Emergence of the Canadian Rockies and adjacent plains: A comparison of physiography between end-of-Laramide time and the present day

The landscape of the Canadian Rockies in southern Alberta is not a direct result of constructional processes; that is, the ridges and peaks have not been pushed into the positions in which we see them today. Tectonic activity provided original elevation but not mountains: at the end of Laramide time, what are now the front ranges and foothills of the Rockies comprised a high-elevation upland of relatively low relief. The present mountain physiography is the result of 55–60 million years of post-orogenic differential erosion, in which more resistant rocks have been left at higher elevations than less-resistant rocks.The Canadian Rockies and the foothills are developed in a thin-skinned, thrust-and-fold belt created during the Laramide Orogeny; the adjacent Interior Plains cut across foreland basin sediments derived from the mountains. The mountains currently consist of large parts of ridges of well-indurated Paleozoic and, locally, Proterozoic rock alternating with valleys developed in soft Mesozoic clastic rock. In the foothills, where the soft Mesozoic rock is at the surface, relief is subdued, but ridges of more-resistant sandstone rise above shaley lowlands. The plains are relatively flat but also contain erosional outliers of higher paleo-plains-surfaces.Numerous lines of evidence suggest that the mountains and foothills have lost several kilometers of overburden since the end of the Laramide Orogeny, while the western plains have lost at least 2 km, requiring that the local relief of the mountains and foothills that we see is erosional in origin. Local physiography is adjusted to lithology: the mountains have high relief because the exposed sub-Mesozoic rocks can hold up high, steep slopes, whereas the foothills have low relief because the underlying Cretaceous rocks cannot hold up high, steep slopes. The east-facing escarpment at the mountain front is a fault-line scarp along a low-angle thrust.Mesozoic rocks involved in the deformation originally extended all the way across the thrust and fold belt, and physiography of the belt at the end of Laramide time (60–55 Ma) depended mainly on whether Mesozoic or Paleozoic/Proterozoic rocks were exposed at the surface at that time. A reconstruction using critical-taper theory generally agrees with reconstructions from earlier stratigraphic and paleothermometry studies: what are now the front ranges at the eastern edge of the Rocky Mountains were mostly or perhaps entirely covered with Mesozoic rocks and despite that high elevation had a hilly, not mountainous, character. The main ranges, in the central Rocky Mountains, were in part stripped of Mesozoic cover by then and more mountainous. Treeline was higher then, and the thrust belt may have been largely or entirely vegetated. Generation of modern relief in the front ranges, including the escarpment at the mountain front, had to await stripping of Mesozoic rocks and incision of rivers into harder substrates in post-Laramide time.The Interior Plains are an erosional surface that was cut 1 to 3 km below the aggradational top of the foreland basin sediments. Although some of the present low local relief of the plains results from weakness of underlying Cretaceous/Tertiary rocks, the low relief is probably largely related to the process of denudation.     

Cooling of the Sverdrup Basin during Tertiary basin inversion: implications for hydrocarbon exploration

ABSTRACT The regional thermal history of the north‐eastern Sverdrup Basin, Canadian Arctic Archipelago, has been assessed using apatite fission‐track thermochronology and vitrinite reflectance data. Fission‐track data for 27 samples from six wells through the Mesozoic section on Axel Heiberg and Ellesmere Islands reveal significant Palaeocene cooling associated with basin inversion during the Eurekan Orogeny. Fission‐track data for 29 outcrop samples, ranging in stratigraphic age from Cambrian to Tertiary, also reveal significant Palaeocene cooling. Vitrinite reflectance data from carbonaceous shales and coal seams in well and outcrop samples are consistent with these conclusions. The degree of Palaeocene cooling observed is greatest for well and outcrop samples in the cores of anticlines or the hanging walls of thrust faults, such as the Fosheim anticline, and faults, such as the Lake Hazen fault system, and the East Cape and Vesle Fiord thrust faults. Palaeocene cooling is largely attributed to the denudation of structures during the Eurekan Orogeny. At one locality on north‐western Ellesmere Island, which is on the northern flank of the Sverdrup Basin, the underlying Franklinian basement rocks yield Early Cretaceous fission track ages with relatively long mean track lengths. This indicates that this part of the basin was uplifted at this time and that subsequent sedimentation and subsidence in the Cretaceous and early Tertiary were modest. This locality thus appears to be on the rift shoulder, which developed along the flank of the Amerasia Basin in the Lower Cretaceous. At a locality on western Axel Heiberg Island, which is downflank from the rift shoulder, the Upper Jurassic Awingak sandstone has a Late Cretaceous fission track age. This is best explained by heating above the total annealing temperature for fission‐tracks in apatite by extensive Lower Cretaceous intrusions and subsequent heat dissipation and cooling in the Late Cretaceous followed by further substantial cooling due to Tertiary denudation. These results indicate that maximum burial temperatures occurred in the presently exposed Mesozoic section prior to basin inversion during the Eurekan Orogeny. It can therefore be inferred that peak hydrocarbon generation and primary migration predated the formation of structural traps during the Tertiary at shallow depths within the northern Sverdrup Basin.     

