ABSTRACT Deep seismic profiles, recorded in the foothills of the Northern Emirates, image the thrust-belt architecture and document the wide underthrusting of Mesozoic sedimentary units in the footwall of the Hawasina–Sumeini allochthon in the Dibba Zone, beneath the Semail Ophiolite. Integrated structural and geophysical modeling helped to constrain the structural architecture of two regional transects crossing the foreland and adjacent foothills. 2-D forward kinematic and thermal modeling was performed with Thrustpack® along the transects, whereas CERES2D® complete petroleum system modeling was subsequently performed along the northern transect. One hundred twenty kilometers (75 mi) of convergence occurred from the Santonian to the end of Early Miocene, of which about 80 km (50 mi) correspond to the obduction of the Semail Ophiolite and Sumeini–Hawasina units over the Arabian margin, whereas the remaining approximately 40 km (25 mi) were accommodated by the fold-and-thrust structures of the Oman belt. Paleogene source rocks of the foredeep only reached the beginning of the oil window. In contrast, Mesozoic source rocks of the underthrusted foreland are overmature or in the gas window in the foothills, but still preserve hydrocarbon (HC) potential further west in the foreland. Frozen kitchens may still be preserved in the hinterland, due to the high thermal conductivity of its former ophiolitic cover.

ABSTRACT The Papua New Guinea fold and thrust belt petroleum system is studied along a 200-km (124-mi)-long transect. The kinematic scenario includes the Jurassic rifting and passive margin, the erosion during the Upper Cretaceous related to the Coral Sea rifting and Pliocene–Pleistocene shortening, with an early growth of the Hedinia Anticline limiting lateral migration of oil in the adjacent Darai Plateau. Data from seven wells and two fields were used to calibrate section boundary conditions and properties. Apart from the high-pressure trend in the Kutubu/Moran structures, all data are well reproduced, and the modeled section appears quantitatively predictive. The modeling demonstrates three major pathways for water: (1) topographically driven flow from the onset of mountain building; (2) deep updip basinal flux, flowing along the tilted reservoirs; and (3) across fault escape from connected reservoir bodies. Type II or mixed type II/III is used to model the Triassic and Jurassic source rock. Maturation starts in the Middle Cretaceous and increases strongly during the late tectonic burial, with three main accumulations: (1) the deep part of the Mubi zone, with vertical migration along faults; (2) the Hedinia and Kutubu anticlines charged during Orubadi and Era deposition; and (3) the Darai Plateau.

Abstract Continental margins and their fossilized analogues are important repositories of natural resources. With better processing techniques and increased availability of high-resolution seismic and potential field data, imaging of present-day continental margins and their embedded sedimentary basins, in which the majority of these resources are located, has reached unprecedented levels of refinement and definition, as illustrated by papers in this volume. This, in turn, has led to greatly improved geological, geodynamic and numerical models for the crustal and mantle processes involved in continental-margin formation from the initial stages of rifting through to continental rupture and break-up, to the eventual development of a new ocean basin. Further informing these models, and contributing to a better understanding of the features imaged in the seismic and potential field data, are observations made on fossilized fragments of exhumed subcontinental mantle lithosphere and ocean–continent transition zones preserved in ophiolites and orogenic belts of both Palaeozoic and Mesozoic age from several different continents, including Europe, South Asia and Australasia.

Abstract Several topics are covered including: *the use of hydrocarbon-bearing fluid inclusions and apatite fission tracks as paleothermometers for reconstructing P-T evolution of subthrust reservoirs *the use of hydrocarbon-bearing fluid inclusions and apatite fission tracks as paleothermometers for reconstructing P-T evolution of subthrust reservoirs *the coupling of kinematic and thermal modeling performed to trace the burial (P-T) evolution of potential source rocks and reservoirs in three cases studies in the southern Apennines, Colombia, and Pakistan *analytical results and integrated studies, which link deformation and fluid circulation in various fold and thrust belts, with the Sierra Madre in Mexico, the Central Brooks Range, the Arctic in Alaska, the Coastal belt in northern Spain, and the Ukraine featured. Links between deformation, fluid flow, diagenesis, and reservoir characteristics are discussed in depth and descriptions of petrographic techniques integrated with basin modeling are discussed in case studies for carbonate reservoirs in the Apennines, the Canadian Rockies, and the Polish Carpathians, and for sandstone reservoirs in Eastern Venezuela. Sixteen of the twenty-one chapters illustrate the influence of thrust-belt evolution on regional petroleum systems. The petroleum potential in the Tunisian Atlas and in Sicily, close to where the Hedberg Conference and post-conference field trip were held, is described. An older example is documented, for the Gaspé Appalachians, where multiphase Paleozoic deformation had a strong control on the burial history of potential source rocks, petroleum generation and migration, and oil charge of the traps. As the first in the brand-new Hedberg Series of publications, this volume is a comprehensive look at understanding petroleum systems in fold and thrust belts.

