around 55%. Within the Adelaidean basin, shortening is dominantly accommodated by major mylonitic shear zones and reverse faults. In the Cambrian Kanmantoo basin, several thrusts are demonstrably reactivated growth faults, across which thickness changes of Cambrian sedimentary rocks are revealed by balancing and restoration of cross sections. Owing to the steep easterly dips of these faults, in the western part of the Kanmantoo basin lateral shortening of up to 58% is largely accommodated by intense folding and fold axial-planar flattening strain. Further east, strain magnitudes wane and the overall shortening of around 30% is accommodated by both, discrete thrust zones and regional folds. The basin reactivation along the Australian part of Gondwana's palaeo-Pacific craton margin reflects the rapid transition from passive to active tectonism, which possibly is a consequence of the disintegration of the formerly conjugate margins of Gondwana and Laurentia.

Structure of the high-grade Opatica Belt and adjacent low-grade Abitibi Subprovince, Canada: an Archaean mountain front

Plutonic gneisses in the Archaean Opatica Belt in Canada record crustal-scale, WSW-vergent (i.e. longitudinal), high-temperature D 1 ductile shearing. Similar structures occur in adjacent greenstone belts, but only in narrow amphibolite facies zones near the base; metavolcanics in the greenschist facies portions do not record penetrative D 1 structures. The greenstone belts are interpreted as allochthonous on the gneisses; they form the uppermost D 1 thrust sheets. SSE-vergent (i.e. approximately transverse) D 2 thrusting imbricated the Opatica Belt and formed a large antiform—either a culmination or an antiformal stack—in the southern Opatica. The imbricated Opatica Belt wedged under the northern part of the Abitibi Subprovince and created a tight syncline with NNW-vergent D 2 thrusts (interpreted as backthrusts) on its northern limb. The Opatica Belt-Abitibi Subprovince contact, therefore, resembles some Phanerozic mountain fronts. Further southwards-propagating imbrication of the crust beneath the Abitibi Subprovince created a S-vergent foreland fold and thrust belt in the overlying greenstones. High-grade Opatica gneisses and adjacent low-grade Abitibi greenstones contain complementary structural patterns and may therefore represent the internal and foreland parts of an Archaean mountain belt, similar in many respects to Phanerozoic examples.

The origin of the West Spitsbergen Fold Belt from geological constraints and plate kinematics: Implications for the Arctic

The West Spitsbergen Fold Belt, which extends for 300 km along the western margin of Svalbard and is up to 80 km wide, has much in common with foreland fold and thrust belts. The near-foreland segment of the fold belt exhibits ramp-flat thrust trajectories whilst structurally higher nappes are typified by more listric thrusts. Higher nappes to the west contain imbricated Carboniferous and basement rocks showing that the latter were actively involved in the fold belt deformation and a minimum of 80 km total shortening perpendicular to the western margin of Svalbard is estimated. The early stages of the Eurekan deformation in North Greenland can be linked to that of the West Spitsbergen Fold Belt and the combined shortening across the two fold belts may exceed 80 km. In Ellesmere Island Eurekan structures are distributed in an arc-like belt which records between 50 to 100 km of shortening since the Late Cretaceous. Kinematic reconstructions suggest that before the opening of the Eurasian Basin and Norwegian-Greenland Sea (Chron 25), Svalbard was linked to North America. In the Late Cretaceous-Palaeocene interval the motion across the Greenland-Svalbard margin, was mainly convergent giving rise to the West Spitsbergen Fold Belt and the Eurekan structures of North Greenland. The dextral separation of Greenland and Svalbard in post-Chron 24 time was accompanied by extension followed by pure extension in post-Chron 13 time.

Tectonic Control of the Sedimentary Record: Constraints from Quantitative Basin Modeling

The incorporation of finite strength of the lithosphere during rifting in models for extensional basin formation in conjunction with temporal changes in tectonic stress levels leads to the prediction of rapid vertical motions in these basins with a rate and magnitude comparable to second- and third-order changes in relative sea level. We present results of modeling simulations, incorporating the interplay of flank uplift and erosion for rifted basins in the northern Atlantic/North Sea area. The incorporation of the mechanical properties of the lithosphere in forward stratigraphic modeling appears also to be of key importance for an accurate prediction of the record of vertical motions in foreland fold and thrust belts. Models invoking the mechanical coupling between plate flexure and near-surface brittle tectonics are capable of producing onlap/offlap patterns in syntectonic basins sometimes strikingly similar to the basin-fill signatures attributed to third-order glacio-eustatic signals. The full incorporation of structural geological constraints in forward modeling of basin stratigraphy proves to be a powerful instrument in linking different temporal and spatial scales in the sedimentary record. This approach also leads to a quantification of the tectonic control of the sedimentary record in frequency bands hitherto primarily attributed to external forcing functions.

