East-directed Thrust Research Articles

Discussion on ‘Paleozoic sedimentation and Caledonian terrane architecture in NW Svalbard: indications from U–Pb geochronology and structural analysis’ by Koglin et al . 2022 ( JGS , 179, jgs2021-053)

Koglin et al. (2022) present valuable new isotopic and field data regarding northwest Spitsbergen's tectonic history, and propose the existence of the Germaniahalvøya terrane with Scandian (Late Caledonian) igneous activity and deformation. Within this terrane, the Lerner Deformation zone (LDZ), is cast as an east-directed thrust, with the Liefdefjorden Migmatite Complex in the footwall and Lenerøyane Group meta-sediments above. The LDZ occurs on the west limb of the major, shallowly north plunging, Bockfjorden anticline. We suggest their data supports the unmentioned and discussed idea (Dallmann & Piepjohn 2018, Braathen et al. 2018) that a Scandian metamorphic core complex exists here and that the LDZ is part of the Keisarhjelmen extensional detachment (Braathen et al. 2017, Maher et. al. 2022). The detachment consists of ductile to brittle, retrogressive, fault rocks up to several hundred meters thick along the contact between metamorphic basement and overlying Devonian basin strata. In addition, we note that Koglin et al's (2022) evidence for Scandian activity is consistent with core complex dynamics.

Zircon geochronology and paleomagnetism of an Archean harzburgite intrusion, eastern Bighorn Mountains, Wyoming

A harzburgite intrusion, which is part of the trailside mafic complex) intrudes ~2900-2950 Ma gneisses in the hanging wall of the Laramide Bighorn uplift west of Buffalo, Wyoming. The harzburgite is composed of pristine orthopyroxene (bronzite), clinopyroxene, serpentine after olivine and accessory magnetite-serpentinite seams, and strike-slip striated shear zones. The harzburgite is crosscut by a hydrothermally altered wehrlite dike (N20°E, 90°, 1 meter wide) with no zircons recovered. Zircons from the harzburgite reveal two ages: 1) a younger set that has a concordia upper intercept age of 2908±6 Ma and a weighted mean age of 2909.5±6.1 Ma; and 2) an older set that has a concordia upper intercept age of 2934.1±8.9 Ma and a weighted mean age 2940.5±5.8 Ma. Anisotropy of magnetic susceptibility (AMS) was used as a proxy for magmatic intrusion and the harzburgite preserves a sub-horizontal Kmax fabric (n=18) suggesting lateral intrusion. Alternating Field (AF) demagnetization for the harzburgite yielded a paleopole of 177.7 longitude, -14.4 latitude. The AF paleopole for the wehrlite dike has a vertical (90°) inclination suggesting intrusion at high latitude. The wehrlite dike preserves a Kmax fabric (n=19) that plots along the great circle of the dike and is difficult to interpret. The harzburgite has a two-component magnetization preserved that indicates a younger Cretaceous chemical overprint that may indicate a 90° clockwise vertical axis rotation of the Clear Creek thrust hanging wall, a range-bounding east-directed thrust fault that accommodated uplift of Bighorn Mountains during the Eocene Laramide Orogeny.

Thick-skinned tectonic style resulting from the inversion of previous structures in the southern Cordillera Oriental (NW Argentine Andes)

Abstract Structures mapped in the southern Cordillera Oriental of the Andes show an unexpected geometry in an east–west cross-sectional view, with a remarkable predominance of west-directed thrusts. Although some of the Andean structures trend north–south perpendicular to the main east–west direction of Andean shortening, many of them clearly differ from this expected orientation. This peculiar structural style has been largely related to the inversion of the Cretaceous Salta Rift Basin; however, some of these anomalously trending Andean folds and faults do not result from the inversion of Cretaceous faults. This lack of inversion of some Cretaceous structures becomes evident where west-dipping extensional faults rest in the footwall of west-directed thrusts instead of developing east-directed thrusts, as would be expected. Detailed study of several structures and examination of the geometry and facies distribution of several basins highlight not only the role played by the inversion of Cretaceous extensional faults on the geometry of the Andean structures, but also that played by basement anisotropies on the development of both the Cretaceous extensional faults and the Andean contractional structures.

