Adjacent Foreland Basin Research Articles

Upright emplacement of the Gibraltar slab and the origin of rifting in adjacent foreland and Backarc Basins

Back-arc basins in subduction systems usually form following discrete subduction-related extension stages. However, in the westernmost Mediterranean, the initiation of Cenozoic extension associated with the Valencia Trough and Algerian Sea Basin introduced two extension centers nearly simultaneously in the Gibraltar subduction system, and this presents a challenge to understanding their underlying dynamics. We built three-dimensional time-dependent geodynamic models constrained by tectonic reconstructions to determine the role that the Gibraltar subduction played in shaping unconventional extensional basins. Our results suggest that the Algerian Sea Basin is a typical back-arc basin, whereas the Valencia Trough opened on the subducting plate during the rotational rollback of the Gibraltar slab. Moreover, the Valencia Trough extension likely delayed slab tearing and guided slab sinking before its final detachment, facilitating its ultimate upright positioning. We show how plates and the mantle interact dynamically in a subduction system where the slab asymmetrically rolls back.

Geochronology of reworked ash and its implications for accommodation space variations in distal foreland basins, McMurray Formation, Alberta, Canada

A bentonite bed consisting of reworked volcanic ash from the Barremian to Aptian lower McMurray Formation is dated using chemical abrasion thermal ionization mass spectrometry (CA-TIMS) on zircon. The bentonite bed is from a cored interval in the McKay Paleovalley in the northern Athabasca Oil Sands Region of Alberta, Canada. Deposition of the lower McMurray Formation took place in the distal portion of the Western Canada Sedimentary Basin early in the Cretaceous-to-Paleocene phase of widespread contractional deformation in the Canadian Cordillera. A 30 g sample of volcanic ash yielded >10,000 very small (<50 µm) zircon grains with the vast majority (~99.7%) being highly abraded and clearly detrital. Twenty-six sharply faceted grains were sufficiently large to be mounted for laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analysis, which yielded seven Lower Cretaceous dates. These Lower Cretaceous grains were further analyzed using CA-TIMS. The five youngest CA-TIMS dates agree with a weighted mean of 121.51 ± 0.11 Ma which is conservatively interpreted as a maximum depositional age (MDA). The good agreement between the five zircon dates indicates the MDA is probably the depositional age. This study demonstrates that with careful analysis of zircon populations, high-confidence MDAs can be derived even from reworked ash beds that are dominated overwhelmingly by detrital zircon. The date from this study overlaps a previously published MDA of 121.39 ± 0.27 Ma for the top of the lower McMurray Formation in Firebag Tributary in the northeastern Athabasca Oil Sands Area. The overlapping dates establish a widespread chronostratigraphic surface within the northern Athabasca Oil Sands Area. Both ages are from ash beds preserved in coal seams, suggesting that the coal-bearing interval is a regional marker bed and that the ash beds may form an intraformational datum. Further, the occurrence of the ash beds in the McMurray Formation in close proximity to regional flooding surfaces suggests that tectonic activity in the emerging Canadian Cordillera influenced accommodation space variations in the distal portions of the adjacent foreland basin.

Cenozoic Shortening and Propagation in the Eastern Kuqa Fold‐And‐Thrust Belt, South Tian Shan, NW China

AbstractIn an orogen‐foreland basin system, syn‐orogenic sedimentation and deformation records within the foreland basin provide critical evidence for understanding uplift and erosion processes of adjacent orogenic belts. Foreland fold‐and‐thrust belts (FTBs) are widely developed around the Tian Shan range, Central Asia. Problem of how crustal shortening accumulates and propagates from the Tian Shan to adjacent foreland basins is essential for understanding the overall dynamics of Tian Shan. In this study, we interpreted three high‐resolution seismic reflection profiles to estimate the magnitude and distribution of Cenozoic shortening across the Eastern Kuqa FTB, South Tian Shan foreland. Combined with well‐dated syntectonic stratigraphy, we further evaluate the timing, rate, and migration of the deformation front of the Kuqa FTB. Our results suggest that the Kuqa FTB's total crustal shortening increases westward. The distribution of crustal shortening across secondary structural belts shows a gradual, basinward decrease, which can be explained by the telescoped mechanism. Propagation rates of the Kuqa FTB's deformation front indicate an episodic propagation of the foreland thrust wedge, with an acceleration after ∼12 Ma. Similar episodic propagation pattern has also been observed in other foreland FTBs around the Tian Shan, but they show significant spatio‐temporal variations in the propagation stages and related rates.

