Reactivation Of Basement Structures Research Articles

Influence of reactivated basement structures on evolving orogens: Along-strike diachronous Himalayan metamorphism in far west Nepal

Determining the geometry and evolution of a basal detachment and its influence on orogenesis is a challenging, but important, aspect to understanding orogenic evolution. The basal detachment of the Himalayan orogen in far west Nepal is presently segmented by a documented tear fault. New pressure-temperature-time-deformation paths from the Himalayan metamorphic core along the Seti Khola river transect were integrated to compare the tectonometamorphic evolution on either side of the basal detachment tear fault to outline its history. Peak metamorphic conditions of 645−745 °C and 0.85−1.1 GPa were reached in the Seti Khola Himalayan metamorphic core rocks during the Oligocene to earliest Miocene, 10−14 m.y. prior to equivalent along-strike rocks in the adjacent Karnali valley, which indicates segmentation of the Himalayan metamorphic core across the tear fault. We interpret the segmentation of the orogen to have been caused by the development of the tear fault in the basal detachment of the Himalayan orogen and differing ramp-flat geometries on either side. The segmentation and change in basal detachment geometry is consistent with the reactivation of an underthrusted Indian plate inherited basement structure, the Great Boundary Fault, during the Oligocene to earliest Miocene. The comparison of tectonometamorphic histories along-strike in far west Nepal highlights the basal detachment geometry through time and the need to consider the pre-orogenic structural features of the plates involved in orogenesis. This study reinforces the importance of combining tectonometamorphic studies with geophysical and geomorphological data to fully understand the causes of along-strike segmentation of orogenic systems through time.

Intra-basement structures and their implications for rifting of the northeastern South China Sea margin

• Intra-basement structures in the northeastern South China Sea margin are imaged by reflection seismic data . • A group of intra-basement reflectors are shear zones inherited from the former Andean-type orogeny . • Offshore shear zones are similar to onshore counterparts in the footwall of extensional detachments. • Late Mesozoic orogenic collapse reactivated basement structures and changed basement rheology. The present-day passive northern South China Sea (SCS) margin has evolved from an Andean-type margin since the Late Cretaceous and subsequently undergone multiple rifting events. However, few studies focused on the role of pre-existing basement structures and their influence on the Early Cenozoic SCS rifting. This paper addresses the nature and tectonic significance of intra-basement structures in outer northeastern SCS margin, based on 2D seismic reflection, well data and onshore geological studies. We identified three subunits (SF1, SF2 and SF3) of intra-basement reflections by their seismic amplitude and geometrical characteristics. By correlating to drilling and waveform modelling results, we interpret that these subunits represent magmatic and/or metamorphic rocks, residual Mesozoic strata, and layered deformation zone in the basement, respectively. The SF3 shows various spatial contacts with the rift-related faults and has influences on rifting fillings. For faults merging downwards with SF3 shear zones, obvious syn -rift depocenters are located in their hanging walls, while faults with higher dip angles above SF3 almost kept inactive during the rifting. The SF3 extend and rise upward to the shallow level, and form a preferential zone for fault activity. In analogy to the onshore extensional structures, structures in SF3 are most likely mylonitized shear zones in the footwall of detachment faults. The interaction of SF3 with the faults produces antiformal structures in the footwall of extensional detachments, recording ductile to brittle transition of the basement. The intra-basement structures inherited from the Early Mesozoic subduction were probably reactivated with significant rheological change following the Late Cretaceous orogenic collapse.

Assessment of tectonic control on the development of low mountains moderate relief in the Outer Carpathians (Southern Poland)

Inherited tectonic structures, ongoing tectonic deformation, and variations in relative rock uplift rates play an important role in conditioning the processes of relief development. Their influence among other factors, such as climate and lithology, can be quantified using landscape analysis, and geomorphometric indices, in particular. The usage of landscape analysis in recent years is increasing systematically due to the constant improvement of the digital elevation models and GIS software that significantly facilitate this approach. In this study, we aim to recognize the influence of tectonic structures and processes on relief development in the low mountains with moderate relief of the Soła River catchment in the Western Outer Carpathians. To this end, we calculated geomorphometric indices (river longitudinal profile, stream-length gradient index, minimum bulk erosion, relief ratio, circulatory ratio, elongation ratio, and hypsometric integral) for the Sola River and its 47 sub-catchments using a 25-m spatial resolution Digital Terrain Elevation Data Level 2. Additionally, we identified lineaments and knickpoints and correlated the computed results with local and regional fault networks, variations in lithology, and climate fluctuations. Obtained results indicate a significant impact of inherited tectonic structures on the relief development of the Soła River catchment, i.e., directions of principal ridges and valleys follow the orientation of main folds and faults recorded in this area. Anomalously high values of minimum bulk erosion, river gradient, and stream-length gradient index allowed us to define two areas with higher relative uplift rates: 1) the Sola Gorge and 2) the Beskid Żywiecki Mts. Polish Outer Carpathians are generally considered as an area of low strain rate and low seismic activity. However, the possibility of neotectonic processes should be considered in geohazard estimations. Observed bends in the direction of river valleys that do not correspond with changes in lithology could be related to active strike-slip faults. These are probably the reactivated basement structures, copied in the thin-skinned nappe cover, as a result of the accommodation of the Mur-Žilina Fault Zone resulting from the tectonic push of the Alcapa (Alpine-Carpathian-Pannonian) microplate against the European plate. Thus, the role of recent tectonic activity in relief development of the Sola River catchment even though appears to be subsidiary at the most, should not be excluded.