Western Newfoundland, Canada, a Lower Paleozoic Basin with Hydrocarbon Potential

Western Newfoundland hosts an early Paleozoic passive margin that was destroyed by later convergence and is now preserved in a foreland fold and thrust belt. Autochthonous Cambrian-Ordovician carbonate and siliciclastic rift, shelf, and foreland basin cover sequences are structurally overlain by allochthonous comprising of coeval deep-water periplatform and oceanic sedimentary, volcanic, and ophiolitic rocks. Geologic relationships indicate that allochthonous deep-water sediments were partly emplaced above the autochthon by Late Ordovician Taconian orogenesis. Recent mapping of a post-Silurian, pre-Mississippian triangle zone in the offshore, and of an imbricated thrust shelf carbonates in the onshore indicates that the present disposition of lower Paleozoic rocks is the product of Acadian foreshortening of at least 100 km. The stratigraphy, geologic setting, and history of the lower Paleozoic rocks and the presence of oil shows in wells, seeps, and bitumin throughout the area mark western Newfoundland as a potential hydrocarbon basin similar to other lower Paleozoic examples dolomite reservoirs are common in the autochthon and occur locally in the allochthon. Source rocks occur in both the deep-water and foreland basin sequences. Allochthonous deep-water shales have total organic carbon values of up to 8.37%, whereas foreland basin black graptolitic shales range up to 2%. Pyrolysis datamore » and conodont color alteration indices document immature to mature thermal maturation in the southern part of the area and mature to overmature maturation in the northern part of the area.« less

Plate Boundary Forces in the Vicinity of Trinidad: the Transition from Transpression to Transtension in the Southern Caribbean Plate Boundary Zones

Deformation in the southern Caribbean plate boundary zones as recorded in the Northern Range of Trinidad initiated in the Oligocene with northward vergent gravity sliding of Northern Range sediments due to uplift and oversteepening of the previously passive margin by the eastward migration of the Caribbean flexural forebulge. Progressive east-southeast transvergence of the Caribbean Plate with respect to South America overthrust incorporated the Northern Range sediments into the Caribbean accretionary prism, thrusting them south-southeast to produce a Middle Miocene transpressive foreland fold and thrust belt in southern Trinidad. Late Miocene deformation within Trinidad was increasingly dominated by right-lateral strike-slop (RLSS) faulting, at the expense of transpressive compressional features. Right-stepping of RLSS motion initiated the Gulf of Paria and Caroni pull-apart basins, Since Early Pliocene these basins and other areas to the north of Trinidad have undergone north-south extension in addition to east-west trending RLSS. Such extension caused the northward withdrawal of Caribbean terranes from atop of the Northern Range, Resulting in rapid isostatically induced uplift (approximately 0.5 mmyr[sup -1]). This change in deformation style may relate to a hitherto unrecognized shift in the relative motion of the eastern Caribbean Plate with respect to South America: from east-southeast-directed transpression to east-northeast-directedmore » transtension.« less

87Sr/ 86Sr dating of coralline algal limestones and its implications for the tectono-stratigraphic evolution of the eastern Prebetic (Spain)

The use of 87Sr/ 86Sr chronostratigraphy for dating marine carbonates is a powerful method in tectono-stratigraphic analysis of fold-and-thrust belts. In this paper, we present the results of 87Sr/ 86Sr dating of Early Miocene coralline algal limestones, which constrain the age of foreland folding in the eastern Prebetic Zone of the Betic Cordilleras (Spain). The first folding phase took place between 23.6 and 19.0 Ma, associated with erosion and the subsequent deposition of coralline algal limestones. Deposition of these limestones was terminated by drowning and subsequent folding between 21.5 and 17.2 Ma. Inferred regional variations in the ages of folds in the order of 1–2 m.y. are thought to reflect the sequential, and therefore intrinsically diachronous growth of fold belts. The depositional history was primarily controlled by these tectonic movements, even though eustasy may have contributed to the drowning of the algal carbonate platforms. The unconformities which bound the algal limestones probably formed in direct response to the thrust emplacement of the Betic nappe pile and reflect the northward migration of the peripheral bulge and foreland basin.