Oligocene Laramide deformation in southern New Mexico and its implications for Farallon plate geodynamics

The Silver City Range in southwest New Mexico contains Proterozoic basement rocks that are overlain by a sequence of Paleozoic, Mesozoic, and Paleogene strata. These rocks are folded in a broad, NW-SE–trending, east-facing monocline that lies structurally above an east-directed thrust fault. The youngest rocks folded in the Silver City monocline are similar to other late Eocene and early Oligocene volcanic rocks of the Mogollon-Datil volcanic field; an ash-flow tuff near the bottom of the volcanic sequence gives an 40 Ar/ 39 Ar age on sanidine of 34.9 ± 0.4 Ma (2σ), and another tuff near the top of the section contains zircons that yield a weighted 206 Pb/ 238 U age of 34.6 ± 0.6 Ma (2σ). We interpret similar structures in the Little Burro Mountains, Lone Mountain, and Bayard area, immediately east and west of the Silver City monocline, to all be genetically related to a system of basement-involved thrust faults. Modeling of these structures from the Mangas Valley in the southwest to the Mimbres Valley in the northeast, suggests ∼17% total shortening. We conclude that Laramide shortening was active in southwest New Mexico generally, and the Silver City region in particular, from the Cretaceous until the earliest phase of Mogollon-Datil volcanism beginning at ∼37 Ma, during which time the earliest extension in the southern Rio Grande rift was initiated. The final stage of Laramide shortening, recorded in the Silver City monocline, took place during a lull of volcanism (and extension) from ∼31.5 to ∼29.3 Ma. We explain the contemporaneity of shortening, significant ignimbrite eruptions, and crustal extension as the consequence of intermittent slab breakoff and renewed underthrusting of the downgoing Farallon plate.

Structural geology and Zn-Pb mineral occurrences of northeastern Cornwallis Island: implications for exploration of the Cornwallis Fold Belt, northern Nunavut

Abstract The Stuart River and Caribou Lake areas of northeastern Cornwallis Island, Nunavut are dominated by Middle Ordovician to Upper Silurian carbonate and shale, and Lower to upper Middle Devonian carbonate and sandstone. These units have been thrust faulted and have undergone multiple phases of folding. Four deformational events are recognized. D1 develops NNW-SSE trending F1 folds and associated thrusts in Ordovician and Silurian strata. D1 is correlative with movement on the Boothia Uplift (Late Silurian-Early Devonian). D2 develops F2 folds and west-directed thrust faults that are co-axial with F1 but affect late Middle Devonian and older strata. D3 exhibits east-west trending F3 folds and is correlated with the Ellesmerian Orogeny (Late Devonian to Early Carboniferous). F3 folds re-fold F1/F2 folds creating interference patterns at a variety of scales. D4 is defined by northwest-southeast trending F4 folds. F4 folds overprint F1/F2 folds, and are unaffected when axial surfaces of F3 are crossed. D4 may possibly be associated with the Paleogene Eurekan Orogeny. Hydrothermal dolomite and Zn-Pb mineralization are strongly controlled by pre-exisiting structures. Specifically, they correlate most commonly with thrusts (preferentially east-directed thrusts) and along the hinge surface traces of regional F3 folds.

Structure and regional significance of the Late Permian(?) Sierra Nevada–Death Valley thrust system, east-central California

An imbricate system of north-trending, east-directed thrust faults of late Early Permian to middle Early Triassic (most likely Late Permian) age forms a belt in east-central California extending from the Mount Morrison roof pendant in the eastern Sierra Nevada to Death Valley. Six major thrust faults typically with a spacing of 15–20 km, original dips probably of 25–35°, and stratigraphic throws of 2–5 km compose this structural belt, which we call the Sierra Nevada–Death Valley thrust system. These thrusts presumably merge into a décollement at depth, perhaps at the contact with crystalline basement, the position of which is unknown. We interpret the deformation that produced these thrusts to have been related to the initiation of convergent plate motion along a southeast-trending continental margin segment probably formed by Pennsylvanian transform truncation. This deformation apparently represents a period of tectonic transition to full-scale convergence and arc magmatism along the continental margin beginning in the Late Triassic in central California.