ABSTRACT The lithostratigraphic column of the Outer Albanides records a long geodynamic evolution. It began with a Liassic rifted margin made up of tilted fault blocks, carbonate platforms, and euxinic basins (Posidonia Schist) and evolved into a Paleogene flexed foreland, part of which inverted during the Neogene. The complex Mesozoic paleogeography accounts for the distribution of potential décollement levels and lateral changes in structural styles. Petroleum plays are numerous and result from the occurrence of various source rocks with contrasting burial histories and migration pathways. To document the respective timing of thrusting, petroleum generation, and trapping, that is, the critical timing of Albanian petroleum systems, we have reconstructed the kinematic and thermal evolution of two representative regional transects that cross the Peri-Adriatic Depression and the Kruja Zone, as well as the Ionian Basin, in the northern and southern segments of the Outer Albanides, respectively.

Abstract This volume constitutes the proceedings of the American Association of Petroleum Geologists-Institut Francats du Petrole Hedberg conference entitled Deformation, Fluid Flow, and Reservoir Appraisal in Foreland Foldand Thrust Beltsheld on May 14-18, 2002, in Mondello, Sicily,with the participation of 80 earth scientists from the petroleum industry and universities. It comprises 21 papers, selected from among 38 oral and 13 poster presentations. Reservoir risk remains one of the key concerns to the exploration of deep subthrust prospects in foothill areas of fold and thrust belts. Case studies that have integrated structural geology, geochemistry, and reservoir petrology not only now provide a better understanding of the multiscale processes involved in reservoir damage (particularly small-scale deformation and cementation) but also have begun to identify the critical parameters that are likely to enhance secondary porosity and permeability in deeply buried sandstone and carbonate series. Recent progress in analytical resolution makes it more and more possible to approach the problems on a microscale level and link the evolution of rock properties to the geological history on a broader scale. Basin-scale numerical simulations now incorporate the kinematics and thermal evolution of fold and thrust belts, as well as reconstructions of the history of fluid flow and pore-fluid pressure. Basinmodeling techniques provide quantitative and predictive results based on both thermodynamics and mass-balance calculations of fluid-rock interactions at reservoir scale. Instantaneous fluid-flow values derived from such models can be used as boundary conditions for forward diagenetic simulations applied to a specific reservoir layer, particularly for those episodes of

Abstract Several advances have been made for the reconstruction of fluid circulations and diagenetic history in subthrusted petroleum reservoirs because of the combination of the in-situ microanalysis of hydrocarbon fluid inclusions by Synchrotron Fourier transform infrared spectroscopy and PVTX modeling coupled to diagenetic history and tectonic setting. Integrated study has been made in the Eocene Chorgali formation (North Potwar Basin, Pakistan), where the shallow-marine carbonates formed important fractured reservoirs. Hydrocarbon fluid inclusions recognized in authigenic quartz and calcite from hydroveins show atypical association of CO 2 -rich light oil depleted in H 2 O in sulfates-quartz-calcite along simultaneous dissolution recrystallization processes at micrometer scale. Synchrotron Fourier transform infrared spectroscopy analyses, microthermometry, and pressure-volume-temperature modeling led to the beginning of quartz and calcite recrystallization at no more than 75–85°C and 150–180 bar in conditions of sulfate-calcite transformation. Temperatures of 150°C measured in aqueous fluid inclusions from calcite hydroveins are in favor of a thermosulfatoreduction mechanism. Early diagenetic sulfates are reduced by organic acids, and CO 2 comes from organic matter decomposition and/or previous decarbonation. A second phase of quartz growth is evidenced by the homogeneous entrapment in fluid inclusions of more mature oil in 60% CH 4 and a large amount of water at temperatures reaching 150–170°C. This late production of CH 4 agrees with δ 13 C depletion (−20 and −36%o) measured in veins and the crystallization of saddle dolomite. Thrustpack ® modeling shows that the onset of hydrofracturing and quartz precipitation at 1.5 km (1 mi) depth and 15–10.8 Ma (middle Siwalik) began when temperatures of 65 ± 10°C were reached at the end of sedimentation in the basin. It lasted until 4–6 km (2.5–4 mi) depth at temperatures as much as 170°C and reached the development of the thrust sheet at 5 Ma. Thus, circulations of hydrocarbon-rich fluids may be considered in thermal equilibrium with host rocks in both cases. The oil could then be derived from source rocks in the deep Mesozoic formation for the first input. The second input originated from the deep part of the basin itself and mixed with tectonic and meteoric water along the circulation pathways. The fluids are mainly driven by tectonics. They are expelled from the hinterland farther to the north and move updip toward the south in the Chorgali conduits, below the Kuldana seals. The potential source rock for organic matter is known as type II and type III kerogens in coal and black shales from the Paleocene.

