Older than you think: using U–Pb calcite geochronology to better constrain basin-bounding fault reactivation, Inner Moray Firth Basin, western North Sea
Alexandra TămașR. E. HoldsworthDan Mircea TămașE. DempseyK. HardmanAnna BirdNick M.W. RobertsJack LeeJohn R. UnderhillDavid McCarthyKen McCaffreyDavid Selby
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Abstract:
Like many rift basins worldwide, the Inner Moray Firth Basin (IMFB) is bounded by major reactivated fault zones, including the Helmsdale Fault and the Great Glen Fault (GGF). The Jurassic successions exposed onshore close to these faults at Helmsdale and Shandwick preserve folding, calcite veining and minor faulting consistent with sinistral (Helmsdale Fault) and dextral (GGF) transtensional movements. This deformation has been widely attributed to Cenozoic post-rift fault reactivation. Onshore fieldwork and U–Pb calcite geochronology of five vein samples associated with transtensional movements along the Helmsdale Fault and a splay of the GGF show that faulting occurred during the Early Cretaceous ( c. 128–115 Ma, Barremian–Aptian), while the Helmsdale Fault preserves evidence for earlier Late Jurassic sinistral movements ( c. 159 Ma, Oxfordian). This demonstrates that both basin-bounding faults were substantially reactivated during the episodic NW–SE-directed Mesozoic rifting that formed the IMFB. Although there is good evidence for Cenozoic reactivation of the GGF offshore, the extent of such deformation along the north coast of the IMFB remains uncertain. Our findings illustrate the importance of oblique-slip reactivation processes in shaping the evolution of continental rift basins given that this deformation style may not be immediately obvious in interpretations of offshore seismic reflection data. Supplementary Material: Appendix A – orthomosaic model obtained from unmanned aerial vehicle (UAV) photography of the Helmsdale locality (GeoTiff format); Appendix B – orthomosaic model obtained from UAV photography of the Shandwick locality (GeoTiff format); Appendix C – geochronology data; and Appendix D – additional thin section microphotographs of sample HD1 showing repeated cycles of syntaxial grain growth are available at https://doi.org/10.6084/m9.figshare.c.6708518Keywords:
Geochronology
The Chihuahua trough is a right-lateral pull-apart basin that began to form ~159 to ~156 Ma (Oxfordian) during a period of relative counterclockwise rotation of the North American plate.Jurassic seas were well established by latest Oxfordian time and there was little change in basin configuration throughout the remainder of Late Jurassic, Neocomian and Aptian time.Elements of a broad zone of intersecting pre-existing northwest-trending and north-trending lineaments, along the southwest border of the North American craton, provide the fabric for development of the pullapart basin between the Diablo and Aldama platforms.During Tithonian and Neocomian time sedimentation eventually outpaced tectonic subsidence and, as an ensuing "regressive" event commenced, the eastern area of the Chihuahua trough was the locus of extensive evaporite (including halite) deposition.Near the end of Aptian time, during deposition of the Cuchillo and equivalent formations, faulting along the margins of the Chihuahua trough ceased and the seas began to transgress onto adjacent platform areas.By middle Albian time seas had advanced onto previously emergent areas and the Chihuahua trough became a site of shallow-water carbonate deposition that prevailed, with minor interruptions, until early Cenomanian time.The Ojinaga Formation (early Cenomanian -Santonian?) records a marine clastic influx into the Chihuahua trough, coeval with Upper Cretaceous clastic wedges in the Western Interior Cretaceous Seaway of the United States.Retreat of the Cretaceous sea is reflected in the transition from marine to non-marine beds in the Santonian San Carlos Formation and overlying non-marine El Picacho Formation.During the Laramide orogeny (84 to 43 Ma) the Chihuahua trough was inverted to form the Chihuahua tectonic belt.Laramide deformation is the result of left-lateral transpressional tectonics involving renewed movement along the pre-existing fabric that controlled the location of the Jurassic-Aptian basin.In the evaporite basin portion of the trough (eastern area) reactivation of basin-boundary-faults as Laramide reverse faults, with possible left-lateral components of motion, accompanied by development of gentle "ancestral" folds, was followed by amplification of folds in postevaporite rocks caused by flow of evaporites toward the crests of anticlines.As deformation progressed, structural development involved thrust faulting (principally toward the Diablo Platform) and diapiric injection of evaporites along the margins of the evaporite basin.In the northwestern area of the trough, structure reflects northeast-southwestoriented compression and includes relatively minor southwest-directed thrusting toward and onto the Jurassic Aldama platform.Paleozoic formations are involved in the thrusts and all thrusting can be interpreted as a consequence of faulted basement rather than regional-scale décollement.Post-Laramide tectonic activity includes a continuation of evaporite tectonism, scattered igneous intrusion, minor volcanism, gravity tectonics and late Oligocene-Miocene to Quaternary block faulting.In the eastern area of the Chihuahua trough, erosion, after formation of Laramide structure and before emplacement of Oligocene volcanic rocks, created a topography that was similar to that of the present day.During this interval, gravity-induced flaps and detached flaps developed on flanks of several large anticlines.Collapse structures, related to evaporite solution, have deformed Tertiary and Cretaceous formations in areas of diapiric intrusion along tear fault zones.Tertiary normal faulting occurred after realignment of the regional stress system from east-northeast compression to east-northeast extension ca. 31 Ma.Initial faulting in Chihuahua is probably coeval with inception of block faulting in Trans-Pecos Texas (about 24 Ma).Seismic data in the northwestern area of the trough shows that a large part of the area has been affected by Miocene normal faults that are probably coeval with some of the faulting described in the Rio Grande rift.1,644 + TD in Unit "A".Lithology and electric logs.