Seismic stratigraphy and structural evolution of the Adana Basin, eastern Mediterranean

Detailed interpretation and mapping of multi-channel seismic reflection profiles from the Adana Basin show that a south-verging linked imbricate thrust stack existed across the northern portion of the Adana Basin prior to the deposition of early Miocene sedimentary successions. Following the development of the basement-cored fold–thrust belt the region was irregularly denuded. Paleohighs and lows delineate the axis of hanging wall culminations and intervening structural depressions between forelimbs and backlimbs of the stacked sheets, with residual topographic relief locally reaching 1500 m. The stratigraphic and structural architecture of the early to middle Miocene successions in the Adana Basin records a significant phase of tectonic subsidence that initiated in the early Miocene, followed by deposition of a thick deep water succession in an underfilled basin. Then, relatively rapid uplift gave rise to basin inversion and erosion prior to or during the early Tortonian. The upper Miocene global eustatic sea-level fall initiated in the Tortonian. The transition from deep water to fluvio-deltaic sedimentation in the early Tortonian recorded the combined effects of a global eustatic sea-level fall and regional tectonic uplift, presumably related to renewed activity in the Tauride orogen to the north. It led to widespread erosion giving rise to a smooth unconformity surface in most of the Adana Basin. The stratigraphic and structural relationships suggest that the onset of east-directed thrusting in the trailing portion of the Misis fold–thrust belt occurred sometime during the lower Tortonian, coinciding with the switch from marine to terrestrial sedimentation. The truncation by this unconformity of the folded–thrusted successions of both Miocene successions across the trailing thrust sheets marks the effect of progressive erosion as the thrust sheets are uplifted above base level. A prominent progressive syntectonic unconformity is developed extending from the region of main thrust culminations westward into the depocentre situated on the backlimb of the leading thrust of the Misis fold–thrust belt. Much of the Tortonian deposition took place within this trailing piggy-back basin and the easterly onlap and the westerly tilt of this succession mark the progressive uplift of the fold–thrust belt during the remainder of the Tortonian. Terrestrial and fluvio-deltaic deposition in the Tortonian was succeeded in the Messinian by the deposition of mixed evaporite–carbonate–clastic succession in the Adana Basin. The base of this succession is a local unconformity across the Tortonian succession that marks the cessation of the fold–thrust activity in the east and ensuing erosion across the fill of the Tortonian piggy-back basin. The subcrop of the Messinian succession marks the edge of deposition in a paleotopographic depression situated between the Tortonian Misis fold–thrust belt in the east and the paleoslope over the front of the reactivated Tauride fold–thrust belt in the north. The western margin of deposition of the Messinian succession was probably controlled by the development of the Kozan Fault zone. Plio-Quaternary subsidence created the accommodation space that allowed rapid delta progradation.

The role of felsic and mafic igneous rocks in deciphering the evolution of thrust-stacked terranes: An example from the north Norwegian Caledonides