Abstract Apatite fission-track (AFT) data from rocks above and below Lewis thrust fault lying in the footwall and hanging wall of Flathead normal fault record different thermal-history components, depending on individual structural and stratigraphic positions. Apatite fission-track temperature-history models (THMs) indicate that rapid cooling of the Lewis thrust sheet began at about 75 Ma. This cooling coincided with major displacement on the Lewis thrust. Subsequently, folding of the Lewis thrust sheet by underlying thrust duplex culminations formed the Akamina syncline, and a fossil AFT partial annealing zone was superimposed on the syncline. Apatite fission-track data from east of the Flathead graben record a subsequent cooling event during the middle Eocene onward that was coeval with extensional displacement on the Flathead fault and with accompanying uplift and erosion of its footwall. Apatite fission-track data from lower Oligocene sediments in the Flathead graben preserved the temperature history of the sediment source regions in the Lewis thrust sheet without significant subsequent annealing. A set of similar THMs that are consistent with the regional structural history can account for observed variations in AFT parameters at various levels, which are exposed in the Lewis thrust sheet and are penetrated below the thrust sheet by deep wells. From the onset of displacement on the Lewis thrust until the early Oligocene, paleogeothermal gradients in the thrust sheet (8.6–12°C/km) were lower than present values (~17°C/km). The changes in geothermal gradients are attributed to advective heat transfer by tectonically induced, topographically driven, deeply penetrating meteoric water flow. This is a complicated heat-transfer mechanism that can affect organic maturation history and petroleum systems in overthrust belts.

Abstract The structural deformation and the petroleum system of the southern Apennines thrust belt (SATB) are studied along a regional cross section traversing the Monte Alpi–Tempa Rossa fields. The SATB is interpreted as a system of three major structural blocks incorporating the basement and the sediments up to the Apulian platform deposits beneath an allochthonous complex. The Thrustpack® software has been used to reconstruct the successive geometries and their progressive burial under foredeep sediments and the allochthonous complex. The bottom of the Apulian platform and the basement are involved in the deformation, and the thickness of the Permian interval, drilled in the foreland, is extended regionally. The timing of the deformation is constrained by the ages of the Pliocene foredeep sediments drilled on top of the Apulian platform. This record was also instrumental to propose a flexure scenario of the migrating foredeep-forebulge system, in which the slope of the topography had to be maintained to a realistic value. These assumptions and boundary conditions were tested by successive, two-dimensional kinematic and thermal reconstructions until a satisfactory match could be obtained with the available temperatures and vitrinite reflectance data. A final good thermal calibration has been obtained for the structural blocks of Monte Alpi and Tempa Rossa. However, the relatively poor quality of the temperature and vitrinite data available for the most hinterland structure questions the conclusions about the validity of our proposed geometry and assumed accumulated thrust displacement. The methodology used in this work is a useful tool in exploration, because it forces one to improve and update structural scenarios and to provide the grounds for highlighting important data gathering to further enhance an evaluation of the hydrocarbon potential at a basin scale. This latter point will be described in a companion chapter.