STRUCTURE AND EVOLUTION OF THE TEREK‐CASPIAN FOLD‐AND‐THRUST BELT: NEW INSIGHTS FROM REGIONAL SEISMIC DATA

The Terek‐Caspian fold‐and‐thrust belt along the northern flank of the Greater Caucasus Mountain Range together with the adjacent foreland basin is one of the oldest oil‐producing regions in Russia. Despite the long history of exploration, recently acquired seismic data has provided new insights about the structural architecture and evolution of this area. Its structural development during the Neogene was constrained by a syn‐extensional tectonic fabric inherited from Jurassic rifting and extension of the Great Caucasus Basin. The structural framework of this basin controlled the distribution of syn‐extensional deposits, and the Cenozoic reactivation of lateral ramps resulted in along‐strike variations in structural style. Thus western, central and SE segments of the Terek‐Caspian foldbelt are recognised and are referred to here as the Terek‐Sunzha fold zone, the Dagestan Promontory, and the Maritime Zone in southern Dagestan.Three principal episodes of Cenozoic compression in the Terek‐Caspian fold‐and‐thrust belt took place. The first episode in the Oligocene resulted in the inversion of pre‐existing normal faults with the coeval development of a foreland basin to the north of the thrust belt. The dominance of sediments of northerly provenance in the foreland basin suggests there was only moderate uplift of the Greater Caucasus at this time. However, significant uplift of the orogenic belt took place during later phases of Sarmatian (Late Miocene) and Akchagylian (Late Pliocene) compression. Erosion of the uplifting Greater Caucasus gave rise to the development of large‐scale, northerly prograding clinoforms which are clearly observed on seismic profiles in the foreland basin. Shortening was largely accommodated by wedge‐shaped thrusting facilitated by the presence of mechanically weak stratigraphic units.Structural development of the Terek‐Sunzha fold zone in the west of the Terek‐Caspian fold‐and‐thrust belt was largely controlled by a Tithonian salt layer which provided an efficient basal detachment surface and which also supplied material to squeezed diapirs in front of the belt. To the east, the plan‐view shape and internal architecture of the Dagestan Promontory were influenced by the areal extent of the Lower‐Middle Jurassic depocentre of the palaeo‐Volga delta which is up to 10 km thick. In the Maritime Zone, the style of shortening was mostly controlled by the presence of a pre‐existing structural high composed of folded Palaeozoic‐Triassic strata in front of the fold‐thrust belt.

EXPLORATION PLAYS IN THE CAUCASUS REGION

Exploration efforts around the Greater Caucasus region started towards the end of the 19th century and established a wide range of petroleum play types in various basin segments around the orogen. All these plays are associated with the flanks of the inverted thrust‐fold belt and the adjacent foreland basin systems, but display significant variation among the basin segments depending on the tectonostratigraphic units involved and the degree of exploration maturity. Whereas the same main source rocks have generated most of the hydrocarbons in all the basins (namely organic‐rich shales in the Oligocene – Lower Miocene Maykop Group and the Eocene Kuma Formation), it is primarily the trapping style, both proven and speculative, which is responsible for the broad spectrum of play types observed. Eleven play type diagrams across six main petroleum provinces of the Greater Caucasus region are presented in this paper and summarize the current exploration understanding of the existing discoveries and potential new play targets. These play cartoons offer a prospect‐scale summary of both mature producing and underexplored basin segments in a coherent visual manner, and are therefore intended to promote future exploration efforts in the Caucasus region. The testing of new play types requires the proper risking of the two most critical elements in the region: hydrocarbon kitchen effectiveness, and post‐charge trap modification. The de‐risking of these factors will require properly designed, fit‐for‐purpose acquisition of modern geological and geophysical data sets.