Excellent exposure of appropriate structural levels across the trace of the regional Ofoten synform, north Norway, make this an exceptional area in which to investigate the role of felsic and mafic igneous rocks in the amalgamation and final emplacement of a stack of far-traveled terranes. The nappe sequence in Ofoten is composed, from bottom to top, of the Narvik, Ofoten, and Niingen nappe complexes. Each of these nappe complexes contains multiple, internal thrust sheets, or nappes, that were emplaced onto Precambrian-Cambrian rocks of the Baltic shield along east-directed thrusts during the Caledonian orogeny. Distinct contrasts in rock types, structural arguments, and regional correlations indicate that the nappes are suspect or exotic terranes. Whole-rock chemistry, petrography, and field relations of felsic and mafic igneous rocks within each allochthon are used to explore the tectonic setting of each terrane, the crustal levels of their amalgamation, and the relative and absolute timing of their amalgamation. Three different types of felsic intrusions were distinguished in each lithologic package, with the exception of the Evenes Group (Ofoten nappe complex) which has none. Caledonian plutons also are lacking in the underlying Baltic basement complex. The intrusions are separated temporally into early-phase, nappe-specific plutons and late-phase intrusions, some of which cut across nappe boundaries. Most are S- and I-type collisional granitoidal rocks. Early-phase granitoids in the Bogen Group (Ofoten nappe complex), however, have an A-type chemistry and mineralogy, probably indicating either second-cycle melting of a granulite terrane or melting of calc-alkaline rocks. The lack of Caledonian intrusives in both the Evenes Group and the basement requires that allochthons overlying these units rooted to the west. Discrimination of the various tectonic settings based on the geochemistry of the felsic intrusions dovetails closely with that determined from a sampling of mafic rocks. Each nappe originated as a laterally-distinct entity that rode piggyback with the Niingen nappe complex being emplaced first and the composite nappe stack thrust lastly upon the basement complex. Previously reported lithologic correlations with fossiliferous units and U-Pb and 40Ar/39Ar mineral cooling dates combine with new Rb/Sr and U-Pb dates to place the relative nappe-stacking chronology into an absolute timeframe. Metamorphism of the Narvik nappe complex was first at ∼432 Ma, followed by metamorphism of the Niingen around ∼430 Ma, which later metamorphosed the Ofoten by down heating between ∼430 and 425 Ma. The stacking process took only ∼7 my and resulted from the Silurian-Devonian Scandian orogeny. Pre-Scandian relics occur within each of the nappe complexes and were strongly reworked and emplaced during the Scandian event.

Widespread effects of middle Mississippian deformation in the Great Basin of western North America

Stratigraphic analyses in central and eastern Nevada reveal the importance of a deformation event in middle Mississippian time that caused widespread deformation, uplift, and erosion. It occurred between middle Osagean and late Meramecian time and resulted in deposition of both synorogenic and postorogenic sediments. The deformation resulted in east-west shortening, expressed as east-vergent folding and east-directed thrusting; it involved sedimentary rocks of the Antler foredeep as well as strata associated with the Roberts Mountains allochthon. A latest Meramecian to early Chesterian unconformity, with correlative conformable lithofacies changes, postdates this deformation and occurs throughout Nevada. A tectonic highland—created in the middle Mississippian and lasting into the Pennsylvanian and centered in the area west and southwest of Carlin, Nevada—shed sediments eastward across the Antler foreland, burying the unconformity. Post oro genic strata are late Meramecian to early Chesterian at the base and are widespread throughout the Great Basin. The tectonism therefore occurred 20 to 30 m.y. after inception of the Late Devonian Antler orogeny, significantly extending the time span of this orogeny or representing a generally unrecognized orogenic event in the Paleozoic evolution of western North America. We propose a revised stratigraphic nomenclature for Mississippian strata in Nevada, based on detailed age control and the recognition of unconformities. This approach resolves the ambiguity of some stratigraphic names and emphasizes genetic relationships within the upper Paleozoic section. We take advantage of better stratigraphic understanding to propose two new stratigraphic units for southern and eastern Nevada: the middle Mississippian Gap Wash and Late Mississippian Captain Jack Formations.

Early Paleozoic and Andean structural evolution in the Rio Jáchal section of the Argentine Precordillera

In the fold-and-thrust belt of the northern Argentine Precordillera, Early Paleozoic basin and slope sediments are affected by a folding event which was combined with a slight greenschist facies metamorphism. The structural geometries are influenced by the former normal faulted boundary towards the eastern carbonate platform. To the east of the slope, Early Paleozoic marine deposits record a ~ W-vergent folding without a clear metamorphic overprint. This deformation probably took place in the Devonian to pre-Upper Carboniferous interval while in the west an onset during the Late Silurian is reasonable. During Andean (Late Tertiary) compression, the escarpment was again reactivated as an important, east-directed thrust fault, and the folded strata to the east were juxtaposed along distinct, east-directed high-angle reverse faults with some ~ N-S fold structures interfering with pre-Tertiary folds. Hence, the present architecture of this part of the orogen was largely influenced by different Early Paleozoic depositional realms and structures of one pre-Tertiary compressional event. The latter can be linked with the collision of the Sierras Pampeanas basement complex at the eastern margin of the Precordillera and be related to the collision with the Chilenia Terrane in the west.