Abstract The structural deformation and the source rock system evolution of the southern Apennines thrust belt (SATB) are studied along a regional structural profile traversing the Monte Alpi–Tempa Rossa oil fields. In part I (the accompanying chapter), the reconstruction of the structural evolution and the thermal history was addressed to calibrate the burial history of the source rocks along the cross section. Here in part II, the generation and expulsion of hydrocarbons were modeled to test a potential source rock interval and identify geometric factors explaining the observed differences in the nature of the oil found in the three major structural trends. Organic-rich, laminated limestones that were penetrated by a few wells in the region represent the best source rock candidate to date. The source interval shows total organic carbon (TOC) values as much as 4% and hydrogen index as much as 632 mg HC/g TOC. This source rock also contains high amounts of sulfur (3–6% in kerogen). Rock samples and asphaltenes isolated from the oil were analyzed to determine both primary bulk kerogen decomposition and compositional kerogen decomposition products. For the latter, the results include determination of the kinetics of dry gas (C 1 ), wet gas (C 2 –C 5 ), light oil (C 5 –C 14 ), and heavy oil (C 15+ ) components. The southern Apennines Cretaceous source rock behaves as a type I kerogen equivalent, consistent with the distribution of the activation energies dominated by a single activation energy. Most of the predicted generated and expelled hydrocarbons are heavy and light oils. Thermal conditions for secondary cracking of the generated oil into gas could have been reached only in the footwall of the major thrusts. The measured kinetic parameters allow the modeling of a favorable timing of trap formation with respect to hydrocarbon generation and expulsion. When the measured bulk and compositional kinetics are used in the modeling, no oil generation is reached in the Tempa Rossa trend. The model shows that the Tempa Rossa heavy-oil field has been filled by oil that was generated deeper in the surrounding of the structure. Compositional kinetic simulation is consistent with the results of the geochemical analyses performed on several oils from the region. The original oils in the reservoirs should have an API gravity of about 25° API. Only subsequent geological processes (uplift and erosion) provide the pressure-volume-temperature variation responsible for the compositional grading column at the present time. Finally, kerogen transformation ratio vs. depth shows that the three different transformation ratio-depth zones should be considered to fit the thermal history of the southern Apennines. This two-dimensional information can be used to predict the distribution of potential source rock kitchen areas in the surroundings of the modeled section to guide future exploration.

Abstract We modeled the kinematic evolution of two regional-scale transects through the Eastern Cordillera fold and thrust belt of Colombia and then calculated the conductive thermal state of key steps of the kinematic history using Thrustpack ® 4.0. The models were constrained by well, seismic, apatite fission-track, and thermal-maturity data. The main compressional structures in the Cordillera are controlled by Jurassic–Early Cretaceous normal faults of the Bogotá, Cocuy, and the paleo-Magdalena basins. The location of these Mesozoic extensional features strongly influenced thermal evolution. Although shortening and basin inversion started in the early Tertiary, the bulk of the deformation occurred during the Miocene to Holocene Andean orogeny. Rocks in different structural positions in the thrust belt have distinct thermal and maturation histories that determine the timing of hydrocarbon source rock maturation and the quality of sandstone reservoirs. The internal part of the Cordillera had high heat flow, with peak sedimentary burial and peak maturation during the Oligocene flexural phase. Local structures formed during this time and were followed by major uplift and denudation during the Andean orogeny. Hydrothermal circulation of basinal fluids, which was probably expulsed at the onset of structural inversion, led to extensive cementation of Albian reservoirs. In contrast, the Llanos foreland is characterized by continued flexural subsidence and syntectonic sedimentation up to the present time. Thermal maturation results from the combination of syntectonic sedimentation and tectonic burial. Quartz cementation appears to be linked to the appearance of abundant silica in the system from pressure solution during Andean shortening. The thermal regime of the western flank of the Cordillera is cooler than the interior of the range, whereas the structural history is more complex. Along our transect, an active kitchen is located in the west-vergent thrust belt of the Eastern Cordillera. In the Magdalena Valley, there are local kitchens only where a thick stratigraphic section is preserved. The main limitations of our thermal models are (1) the lack of constraints on the thickness and timing of deposition of the Eocene-Oligocene flexural deposits, which are sparsely preserved in the Eastern Cordillera; (2) the paucity of good-quality thermochronologic data to constrain the timing of erosion and rates of fault motion; and (3) the difficulty in modeling the effects of fluid circulation over this large and structurally complex region.

Abstract A study of the southern Kirthar fold belt in Pakistan was undertaken to elucidate the hinterland structure and hydrocarbon prospectivity. Interpretation of structure and stratigraphy is difficult because of suboptimal seismic data, a lack of hinterland well data, and a transition from shelfal to basinal stratigraphy. An interpretation of two cross sections was made using outcrop and seismic data and well data from foreland discoveries. The Institut Français du Pétrole Thrustpack ® software was used to validate the structural model and provide data on the maturity of the source rock. The Kirthar fold belt is dominated by open and symmetrical folds that are driven by inversion of basement-involved Jurassic extensional faults. Thrusts have been interpreted with two detachments, thrusts with a shallow detachment in the Eocene mudstones and thrusts with a deeper detachment in the Lower Cretaceous source rock interval that involve the reservoir during deformation. The major mountain-building episode is interpreted as late Pliocene–Pleistocene, but there is evidence for earlier inversion dating from the late Paleocene associated with the emplacement of the Bela ophiolite and constrained by maturity data obtained from outcrop. Early inversion and uplift impacts the burial curve and, thus, the prospectivity of the area.