Geometry and kinematic evolution of passive roof duplexes deduced from cross section balancing: Example from the foreland thrust system of the southern Bolivian Subandean Zone

The Subandean Zone of Bolivia is a foreland fold and thrust belt which forms the eastern edge of the central Andes mountains. Between 19°S and 22°S latitude, the construction of five balanced cross sections shows that the N‐S trending Subandean Zone is characterized by the existence of passive roof duplexes. These complex structures can be distinguished by the lithotectonic unit within which duplexing occurs. The five balanced cross sections permit the geometric and kinematic analyses of these passive roof duplexes. The sequential restorations of certain cross sections reveal a possible development of a piggy back sequence of three passive roof duplexes. Apparently, these passive roof duplexes propagated toward the foreland from deeper and deeper lithotectonic units. While a passive roof duplex was developing, the sole thrust stuck and the major horizontal displacements were then transferred either to out‐of‐sequence thrusts or to a new sole thrust, in a deeper detachment horizon. Therefore each passive roof duplex would correspond to the orogenic front of the Andean range at one very particular time in the history of the Subandean Zone of southern Bolivia. From south to north, the quantitative analysis by cross section balancing shows a transfer of displacement from the hinterland structures to the passive roof duplex that forms the present orogenic front. Available data do not permit us to explain completely this phenomenom.

Seismogenic structures activated during the 1987 seismic sequences along the Adriatic coast

SUMMARY In order to obtain a better spatial definition of the structures which were activated during the Marche Adriatic coast seismic sequences in summer 1987 we relocated the earthquakes epicentres by means of an original computer code based on the ‘joint hypocentre determination’ technique. The sequences took place at sites which are considered of moderate seismicity although historical files contain information about stronger events that occurred in the surrounding areas. The activated structures belong to the outer Apennines front range and Adriatic foreland folds, respectively. The spatial trend shown in the results of our seismological analysis agrees with the main geological features known from literature.

Organic maturity in the western Canada sedimentary basin

Organic maturity of strata in the Western Canada Sedimentary Basin has been investigated in numerous studies using petrological, chemical and physical methods. Compilation of these data has facilitated a regional analysis of the maturity patterns and maturation history. Variation in maturity patterns can be divided into three levels; basin wide (1st order), regional (2nd order) and local (3rd order). First order variations are manifested by an overall increase in maturity of strata of the same age from east to west from the Plains to the Rocky Mountain Foothills and Front Ranges in response to progressively deeper burial and higher paleo-geothermal gradients. Superimposed on this first order (basinal scale) variation in maturity are second and third order variations which reflect local differences in depth of burial, conductive and advective heat transport or effects of thrust faulting. Maturity gradients in the axis of the basin are low, averaging 0.10 log% R omax/km, whereas in the Front Ranges and Foothills the gradients are substantially greater (average 0.25 log% R omax/km. Variations in maturity gradients reflect lower paleo-geothermal gradients resulting from rapid sediment loading and subsequent unloading in the Plains, and higher conductive heat transport in the deformed belt. The main component of maturity of most Phanerozoic strata occurred during deep burial under Upper Cretaceous and/or Paleogene molasse in foredeeps developed in response to crustal loading during the easterly migration of the foreland fold and thrust belt. As a result of the west to east propagation of deformation during the Laramide Orogeny, deep burial, maturation, hydrocarbon generation and uplift occurred earlier in the foreland belt (Late Cretaceous) than in the Plains to the east where the main component of maturity and hydrocarbon generation occurred as late as Eocene. In the Plains, because of the low maturity gradients, a thick succession of strata currently is within the oil window. In the deformed belt however, higher maturity gradients have resulted in a thinner succession of strata within the oil window. In response to higher paleo-geothermal gradients, strata in the deformed belt matured more quickly leading to more rapid hydrocarbon generation and migration than areas to the east. In the southern part of the Cordillera a significant component of maturation post-dates structural deformation of the strata whereas in northeastern British Columbia and adjacent parts of Alberta, maturity primarily pre-dates structural deformation. Data from some deep bore holes and surface samples in the southern part of the Cordillera provide evidence for maturity post-dating or accompanying emplacement of major overthrust sheets (tectonic burial). In most areas however, there is no clear evidence for the timing of maturity relative to faulting or evidence of frictional heating. The maturity gradients and levels of maturity of surface and near surface strata in the western Plains and Rocky Mountain Foothills indicate that a succession (several kilometres) of Paleogene strata were deposited in these regions (foredeep) and subsequently eroded in response to post-Laramide uplift.