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1.
Abstract— The results of a systematic field mapping campaign at the Haughton impact structure have revealed new information about the tectonic evolution of mid‐size complex impact structures. These studies reveal that several structures are generated during the initial compressive outward‐directed growth of the transient cavity during the excavation stage of crater formation: (1) sub‐vertical radial faults and fractures; (2) sub‐horizontal bedding parallel detachment faults; and (3) minor concentric faults and fractures. Uplift of the transient cavity floor toward the end of the excavation stage produces a central uplift. Compressional inward‐directed deformation results in the duplication of strata along thrust faults and folds. It is notable that Haughton lacks a central topographic peak or peak ring. The gravitational collapse of transient cavity walls involves the complex interaction of a series of interconnected radial and concentric faults. While the outermost concentric faults dip in toward the crater center, the majority of the innermost faults at Haughton dip away from the center. Complex interactions between an outward‐directed collapsing central uplift and inward collapsing crater walls during the final stages of crater modification resulted in a structural ring of uplifted, intensely faulted (sub‐) vertical and/or overturned strata at a radial distance from the crater center of ?5.0–6.5 km. Converging flow during the collapse of transient cavity walls was accommodated by the formation of several structures: (1) sub‐vertical radial faults and folds; (2) positive flower structures and chaotically brecciated ridges; (3) rollover anticlines in the hanging‐walls of major listric faults; and (4) antithetic faults and crestal collapse grabens. Oblique strike‐slip (i.e., centripetal) movement along concentric faults also accommodated strain during the final stages of readjustment during the crater modification stage. It is clear that deformation during collapse of the transient cavity walls at Haughton was brittle and localized along discrete fault planes separating kilometer‐size blocks. 相似文献
2.
Gordon R. Osinski Pascal Lee John G. Spray John Parnell Darlene S. S. Lim Theodore E. Bunch Charles S. Cockell Brian Glass 《Meteoritics & planetary science》2005,40(12):1759-1776
Abstract— The Haughton impact structure has been the focus of systematic, multi‐disciplinary field and laboratory research activities over the past several years. Regional geological mapping has refined the sedimentary target stratigraphy and constrained the thickness of the sedimentary sequence at the time of impact to ?1880 m. New 40Ar–39Ar dates place the impact event at ?39 Ma, in the late Eocene. Haughton has an apparent crater diameter of ?23 km, with an estimated rim (final crater) diameter of ?16 km. The structure lacks a central topographic peak or peak ring, which is unusual for craters of this size. Geological mapping and sampling reveals that a series of different impactites are present at Haughton. The volumetrically dominant crater‐fill impact melt breccias contain a calcite‐anhydrite‐silicate glass groundmass, all of which have been shown to represent impact‐generated melt phases. These impactites are, therefore, stratigraphically and genetically equivalent to coherent impact melt rocks present in craters developed in crystalline targets. The crater‐fill impactites provided a heat source that drove a post‐impact hydrothermal system. During this time, Haughton would have represented a transient, warm, wet microbial oasis. A subsequent episode of erosion, during which time substantial amounts of impactites were removed, was followed by the deposition of intra‐crater lacustrine sediments of the Haughton Formation during the Miocene. Present‐day intra‐crater lakes and ponds preserve a detailed paleoenvironmental record dating back to the last glaciation in the High Arctic. Modern modification of the landscape is dominated by seasonal regional glacial and niveal melting, and local periglacial processes. The impact processing of target materials improved the opportunities for colonization and has provided several present‐day habitats suitable for microbial life that otherwise do not exist in the surrounding terrain. 相似文献
3.
Jillian A. Hudgins John G. Spray Randy L. Korotev 《Geochimica et cosmochimica acta》2008,72(23):5781-5798
Infrared laser probe 40Ar/39Ar geochronology, instrumental neutron activation analysis (INAA) and analytical electron microscopy have been performed on four 0.5 × 1.0 × 0.3 cm polished rock tiles of Apollo 16 and 17 granulitic breccias (60035, 77017, 78155, and 79215). Pyroxene thermometry indicates that these samples were re-equilibrated and underwent peak metamorphic sub-solidus recrystallization at 1000-1100 °C, which resulted in homogeneous mineral compositions and granoblastic textures.40Ar/39Ar data from this study reveal that three samples (60035, 77017, and 78155) have peak metamorphic ages of ∼4.1 Ga. Sample 79215 has a peak metamorphic age of 3.9 Ga, which may be related to Serenitatis basin formation. All four samples contain moderately high concentrations of meteoritic siderophiles. Enhanced siderophile contents in three of the samples provide evidence for projectile contamination of their target lithologies occurring prior to peak metamorphism.Post-peak metamorphism, low-temperature (<300 °C) events caused the partial resetting of argon in the two finer-grained granulites (60035 and 77017). These later events did not alter the mineralogy or texture of the rocks, but caused minor brecciation and the partial release of argon from plagioclase. Interpretation of the low-temperature data indicates partial resetting of the argon systematics to as young as 3.2 Ga for 60035 and 2.3 Ga for 77017. Cosmic ray exposure ages range from 6.4 to ∼339 Ma.Our results increase the amount of high-precision data available for the granulitic breccias and lunar highlands crustal samples. The results demonstrate the survival of pre-Nectarian material on the lunar surface and document the effects of contact metamorphic and impact processes during the pre-Nectarian Epoch, as well as the low-temperature partial resetting of ages by smaller impact events after 3.9 Ga.The mineralogy and chemical composition of these rocks, as well as exhumation constraints, indicate that the source of heat for metamorphism was within kilometers of the surface via burial beneath impact-melt sheets or hot ejecta blankets. 相似文献
4.