Variation of subsidence in the Dezful Embayment, SW Iran: influence of reactivated basement structures

Subsidence analyses over the past 140 Ma in 15 wells in the Dezful Embayment, which is the main oil-producing area in the Zagros fold and thrust belt, SW Iran, have significantly improved our understanding of the post-Jurassic evolution of the basin. A number of regional-scale tectonic events since the Early Cretaceous were separated by tectonically quiescence periods, which controlled the basin architecture of Central Zagros basin evolution. Subsidence patterns in the Dezful Embayment during tectonically active periods are evidence for the influence of the Precambrian basement tectonics on sedimentation and deformation. This study shows that basement paleo-highs in the Central Zagros are regional-scale features, which may be related to regional upwarps, and are not restricted to the reactivation of deep-seated faults. However, when deep-seated faults developed over regional basement highs, their impact on sedimentation could intensify and be easily recognized in seismic by truncation/onlap features. The impacts of subsidence variations in the Dezful Embayment to the deposition and maturation of the Albian source rock, and on the development of migration path ways for hydrocarbons, are discussed.

CRETACEOUS – NEOGENE STRUCTURAL EVOLUTION OF SE ABU DHABI, UNITED ARAB EMIRATES

Regional 2D seismic profiles and 2D cross‐sections extracted from high‐resolution 3D seismic data were interpreted together with well data to reconstruct the sequence stratigraphy of the Late Cretaceous – Miocene sedimentary succession in SE Abu Dhabi. This region is a major hydrocarbon province with numerous giant oilfields producing from large‐scale anticlines. The paper focusses on the structural development of the Sahil, Asab and Shah fields.Six sequence boundaries within the Aptian to Miocene succession were identified on the basis of erosional truncations and onlap patterns. The deepest‐lying erosional surface is the top‐Albian sequence boundary. This was identified by angular truncation of the Nahr Umr Formation in the SE of the study area near an uplifted domal structure which hosts the Mender and nearby Lekhwair fields. The unconformity is interpreted to result from reactivation of the basement together with a eustatic fall in sea‐level which resulted in regional uplift and erosion. A second phase of uplift occurred during the mid‐Turonian and resulted in significant erosion of the Shilaif Formation along structural highs. The top‐ Shilaif Formation unconformity forms a sequence boundary and is observed at the Shah, Zararrah and Sahil anticlines. The unconformity is coeval with the onset of obduction of the Semail Ophiolite onto the Arabian margin, and the associated flexural bulge probably resulted in structural uplift and reactivation of basement structures. A global sea‐level fall is also reported during middle Turonian time. The third sequence boundary occurs in the lower Campanian as an angular unconformity surface between the Halul and Fiqa Formations. This event is most obvious along the Shah and Zararrah anticlines and is interpreted to have occurred as a result of flexural uplift related to a further episode of obduction of the Semail Ophiolite. The fourth sequence boundary occurs in the Campanian – Maastrichtian and was identified by angular truncations of the Fiqa Formation against the Simsima Formation, an event which caused widespread erosion of a significant portion of the Fiqa Formation. This tectonic activity was most likely controlled by the final emplacement of the Semail Ophiolite, and probable reactivation of basement faults. The Simsima Formation is bounded above by the fifth sequence boundary, which coincides with the Cretaceous/Tertiary boundary and which is associated with the final stages of Late Cretaceous tectonic activity. The final sequence boundary is the supra‐Dammam unconformity, which resulted in a peneplain surface associated with widespread erosion and an extended hiatus.Major trap formation in the study area occurred from the Cenomanian to the Maastrichtian, and coincided with obduction‐related emplacement of allochthonous thrust sheets onto the plate margin in Oman. However, the structural growth history of the Shah, Asab and Sahil anticlines suggest multiple episodes of uplift. Growth of the Shah anticline was initiated during deposition of the Shilaif Formation in Cenomanian – early Turonian times, but neither the Asab nor Sahil anticlines show significant growth or faulting older than Campanian. In all the anticlines, the most pronounced tectonic activity observed was in the Campanian during deposition of the Fiqa Formation. Significant growth of structures was completed by Paleocene time, suggesting that the Zagros orogeny had little structural influence in SE Abu Dhabi.