3D crustal structure of the Eastern Alpine region from ambient noise tomography

The tectonic evolution of the European Eastern Alps within the Alpine orogeny is still under debate. Open questions include: the link between surface, crustal and mantle structures; the nature of the Moho gap between the two plates; the relationship between the Alps, the adjacent foreland basin and the Bohemian Massif lithospheric blocks. We collected one year of continuous data recorded by ~250 broadband seismic stations –55 of which installed within the EASI AlpArray complementary experiment– in the Eastern Alpine region. Exploiting surface wave group velocity from seismic ambient noise, we obtained an high-resolution 3D S-wave crustal model of the area.The Rayleigh-wave group-velocity from 3 s to 35 s are inverted to obtain 2-D group velocity maps with a resolution of ~15 km. From these maps, we determine a set of 1D velocity models via a Neighborhood Algorithm, resulting in a new 3D model of S-wave velocity with associated uncertainties. The vertical parameterization is a 3-layer crust with the velocity properties in each layer described by a gradient. Our final model finds high correlation with specific geological features in the Eastern Alps up to 20 km depth, the deep structure of the Molasse basin and important variations of crustal thickness and velocities as a result of the Alpine orogeny post-collisional evolution. The strength of our new information relies on the absolute S-wave crustal velocity and the velocity gradient unambiguously sampled along the Moho, only limited by the amount and quality distribution of the data available.

A review of the crustal architecture and related pre-salt oil/gas objectives of the eastern Maghreb Atlas and Tell: Need for deep seismic reflection profiling

The Atlas and Tell domains constitute a major segment of the Circum-Mediterranean Alpine fold and thrust belts in the Maghreban part of North Africa. South of the Tell, the Atlas results from a progressive Late Cretaceous to present foreland inversion of former Jurassic to Cretaceous extensional tectonic features inherited from the opening of the Tethys along the northern African margin. In contrast, the Tell is made up of allochthonous Cretaceous to Neogene basinal units detached along a tectonic sole made up of remobilized Triassic salt and tectonic mélanges.Since the last decade, the major tectonic steps accounting for the development of the eastern Maghreb structures are becoming more precisely constrained by seismic data and associated structural analyses. However their overall configuration at depth, as well as the deep architecture of the underlying basement remain poorly understood because only few wells yet penetrated deeper than the Triassic series in the Atlas and adjacent foreland basin. Therefore, this paper is intended to give an overview on the deep structural features affecting the Pan-African basement in the eastern Maghreb by means of tentative regional cross-sections. Our purpose is to stimulate exploration of deep hydrocarbon plays and give some insights to help both the industry, universities and other public research institutes to undertake jointly a major research program on the deep crustal configuration in the eastern Maghreb with particular focus on the pre-Triassic series beneath the Tellian allochthon in the north, the parautochthonous inverted Atlasic structures in the center, and in the adjacent autochthonous foreland domain extending southeastward in the offshore.

Quantifying vertical movements in fold and thrust belts: subsidence, uplift and erosion in Kurdistan, northern Iraq

Abstract Traditional structural analysis in fold and thrust belts has focused on quantifying horizontal movements. In this paper, the importance of quantifying vertical movements is illustrated using a case study from Kurdistan, northern Iraq. The subsidence history of this area can be determined by analysis of the stratigraphic record from deep exploration wells. A phase of thermal subsidence from Middle Permian to Late Cretaceous (tectonic subsidence 1.8–1.9 km) was followed by flexural subsidence in the Late Cretaceous and Cenozoic (tectonic subsidence >0.6 km) in response to the closure of the Neo-Tethys Ocean. The main phase of continental collision during the Neogene resulted in the development of the Zagros fold and thrust belt; the amount of uplift at individual anticlines can be estimated from their amplitude (up to 3 km), but regional cross-sections indicate that approximately 1 km of additional basement-involved uplift is present NE of the Mountain Front. The timing of basement-involved uplift is interpreted to be coeval with the deposition of a Pliocene–Quaternary growth sequence adjacent to the Mountain Front. The amount of erosion resulting from the uplift can be estimated from vitrinite reflectance and cross-sections; these estimates show a similar pattern, with maximum erosion in the mountains NE of the Mountain Front (>1.5 km) and lesser erosion in the adjacent foreland basin (generally <0.8 km). The results provide a quantitative understanding of subsidence, uplift and erosion, and have been used to define prospective and high-risk areas for petroleum exploration.