Over 70 new Nd isotope analyses are presented for the Manicouagan area of the Grenville Province to estimate the crustal age of target rocks involved in the 214 Ma Manicouagan Impact Structure, and to reconstruct the Precambrian geological evolution of this crustal segment. The rocks fall into two main groups: Samples from the Archean-aged Gagnon Terrane to the north and west of the impact give TDM ages averaging 2.70 Ga. Samples from the Manicouagan Imbricate Zone (MIZ) and other allochthonous lithotectonic domains to the south of the impact yield Paleoproterozoic TDM ages averaging 2.01 Ga for the MIZ and 1.86 Ga for the southern domains. These Paleoproterozoic terranes are correlated with Makkovik-age crust in Labrador that was heavily reworked by Labradorian magmatism that increased in intensity southwards. The target rocks involved in the impact event would have consisted almost entirely of the MIZ, which formed a layer several kilometres thick, overlying Archean crust at depth. 相似文献
5.
Non-cylindrical, flexural slip folding is described from the well-known coastal section between Ardwell Bay and Kennedy's Pass, near Girvan, southwest Scotland. Bedding plane slip is recognised by the ubiquitous slickenside striations on bedding surfaces, and these linear elements define the ac kinematic plane of fold formation. This intersects the axial surfaces of folds (ab) in the common direction a which is the movement direction during fold propagation. Statistical treatment of the orientation of structural and kinematic elements yields important conclusions concerning fold formation: the Ardwell folds are markedly non-cylindrical and this is a primary feature amplified during the Ardwell Fold Phase. 相似文献
6.
John G. Spray 《Journal of Structural Geology》1985,7(2):187-203
A 500 m wide shear zone occurs between the base of an Archaean greenstone sequence and adjacent granitoid gneiss complex on the shores of Lake Dundas, Western Australia. The dynamothermal margin remains distinguishable due to the preservation of upper amphibolite facies assemblages, related granitoid anatectites and mylonitic, schistose and gneissose fabrics developed parallel to the contact, which itself lies subparallel to the greenstone bedding surface. The margin contrasts with less deformed greenschist to low amphibolite facies assemblages which characterize lithologies within the greenstone belt, many of which retain igneous textures and relict primary phases. Structural, petrological and textural evidence indicates that the dynamothermal contact originally evolved as a subhorizontal ductile shear zone during juxtaposition of the greenstone pile with granitoid gneiss and that its formation preceded regional folding, greenschist facies overprinting and granitoid intrusion which occurred at about 2700 Ma. The amount of heat generated within the transition zone during thrusting was limited to maximum temperatures of c. 650°C due to the buffering effect of granitoid anatexis. 相似文献
7.
Natalie?DesetaEmail author Suporn?Boonsue Roger?L.?Gibson John?G.?Spray 《Contributions to Mineralogy and Petrology》2017,172(10):84
We present a compositional and textural analysis of shock-induced microtextures in garnet porphyroblasts in migmatitic garnet–cordierite–biotite paragneisses from the centre of the Vredefort impact structure, South Africa. Detailed imaging and major element analysis of deformation features in, and adjacent to, the garnet porphyroblasts record a complex, heterogeneous distribution of shock effects at the microscale. As the most competent silicate mineral in the assemblage, with the highest Hugoniot Elastic Limit and a wide pressure–temperature stability field, the porphyroblastic garnet preserves a more diverse shock deformation response compared to minerals such as quartz and feldspar, which underwent more comprehensive shock metamorphism and subsequent annealing. The garnet porphyroblasts display pre-impact fractures that are overprinted by later intra-granular Hertzian and distinctive planar fractures associated with the impact event. Shock-induced strain localization occurred along internal slip planes and defects, including pre-existing fractures and inclusion boundaries in the garnet. Symplectitic (kelyphitic) coronas commonly enclose the garnet porphyroblasts, and inhabit intra-granular fractures. The kelyphite assemblage in fractures with open communication beyond garnet grain boundaries is characterized by orthopyroxene—cordierite—sapphirine. Conversely, the kelyphite assemblage in closed-off intra-granular fractures is highly variable, comprising spatially restricted combinations of a secondary garnet phase with a majoritic component, Al-rich orthopyroxene, sapphirine and cordierite. The impedance contrast between garnet porphyroblasts and their inclusions further facilitated the formation of shock-induced features (Al-rich orthopyroxene coronas). Together, the textural and mineralogical data suggest that these features provide a record of oscillatory shock perturbations initiated under confining pressure beneath the transient crater floor. This occurred as the shocked rock volume underwent post-shock expansion, forming the core of the central uplift, and was followed by variable textural re-equilibration. This study thus provides a microtextural and mineralogical perspective of the shock regime within confined crust immediately prior to and during central uplift formation. 相似文献
8.
