New onshore insights into the role of structural inheritance during Mesozoic opening of the Inner Moray Firth Basin, Scotland

The Inner Moray Firth Basin (IMFB) forms the western arm of the North Sea trilete rift system that initiated mainly during the Late Jurassic–Early Cretaceous with the widespread development of major NE–SW-trending dip-slip growth faults. The IMFB is superimposed over the southern part of the older Devonian Orcadian Basin. The potential influence of older rift-related faults on the kinematics of later Mesozoic basin opening has received little attention, partly owing to the poor resolution of offshore seismic reflection data at depth. New field observations augmented by drone photography and photogrammetry, coupled with U–Pb geochronology, have been used to explore the kinematic history of faulting in onshore exposures along the southern IMFB margin. Dip-slip north–south- to NNE–SSW-striking Devonian growth faults are recognized that have undergone later dextral reactivation during NNW–SSE extension. The U–Pb calcite dating of a sample from the synkinematic calcite veins associated with this later episode shows that the age of fault reactivation is 130.99  ±  4.60 Ma (Hauterivian). The recognition of dextral-oblique Early Cretaceous reactivation of faults related to the underlying and older Orcadian Basin highlights the importance of structural inheritance in controlling basin- to sub-basin-scale architectures and how this influences the kinematics of IMFB rifting

New onshore insights into the role of structural inheritance during Mesozoic opening of the Inner Moray Firth Basin, Scotland

The Inner Moray Firth Basin (IMFB) forms the western arm of the North Sea trilete rift system that initiated mainly during the Late Jurassic–Early Cretaceous with the widespread development of major NE–SW-trending dip-slip growth faults. The IMFB is superimposed over the southern part of the older Devonian Orcadian Basin. The potential influence of older rift-related faults on the kinematics of later Mesozoic basin opening has received little attention, partly owing to the poor resolution of offshore seismic reflection data at depth. New field observations augmented by drone photography and photogrammetry, coupled with U–Pb geochronology, have been used to explore the kinematic history of faulting in onshore exposures along the southern IMFB margin. Dip-slip north–south-to NNE–SSW-striking Devonian growth faults are recognized that have undergone later dextral reactivation during NNW–SSE extension. The U–Pb calcite dating of a sample from the synkinematic calcite veins associated with this later episode shows that the age of fault reactivation is 130.99 ± 4.60 Ma (Hauterivian). The recognition of dextral-oblique Early Cretaceous reactivation of faults related to the underlying and older Orcadian Basin highlights the importance of structural inheritance in controlling basin-to sub-basin-scale architectures and how this influences the kinematics of IMFB rifting

Using UAV-Based Photogrammetry Coupled with In Situ Fieldwork and U-Pb Geochronology to Decipher Multi-Phase Deformation Processes: A Case Study from Sarclet, Inner Moray Firth Basin, UK

Constraining the age of formation and repeated movements along fault arrays in superimposed rift basins helps us to better unravel the kinematic history as well as the role of inherited structures in basin evolution. The Inner Moray Firth Basin (IMFB, western North Sea) overlies rocks of the Caledonian basement, the pre-existing Devonian–Carboniferous Orcadian Basin, and a regionally developed Permo–Triassic North Sea basin system. IMFB rifting occurred mainly in the Upper Jurassic–Lower Cretaceous. The rift basin then experienced further regional tilting, uplift and fault reactivation during the Cenozoic. The Devonian successions exposed onshore along the northwestern coast of IMFB and the southeastern onshore exposures of the Orcadian Basin at Sarclet preserve a variety of fault orientations and structures. Their timing and relationship to the structural development of the wider Orcadian and IMFB are poorly understood. In this study, drone airborne optical images are used to create high-resolution 3D digital outcrops. Analyses of these images are then coupled with detailed field observations and U-Pb geochronology of syn-faulting mineralised veins in order to constrain the orientations and absolute timing of fault populations and decipher the kinematic history of the area. In addition, the findings help to better identify deformation structures associated with earlier basin-forming events. This holistic approach helped identify and characterise multiple deformation events, including the Late Carboniferous inversion of Devonian rifting structures, Permian minor fracturing, Late Jurassic–Early Cretaceous rifting and Cenozoic reactivation and local inversion. We were also able to isolate characteristic structures, fault kinematics, fault rock developments and associated mineralisation types related to these event

Unravelling the Sequence and Timing of Fault-Related Deformation in Superimposed Rift Basins, Inner Moray Firth, NE Scotland