Geochronologic and isotopic evidence for Triassic-Jurassic emplacement of the eugeoclinal allochthon in the Mojave Desert region, California

The geologic history of the outer continental margin (eugeoclinal) rocks in the El Paso Mountains and northern Mojave Desert has long been important in models for the development of the active continental margin in the western Cordillera. Current interpretations call for either strike-slip or thrust juxtaposition of eugeoclinal rocks against miogeoclinal/cratonal (platformal) rocks, or some combination of both strike-slip faulting and thrusting. Two broad and interrelated aspects of the history of the eugeoclinal rocks are at issue: (1) How much primary displacement is necessary to account for the present position of the eugeoclinal rocks? and (2) When were the eugeoclinal rocks thrust against platformal rocks? This study primarily addresses the second issue. Lithologic correlation indicates that the outcrop belt of eugeoclinal rocks is bounded by platformal rocks to the east and south. Platformal rocks are also present to the west, but many of these exposures restore to a position structurally beneath the eugeoclinal rocks when Tertiary extension is restored, implying stacking by a thrust. New U-Pb zircon geochronology and whole rock geochemistry and Sr, Nd, and Pb isotopic data for plutons in the El Paso Mountains and northern Mojave Desert lend insight into the timing of this thrusting. The outcrop belt of eugeoclinal rocks coincides with the only known Permian and Triassic plutons in the El Paso Mountains and northern Mojave Desert, which are dated at 260 to 240 Ma by U-Pb zircon in this report. Initial Sr and ϵ Nd(t) isotopic values from these plutons are distinct ≤0.704 and ≥+2, respectively) from continental lithosphere isotopic signatures ≥0.705 and ≤−2) of both Middle Jurassic plutons in the same area and Triassic plutons in the southern and eastern Mojave Desert. Feldspar common lead data for the Permian and Triassic plutons within the eugeoclinal outcrop belt also indicate limited crustal involvement and do not overlap previously reported values for common lead data from Mesozoic plutons in the eastern Mojave Desert region where Proterozoic basement is widespread. The observations and data reported here indicate that Late Permian–Early Triassic plutons in the northern Mojave Desert and El Paso Mountains were generated within or passed primarily through oceanic lithosphere, but later Jurassic plutons were derived from and/or interacted extensively with continental lithosphere. We hypothesize that the eugeoclinal rocks were deposited on oceanic crust that was thrust eastward over Precambrian cratonal basement and overlying strata between approximately 240 Ma and 175 Ma, a time of little documented tectonic activity in the Mojave Desert region. Postulated Permian or late Middle Jurassic east-directed thrusting is incompatible with our data and observations. The data also fill an important gap in palinspastic reconstructions of the early Mesozoic arc and indicate that this arc was northwest-trending and oblique to Paleozoic facies trends in the Mojave Desert.

A structural and palaeomagnetic study of a section through the eastern Subbetic, Southern Spain

A well-exposed 2 km section through the Eastern Subbetic, immediately to the south of the Crevillente wrench fault zone can be divided into an upper thrust sheet of Triassic rocks, a middle thrust sheet of Cenomanian-Turonian basinal sediments, and a lower thrust sheet of Jurassic limestones unconformably overlain by Upper Cretaceous-Palaeocene sediments. Structural relationships suggest that the middle sheet overthrust the lower sheet; and both units were subsequently folded and imbricated. This sequence was then overthrust by the upper sheet, incorporating a footwall duplex of Cenomanian-Turonian sediments. These structures can be related to the complex deformation associated with a single overthrusting event. The dominant thrust transport direction is to the east, strongly oblique to the overall trend of the orogen. However, a palaeomagnetic study isolates stable components which indicate no cumulative vertical axis rotation of the thrust sheets. The section is cut by strike-slip faults whose displacements appear to be relatively minor. The Triassic rocks of the upper thrust sheet form a highly disrupted thrust sheet extending over much of the Subbetic, rather than being local to the Crevillente fault zone. This section through the Subbetic can be interpreted as the result of a major east-directed thrust of basinal sediments of the Median Subbetic over marginal shelf facies of the Internal Subbetic, strongly oblique to the general ESE trend of the orogen. The shortened sequence is cut by a later set of strike slip faults which are probably related to movement on the Crevillente fault.