Abstract— Impact structures developed on active terrestrial planets (Earth and Venus) are susceptible to pre‐impact tectonic influences on their formation. This means that we cannot expect them to conform to ideal cratering models, which are commonly based on the response of a homogeneous target devoid of pre‐existing flaws. In the case of the 1.85 Ga Sudbury impact structure of Ontario, Canada, considerable influence has been exerted on modification stage processes by late Archean to early Proterozoic basement faults. Two trends are dominant: 1) the NNW‐striking Onaping Fault System, which is parallel to the 2.47 Ga Matachewan dyke swarm, and 2) the ENE‐striking Murray Fault System, which acted as a major Paleoproterozoic suture zone that contributed to the development of the Huronian sedimentary basin between 2.45–2.2 Ga. Sudbury has also been affected by syn‐ to post‐impact regional deformation and metamorphism: the 1.9–1.8 Ga Penokean orogeny, which involved NNW‐directed reverse faulting, uplift, and transpression at mainly greenschist facies grade, and the 1.16–0.99 Ga Grenville orogeny, which overprinted the SE sector of the impact structure to yield a polydeformed upper amphibolite facies terrain. The pre‐, syn‐, and post‐impact tectonics of the region have rendered the Sudbury structure a complicated feature. Careful reconstruction is required before its original morphometry can be established. This is likely to be true for many impact structures developed on active terrestrial planets. Based on extensive field work, combined with remote sensing and geophysical data, four ring systems have been identified at Sudbury. The inner three rings broadly correlate with pseudotachylyte (friction melt) ‐rich fault systems. The first ring has a diameter of ?90 km and defines what is interpreted to be the remains of the central uplift. The second ring delimits the collapsed transient cavity diameter at ?130 km and broadly corresponds to the original melt sheet diameter. The third ring has a diameter of ?180 km. The fourth ring defines the suggested apparent crater diameter at ?260 km. This approximates the final rim diameter, given that erosion in the North Range is <6 km and the ring faults are steeply dipping. Impact damage beyond Ring 4 may occur, but has not yet been identified in the field. One or more rings within the central uplift (Ring 1) may also exist. This form and concentric structure indicates that Sudbury is a peak ring or, more probably, a multi‐ring basin. These parameters provide the foundation for modeling the formation of this relatively large terrestrial impact structure. 相似文献
9.
Whole rock, electron microprobe analyses and 40Ar/39Ar geochronology of certain ophioliterelated metamorphic rocks from beneath the Pindos, Vourinos, Othris and Euboea ophiolites of Greece show that they were formed mainly from ocean-type basalts, in part under P-T conditions of the upper mantle and that they have ages between 170–180 m.y. The evidence presented is inconsistent with the view that these sub-ophiolite metamorphic rocks were produced by the obduction of ocean-type crust onto a continental margin, or that they are remnant slices of Palaeozoic ‘basement’, but is consistent with their formation by thrusting and related metamorphism occurring within ocean lithosphere during the Lower to Middle Jurassic. It is proposed that this intraoceanic metamorphism was caused by the inception of a fault zone which subsequently became the transport surface for the main phase of ophiolite emplacement that occurred in the Hellenides from the Late Jurassic to Early Cretaceous. 相似文献
10.
Amphibolitic metamorphic rocks are associated with the Thetford, Asbestos and Orford ophiolites as well as the Mont Albert and Pennington Sheet peridotites of the Québec Appalachians. An augmented compilation of the existing data on their field relations, mineralogy, geochronology, structural features and geothermobarometry is presented in order to help reconstruct the timing and processes of marginal basin closure and ophiolite emplacement during the Ordovician. A new, refined 40Ar/39Ar incremental release spectrum and isochron age is presented for the Thetford Mines ophiolite dynamothermal sole. The new 477 ± 5 Ma age resolves the existing disparity between the crustal (plagiogranite) and sole ages. The sole was formed shortly after crustal formation, suggesting that the ophiolite was decoupled at or near a spreading centre. A hitherto undescribed ultramafic-mafic amphibolitic sole beneath the Asbestos ophiolite was decoupled and rotated during the continental emplacement of the overlying ophiolite. Dating of the sole at Asbestos was hampered by presence of low K2O amphiboles, but an Acadian (377 Ma) age was obtained from orthoclase. The Orford sole was dismembered and incorporated within a serpentinitic mélange that contains other ophiolitic lithologies. Sheared amphibolites from alongside the Pennington Sheet in the Flintkote Mine are reinterpreted as a dynamothermal sole, rather than a metasomatically generated amphibole-bearing metasediment. 相似文献