Saglek Basin in the Labrador Sea, east coast Canada; stratigraphy, structure and petroleum systems

Abstract Saglek Basin is the more northerly of the two major sedimentary basins along the Labrador margin of the Canadian east coast in which exploration drilling was undertaken in the 1970s-early 1980s. Saglek Basin has an area of over 100,000 km2, and contains a clastic-dominated succession of late Cretaceous to Pleistocene age. Nine wells were drilled in Saglek Basin, with only one significant discovery. The Hekja O-71 well, drilled in the northern half of the basin just east of Frobisher Bay, flowed natural gas with condensates and has reserves estimated at approximately 6.51 × 1011 m3 (2.3 Tcf). A new interpretation of seismic and marine Bouguer gravity data shows the very strong influence of reactivated basement structures on the overlying section. Earthquake activity along the Labrador Sea seismic zone is proposed as a possible cause for episodic venting of thermogenic natural gas through pre-existing fracture networks. This may account for the observations of persistent oil slick features on the sea surface observed in satellite radar data. The presence of these oil seep indicators in the southern part of the basin implies the existence of a second petroleum system that is more oil prone. Source rock analysis indicates that three formations in the post-rift succession of the basin have petroleum source potential. Previous studies have raised the issue of inadequate thermal maturation of these rocks in downgrading the petroleum potential of the basin. An integrated 4-D basin modelling study, that shows significant natural gas generation, estimated that volumes of 2.83 × 109 m3 (100 Tcf) could occur, along with a minor oil component, in self-sourced petroleum kitchens in some reservoirs and from sites deeper in the basin. Three separate prospect fairways are interpreted along the western side of the basin. Collectively, these observations suggest the petroleum potential of the Saglek Basin may be significantly higher than estimated previously.

Basement reactivation in the development of rift basins: an example of reactivated Caledonide structures in the West Orkney Basin

The West Orkney Basin is a poorly studied Palaeozoic and Mesozoic rift basin that overlies a highly seismic-reflective Caledonian orogenic basement terrane. This study presents a novel approach of using 2D seismic data to map single intra-basement reflection packages and their relationship to overlying normal faults in the West Orkney Basin. It is demonstrated that the rift architecture of the West Orkney Basin is more linear with a prominence of relay zones relative to previous interpretations, and that rift structures form complex discordant and concordant relationships with pre-rift Caledonide basement structures. Restoration of basement fabrics to their pre-extensional geometry indicates that the reactivation of basement structures as normal faults has occurred only where the pre-extensional-dip of basement structures is greater than 30°. The relatively high density of relay zones mapped in the West Orkney Basin is proposed to be the result of the rift system forming a partially exploitative relationship with basement structure, where extension has been accommodated between segments that have or have not reactivated basement fabrics.

Tectonic reactivation of transverse basement structures in the Grenville orogen of SW Quebec, Canada: Insights from gravity and aeromagnetic data

NW-SE trending, transverse lineaments, including the lithospheric-scale Mont-Laurier lineament, are interpreted from regional Bouguer gravity of the Grenville orogen of SW Quebec and adjacent Superior Craton in southeastern Canada. These lineaments, transverse to the ENE trending Grenville orogen, are inferred to correspond to Palaeoproterozoic structures in Archaean basement that have played an important role: (i) in the development of volcano-sedimentary back-arc basins along this segment of the Laurentian margin; (ii) on the geometry of thrust sheets and folds formed during thrusting in the ca. 1.23–1.2Ga Elzevirian orogeny and incorporation of the basins within the orogen; (iii) on reorientation of early-formed structures in the Central Metasedimentary Belt of Quebec (CMB-Q) during ca. 1.19–1.17Ga post-Elzevirian orogenic collapse; and (iv) for development of syn-plutonic deformation corridors and shear zones at the onset of the emplacement of the Morin anorthosite-mangerite-charnockite-granite (AMCG) suite. In the CMB-Q, a 100km wide megakink zone developed during ca. 1.19–1.17Ga differential, post-Elzevirian orogenic collapse in the upper-most nappe above transverse sinistral shear corridors 10–20km wide located in an underlying thrust sheet or “lower-deck”. Emplacement of 1.17Ga Chevreuil intrusive suite preferentially occurred within the megakink zone, starting late in the post-Elzevirian collapse and culminating during a switch to local shortening early in (and in part as a consequence of) the emplacement of the voluminous Morin anorthosite and associated AMCG-suite plutons. The Labelle deformation zone separating the CMB-Q and Morin terrane is interpreted as a post-Shawinigan, reverse shear zone that truncates folded lithological layering in the eastern CMB-Q and western Morin terrane that is either subsequently folded above the Mont-Laurier lineament during its further reactivation, or developed as a curved shear zone stepping across the Mont-Laurier lineament. The Grenville Province of SW Quebec therefore provides an example of strain partitioning and distinct deformation responses at different crustal levels during reactivation of basement structures.