Abstract A structural analysis and petrographic investigation has been performed on veins in Cretaceous carbonates in the Cordoba Platform in eastern Mexico, which is part of the Laramide foreland fold and thrust belt (FFTB). This chapter focuses on the different episodes of vein formation, vein morphology, and possible mechanisms of vein formation. Vein (fracture) formation is interpreted in relation to the kinematic evolution of the FFTB. Evidence for the development of hydrofractures during this evolution is given. This study documents veins (fractures) related to Laramide FFTB development in the Cordoba Platform. These veins (fractures) are related to the kinematic evolution of the area and the inferred paleostress conditions. The kinematic evolution can be split up into three major stages: a precompression phase with platform development; a Laramide compressional stage, during which the FFTB developed; and finally, a late Basin and Range-related extension phase. Compound veins and densely spaced microveins record multiple fracturing events in a cyclic stress field during burial, most probably caused by changes in fluid pressure. They are interpreted in relation with early foreland flexuring. With rising compressional stress, less well-oriented veins and breccia veins develop because of a lowered differential stress in the prefolding stage. Progressive layer-parallel shortening (LPS) leads to a caterpillar-type scenario of fluid migration toward the foreland, eventually causing hydrofracturing, succeeded by pressure solution and development of vertical stylolitic planes. These LPS stylolites have the potential to be reopened during subsequent folding of the strata. In addition, older LPS-parallel planes and extrados fractures may open in anticlinal hinges. Shear-associated, shallow-dipping veins develop after LPS development, possibly because of bedding-parallel shear and/or thrust migration. Other post-LPS veins are steeply dipping and commonly reuse older vein orientations. Dark, banded veins, which are filled with a silt-sized and clay-sized material and lack significant cementation, are interpreted to reflect fracture planes along which recrystallization of matrix occurred. Many post-LPS dissolution-enlarged veins and breccias relate to telogenetic karstification. Post-LPS multiple brecciation just above a major thrust plane in the buried tectonic front area is interpreted to reflect the damage zone of that fault.

Abstract The Brooks Range is a north-directed fold and thrust belt that forms the southern boundary of the North Slope petroleum province in northern Alaska. Field-based studies have long recognized that large-magnitude, thin-skinned folding and thrusting in the Brooks Range occurred during arc-continent collision in the Middle Jurassic to Early Cretaceous (Neocomian). Folds and thrusts, however, also deform middle and Upper Cretaceous strata of the Colville foreland basin and thus record a younger phase of deformation that apatite fission-track data have shown to occur primarily during the early Tertiary (~60 and ~45 Ma). A structural and kinematic model that reconciles these observations is critical to understanding the petroleum system of the Brooks Range fold and thrust belt. New interpretations of outcrop and regional seismic reflection data indicate that from the modern mountain front northward to near the deformation front under the coastal plain, the basal thrust detachment for the orogen is located in the Jurassic and Lower Cretaceous Kingak Shale in the upper part of the regionally extensive, gently south-dipping, north-derived Mississippian to Early Cretaceous Ellesmerian sequence. The frontal part of the orogen lies in middle Cretaceous foreland basin strata and consists of a thin-skinned fold belt at the deformation front and a fully developed passive-roof duplex to the south. Near the mountain front, the orogen is composed of a stacked series of allochthons and thrust duplexes and associated Neocomian syntectonic deposits that are unconformably overlain by proximal foreland basin strata. The foreland basin strata and underlying deformed rocks are truncated by a younger generation of folds and thrusts. Vitrinite reflectance and stable isotope compositions of veins provide evidence of two fluid events in these rocks, including an earlier higher temperature (~250–300°C) event that was buffered by limestone and a younger, lower temperature (~150°C) event that had distinctly lower δ 13 C values as a result of oxidation of organic matter and/or methane. Zircon fission-track data from the host rocks of the veins show that the higher temperature fluid event occurred at 160–120 Ma, whereas the lower temperature event probably occurred at about 60–45 Ma. It is proposed that the Brooks Range consists of two superposed contractional orogens that used many of the same mechanically incompetent stratigraphic units (e.g., Kayak Shale, Kingak Shale) as sites of thrust detachment. The older orogen formed in a north-directed arc-continent collisional zone that was active from 160 to 120 Ma. This deformation produced a thin-skinned deformational wedge that is characterized by fartraveled allochthons with relatively low structural relief, because it involved a thin (1–4-km [0.6–2.5-mi]-thick) stratigraphic section. Deeper parts of the deformational wedge are envisioned to have contained relatively high-temperature fluids that presumably migrated from or through limestone-rich source areas in the underlying autochthon or from deeper parts of the orogen. The younger orogen, which formed initially at about 60 Ma and reactivated at 45 Ma, produced a thrust belt and frontal triangle zone with low amounts of shortening and relatively high structural relief, because it involved a structural section 5–10 km (3–6 mi) thick. Fluids associated with this deformation were relatively of lower temperature and suggest that hydrocarbon migration occurred at this time. We conclude that hydrocarbon generation from Triassic and Jurassic source strata and migration into stratigraphic traps occurred primarily by sedimentary burial principally at 100–90 Ma, between the times of the two major episodes of deformation. Subsequent sedimentary burial caused deep stratigraphic traps to become overmature, cracking oil to gas, and initiated some new hydrocarbon generation progressively higher in the section. Structural disruption of the traps in the early Tertiary released sequestered hydrocarbons. The hydrocarbons remigrated into newly formed structural traps, which formed at higher structural levels or were lost to the surface. Because of the generally high maturation of the Colville basin at the time of the deformation and remigration, most of the hydrocarbons available to fill traps were gas.