Devonian rocks of the Palaeozoic Orcadian Basin are well exposed along the northern flanks of the younger Mesozoic to Cenozoic Inner Moray Firth Basin in Scotland. These rocks preserve a succession of structures related to superimposed rifting and inversion events spanning nearly 400 Myrs. We combine new detailed field observations augmented by drone photography and the creation of 3D digital outcrops, coupled with U-Pb geochronology of syn-faulting calcite-mineralized veins to better constrain the absolute timing of fault movements and decipher the kinematic history of basin opening and inversion.Using this approach, we were able to isolate characteristic structures, fault kinematics, fault rock development and associated mineralization types related to five regional deformation events: (1) Devonian transtensional rifting associated to sinistral Great Glan Fault movements leading to the development of the Orcadian Basin; (2) Late Carboniferous inversion related to dextral Great Glen Fault reactivation; (3) minor N-S, possibly Permian calcite veins; (4) Late Jurassic–Early Cretaceous rifting related to the development of the IMFB; and finally, (5) Cenozoic uplift, reactivation, and local inversion. Our study demonstrates the utility of microstructurally constrained U-Pb geochronology of fault-related calcite mineralization. Applied elsewhere, our methodology has the potential to give consistent and regionally significant new insights into the nature and timing of superimposed rift-related deformation processes worldwide

The nature and significance of rift-related, near-surface fissure fill networks in fractured carbonates below regional unconformities

Fissure-fill networks are a widely recognized, but relatively little described, near-surface phenomenon (<1–2 km) hosted in carbonate and crystalline basement rocks below regional unconformities. Faults and fractures in otherwise tight Devonian carbonate basement rocks of the Tor Bay region, Devon, SW England are associated with the development of millimetre- to decametre-wide fissures containing red-coloured early Permian sedimentary material, vuggy calcite mineralization and wall rock collapse breccia. These features preserve evidence about the style and history of fault deformation and reactivation in near-surface settings and on fluid-related processes, such as elutriation and/or mineralization. Field observations, palaeostress analysis and fracture topology analyses show that the rift-related faults and fractures created a network of long-lived open cavities during the development of the Portland–Wight Basin in the early Permian. Once formed, they were subjected to episodic, probably seismically induced, fluid fluxing events and local karstification. The large, well-connected networks of naturally propped fractures were (and possibly still are) important fluid migration pathways within otherwise low-permeability host rocks. These structures are probably equivalent to those observed in many other rift-related, near-surface tectonic settings and suggest that the Tor Bay outcrops can be used as a global analogue for sub-unconformity open fissure systems hosted in low-permeability basement rocks

Unravelling the enigmatic Grampian Shear Zone:&amp;#160;In-situ monazite and titanite U-Pb analysis of the juxtaposed Badenoch and Grampian Groups

&amp;lt;p&amp;gt;The Grampian Shear Zone (GSZ) represents a highly deformed tectonostratigraphic contact between the Proterozoic metamorphic rocks of the Dalradian Group from the underlying high grade metamorphic Neoproterozoic rocks of the Badenoch Group within the Grampian Highlands. The nature (tectonic suture or palaeo-unconformity), age and structure of the GSZ and indeed the underling Badenoch Group are poorly constrained. Previous studies of the GSZ and synkinematic (intruded during shearing) pegmatites found therein, yielded metamorphic/deformation (and magmatic) ages ranging from c.a. 808 to 440 M. This study reinvestigates this shearzone using in-situ (within section) petrochonological analysis on a range of U-Pb and Rb-Sr chronometers &amp;amp;#8211; Monazite, zircon, titanite, rutile and mica. Carrying out this analysis in-situ and using a variety of minerals allows us to directly date deformation fabrics over a wide range of deformation temperatures, giving us a far more detailed picture of the events recorded within these rocks. Large monazite grains (&amp;amp;#8805;100&amp;amp;#956;m) were mapped using in-situ LA-ICP-MS to show within grain variation of major elements and REEs. Monazite U-Pb spot analysis from the GSZ has yielded ages ranging from 784.11 &amp;amp;#177; 1.2Ma to 442.58 &amp;amp;#177; 0.58Ma. The same analysis was performed on a sample from the Grampian group which yielded an age of 441.34 &amp;amp;#177; 037Ma. In addition to this monazite data, in-situ U-Pb Titanite analysis from the Badenoch Group gave ages of 526.96 &amp;amp;#177; 1.33 Ma from a metabasite sample, with a metasedimentary sample giving a range of titanite U Pb ages from 540 to 460Ma. These age ranges show that the Badnoch Group and the GSZ have recorded a complex polyorogenic history relative to the &amp;amp;#8220;simple&amp;amp;#8221; overlying Dalradian metasediments. We propose that the Grampian Shear Zone represents a deep-seated Knoydartian (808 to 784Ma) age shear zone within the meso-Neoproterozoic Badenoch Group. This shear zone was then reactivated during the Grampian phase of the Caledonian Orogeny resulting in the tectonic emplacement of the Dalradian metasediments above the Badenoch group.&amp;lt;/p&amp;gt;</jats:p