Timing of Late Cretaceous shortening and basin development,Little Hatchet Mountains,southwestern New Mexico,USA – implications for regional Laramide tectonics

Laser ablation‐multi collector‐inductively coupled mass spectrometry U‐Pb geochronology, detailed field mapping and stratigraphic data offer improved insights into the timing and style of Laramide deformation and basin development in the Little Hatchet Mountains, southwestern New Mexico, USA, a key locality in the ‘southern Laramide province.’ The Laramide synorogenic section in the northern Little Hatchet Mountains comprises upper Campanian to Maastrichtian strata consisting of the Ringbone and Skunk Ranch formations, with a preserved maximum thickness of >2400 m, and the correlative Hidalgo Formation with a total thickness >1700 m. The Ringbone Formation and superjacent Skunk Ranch Formation are each generally composed of (1) a basal conglomerate member; (2) a middle member consisting of lacustrine shale, limestone, sandstone, and interbedded ash‐fall tuffs; and (3) an upper sandstone and conglomerate member. Basaltic andesite flows are intercalated with the upper member of the Ringbone Formation and the middle member of the Skunk Ranch Formation. The Hidalgo Formation, which crops out in the northern part of the range, is dominantly composed of basaltic andesite breccias and flows equivalent to those of the Ringbone and Skunk Ranch formations. The Laramide section was deposited in an intermontane basin partitioned across intrabasinal thrust structures, which controlled growth‐stratal development. U‐Pb zircon ages from five tuffs indicate that the age range of the Laramide sedimentary succession is ca. 75–70 Ma. U‐Pb detrital‐zircon age data (n = 356 analyses) from the Ringbone Formation and a Lower Cretaceous unit indicate sediment contribution from uplifted Lower and Upper Cretaceous rocks adjacent to the basin and the contemporary Tarahumara magmatic arc in nearby northern Sonora, Mexico. The new ages, combined with published data, indicate that uplift, basin development, and magmatism in the region proceeded diachronously northeastwards as the subducting Farallon slab flattened under northern Mexico and southern New Mexico from Campanian to Palaeogene time.     

Fluvial deposition during transition from flexural to dynamic subsidence in the Cordilleran foreland basin: Ericson Formation,Western Wyoming,USA

The Ericson Formation was deposited in the distal foredeep of the Cordilleran foreland basin during Campanian time. Isopach data show that it records early dynamic subsidence and the onset of basin partitioning by Laramide uplifts. The Ericson Formation is well exposed around the Rock Springs uplift, a Laramide structural dome in southwestern Wyoming; the formation is thin, regionally extensive, and does not display the wedge‐shaped geometry typical of foredeep deposits. Sedimentation in this area was controlled both by activity in the thrust belt and by intraforeland tectonics. The Ericson Formation is ideally situated both spatially and temporally to study the transition from Sevier to Laramide (thin‐ to thick‐skinned) deformation which corresponded to the shift from flexural to dynamic subsidence and the demise of the Cretaceous foreland basin system. We establish the depositional age of the Ericson Formation as ca. 74 Ma through detrital zircon U–Pb analysis. Palaeocurrent data show a generally southeastward transport direction, but northward indicators near Flaming Gorge Reservoir suggest that the intraforeland Uinta uplift was rising and shedding sediment northward by late Campanian time. Petrographic data and detrital zircon U–Pb ages indicate that Ericson sediment was derived from erosion of Proterozoic quartzites and Palaeozoic and Mesozoic quartzose sandstones in the Sevier thrust belt to the west. The new data place temporal and geographic constraints on attempts to produce geodynamic models linking flat‐slab subduction of the oceanic Farallon plate to the onset of the Laramide orogenic event.     