Abstract Beneath the Arctic coastal plain (commonly referred to as "the 1002 area") in the Arctic National Wildlife Refuge, northeastern Alaska, United States, seismic reflection data show that the northernmost and youngest part of the Brookian orogen is preserved as a Paleogene to Neogene system of blind and buried thrust-related structures. These structures involve Proterozoic to Miocene (and younger?) rocks that contain several potential petroleum reservoir facies. Thermal maturity data indicate that the deformed rocks are mature to overmature with respect to hydrocarbon generation. Oil seeps and stains in outcrops and shows in nearby wells indicate that oil has migrated through the region; geochemical studies have identified three potential petroleum systems. Hydrocarbons that were generated from Mesozoic source rocks in the deformed belt were apparently expelled and migrated northward in the Paleogene, before much of the deformation in this part of the orogen. It is also possible that Neogene petroleum, which was generated in Tertiary rocks offshore in the Arctic Ocean, migrated southward into Neogene structural traps at the thrust front. However, the hydrocarbon resource potential of this largely unexplored region of Alaska’s North Slope remains poorly known. In the western part of the 1002 area, the dominant style of thin-skinned thrusting is that of a passive-roof duplex, bounded below by a detachment (floor thrust) near the base of Lower Cretaceous and younger foreland basin deposits and bounded above by a north-dipping roof thrust near the base of the Eocene. East-west-trending, basement-involved thrusts produced the Sadlerochit Mountains to the south, and buried, basement-involved thrusts are also present north of the Sadlerochit Mountains, where they appear to feed displacement into the thin-skinned system. Locally, late basement-involved thrusts postdate the thin-skinned thrusting. Both the basement-involved thrusts and the thin-skinned passive-roof duplex were principally active in the Miocene. In the eastern part of the 1002 area, a northward-younging pattern of thin-skinned deformation is apparent. Converging patterns of Paleocene reflectors on the north flank of the Sabbath syncline indicate that the Aichilik high and the Sabbath syncline formed as a passive-roof duplex and piggyback basin, respectively, just behind the Paleocene deformation front. During the Eocene and possibly the Oligocene, thin-skinned thrusting advanced northward over the present location of the Niguanak high. A passive-roof duplex occupied the frontal part of this system. The Kingak and Hue shales exposed above the Niguanak high were transported into their present structural position during the Eocene to Oligocene motion on the long thrust ramps above the present south flank of the Niguanak high. Broad, basement-cored subsurface domes (Niguanak high and Aurora dome) formed near the deformation front in the Oligocene, deforming the overlying thin-skinned structures and feeding a new increment of displacement into thin-skinned structures directly to the north. Deformation continued through the Miocene above a detachment in the basement. Offshore seismicity and Holocene shortening documented by previous workers may indicate that contractional deformation continues to the present day.