Re-evaluation of polyphase kinematic and 40Ar/39Ar cooling history of Moldanubian hot nappe at the eastern margin of the Bohemian Massif

A structural and geochronological 40Ar/39Ar study was performed in kilometre-scale middle and lower crustal lens-shaped domains dominated by a preserved subvertical foliation, surrounded by horizontally foliated migmatites. These domains occur within the Moldanubian nappe overlying the Brunia microcontinent at the eastern margin of the European Variscides. Three main deformation phases were recognized: subvertical S2 fabric trending NW–SE in lower crustal rocks and NE–SW in mid-crustal rocks. It is reworked by HT/MT horizontal fabric S3 along margins of crustal domains and in surrounding migmatites. S3 bears a prolate NE lineation parallel to the S2–S3 intersection in the lower crustal domain. In the middle crustal units, L3 is weak, connected to oblate strain and trends NE–SW parallel to the S2–S3 intersection. D4 non-coaxial shear deformation is mainly localized at the boundary between the Moldanubian nappe and Brunia and bears strong top to the NNE shear criteria. In order to constrain kinematics of the D3 deformation, strain modelling was performed to show that the Moldanubian hot nappe was frontally thrust over the Brunia indentor. The renewed D4 tangential movement only heterogeneously reactivates the horizontal S3. This evolution is recorded in 40Ar/39Ar amphibole cooling ages, which show two statistically significant Carboniferous peaks at ~342 and 332 Ma, which are also reflected by published detrital muscovite 40Ar/39Ar ages in the adjacent foreland basin. This geochronological record is correlated with progressive erosion of the topographically elevated upper crustal part of the Moldanubian nappe during D3 frontal thrusting, followed by greenschist facies D4 transpressive reactivation and subsequent erosion of high-grade parts of the nappe.

Flexural bending of southern Tibet in a retro foreland setting.

The highest elevation of the Tibetan Plateau, lying 5,700 m above sea level, occurs within the part of the Lhasa block immediately north of the India-Tibet suture zone (Yarlung Zangbo suture zone, YZSZ), being 700 m higher than the maximum elevation of more northern parts of the plateau. Various mechanisms have been proposed to explain this differentially higher topography and the rock uplift that led to it, invoking crustal compression or extension. Here we present the results of structural investigations along the length of the high elevation belt and suture zone, which rather indicate flexural bending of the southern margin of the Lhasa block (Gangdese magmatic belt) and occurrence of an adjacent foreland basin (Kailas Basin), both elements resulting from supra-crustal loading of the Lhasa block by the Zangbo Complex (Indian plate rocks) via the Great Counter Thrust. Hence we interpret the differential elevation of the southern margin of the plateau as due originally to uplift of a forebulge in a retro foreland setting modified by subsequent processes. Identification of this flexural deformation has implications for early evolution of the India-Tibet continental collision zone, implying an initial (Late Oligocene) symmetrical architecture that subsequently transitioned into the present asymmetrical wedge architecture.

Cenozoic sedimentation and exhumation of the foreland basin system preserved in the Precordillera thrust belt (31-32°S), southern central Andes, Argentina