Magnetostratigraphy,age and depositional environment of the Lobo Formation,southwest New Mexico: implications for the Laramide orogeny in the southern Rocky Mountains

The Lobo Formation of southwestern New Mexico consists of spatially variable continental successions attributed to the Laramide orogeny (80–40 Myr), although its age and provenance are virtually undocumented. This study combines sedimentological, magnetostratigraphical and geochronological data to infer the timing and origin of the Lobo Formation. Measured sections of Lobo strata at two locations, Capitol Dome in the Florida Mountains and in the Victorio Mountains, indicate significant differences in depositional environments and sediment provenance. At Capitol Dome, where Lobo strata were deposited above a syncline developed in Palaeozoic strata, deposition took place in fluvial, palustrine and marginal lacustrine settings, with alluvial‐fan deposits only at the top of the formation. Combined magnetostratigraphy and a young U–Pb detrital zircon age suggest deposition of the section at Capitol Dome from ~60 to 52 Ma. The Lobo Formation in the Victorio Mountains was deposited in alluvial‐fan and fluvial settings; the age of deposition is poorly bracketed between 66 ± 2 Ma, the weighted‐mean age of two young zircons, and middle Eocene (~40 Ma), the approximate age of overlying volcanic rocks. U–Pb zircon ages from sandstones at the Victorio and Capitol Dome localities indicate that different source rocks provided sediment to the Lobo Formation. Local Proterozoic basement (~1.47–1.45 Ga) dominated the source of the Lobo Formation in the Victorio Mountains, consistent with abundant granitic clasts that are present in the proximal facies there; a diverse range of grain ages suggest that recycled Lower Cretaceous strata provided the dominant source for Lobo Formation sediment at the Capitol Dome locality. The U–Pb data suggest that the depositional systems at the two sites were not connected. Contrasts in depositional setting and detrital zircon provenance indicate that the Palaeogene Lobo Formation in southwest New Mexico was deposited in an assemblage of local depositional settings, possibly in separate structural basins, as a consequence of Laramide tectonics in the region.     

Post‐breakup burial and exhumation of the southern margin of Africa

Despite many years of study, the processes involved in the development of the continental margin of southern Africa and the distinctive topography of the hinterland remain poorly understood. Previous thermochronological studies carried out within a monotonic cooling framework have failed to take into account constraints provided by Mesozoic sedimentary basins along the southern margin. We report apatite fission track analysis and vitrinite reflectance data in outcrop samples from the Late Jurassic to Early Cretaceous sedimentary fill of the Oudtshoorn, Gamtoos and Algoa Basins (Uitenhage Group), as well as isolated sedimentary remnants further west, plus underlying Paleozoic rocks (Cape Supergroup) and Permian‐Triassic sandstones from the Karoo Supergroup around the Great Escarpment. Results define a series of major regional cooling episodes. Latest Triassic to Early Jurassic cooling which began between 205 and 180 Ma is seen dominantly in basement flanks to the Algoa and Gamtoos Basins. This episode may have affected a wider region but in most places any effects have been overprinted by later events. The effects of Early Cretaceous (beginning between 145 and 130 Ma) and Early to mid‐Cretaceous (120–100 Ma) cooling are both delimited by major structures, while Late Cretaceous (85–75 Ma) cooling appears to have affected the whole region. These cooling events are all interpreted as dominantly reflecting exhumation. Higher Late Cretaceous paleotemperatures in samples from the core of the Swartberg Range, coupled with evidence for localised Cenozoic cooling, are interpreted as representing Cenozoic differential exhumation of the mountain range. Late Cretaceous paleotemperatures between 60°C and 90°C in outcropping Uitenhage Group sediments from the Oudtshoorn, Gamtoos and Algoa Basins require burial by between 1.2 and 2.2 km prior to Late Cretaceous exhumation. Because these sediments lie in depositional contact with underlying Paleozoic rocks in many places, relatively uniform Late Cretaceous paleotemperatures across most of the region, in samples of both basin fill and underlying basement, suggest the whole region may have been buried prior to Late Cretaceous exhumation. Cenozoic cooling (beginning between 30 and 20 Ma) is focussed mainly in mountainous regions and is interpreted as representing denudation which produced the modern‐day relief. Features such as the Great Escarpment are not related to continental break up, as is often supposed, but are much younger (post‐30 Ma). This history of post‐breakup burial and subsequent episodic exhumation is very different from conventional ideas of passive margin evolution, and requires a radical re‐think of models for development of continental margins.     

西南极利文斯顿岛百耳斯半岛火山岩的同位素年龄

以Ｋ－Ａｒ稀释法和４０Ａｒ／３９Ａｒ阶段加热法对利文斯顿岛百尔斯半岛的熔岩和次火山岩测定了同位素年龄。新的年龄数据表明,该区火山活动在晚侏罗纪末即已开始,一直持续到晚白垩世,存在着三期火山活动的产物。侏罗纪末～早白垩世初的火山活动除有熔岩（１４６Ｍａ,１３７Ｍａ）喷出外,还发育有辉长岩侵入体（１３５Ｍａ）和辉绿岩岩墙（１２９Ｍａ）等次火山岩。安山岩（１１４Ｍａ）和石英斑岩（９４Ｍａ）则是早白垩世火山活动的结果。半岛东部具沉积夹层的玄武岩（７１Ｍａ）是区域上晚白垩世～早第三纪火山活动的代表。     