Upper Mississippian (Lower Carboniferous) carbonate stratigraphy and syndepositional faulting reveal likely Ouachita flexural forebulge effects, eastern Kentucky, U.S.A.

Stratigraphic analysis of late Middle to early Late Mississippian (Early Carboniferous; late Meramecian–early Chesterian) carbonates in east-central and northeastern Kentucky, U.S.A., shows six unconformity-bound sequences. The earliest, late Meramecian, St. Louis sequence shows a relatively uniform distribution and a typical eastward Appalachian gradient along the ENE-trending outcrop belt on the western margin of the Appalachian Basin. However, abrupt uplift and exposure of deeper water St. Louis deposits, indicated by erosional thinning and well-developed exposure features, at the Meramecian–Chesterian transition signal inception of regional early Chesterian uplift that intensively altered the nature and distribution of the five overlying early Chesterian carbonate sequences. Uplift was related to the syndepositional reactivation of WNW-trending basement faults that broke up the St. Louis platform and generated a mosaic-like distribution of the overlying five sequences due to structurally related erosional incision and paleoslope change to the southwest. These early Chesterian events coincide with a major phase of northeast-directed Ouachita Orogeny on the southern margin of Laurussia and concomitant forebulge migration to the ENE. Modeling of regional stress distribution, as well as timing and location during bulge migration, strongly indicate that a migrating Ouachita bulge reactivated basement structures and generated the noted stratigraphic anomalies in eastern Kentucky and nearby areas. The likelihood of low-amplitude flexural waves, like the Ouachita bulge, migrating across areas where they were not previously considered, has major implications for understanding structural, stratigraphic, and eustatic anomalies in supposedly stable cratonic areas far removed from active tectonism.

Arc–continent collision and orogenesis in western Tasmanides: Insights from reactivated basement structures and formation of an ocean–continent transform boundary off western Tasmania

Crustal architecture in formerly contiguous basement terranes in SE Australia, Tasmania and northern Victoria Land is a legacy of late Neoproterozoic–Cambrian subduction-related processes, culminating in formation of the Delamerian–Ross orogen. Structures of Delamerian–Ross age were subsequently reactivated during late Mesozoic–Cenozoic Gondwana breakup, strongly influencing the geometry of continental rifting and providing clues about the origins and configuration of the pre-existing basement structures. An ocean–continent transform boundary developed off western Tasmania follows the trace of an older Paleozoic strike-slip structure (Avoca–Sorell fault system) optimally oriented for reactivation during the final separation of Australia from Antarctica. This boundary cuts across rocks preserving an earlier record of arc–continent collision during the course of which continental crust was subducted to mantle depths and Cambrian mafic–ultramafic island arc rocks were thrust westwards over late Neoproterozoic–Cambrian passive margin sequences. Collision was accompanied by development of a foreland basin into which 520–600 Ma arc-derived detrital zircons were shed. Following a reversal in subduction polarity, and change to transcurrent motion along the Gondwana margin, Tasmania migrated northward along the proto-Avoca fault system before entering a subduction zone located along the Heathcote–Governor fault system, precipitating a second collision, south-vergent thrusting, and tectonic reworking of the already accreted Cambrian arc–forearc assemblages and underlying passive margin sequences.

The Bedrock Surface of the Western Lake Ontario Region: Evidence of Reactivated Basement Structures?

Lower Paleozoic bedrock strata, in south-central Ontario and the adjacent part of New York State are covered by a thick (100m+) blanket of Pleistocene glacial and interglacial sediments. The form of the buried bedrock surface has been reconstructed from 70,000 waterwell boreholes that extend through the entire Pleistocene cover using GIS data processing techniques. The sub-drift bedrock surface shows linear channels that connect the basins of lakes Huron, Ontario and Erie and which form part of an ancestral mid-continent Great Lake drainage system prior to modification and infilling during successive Pleistocene glaciations. This relict drainage system is cut across Lower Paleozoic carbonates and elastics up to 500 m thick, but the position of several channels is aligned above terrane boundaries, faults and other deep-seated and poorly understood geophysical anomalies in underlying mid-Proterozoic Grenville basement rocks. Other channels are controlled by a dominant northwest and northeast trending regional joint system. A close relationship among deeply seated geophysical lineaments, basement structures and topographic lineaments cut across thick Paleozoic cover strata suggests a history of Phanerozoic reactivation and upward propagation of fractures from the Precambrian basement. Several basement structures and lineaments are seismically active suggesting ongoing neotectonic activity across the 'stable' craton of south-central Ontario.