Andean retroarc compression associated with subduction and shallowing of the oceanic Nazca plate resulted in thin-skinned thrusting that partitioned and uplifted Cenozoic foreland basin fill in the Precordillera of west-central Argentina. Evolution of the central segment of the Precordillera fold-thrust belt is informed by new analyses of clastic nonmarine deposits now preserved in three intermontane regions between major east directed thrust faults. We focus on uppermost Oligocene-Miocene basin fill in the axial to frontal Precordillera at 31–32°S along the Rio San Juan (Albarracin and Pachaco sections) and the flank of one of the leading thrust structures (Talacasto section). The three successions record hinterland construction of the Frontal Cordillera, regional arc volcanism, and initial exhumation of Precordillera thrust sheets. Provenance changes recorded by detrital zircon U-Pb age populations suggest that initial shortening in the Frontal Cordillera coincided with an early Miocene shift from eolian to fluvial accumulation in the adjacent foreland basin. Upward coarsening of fluvial deposits and increased proportions of Paleozoic clasts reflect cratonward (eastward) advance of deformation into the Precordillera and resultant structural fragmentation of the foreland basin into isolated intermontane segments. Apatite (U-Th)/He thermochronometry of basin fill constrains to 12–9 Ma the most probable age of uplift-induced exhumation and cooling of Precordillera thrust sheets. This apparent pulse of exhumation is evident in each succession, suggestive of rapid, large-scale exhumation by synchronous thrusting above a single decollement linking major structures of the Precordillera.

A model for post‐orogenic development of a mountain range and its foreland

AbstractDecaying mountain ranges often show a surprisingly dynamic pattern of landscape evolution. Although one might expect a simple, monotonic decline in relief over time, evidence from several inactive mountain ranges shows alternating sequences of deposition and erosion in the associated basins, suggesting variations in relief and exhumation rate in the ranges themselves. Examples include the Southern Rocky Mountains, the Pyrenees, the European Alps and the Atlas Mountains. In this paper, we explore the possible origins of post‐orogenic landscape dynamics using a simple mathematical model of a mountain range and an adjacent foreland basin. The analysis highlights the importance of mass balance. In particular, a switch from basin exhumation to renewed sedimentation requires either an increase in sediment influx from the range or a decrease in sediment outflux beyond the basin margin. Although it is widely understood that post‐orogenic changes in erosion and sediment flux can have multiple causes (including climate change, regional tectonic uplift or tilting, or exhumation of variable lithologies), an important implication of our analysis is that the impact of such changes must differ in sign or magnitude between the range and the basin to be recorded. This requirement places an important constraint on viable explanations for alternating sequences of deposition and erosion in a decaying mountain‐basin pair.

Miocene paleotopography of the Central Alps

Reconstructing surface elevation, surface uplift, and relief histories is fundamental to understanding the growth of mountain ranges, to explore their topographic limits, and relate these to the interplay between geodynamic and Earth surface processes. Here, we aim to quantify Miocene paleoelevation of the Central European Alps through stable isotope paleoaltimetry. The novelty of our approach, which renders our analysis less sensitive to past climate change, is that stable isotope proxies of identical age are analyzed from both the high internal parts of the Alpine orogen and the adjacent foreland basin. Whereas the foreland basin was at or near sea level and traces the combined effects of upstream changes in rainfall amount and composition, the high Alpine site additionally records the effect of elevation on stable isotopes in precipitation. We compare hydrogen isotopic ratios (δD) in mica and chlorite that interacted with meteoric water along the Simplon detachment, a major normal fault that developed at high elevations, with meteoric water compositions deduced from carbonate-bearing paleosols of the North-Alpine foreland basin.δD values of muscovite (−126‰) and chlorite (−135‰) from the brittle hanging wall and of recrystallized muscovite (−108‰) and biotite (−140‰) from adjacent footwall mylonites provide unequivocal evidence for localized syntectonic meteoric water interaction along the Simplon detachment. Detailed 40Ar/39Ar and fission track geochronology constrains the timing of isotopic exchange to ca. 14.5Ma, when the footwall mylonites passed through the ductile to brittle transition. Age-equivalent oxygen isotope ratios (δ18O) measured within pedogenic carbonate from North-Alpine foreland paleosols that developed near Miocene sea level serve as our low-elevation point of reference. δ18O values in these paleosols, dated with ca. 100ka precision, vary between +19 and +25‰ (SMOW) with average values of +20‰ at ca. 14.5Ma. Using the relative differences between meteoric water compositions in the foreland basin and the high Alpine Simplon detachment, our isotope data are consistent with a mid-Miocene minimum average elevation difference of 2350 (+700/−500) m for the Simplon region. Our results indicate that Miocene Alpine elevations were at least comparable to those of today and are likely to have acted as an important barrier to Atlantic-derived moisture transport into central Europe and Eurasia since that time.