Detrital record of Mesozoic shortening in the Yanshan belt, NE China: testing structural interpretations with basin analysis

The Yanshan fold‐thrust belt is an exposed portion of a major Mesozoic orogenic system that lies north of Beijing in northeast China. Structures and strata within the Yanshan record a complex history of thrust faulting characterized by multiple deformational events. Initially, Triassic thrusting led to the erosion of a thick sequence of Proterozoic and Palaeozoic sedimentary strata from northern reaches of the thrust belt; Triassic–Lower Jurassic strata that record this episode are deposited in a thin belt south of this zone of erosion. This was followed by postulated Late Jurassic emplacement of a major allochthon (the Chengde thrust plate), which is thought to have overridden structures and strata associated with the Triassic event and is cut by two younger thrusts (the Gubeikou and Chengde County thrusts). The Chengde allochthon is now expressed as a major east–west trending, thrust‐bounded synform (the Chengde synform), which has been interpreted as a folded klippe 20 km wide underlain by a single, north‐vergent thrust fault. Two sedimentary basins, defined on the basis of provenance, geochronology and palaeodispersal trends, developed within the Yanshan belt during Late Jurassic–Early Cretaceous time and are closely associated with the Chengde thrust and allied structures. Shouwangfen basin developed in the footwall of the Gubeikou thrust and records syntectonic unroofing of the hanging wall of that fault. Chengde basin developed in part atop Proterozoic strata interpreted as the upper plate of the Chengde allochthon and records unroofing of the adjacent Chengde County thrust. Both the Chengde County thrust and the Gubeikou thrust are younger than emplacement of the postulated Chengde allochthon, and structurally underlie it, yet neither Shouwangfen basin nor Chengde basin contain a detrital record of the erosion of this overlying structure. In addition, facies, palaeodispersal patterns and geochronology of Upper Jurassic strata that are cut by the Chengde thrust suggest only limited (ca. 5 km) displacement along this fault. We suggest that the units forming the Chengde synform are autochthonous, and that the synform is bounded by two limited‐displacement faults of opposing north and south vergence, rather than a single large north‐directed thrust. This conclusion implies that the Yanshan belt experienced far less Late Jurassic shortening than was previously thought, and has major implications for the Mesozoic evolution of the region. Specifically, we argue that the bulk of shortening and uplift in the Yanshan belt was accomplished during Triassic–Early Jurassic time, and that Late Jurassic structures modified and locally ponded sediments from a well‐developed southward drainage system developed atop this older orogen. Although Upper Jurassic strata are widespread throughout the Yanshan belt, it is clear that these strata developed within several discrete intermontane basins that are not correlable across the belt as a single entity. Thus, the Yanshan has no obvious associated foreland basin, and determining where the Mesozoic erosional products of this orogen ultimately lie is one of the more intriguing unresolved questions surrounding the palaeogeography of North China.     

A flexural model for the Paradox Basin: implications for the tectonics of the Ancestral Rocky Mountains

The Paradox Basin is a large (190 km × 265 km) asymmetric basin that developed along the southwestern flank of the basement‐involved Uncompahgre uplift in Utah and Colorado, USA during the Pennsylvanian–Permian Ancestral Rocky Mountain (ARM) orogenic event. Previously interpreted as a pull‐apart basin, the Paradox Basin more closely resembles intraforeland flexural basins such as those that developed between the basement‐cored uplifts of the Late Cretaceous–Eocene Laramide orogeny in the western interior USA. The shape, subsidence history, facies architecture, and structural relationships of the Uncompahgre–Paradox system are exemplary of typical ‘immobile’ foreland basin systems. Along the southwest‐vergent Uncompahgre thrust, ～5 km of coarse‐grained syntectonic Desmoinesian–Wolfcampian (mid‐Pennsylvanian to early Permian; ～310–260 Ma) sediments were shed from the Uncompahgre uplift by alluvial fans and reworked by aeolian‐modified fluvial megafan deposystems in the proximal Paradox Basin. The coeval rise of an uplift‐parallel barrier ～200 km southwest of the Uncompahgre front restricted reflux from the open ocean south and west of the basin, and promoted deposition of thick evaporite‐shale and biohermal carbonate facies in the medial and distal submarine parts of the basin, respectively. Nearshore carbonate shoal and terrestrial siliciclastic deposystems overtopped the basin during the late stages of subsidence during the Missourian through Wolfcampian (～300–260 Ma) as sediment flux outpaced the rate of generation of accommodation space. Reconstruction of an end‐Permian two‐dimensional basin profile from seismic, borehole, and outcrop data depicts the relationship of these deposystems to the differential accommodation space generated by Pennsylvanian–Permian subsidence, highlighting the similarities between the Paradox basin‐fill and that of other ancient and modern foreland basins. Flexural modeling of the restored basin profile indicates that the Paradox Basin can be described by flexural loading of a fully broken continental crust by a model Uncompahgre uplift and accompanying synorogenic sediments. Other thrust‐bounded basins of the ARM have similar basin profiles and facies architectures to those of the Paradox Basin, suggesting that many ARM basins may share a flexural geodynamic mechanism. Therefore, plate tectonic models that attempt to explain the development of ARM uplifts need to incorporate a mechanism for the widespread generation of flexural basins.     