Tertiary stratigraphy and basin evolution of southern Sabah: implications for the tectono-stratigraphic evolution of Sabah, Malaysia

Integrated surface mapping, dating and radar image interpretation of strata in southern Sabah (northern Borneo) has made it possible to revise the lithostratigraphy, chronostratigraphy, structure and tectonic evolution of the area. The recognition in the field of an Early Miocene regional unconformity, which may be equivalent to the Deep Regional Unconformity recognised offshore, has allowed the development of a stratigraphic framework of groups and formations which correspond to stages in the sedimentary basin development of the area. Below the Early Miocene unconformity the succession can be resolved into deposits of an accretionary complex of Eocene age overlying an ophiolitic basement, and late Palaeogene deep water succession which formed in a forearc basin. The late Palaeogene deposits underwent syn-depositional deformation, including the development of extensive m~langes, all of which can be demonstrated to lie below the unconformity in this area. Some localised limestone deposition occurred during the period of uplift and erosion in the Early Miocene, following which there was an influx of clastic sediments deposited in delta and pro-deltaic environments in the Middle Miocene. These deltaic to shallow marine deposits are now recognised as forming two coarsening­ upward successions, mapped as the Tanjong and Kapilit Formations. Their map distribution have been revised. The total thickness of these two formations in the southern Sabah Basin amounts to 6,000 m, only half of the previous estimates. The Tanjong and Kapilit Formations are deformed into broad NW-SE-trending synclines separated by narrow anticlines. The anticlines are sub-parallel to major faults and associated with high angle reverse faults, and positive flower structures. Secondary fold-faults formed oblique to the major faults. The structural style suggests that the NW-SE trending faults acted as major left-lateral transpressional zones. The faults may in part be reactivated basement structures. The Early Miocene unconformity is interpreted to be the result of deformation and uplift following underthrusting of continental crust of the South China Sea which terminated Paleogene subduction beneath North Borneo. Renewed subsidence is related here to rifting in the Sulu Sea which led to the development of a major Miocene depocentre above the older forearc accretionary complex. The major transpressional deformation probably occurred during the Late Pliocene, and is possibly related to propagation of deformation from Sulawesi towards NW Sabah. This strike-slip deformation which uplifted the area is termed here the Meliau Orogeny. Renewed extension during the Quaternary has caused some sequence repetition. The 'circular basins' of the Meliau, Malibau and Tidung areas, are interpreted as remnants of a single large basin, deformed in the NW-SE trending transpressional fault zones.

The origin of the 'circular basins' of Sabah, Malaysia

Surface mapping, dating and radar image study of strata in southern Sabah (northern Borneo) have made it possible to revise the stratigraphy and reinterpret the structure and tectonic evolution of the area. Early Miocene regional unconformity may be equivalent to the Deep Regional Unconformity recognised offshore, below which the succession can be resolved into an Eocene accretionary complex age overlying an ophiolitic basement, and an upper Paleogene deep-water succession which formed in a forearc. The Paleogene deposits underwent syn-depositional deformation, including the development of extensive melanges, all of which lie below the unconformity. Localised limestones were deposited followed after a period of uplift and erosion in the Early Miocene, followed by an influx of clastic sediments deposited in delta and pro-deltaic environments in the Middle Miocene. These deltaic to shallow marine deposits form two coarsening-upward successions, mapped as the Tanjong and Kapilit Formations. The total thickness of these two formations remaining in the southern Sabah Basin amounts to 6,000 m, about half of previous estimates. The Early Miocene unconformity is interpreted to be the result of deformation and uplift following underthrusting of continental crust of the South China Sea which terminated Paleogene subduction beneath North Borneo. Renewed subsidence led to the development of a major Miocene depocentre above the older forearc accretionary complex. A new tectonic model is proposed for southern Sabah whereby a major transpressional deformation probably occurred during the Late Pliocene and the Tanjong. and KapilitFormations were deformed into broad NW-SE-trending synclines separated by narrow anticlines. The anticlines are sub-parallel to major faults and associated with high angle reverse faults, and positive flower structures. Secondary fold-faults formed oblique to the major faults. The structural style suggests that the NW-SE trending faults acted as major left-lateral transpressional zones and possibly produced large­ scale contractional duplexes. The faults may in part be reactivated basement structures. This deformation uplifted the area and is termed here the Meliau Orogeny. Renewed extension during the Quaternary caused some sequence repetition and widened the original synclines. The 'subcircular- to· elliptical-shaped basins' of the Meliau, Malibau and Tidung areas are structurally controlled synclines and interpreted as remnants of a single large basin, deformed in the NW -SE trending transpressional fault zones.