Early Cenozoic faulting of the northern Tibetan Plateau margin from apatite (U–Th)/He ages

Models to explain the distributed nature of continental deformation predict the propagation of strain and high topography away from the plate boundary. Yet a growing body of evidence in the Tibetan orogen suggests that deformation occurred at the far northern extent of the modern plateau early in the orogen's history and thus our current mechanical understanding of orogenic plateau development is incomplete. New apatite (U–Th)/He ages from four elevation transects document periods of rapid exhumation related to erosion pulses in hanging wall rocks of major thrust structures. Accelerated erosion is used as a proxy of fault timing, and is interpreted in a larger context of structural data and sediment accumulation in adjacent foreland basins. Helium results are synthesized with published geologic, thermochronometric, and sedimentologic data from which a growing picture of regional compressional deformation in Middle to Late Eocene time in northern Tibet emerges. We relate the early Cenozoic period of deformation to the initiation of collision between India and Eurasia, despite the fact that the plate boundary was located > 3000 km to the south. Regardless of whether or not high topography was built simultaneously as a result of this deformation, early Cenozoic strain signifies that the modern limit of the orogen has been relatively stationary since continental collision began and that deformation has not significantly propagated farther away from the plate boundary in time.

From source to sink: Preserving the cosmogenic 10Be-derived denudation rate signal of the Bolivian Andes in sediment of the Beni and Mamoré foreland basins

The denudation rate signal of the Bolivian Andes as measured by cosmogenic 10Be in sediment is preserved in the floodplain of adjacent foreland basins even though these basins store the sediment for thousands of years. This conclusion is drawn from comparing published Andean source area denudation rates with new cosmogenic 10Be data as measured in the floodplains of the large Beni and Mamoré basins. For the entire Beni basin including the sediment-producing Andes and the vast flooded plains of the foreland, the cosmogenic nuclide-derived denudation rate is 0.45 ± 0.06 mm/yr, while that of the Mamoré basin is 0.55 ± 0.19 mm/yr. By comparison, the respective Andean source areas erode at averaged rates of 0.37 ± 0.06 mm/yr (upper Beni), and at 0.56 ± 0.09 mm/yr (upper Mamoré). We notice a decrease in variability of denudation rate with increasing spatial scale as small-scale processes are averaged out. Sediment mixing within the floodplain damps scatter of nuclide-derived denudation rates picked up in the source area. On the temporal scale, a remarkable agreement between cosmogenic nuclide-derived rates averaging over a few kiloyears with those from fission track dating averaging over millions of years seems to suggest that cosmogenic nuclide-derived denudation rates capture the long-term erosional characteristics of the mountain belt. The sum of these observations suggests that any sample collected along a river traversing a floodplain will yield the denudation rate of the source area. This finding opens the unique possibility of constraining cosmogenic nuclide-derived paleo-sediment budgets for these large basins as the long-term, spatially-averaged denudation rate signal of the sediment-producing area is preserved in sedimentary archives.

Tectonic control of erosion and sedimentation in the Amazon Basin of Bolivia

AbstractThe western Amazon drainage basin, which extends from southern Colombia to northern Bolivia, comprises the Cordillera Oriental of the Andes and its adjacent foreland basin system. In northern Bolivia, the orogenic wedge of the eastern Andes is very large, and its forward propagation controls the morphology of the Madeira drainage basin. We consider here the erosion and sedimentation mass balance in this part of the Amazon Basin, estimated on the basis of recent sediment yield data, within the current tectonic and geomorphic framework. The total suspended sediment (TSS) flux exported from the present orogenic wedge of northern Bolivia has been estimated at 500–600 million t year−1. More than 50% of the total sediment load crossing the Madeira foreland basin system is deposited. The rest of the sediments (less than 46%) reaches the eastern Amazon Basin, bypassing the Brazilian craton to the north. The average mass of sediment that has been deposited from the late Miocene to the present in the Madeira foreland basin sedimentation system is less than that intercepted today, by a factor of about 2·4. These results can be interpreted as an increase in Bolivian foreland basin flexural subsidence over time, associated with crust thickening and orogenic loading, and accentuated by the growing mass of retained sediments. They are consistent with the uplift rates of the Cordillera Oriental, obtained from fission‐track dating, which began increasing significantly around 10–15 Ma. Copyright © 2009 John Wiley & Sons, Ltd.