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Three geological provinces are recognized, separated by major fault zones: the oceanic Lofoten Basin and the Vestbakken volcanic province in the west; the southwestern Barents Sea basin province; and the eastern region which has largely acted as a stable platform since Late Paleozoic times. Since Middle Jurassic times, two structural stages are recognized in the southwestern Barents Sea: Late Mesozoic rifting and basin formation; and Early Tertiary rifting and opening of the Norwegian–Greenland Sea. This evolution reflects the main plate tectonic episodes in the North Atlantic–Arctic break-up of Pangea. Middle–Late Jurassic and Early Cretaceous structuration were characterized by regional extension accompanied by strike-slip adjustments along old structural lineaments, which developed as the Bjørnøya, Tromsø and Harstad basins. Late Cretaceous development was more complex, with extension west of the Senja Ridge and the Veslemøy High, and halokinesis in the Tromsø Basin. Tertiary structuration was related to the two-stage opening of the Norwegian–Greenland Sea and the formation of the predominantly sheared western Barents Sea continental margin. Tectonic activity shifted towards the west in successive phases. The southwestern Barents Sea basin province developed within the De Geer Zone in a region of rift-shear interaction. Initially, oblique extension linked the Arctic and North Atlantic rift systems (Middle Jurassic–Early Cretaceous). Later, a continental megashear developed (Late Cretaceous–Paleocene), and finally a sheared-rifted margin formed during the opening of the Norwegian–Greenland Sea (Eocene–Recent).
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During the early two decades of third millennium, many Mesozoic and Cenozoic biotas belong to plesiosaur, Titanosauriformes, titanosaurs, theropods, Mesoeucrocodiles, pterosaur, bird, snake, fishes, mammals, eucrocodiles, invertebrates and plants from Pakistan were found. Previously a few were formally published according to nomenclatural rules. Most of the Mesozoic vertebrates were formally published in August 2021, and the remaining Mesozoic and Cenozoic biotas are being formally described here.
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Research Article| April 01, 1985 Structural styles in Mesozoic and Cenozoic mélanges in the western Cordillera of North America DARREL S. COWAN DARREL S. COWAN 1Department of Geological Sciences, University of Washington, Seattle, Washington 98195 Search for other works by this author on: GSW Google Scholar Author and Article Information DARREL S. COWAN 1Department of Geological Sciences, University of Washington, Seattle, Washington 98195 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1985) 96 (4): 451–462. https://doi.org/10.1130/0016-7606(1985)96<451:SSIMAC>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation DARREL S. COWAN; Structural styles in Mesozoic and Cenozoic mélanges in the western Cordillera of North America. GSA Bulletin 1985;; 96 (4): 451–462. doi: https://doi.org/10.1130/0016-7606(1985)96<451:SSIMAC>2.0.CO;2 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 SocietyGSA Bulletin Search Advanced Search Abstract The term "mélange" is currently used to describe several different kinds of mudstone-rich rocks that are broadly characterized by an obscure stratigraphy, stratal disruption, or a chaotic, "block-in-matrix" fabric. Four types of mélange, which can be defined in outcrop on the basis of mesoscopic fabric and lithologic composition, are particularly widespread and distinctive. Type I includes sequences of originally interbedded sandstone and mudstone that record incipient to thorough disruption and fragmentation of strata accomplished largely by layer-parallel extension. Type II consists of similarly deformed, thin layers of green tuff, radiolarian ribbon chert, and minor sandstone originally interbedded with black mudstone. Disruption in both types I and II, which probably occurred while the sediments were incompletely consolidated, has been ascribed to either imbricate faulting in accretionary wedges or gravitationally driven deformation. Type III comprises inclusions of diverse shapes, sizes, and compositions enveloped in a locally scaly, pelitic matrix. The ultimate source of fragments is obscure, because the majority were not derived by either the progressive disruption of interbedded sediments or in situ tectonic plucking and abrasion of adjacent rocks. Although some type III mélanges may have originated deep within accretionary prisms, final emplacement as olistostromes (muddy debris-flow deposits) or mud diapirs seems likely. Type III mélanges are mechanically analogous to scaly, "sheared" serpentinites; many probably have been tectonically remobilized or even intruded into shallow-level fault zones. Type IV consists of lenticular inclusions bounded by an anastomosing network of subparallel faults. Their fabric records progressive slicing in brittle fault zones.Each of the four types of mélange described here could, in theory, have formed in a variety of settings on or within an accretionary wedge at an active convergent margin; none can yet be singled out as a uniquely diagnostic "subduction mélange." 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.
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