Basement-controlled faulting of Paleozoic strata in southern Ontario, Canada: new evidence from geophysical lineament mapping

Basement fault reactivation is now recognized as an important control on sedimentation and fault propagation in intracratonic basins. In southern Ontario, the basement consists of complexly structured mid-Proterozoic (ca. 1.2 Ga) crystalline rocks and metasedimentary rocks that are overlain by up to 1500 m of Paleozoic sedimentary strata. Reactivation of basement structures is suspected to control the location of Paleozoic fault and fracture systems, but evaluation has been hindered by a limited understanding of the regional structural characteristics of the buried basement. New aeromagnetic- and gravimetric-lineament mapping presented in this paper better resolves the location of basement discontinuities and provides further evidence for basement controls on the distribution of Paleozoic fault and fracture systems. Lineament mapping was facilitated by reprocessing and digital image enhancement (micro-levelling, regional residual separation, derivative filtering) of existing regional gravity and aeromagnetic datasets. Reprocessed images identify new details of the structural fabric of the basement below southern Ontario and delineate several previously unrecognized aeromagnetic and gravity lineaments and linear zones. Linear zones parallel the projected trends of mid-Proterozoic terrane boundaries identified by field mapping on the exposed shield to the north of the study area, and are interpreted as zones of shearing and basement faulting. Mapped aeromagnetic and gravity lineaments show similar trends to Paleozoic faults and fracture networks and broad zones of seismicity in southern Ontario. These new data support an ‘inheritance model’ for Paleozoic faulting, involving repeated reactivation and upward propagation of basement faults and fractures into overlying cover strata.

Reactivated basement structures affecting the sedimentary facies in a tectonically “quiescent” epicontinental basin: an example from NW Switzerland

The Jurassic deposits in the southern part of central Europe accumulated in a shallow epicontinental sea, and their deposition has usually been believed to have corresponded to a phase of tectonic quiescence; neither on conventional seismic records nor in outcrops obvious indications of synsedimentary tectonics were found. Nonetheless, subtle variations in lithofacies and thickness occur above faults in the crystalline basement. In fact, preexisting structures became reactivated in the Jurassic during major extensional phases when the Tethys and the North Atlantic opened. This reactivation led to differential subsidence and/or to rotation of fault-bounded blocks, but the sediments were deformed mainly flexurally as vertical movements in the basement were dissipated by Triassic salt. Thus, the depositional area was morphologically differentiated into swells and depressions. In siliciclastic muddy environments, swells were characterized by hardbottoms and hiatus beds; depressions were filled by distal tempestites and gravity deposits. In carbonate settings, reefs nucleated on swells; marls and muds were deposited in depressions (many of them gravity deposits). Reactivation of faults occurred only during short episodes and was not synchronous throughout the study area, and reactivation of individual faults was episodic and probably controlled by the palaeostress field and the faults' orientation. Reactivation of deep-rooted faults is also documented by hydrothermal activity which led to vein mineralization and alteration of minerals—today exposed in nearby basement units. The chronostratigraphic ages of the hydrothermal processes coincide with phases of enhanced subsidence during the Sinemurian, Aalenian, Bajocian/Bathonian, and Oxfordian. In turn, facies changes of the sedimentary cover should be useful to predict basement structures when no seismic records are available.

Forebulge migration in the Cretaceous Western Interior basin of the central United States

ABSTRACT This study combines stratigraphic evidence with geodynamic modelling to demonstrate that a forebulge played an identifiable role in Cenomanian–Turonian erosion and sediment accumulation in the North American Western Interior basin. The early to middle Turonian forebulge migrated progressively eastwards, and by the upper middle Turonian acted as a ‘backstop’ against which barrier islands formed in the axial basin.This paper focuses on the progressive migration of an unconformity on the forebulge. The lengthwise orogen‐parallel orientation and time‐transgressive orogen‐normal migration of the forebulge unconformity are characteristics that differentiate it from unconformities developed on reactivated basement structures. We present a conceptual model in which the unconformity formed as the seafloor was uplifted by forebulge‐related flexure to a water depth at which submarine bypass and erosion occurred. A numerical model that describes forebulge migration in response to load dispersal by erosion of the orogenic front and sedimentation into the foredeep indicates that the distance from the thrust front to the forebulge is within reasonable bounds established using a flexural rigidity of 3×1024 Nm.We identify architecturally similar, coeval unconformities from Montana to New Mexico, and interpret the similar distance from the thrust front to a point where each unconformity dissipates as indicative of a uniform lithospheric flexural response along the orogenic front. Here we ascribe cratonward (west‐to‐east) forebulge migration to erosional load redistribution, whereas orogen‐parallel (north–south) stratigraphic climb of the forebulge unconformities developed in response to depocentre migration. Inherited lithospheric inhomogeneities may have allowed the forebulge in central Colorado to crest farther from the orogen than to the north and south.