An integrated analysis of an orogen-sedimentary basin pair: Latest Cretaceous-Cenozoic evolution of the linked Eastern Cordillera orogen and the Llanos foreland basin of Colombia

The integration of restored basin geometry and internal features of syntectonic units (e.g., stratal architecture, thickness, sandstone composition) with fl exural modeling of the lithosphere constrains the evolution of a basin and its fl exural history related to orogenic growth (spatial/temporal loading confi guration). Using this approach, we determined the Maastrichtian-Cenozoic polyphase growth of the Eastern Cordillera of Colombia, an inverted Mesozoic extensional basin. The record of this growth occurs in an Andean (post‐middle Miocene) thrust belt (the Eastern Cordillera) and in adjacent foreland basins, such as the Llanos Basin to the east. This approach permitted the identifi cation of fitectono-stratigraphic sequences in the foreland basin and fi ve phases of shortening for the Eastern Cordillera. Thermochronological and geochronological data support the spatial and temporal evolution of the orogen‐foreland basin pair. Tectono-stratigraphic sequences were identifi ed in two restored cross sections, one located at a salient and the other in a recess on the eastern fl ank of the Eastern Cordillera. The lower two sequences correspond to late Maastrichtian‐Paleocene fl exural events and record the eastward migration of both tectonic loading and depositional zero in the Llanos Basin. These sequences consist of amalgamated quartzarenites that abruptly grade upward to organic-rich fi ne-grained beds and, to the top, light-colored mudstones interbedded with litharenites in isolated channels. Amalgamated conglomeratic quartzose sandstones of the third sequence record ~15 m.y. of slow subsidence in the Llanos Basin and Llanos foothills during early to middle Eocene time, while shortening was taking place farther west in the Magdalena Valley. The fourth sequence, of late Eocene‐ middle Miocene age, records a new episode of eastward migration of tectonic loads and depositional zero in the Llanos Basin. This sequence begins with deposition of thick fi ne-grained strata to the west, whereas to the east, in the Llanos basin, amalgamated quartzarenites unconformably overlie Cretaceous and older rocks (former forebulge). Apatite fi ssion tracks in the axial zone of the Eastern Cordillera, growth strata in the

Upper Cretaceous to Paleogene Forbulge Unconformity Associated With Foreland Basin Evolution (Kras, Matarsko Podolje and Istria; SW Slovenia and NW Croatia)

A regional unconformity separates the Cretaceous passive margin shallow-marine carbonate sequence of Adriatic Carbonate Platform from the Upper Cretaceous and/or Paleogene shallow-marine sequences of synorogenic carbonate platform in southwestern Slovenia and Istria (a part of southwestern Slovenia and northwestern Croatia). The unconformity is expressed by irregular paleokarstic surface, locally marked by bauxite deposits. Distinctive subsurface paleokarstic features occur below the surface (e.g. filled phreatic caves, spongework horizons…). The age of the limestones that immediately underlie the unconformity and the extent of the chronostratigraphic gap in southwestern Slovenia and Istria systematically increase from northeast towards southwest, while the age of the overlying limestones decreases in this direction. Similarly, the deposits of synorogenic carbonate platform, pelagic marls and flysch (i.e. underfilled trinity), deposits typical of underfilled peripheral foreland basin, are also diachronous over the area and had been advancing from northeast towards southwest from Campanian to Eocene. Systematic trends of isochrones of the carbonate rocks that immediately under- and overlie the paleokarstic surface, and consequently, of the extent of the chronostratigraphic gap can be explained mainly by the evolution and topography of peripheral foreland bulge (the forebulge). The advancing flexural foreland profile was the result of vertical loading of the foreland lithospheric plate (Adria microplate) by the evolving orogenic wedge. Because of syn- and post-orogenic tectonic processes, and time discrepancy between adjacent foreland basin deposits and tectonic (“orogenic”) phases it is difficult to define the exact tectonic phase responsible for the evolution of the foreland complex. According to position and migration of the subaerially exposed forebulge, distribution of the foreland related macrofacies and orientation of tectonic structures, especially of Dinaric nappes, and Dinaric mountain chain I suggest that the foreland basin complex in western Slovenia and Istria was formed during mesoalpine (“Dinaric”) tectonic phase due to oblique collision between Austroalpine terrane/Tisia microplate and Adria microplate when probably also a segmentation of the foreland plate (Adria microplate) occurred.