Paleoproterozoic tectonic assembly of Northeast Laurentia through multiple indentations

Recent detailed geological mapping and UPb geochronological studies in the Torngat, Ungava and Baffin orogens of Northeast Laurentia have provided new information regarding the tectonic assembly of this region between ∼1.92 and 1.74 Ga. In contrast with an earlier hypothesis whereby the geometry of Archean continental fragments was thought to have resulted entirely from northerly indentation of the Superior Province and consequent extrusion tectonics, interpretation of recently acquired data shows that Superior Province indentation was the last of multiple collisional events during the formation of Northeast Laurentia. In the present model, we propose that Southeast Rae and North Rae provinces were separate prior to ∼1.92 Ga, when regional tectonism changed from extension to contraction. Westerly subduction and westward translation of the Archean Disko terrane and Nain Province between the North Rae and Southeast Rae provinces resulted in the development of a continental magmatic arc in the Cumberland batholithic complex and in the Torngat orogen, and deposition of the upper sequences of the Penrhyn-Piling-Karrat sequences in the Foxe and Rinkian belts of the Baffin orogen (1880–1895 Ma). Continental collision between the Disko terrane and North Rae Province produced shortening across the Rinkian and Foxe belts at, or before, 1876 Ma. Meanwhile, early arc magmatism in the Ungava orogen (1861–1898 Ma) accompanied foredeep or transtensional basin formation in the New Quebec orogen (∼1870–1883 Ma) and the onset of deformation in the southeastern Nagssugtoqidian orogen (∼1883 Ma). Nain-Southeast Rae provinces collision at ∼1860 Ma across the Torngat orogen caused renewed granitic magmatism and metamorphism in the Cumberland batholithic complex between 1857 and 1853 Ma. Initiation of felsic volcanism in the Makkovik-Ketilidian orogen at this time indicates a shift in the locus of plate subduction from the Southeast Rae Province to the southern margin of the Nain Province. In the Ungava orogen, northerly subduction changed to southerly subduction resulting in the generation of a second arc suite at between 1844 and 1826 Ma. Formation of the de Pas batholith in the hinterland of the New Quebec orogen at ∼1840 Ma accompanied arc-continent and Superior-Southeast Rae collision from 1845 to 1829 Ma. At this time, accretion of arc material in the Makkovik-Ketilidian orogen was contemporaneous with high-grade sinistral shearing along the reactivated Torngat orogen and the northwestern Nagssugtoqidian orogen. Southeast-directed thrusting along the southeastern margin of the Cumberland batholithic complex, and reactivation of the Rinkian belt is predicted to have also occurred at this time. At ⩽1826 Ma, the onset of arc-continent collision across the Ungava orogen initiated closure of the North Rae-Southeast Rae provinces juncture and generation of the Ford Lake plutonic suite. Subsequent Superior-Southeast Rae continental collision (∼1810 Ma) resulted in dextral “escape tectonics” in the Foxe and Rinkian belts, and thick-skinned folding of basement and cover sequences in the Ungava orogen. High-grade metamorphism and granitoid magmatism in the Makkovik-Ketilidian orogen also occurred. Reactivation of the Nain-Southeast Rae (east-directed thrusting) and Superior-Southeast Rae (west-directed thrusting) margins at ∼1790 Ma resulted in “pop-up” of the Southeast Rae Province between its more rigid neighbors. Final cooling and the emplacement of post-tectonic granitoid rocks took place at ∼ 1740–1790 Ma.