Geological and Isotopic Constraints on the Metallogenic Evolution of the Proterozoic Sediment-Hosted Pb-Zn (Ag) Deposits of Brazil

Integrated studies of seven Proterozoic sediment-hosted, Pb-Zn-Ag sulfide deposits of Brazil, permit the estimation of the age of the hosting sequence and the mineralization, the nature of the sulfur and metal sources, the temperature range of sulfide formation and the environment of deposition. These deposits can be classified into three groups, according to their ages. (a) Archean to Paleoproterozoic: the Boquira deposit, in Bahia state, consists of stratiform massive and disseminated sulfides hosted by parametamorphic sequences of grunnerite-cummingtonite+magnetite that represent a silicate facies of the Boquira Formation (BF). Lead isotope data of galena samples indicate a time span between 2.7 and 2.5 Ga for ore formation, in agreement with the stratigraphic position of the BF. The relatively heavy sulfur isotope compositions for the disseminated and stratiform sulfides (+8.3 to +12.8 ‰ CDT)suggest a sedimentary source for the sulfur. (b) Paleo to Mesoproterozoic: stratiform and stratabound sulfides in association with growth faults are present in the Canoas mine (Ribeira, in Paraná state) and in the Caboclo mineralization (Bahia state). They are hosted by calcsilicates and amphibolites in the Canoas deposit, whereas in the Caboclo area the mineralization is associated with hydrothermally altered dolarenites at the base of the 1.2 Ga Caboclo Formation. The interpreted Pb-Pb age of the Canoas mineralization is coeval with the 1.7 Ga host rocks. Sulfur isotopic data for Canoas sulfides (+1.2 to +16 ‰ CDT) suggest a sea water source for the sulfur. The range between −21.1 and +8.8 ‰ CDT for the Caboclo sulfides could suggest the action of bacterial reduction of seawater sulfates, but this interpretation is not conclusive. (c) Neoproterozoic: stratiform and stratabound sulfide deposits formed during the complex diagenetic history of the host carbonate rocks from the Morro Agudo (Bambui Group), Irecê and Nova Redenção (Una Group), yield heavy sulfur isotope values (+18.9 to +39.4 ‰ CDT). The uniform heavy isotope composition of the barites from these deposits (+25.1 to +40.9 ‰) reflect their origin from Neoproterozoic seawater sulfates. The late-stage, and most important, metallic concentrations represent sulfur scavenged from pre-existing sulfides or from direct reduction of evaporitic sulfate minerals. Lead isotope data from the Bambui Group suggest focused fluid circulation from diverse Proterozoic sediment sources, that probably was responsible for metal transport to the site of sulfide precipitation. (d) Late Proterozoic to Early Paleozoic: lead-zinc sulfides (+pyrite and chalcopyrite) of Santa Maria deposits, in Rio Grande do Sul, form the matrix of arkosic sandstones and conglomerates, and are closely associated with regional faults forming graben structures. Intermediate volcanic rocks are intercalated with the basal siliciclastic members. Lead isotope age of the mineralization (0.59 Ga) is coeval with the host rocks. Sulfur isotopic values between −3.6 and +4.1 are compatible with a deep source for the sulfur. Geological, petrographic and isotopic data of the deposits studied suggest that they were formed during periods of extensional tectonics. Growth faults or reactivated basement structures probably were responsible for localized circulation of metal-bearing fluids within the sedimentary sequences. Sulfides were formed by the reduction of sedimentary sulfates in most cases. Linear structures are important controls for sulfide concentration in these Proterozoic basins.

Alleghanian reactivation of the Acadian fold belt, Meguma Zone, southwest Nova Scotia

Shear zones and northwest-verging folds define a 30 km wide belt of deformation that overprints the Acadian fold belt and telescopes isograds in the Meguma Zone in southwest Nova Scotia. The shear zones appear to form a linked system that accommodated convergence-dominated transpression of the Meguma Zone against an irregular Avalon boundary. Available geochronological data indicate a Mid-Carboniferous (Alleghanian–Variscan) age for the overprinting deformation. The Mid-Carboniferous basement reactivation in southwest Nova Scotia is likely coeval with deformation of Carboniferous strata and reactivation of basement structures (Meguma Group) in the northern Meguma Zone. Together, these Mid-Carboniferous structures may define a wide belt of Alleghanian–Variscan deformation across the northwest (cratonward) margin of the Meguma Zone.