Crustal‐scale fluid flow during the tectonic evolution of the Bighorn Basin (Wyoming,USA)

Stable isotope measurements (O, C, Sr), microthermometry and salinity measurements of fluid inclusions from different fracture populations in several anticlines of the Sevier‐Laramide Bighorn basin (Wyoming, USA) were used to unravel the palaeohydrological evolution. New data on the microstructural setting were used to complement previous studies and refine the fracture sequence at basin scale. The latter provides the framework and timing of fluid migration events across the basin during the Sevier and Laramide orogenic phases. Since the Sevier tectonic loading of the foreland basin until its later involvement into the Laramide thick‐skinned orogeny, three main fracture sets (out of seven) were found to have efficiently enhanced the hydraulic permeability of the sedimentary cover rocks. These pulses of fluid are attested by calcite crystals precipitated in veins from hydrothermal (T > 120 °C) radiogenic fluids derived from Cretaceous meteoric fluids that interacted with the Precambrian basement rocks. Between these events, vein calcite precipitated from formational fluids at chemical and thermal equilibrium with surrounding environment. At basin scale, the earliest hydrothermal pulse is documented in the western part of the basin during forebulge flexuring and the second one is documented in basement‐cored folds during folding. In addition to this East/West diachronic opening of the cover rocks to hydrothermal pulses probably controlled by the tectonic style, a decrease in ^87/86Sr values from West to East suggests a crustal‐scale partially squeegee‐type eastward fluid migration in both basement and cover rocks since the early phase of the Sevier contraction. The interpretation of palaeofluid system at basin scale also implies that joints developed under an extensional stress regime are better vertical drains than joints developed under strike‐slip regime and enabled migration of basement‐derived hydrothermal fluids.     

西南极利文斯顿岛火山岩多种成因过程的地球化学证据

利文斯顿岛火山岩是南极南设得兰岩浆弧的一部分。对岩石的Ｓｒ、Ｎｄ、Ｐｂ同位素组成以及８７Ｓｒ／８６Ｓｒ与１／Ｓｒ、Ｒｂ、Ｋ和ＳｉＯ２关系的研究表明,利文斯顿岛火山岩具有不同的源区特征。百耳斯半岛和史莱夫角的火山岩、西多斯角的粗玄岩和第三纪的英云闪长岩显示出同成因的特点,和依诺特角第四纪橄榄玄武岩一样,它们均具有原生地幔的同位素组分,源区岩浆可能直接来自上地幔。汉那角和中利文斯顿岛火山岩岩浆则受到地壳成分的混染。不相容元素对Ｌａ／Ｓｍ－Ｌａ和Ｃｅ／Ｙｂ－Ｃｅ的研究进一步显示出,上述不同产地的岩石分别为不同源岩浆不同程度部分熔融的产物。结合岩相学和岩石化学特征的研究,提出百耳斯半岛和史莱夫角的火山岩以及西多斯角的粗玄岩是上地幔发生部分熔融所生成的玄武岩岩浆从深部岩浆囊沿构造薄弱带直接喷出形成的。古生代基底岩系的存在对中利文斯顿岛和汉那角火山岩的成因过程有重要的影响,即来自深部岩浆囊的原始岩浆在上地壳中的浅部岩浆囊受到了壳源物质混染。更新世－现代火山岩的源区岩浆直接自上地幔部分熔融生成,火山作用与晚新生代时该区的拉张构造有关。     

南极洲泥盆系概况及其与中国泥盆系之比较

南极洲的泥盆系主要分布于横贯南极山脉的麦克默多和俄亥俄岭－埃尔斯沃思山等两个沉积盆地中。前一个盆地的泥盆系代表从海岸泻湖－河流三角洲到近岸冲积平原的层序；后一个盆地的彭萨科拉山的泥盆系较厚，从非海相冲积扇－冲积平原－浅海相，最后又恢复到非海相沉积环境，但在俄亥俄岭却沉积了厚度不大的浅海相地层，含Ｍａｌｖｉｎｏｋａｆｒｉｃ生物地理大区的海相双壳类、腹足类、三叶虫、竹节石和鱼类等化石。除了上述两个沉积盆地外，在罗斯海两边却出露了火山岩，说明该地当时处于俯冲带附近的火山弧中。中国华南的曲靖型和西北的祁连山型泥盆系也属于滨海相和非海相沉积，它们与南极洲的泥盆系可资比较，但两者的生物地理区系并不相同     