Application of mineral deposit models in exploration: a case study of sediment-hosted gold deposits, Great Basin, Western United States

Abstract This paper emphasizes the development of mineral deposit models for exploration using as a case study the sediment-hosted, structure-controlled, disseminated gold deposits in the Great Basin of the Western United States. More than 60 deposits have been found in this area since the discovery of the Carlin deposit in 1962. Most deposits were discovered by direct prospecting and sampling of outcropping silicified zones (jasperoids). Similar poorly exposed and unexposed deposits remain to be discovered, but future exploration will require improved models that indirectly predict locations of these deposits. Many of these sediment-hosted gold deposits occur within silicified and structurally complex zones in calcareous rocks below regional east-directed thrusts. Geological characteristics that have been successfully employed in exploration include: erosional windows (fensters) through the thrusts; silty to arenaceous calcareous host rocks below the thrusts; silicified zones; anticlines and domes of the lower plate calcareous sediments; alignment of deposits along linear trends; and geochemical anomalies (Au, Ag, As, Sb, Tl, etc.). Although the use of these criteria will discover new deposits, exploration now requires additional criteria that are reliably, but not so directly, associated with the deposits. Identification of these criteria will require the development of genetic models that tie more regional geological characteristics to ore formation and localization. Development of successful mineral deposit models first requires a working definition of the sediment-hosted gold deposit class and its geological characteristics. The characteristics of each deposit included in this class are then documented and validated. Next, the geological processes most likely responsible for the shared characteristics are identified. Finally, the most reliable, informative and accessible criteria, reflective of each ore-forming process, are selected. From these criteria are chosen the most efficient and effective geological features for continued exploration. Processes currently suggested to be important in the formation of sediment-hosted gold deposits include: regional wrench fault tectonics; plutonism; hydrothermal fluid flow driven by intrusions and/or thrusting; fluid flow along regional unconformities; gold precipitation following carbonate dissolution, or fluid mixing; and dissolution of synsedimentary gold from the allochthonous western assemblage sediments and reprecipitation of that gold in the deposits. A general approach for identifying additional possible processes, testing alternative processes and finally selecting exploration criteria is presented.

East-directed imbrication and oblique-slip faulting in the Humber Arm Allochthon of western Newfoundland: structural and tectonic significance

Many of the dominant outcrop-scale structural features in the lower, clastic thrust sheets of the Humber Arm Allochthon were not generated during the westerly emplacement of the allochthonous terranes of western Newfoundland. Two general groups of structures are abundant in the Humber Arm rocks: (1) east-verging folds accompanied by a weakly to moderately developed slaty cleavage and cut by west-dipping thrust faults; and (2) northeast–southwest-striking high-angle faults, with predominantly normal oblique-slip motion and with larger faults down-stepping to the northwest. Evidence of the earlier, west-directed thrusting (refolded and downward-facing folds, folded thrusts, etc.) is uncommon in the Humber Arm area. Slaty cleavage-generation structures, however, appear to overprint the phacoidal fabrics of the mélange zones that exist between and within thrust slices of the allochthon, making the mélange fabrics the most readily identified features associated with the initial east over west imbrication and emplacement of the allochthon.These observations suggest that the original detachment of the rocks of the Humber Arm Supergroup from their basement (early Taconian deformation) occurred with only limited internal deformation. Mélange zones presently define some or all of the early surfaces of movement. The fully assembled and emplaced allochthonous terrane was subsequently reimbricated on a smaller scale through east-directed thrusting, at which time the allochthon was more pervasively deformed (regional slaty cleavage and fold formation). This may represent late Taconian back thrusting or Acadian shortening. The youngest deformation of the Humber Arm region appears to have been a regional extensional event, with a significant northeast–southwest strike-slip component of movement. This may correlate with the development of Carboniferous strike-slip basins in the present Gulf of St. Lawrence and western Newfoundland. Much of the present structural geometry in the Humber Arm region, including the contacts between ophiolitic and clastic thrust sheets, may have originated during these later two deformational sequences, rather than as a consequence of the initial emplacement history.

Collision-related fore-arc and back-arc evolution of the Northern Sunda ARC

Comparison with other arc systems suggests that in the fore-arc area of the northern Sunda Arc emplacement of chromite and jadeite-bearing serpentinite melange diapirs and deposition of olistostromes were caused by Campanian collision with a continental fragment since underthrust eastwards beneath the arc. The age and position of east-directed thrusts and associated tin granites in the continental back-arc area implies that thrusting and generation of granites were genetically related to the collision, and that the back-thrusts were coupled to the fore-arc thrusts by a decollement beneath the fore-arc basin and magmatic arc. Subsidence and sedimentation in the fore-arc basin succeeded collision, and rise of the back-arc Shan Plateau possibly resulted from late Cenozoic relaxation of collision-related compressive stress.