The West-Central Minnesota Earthquake of June 5, 1993: An Opportunity to Re-examine Seismicity Near the Morris Fault

Other| November 01, 1995 The West-Central Minnesota Earthquake of June 5, 1993: An Opportunity to Re-examine Seismicity Near the Morris Fault Val W. Chandler Val W. Chandler Minnesota Geological Survey, 2642 University Avenue, St. Paul, Minnesota 55114-1057 (V. A. L.) Search for other works by this author on: GSW Google Scholar Author and Article Information Val W. Chandler Minnesota Geological Survey, 2642 University Avenue, St. Paul, Minnesota 55114-1057 (V. A. L.) Publisher: Seismological Society of America First Online: 09 Mar 2017 Online Issn: 1938-2057 Print Issn: 0895-0695 © 1995 by the Seismological Society of America Seismological Research Letters (1995) 66 (6): 113–121. https://doi.org/10.1785/gssrl.66.6.113 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Val W. Chandler; The West-Central Minnesota Earthquake of June 5, 1993: An Opportunity to Re-examine Seismicity Near the Morris Fault. Seismological Research Letters 1995;; 66 (6): 113–121. doi: https://doi.org/10.1785/gssrl.66.6.113 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search Abstract The June 5, 1993 west-central Minnesota earthquake (mbLg = 4.1) provides an opportunity to re-investigate Minnesota seismicity and its possible relationship to reactivated basement structures. This earthquake occurred within 25 km of the much larger July 9, 1975 west-central Minnesota earthquake (mbLg = 4.6), and could have a similar geologic cause. Modified Mercalli intensity data from the 1993 earthquake indicate a felt area of about 69,500 km2 and a maximum intensity of V in the epicentral area. Detailed intensity data in the epicentral area generally accord with the position of the instrumentally determined epicenter, although a node of enhanced shaking inferred within the main intensity V area implies an epicenter some 5–10 km north or northwest. The correlations of some local perturbations in both the 1993 and 1975 isoseisms with features in the Precambrian bedrock imply that seismic energy may be locally focused by crustal structure. Intensity-depth relationships indicate that the 1993 focal depth is about the same as the 1975 earthquake, which has been independently estimated to be 7.5 km.The Morris fault, a northeast-striking crustal discontinuity separating two Archean terranes, has been generally thought to be the major seismogenic feature in the region, but neither the 1993 hypocenter inferred by this study nor the 1975 hypocenter agrees with the subsurface trace of the fault as determined by seismic reflection data and magnetic modeling. Alternative sources for the 1975 and 1993 earthquakes are west-northwest-striking, subvertical structures that are interpreted to consist chiefly of fractures that are occupied by mafic dikes. Some of these fractures are also associated with minor faulting, as shown by small offsets in the Morris fault, and the possible disruption of Cretaceous strata. Neither the Morris fault nor the west-northwest structures agrees with a loosely constrained focal mechanism for the 1975 earthquake, but the latter structures are favorably oriented to be reactivated by the east-northeast regional compression estimated for the eastern United States.Minor basement structures, such as mafic dikes and fractures, may influence the seismicity of continental interiors more than is generally believed, because in many parts of the north-central United States, such structures are not resolvable with existing geological and geophysical data sets. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Evolution of the Cockburn Basin: implications for the structural development of the Celtic Sea basins

Abstract The Cockburn Basin is one of a series of sub-parallel basins in the Celtic Sea-Western Approaches area to the south and southwest of Ireland. These basins share broadly similar structural and depositional histories, and are regarded as the remnants of a regionally extensive Triassic-Middle Jurassic basin which was dissected by Late Jurassic-Early Cretaceous tectonics. During the late Paleogene the basin was offset dextrally by 4–5 km across the Cockburn Fault, part of a major Tertiary wrench zone. Displacement on the Cockburn Fault is entirely Tertiary in age; however, the adjacent Fastnet segment of the Goban-Cockburn Fault Zone appears to have exerted a degree of control on the pre-Aptian development of the Fastnet Basin. At least one major basement structure, the Scilly Wrench Zone, appears to have entirely avoided reactivation during the post-Variscan development of the Western Approaches Trough. It would appear that reactivation of basement structures in the Celtic Sea-Western Approaches area has occurred in a patchy and unpredictable manner and, although pre-existing faults locally exert considerable influence, there is no fundamental basement control of the evolution of sedimentary basins in the region.