LA‐ICP‐MS dating of detrital zircon grains from the Cretaceous allochthonous bauxites of Languedoc (south of France): Provenance and geodynamic consequences

The Cretaceous of southern France is characterised by a long erosional hiatus, outlined with bauxite deposits, which represent the only remaining sedimentary record of a key period for geodynamic reconstructions. Detrital zircons from allochthonous karst bauxites of Languedoc (Southern France) have been dated using LA‐ICP‐MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry), in order to specify the age of deposition and to constrain the provenance of the weathered material. We analysed 671 single detrital zircons grains from three karst bauxitic basins, stretching from close to the Variscan Montagne Noire to the present‐day Mediterranean Sea. Analytical results provide Variscan (300–350 Ma) and Late Proterozoic (550–700 Ma) ages as primary groups. In addition, Middle‐, Late Proterozoic and Early Archean (oldest grain at 3.55 Ga) represent significant groups. Mid‐Cretaceous zircons (118–113 Ma) provide a pooled age of 115.5 ± 3.8 Ma, which constitutes the maximum age for bauxite deposition. Results also suggest a dual source for the Languedoc bauxite: one generalised sedimentary source of regional extent and a localised source in the Variscan basement structural high, that has been progressively unroofed during Albian. Integration of these new findings with previously published thermochronological data support the presence of an Early Cretaceous marly cover on the Variscan basement, which has been weathered and then, removed during the Albian. The Languedoc bauxite provide a spatial and temporal link between the uplift of southern French Massif Central to the north, and the Pyrenean rift and its eastward extension to the south. These new results allow to constrain the timing and distribution of uplift/subsidence during the mid‐Cretaceous events in relation with the motion of the Iberian plate relative to Eurasia.     

Devonian–Carboniferous slivers within the basement area of north-east Oscar II Land, Spitsbergen

Formed during an early compressional period in the opening of North Atlantic Ocean, a Tertiary fold-thrust belt extends along the mid-to- southern part of the western coast of Spitsbergen. Complex thrust structures involve the basement (Caledonian and older) and many shallow dipping thrust faults dissect the overlying cover rocks (Devonian and younger) in Oscar II Land in the northern part of the belt. Some of these faults occur within the basement rocks with slivers or fault blocks of the cover rocks from south-western Brøggerhalvøya to innermost St. Jonsfjorden in north-eastern Oscar II Land. Six of the slivers contain Carboniferous rocks and one is a fault-bounded block with Devonian rocks. These steeply west-dipping faults form a complex fault system- EOFC (Engelskbukta-Osbornbreen Fault Complex) - within the basement area. The lithological units of the basement are separated by faults within the EOFC, which is structurally continuous with the Brøggerhalvøya fold-thrust zone to the north and is thought to continue to the fold-thrust zone on the south-eastern coast of St. Jonsfjorden. Some previous authors considered that the two lithologically contrasting Vendian diamictites and intervening Moefjellet Formation are stratigraphically continuous and defined two separate tilloid successions in the present area. This interpretation has been extended over the whole of western Spitsbergen. However, the present study indicates that these two tilloid formations and the Moefjellet Formation are separated by the faults, probably thrusts, within the EOFC and are not in a continuous stratigraphic relation. Therefore, the two-stage history of Vendian glaciation seems questionable.     

Lithoprobe southern Canadian Cordilleran transect: Rocky Mountain thrust belt to Valhalla gneiss complex

Summary. LITHOPROBE has acquired nearly 270 km of crustal seismic reflection data across the eastern portion of the southern Canadian Cordillera, These reflection profiles, obtained during the Fall of 1985, extend from the Rocky Mountain thrust and fold belt, across the Rocky Mountain Trench, Purcell anticlinorium, Kootenay Arc, Nelson batholith and Valhalla gneiss complex. North American basement and its overlying foreshortened miogeoclinal rocks can be traced westward to the Kootenay Arc. The Purcell anticlinorium is carried by a series of west dipping thrust faults which emerge east of the anticlinorium and converge downward and merge with a detachment surface above autochthonous North American basement. Proterozoic supracrustal rocks, thickened by folding and thrusting, occupy the core of the anticlinorium. Steeply dipping surface structures of the western Purcell anticlinorium and Kootenay Arc appear to be truncated at 3 - 4 s (9-12 km) by a gently east-dipping reflection that may delineate the upper boundary of an allochthonous wedge inserted between the near surface rocks and autochthonous basement below. Beneath the Kootenay Arc, at a travel time of 9–10 s (27–30 km), the North American basement seems to be truncated by the major east-dipping Slocan Lake fault zone, which can be traced from its surface exposure at the east edge of the Valhalla gneiss complex eastward to near the base of the crust. A high amplitude, west-dipping reflection underlies the Valhalla complex and may be related to a major compressional shear zone.     