Overthrust model for the Sierra Nevada

The structure of the northern Sierra Nevada, California, is a series of east-directed thrust faults modified by upright and west-directed folds and faults. The highest tectonic unit is a nappe composed of the Smartville ophiolitic complex and related rocks. These structures collectively represent the Middle Jurassic Nevadan orogeny. This overthrust-backfold model resembles Middle Jurassic events in British Columbia and the Cretaceous-Tertiary tectonics of the western European Alpine belt, and it is consistent with the hypothesis that the Nevadan orogeny reflects the collision of an oceanic volcanic arc with North America.

Structural fabric of the Paleozoic Golconda Allochthon, Antler Peak Quadrangle, Nevada: Progressive Deformation of an Oceanic Sedimentary Assemblage

The Golconda allochthon in Nevada consists of structurally complex mid to late Paleozoic basinal sedimentary and volcanic rocks deformed under low temperature and pressure conditions and emplaced onto the North American shelf in the Permo‐Triassic. Above the basal thrust in the Antler Peak area, the allochthon consists (in ascending structural order) of late Early to early Late Permian thick bedded chert (‘Pumpernickel subunit 1 ’); Pennsylvanian and Early Permian thin bedded chert, argillite and siltstone (‘Pumpernickel subunit 2’); Mississippian siliciclastic turbidites (‘Jory unit’) and undated chert and argillite of the ‘Trenton unit.’ Open to tight folds in subunit 1 trend north‐south and are east‐vergent. Folds in thinner bedded subunit 2 vary considerably in style. Fold axes in this unit are distributed within the mean axial plane (oriented N‐S, 60°W) but exhibit a west‐northwest trending data gap. Boudin axes measured in zones of highly disrupted layering (interpreted as zones of movement or shear) exhibit similar patterns without data gaps. These girdled distributions in part reflect rotation of these elements upon greater strain towards the east‐west trending direction of tectonic transport or towards parallelism with the X axis of the strain ellipsoid. Asymmetry of folds indicate movement was west to east. Locally developed late stage folds in subunit 2 are subhorizontal, trend north‐south, and are compatible with fold‐fault solutions that indicate east‐directed thrusting. Striations measured within fault zones yield bidirectional east‐west slip lines. Our fold and fault data indicate that east‐west shortening of these basinal sediments was accomplished by folding and east‐directed thrust faulting during a single progressive deformational event. This conclusion is supported by the presence of a major west‐dipping thrust fault in the map area that places Mississippian over Permian rocks. The allochthon formed as an east‐vergent submarine fold and thrust belt that was subsequently thrust on the continental shelf. Autochthonous rocks beneath the Golconda thrust are virtually undeformed, suggesting that most of the structure in the allochthon formed prior to its ultimate emplacement.

Rio Zulia Field, Colombia: ABSTRACT

The Rio Zulia field in northeastern Colombia lies within the southernmost part of the oil-rich Maracaibo basin. The field measures 3.4 by 0.9 mi (5.5 by 1.5 km) and is located on a faulted anticline whose southeastern flank is cut by an east-directed thrust fault. Cumulative production during the 20 years since Chevron discovered it in March 1962 is 125 million bbl. The oil is brown, paraffinic, and has a gravity of 41.5° API. Maximum production of 35,000 BOPD was reached in 1966; present production is about 4,000 BOPD. Almost all the production has come from the Eocene Mirador Formation, a loosely consolidated fine to coarse-grained, non-calcareous sandstone with irregularly distributed clay and siltstone interbeds. The sandstone is part of the Eocene delta built out to the northeast over the Lake Maracaibo region. The Mirador of the Rio Zulia area belongs to the back-delta fluvial system. End_of_Article - Last_Page 984------------

Geology of Romania

Geology of Romania