Late orogenic vertical movements in the Carpathian Bend Zone – seismic constraints on the transition zone from orogen to foredeep

ABSTRACTPostcollisional tectonic movements in orogens and their adjacent foreland basins related to intraplate stresses and the presence of a remnant slab are likely to induce significant deformations overprinting the existing patterns of nappe emplacement. In the Carpathian Bend Zone, Romania, vertical motions associated with very limited postorogenic intraplate shortening are of similar magnitude as those generally caused by large orogenic deformations. In the Latest Miocene–Pliocene, up to 6 km of postcollisional sediments of remarkably parallel stratification were deposited in a basin extending over a large part of the present‐day orogen. The Early Quaternary featured a dramatical change as the orogen was uplifted while subsidence continued in the basin, tilting the basin flank adjacent to the orogen to a vertical position. The remnant slab presently below the Bend zone in Vrancea is the prime mechanism to have driven the Pliocene subsidence. The Quaternary changes and the eastwards migration of the pattern of vertical motions can be explained by large‐scale folding, in response to the overall compressive regime that is recorded in the whole Pannonian‐Carpathian area.

Sevier orogenesis and nonmarine basin filling: Implications of new stratigraphic correlations of Lower Cretaceous strata throughout Wyoming, USA

Lower Cretaceous nonmarine rocks throughout Wyoming have been used to date movements on major thrusts during the Sevier orogeny, evaluate the sedimentary response in the adjacent foreland basin, reconstruct subsidence histories, evaluate the driving mechanisms of that subsidence, and have been applied to basin modeling studies. However, detailed correlation and dating of these strata have been problematic for a century, making most detailed interpretations equivocal. New age data and correlations presented here have important implications for the evolution of the Sevier orogen and controls on basin fi lling. Lower Cretaceous strata in Wyoming show the former existence of a foreland basin fi ll that displays a thick foredeep depozone to the west (the Gannett Group), a thin forebulge depozone throughout central Wyoming (the Cloverly Formation), and thickening into a backbulge depozone in the Black Hills area to the east (the Lakota Formation). Basin subsidence is attributable not only to orogenic loading, but to dynamic loading as well. New data and correlations indicate that a thrust load was in place during the Neocomian (attributable to movement on the Paris-Willard thrust system) and that sediment supply was high (as represented by the upper Ephraim Formation). Subsequent deposition of the Peterson Limestone in the foredeep during the latest Neocomian and Aptian, and the development of a regional unconformity elsewhere, indicate a reduction in sediment supply that may have been caused by (1) a change to a drier climate, possibly related to progressive mountain-belt uplift and development or intensifi cation of an orographic rain shadow, or (2) a decrease in mountain-belt uplift rate and a reduction in mountain-belt relief. Unconformity development on the forebulge is consistent with the fi rst scenario. Subsequent deposition of the Bechler, upper Cloverly, and upper Lakota Formations during the Early to Middle Albian indicates increased sediment supply, progressive unroofi ng of the mountain belt, and movement on the Meade-LaketownParis-Willard thrust system. These events suggest renewed or accelerated mountainbelt uplift during the early to middle Albian. The Draney Limestone, which overlies the Bechler, correlates with an unconformity at the top of the Cloverly and Lakota Formations and is Albian in age. The Draney Limestone and the unconformity signify a return to conditions similar to those that existed during deposition of the Peterson Limestone. The results presented here differ from those of some researchers who contend that a foredeep did not exist during initial deposition of these strata, and that deposition was not coincident with thrusting during early stages of the Sevier orogeny.