Alluvial fan development and morpho-tectonic evolution in response to contractional fault reactivation (Late Cretaceous–Palaeocene), Provence, France

Along-strike variability within a Late Cretaceous to early Palaeocene contractional growth structure and associated alluvial fan deposits is documented at the northern margin of the Arc Basin (Provence, SE France). This contribution shows that alluvial fans can be used as high-resolution proxies to reconstruct structural segmentation and palaeo-geomorphological evolution of a source/basin margin system. Facies-based reconstruction allows the spatial and temporal distribution of alluvial fan bodies to be mapped. Relationships between fan area and catchment size from modern alluvial fan systems were used to estimate palaeo-catchment size. Combining alluvial fan morphologies with catchment area, pebble provenance analysis and growth structure reconstruction, we show that: (1) fan distribution and related depositional processes were strongly influenced by intrinsic parameters such as drainage basin evolution, local structural inheritance and lateral facies changes in source area lithologies; (2) Inherited structures trending N100 effectively controlled the first-order location of the fold and thrust structures (Montagne Sainte-Victoire Range) and adjacent depositional areas (Arc Basin); (3) Syn-sedimentary faults trending N010-030 influenced the source/basin margin development and interacted with developing growth structures; (4) Facies changes in Jurassic carbonates controlled fold development and consequently the structural evolution of the source area; and (5) the N010-030 faults and along-strike variability of the source/basin margin system were ultimately controlled by basement structures that controlled where Late Cretaceous deformation nucleated. The overall architecture of the source/basin margin system reflects segmentation and strain partitioning along strike, as demonstrated by diachronous alluvial fan distribution.     

Burial/exhumation histories for the Cooper–Eromanga Basins and implications for hydrocarbon exploration, Eastern Australia

The Cooper–Eromanga Basins of South Australia and Queensland are not at their maximum burial depth due to Late Cretaceous–Tertiary, and Late Triassic–Early Jurassic exhumation. Apparent exhumation (maximum burial depth–present burial depth) for the Cooper Basin has been quantified using the compaction methodology. The results show that exhumation of the Cooper Basin for the majority of the wells is greater than the exhumation of the Eromanga Basin. Using the compaction methodology, apparent exhumation of Early to Middle Triassic age Arrabury and Tinchoo Formatios has been quantified. Both units yield similar results and do not support that the Arrabury/Tinchoo boundary represents the Cooper–Eromanga boundary. Hence, the Cooper Basin is believed to have reached its maximum burial depth in Late Triassic times. Sonic log data are not available for the units overlying the Late Cretaceous Winton Formation; thus, it is not possible to date exhumation beyond the Late Cretaceous–Tertiary using the compaction methodology. Tertiary sequences as are preserved are relatively thin and separated by marked unconformities and weathered surfaces; hence, exhumation rather than sedimentation dominated the Tertiary, and in exhumed areas, maximum burial depth was attained in Late Cretaceous times. The burial/exhumation history of representative wells was synthesized using sediment decompaction and establishing porosity/depth relations for the Cooper–Eromanga units. Considering the relative significance of the major periods of exhumation in the Cooper/Eromanga Basins, three broad types of burial/exhumation histories can be distinguished. Maximum burial depth of the Cooper Basin sequence was attained before the deposition of the Eromanga Basin sequence, i.e. Late Triassic–Early Jurassic times; maximum burial depth of the Cooper and Eromanga Basin sequences attained in Late Cretaceous times; and Cooper and Eromanga Basin sequences are currently at maximum burial‐depth. Incorporation of exhumation into burial history has major implications for hydrocarbon exploration.     

The gawler ranges,South Australia: an unusual volcanic massif

The volcanic residuals of the Gawler Ranges together form an extensive massif that in its gross morphology differs markedly from most exposures of silicic volcanic rocks. The upland developed in two stages, the first involving differential fracture‐controlled subsurface weathering, the second the stripping of the regolith. As a result, an irregular weathering front was exposed, with domical projections prominent. These bornhardts are etch forms, and they are of considerable antiquity.

The differential weathering of the rock mass reflects the exploitation of various fracture systems by shallow groundwaters. Orthogonal fracture systems at various scales, sheet fractures and columnar joints control the morphology of the bornhardts in gross and in detail.

The exploitation of the structural base, which was established in the Middle Protero‐zoic, probably took place throughout the Late Proterozoic and the Palaeozoic, though only minor remnants of the Proterozoic land surface remain. The major landscape features developed during the Mesozoic. The weathering which initiated the bornhardts occurred in the Jurassic or earlier Mesozoic, and the landforms were exposed in Late Cretaceous to Early Tertiary times.

Though structural forms dominate the present landscape, some major and some minor landforms are best explained in terms of climatic changes of the later Cainozoic. The palaeodrainage system, established under humid conditions by the Early Tertiary, was alluviated during the Cainozoic arid phases, and salinas were formed. The sand dunes of the region also reflect this aridity.     

Tectonic development of the Drummond Basin, eastern Australia: backarc extension and inversion in a Late Palaeozoic active margin setting

Detailed seismic reflection data combined with regional magnetic, gravity and geological data indicate that the Drummond Basin originated as a backare extensional basin associated with Late Devonian and Early Carboniferous active margin tectonism in the northern New England Fold Belt. Seismic reflection data have been used to generate a two-way time map of seismic basement, providing a clear view of the basinal geometry and structural development. Broadscale structural asymmetry of the basin implies that simple shear along a deep, upper-crustal detachment provided the extensional mechanism and generated an inter-related set of listric normal faults and associated transfer faults, as well as steeply-dipping planat normal faults. The orientation of normal faults near the basin margins appears to have been controlled by regional basement structural trends. Transfer-fault trends were approximarely orthogonal to the line of plate convergence as assessed from the orientation of coeval are, forcare and subduction complex stratorectonic elements. Three distinct phases of infill are represented in the basinal stratigraphic succession. The first consists largely of volcanics and volcaniclastics, indicating that effusive magmatism and extension were closely associated in space and time. The second is quartzose and of basement derivation, but was not derived from footwall blocks at the faulted basinal margins to the east and north. Uplifted hanging-wall crust beyond the western basinal margin, a product of west-directed simple shear detachment, was the likely source terrain. The final infill phase consisted of volcaniclastics considered to have been derived from a coeval volcanic are to the east. Major faults at the basin margins provided conduits for magmatism during extensional basin development, and long after the basinal history was complete. During the Late Carboniferous and mid-Triassic, the basin was affected by two discrete episodes of compressional deformation. This led to inversion with the development of folds, and reverse and wrench faults now seen at the surface.     

Transantarctic Basin: new insights from fission track and structural data from the USARP Mountains and adjacent areas (Northern Victoria Land, Antarctica)

The USARP Mountains comprise two N–S‐aligned mountain ranges (Daniels Range, Pomerantz Tableland) located along the western margin of the Rennick Glacier in Northern Victoria Land (NVL). Four zircon and titanite fission track (FT) ages from granitic samples from the Pomerantz Tableland fall in a common range of 369–392 Ma. The apatite FT ages from 20 Granite Harbour Intrusive rocks sampled throughout the USARP Mountains are distinctively younger (86–270 Ma); their mean track lengths (MTL) vary between 11.0 and 13.9 μm. Six samples from Renirie Rocks and the Kavrayskiy Hills east of the USARP Mountains have even younger, concordant apatite FT ages of 43–71 Ma, and MTL of 12.2–14.0 μm. Thermal history modelling of the thermochronological data indicate that both the Daniels Range and Pomerantz Tableland experienced a common Phanerozoic geologic history consisting of a mid‐Devonian pulse of rapid denudation, followed by a protracted denudation stage between the Carboniferous and Jurassic. This latter period of denudation was contemporaneous with the formation of the Transantarctic Basin to the east. We consequently suggest that the USARP Mountains were one of the major source areas for the Beacon Supergroup that formed the fill of the Transantarctic Basin. Subsequent to the deposition of the Beacon sequence, the now‐outcropping rocks of the USARP Mountains were buried to a maximum depth of 4.2 km. A palaeogeothermal gradient of 25±8°C km^?1 was inferred at the time of maximum burial. Inversion of the Transantarctic Basin due to the breakup of Gondwana, and in response to Cenozoic rifting and uplift of the Transantarctic Mountains, has triggered the final denudation stages recorded in NVL since the Cretaceous. Thereby, the amounts of denudation increase eastward. Whereas 2.4–4.2 km of crustal unloading are recognized for the USARP Mountains since the Cretaceous, more than 4 km of denudation has occurred towards the Rennick Graben alone since the Eocene. This denudation was associated with major fault activities involving early ENE–WSW to E–W‐directed extension. Related structures were reactivated by dominant NW–SE to NNW–SSE‐oriented right‐lateral shear genetically linked to the formation and inversion of the structural depression of the Rennick Graben in Cenozoic times.