Eclogites and basement terrane tectonics in the northern arm of the Grenville orogen, NW Scotland

The presence of eclogites within continental crust is a key indicator of collisional orogenesis and they have been used worldwide to assist in the delineation of ancient collisional sutures. Eclogites within the Eastern Glenelg basement inlier of the Northern Highland Terrane (NHT) have been re-dated in order to provide more accurate constraints on the timing of collision within the northern arm of the Grenville Orogen. The eclogites yield dates of c.1200 Ma which are interpreted to record the onset of convergence, and the NHT as a whole is thought to represent the lower plate in successive 1200-1000 Ma collision events. The Eastern Glenelg basement inlier is viewed as a fragment of the leading edge of the NHT basement that was partially subducted along a suture and then obducted back up the subduction channel. Differences in ages of igneous protoliths and intrusive histories (Storey et al., 2010; Strachan et al., 2020b), and metamorphic events (this paper) between the NHT basement and the Laurentian foreland, suggests that they were separate crustal blocks until after c. 1600 Ma. We therefore suggest that: 1) the NHT represents a fragment of Archean-Paleoproterozoic crust that was reworked within the c. 1600-1700 Ga Labradorian-Gothian belt, although whether it was derived from Laurentia or Baltica (Strachan et al., 2020b) is uncertain (Fig. 3), and 2) amalgamation of the NHT with the Laurentian foreland did not occur until the terminal stages of the Grenville collision at c. 1000 Ma.</jats:p

Geological fissures: linking sub-surface structures to surface processes

&amp;lt;p&amp;gt;Dilatant fissures are a common feature at the Earth&amp;amp;#8217;s surface in active rift systems where faults cut mechanically-strong rocks, such as igneous rocks, metamorphic basement or carbonates. Much attention has focused on modern examples of large-aperture fissures in basaltic rocks, where in most cases, only the near-surface-expression is accessible to depths of ~100 m. Numerous mechanisms have been proposed for the formation of such dilatant fractures, including near-surface tensile fracturing along active normal faults at depth, geometric mismatch along faults, and fault-block rotation. However, fissure system architecture and connectivity in the subsurface, and the depth to which dilatant sections can grow are less well understood, as are the ways in which such structures may interact with surface processes.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;In this presentation, we focus on dilatant faults and fractures from the ancient rock record, including examples hosted in rocks below regional erosional unconformities, commonly on the upfaulted flanks of nearby sedimentary basins. Such fissures are typically sub-vertical Mode I fractures that can be kilometres long, tens of metres wide and can extend to depths of 1 km or more below the palaeosurface. They are filled with a remarkably diverse range of high porosity, high permeability fills which act as natural proppants holding fractures open for tens to hundreds of million years. Fills include: wall rock collapse breccias; clastic or carbonate sediment; fossiliferous materials, and a variety of epithermal mineral deposits with characteristically vuggy forms and cockade-like textures. Alteration related to weathering and/or near-surface epithermal mineralization may extend down fissure systems to depths of many hundreds of metres. The subterranean clastic fills are commonly water-lain and preserve a unique record of the stratigraphic or fossil record that may be missing due to erosion at the overlying unconformity. Fissures can form along active normal faults at depth, as later-stage reactivations of pre-existing exhumed fault zones and along regional joint sets associated with folds. Some fissures form along the margins or interior of pre-existing mafic dykes or may act as sites of subsequent dyke emplacement &amp;amp;#8211; or both. Sub-unconformity fissure systems and their associated fills are likely to be a major influence on both the fluid storage capacity and flow behaviour in subsurface reservoirs including those hosting hydrocarbons, geothermal resources, and in aquifers worldwide.&amp;lt;/p&amp;gt;</jats:p

Geology and evolution of fissure systems in fractured basement rocks, Calabria, southern Italy: implications for sub-unconformity reservoirs and aquifers

Basement-hosted fissure-fill networks in sub-unconformity settings are increasingly recognized globally and have the potential to act as important subsurface reservoirs and/or migration pathways for hydrocarbons, geothermal fluids and groundwater. We examine well-exposed fissures from exhumed crystalline upper Carboniferous basement rocks in southern Italy (Calabria) and describe their nature, origin and evolution. The basement rocks record the emplacement and exhumation of their plutonic protoliths. The evolution of these rocks includes their initial intrusion in the late Carboniferous, followed by veining, folding and rifting events, to eventual exhumation at the surface when fissuring occurred in the mid-Miocene. The fissure network hosts fossiliferous marine sediments, wall rock collapse breccias and limited mineralization with vuggy cavities. In the basement below the main erosional unconformity, fissure-fills form up to 50% by volume of the exposed rock. The fills are notably more porous (up to 15-25% matrix porosity) than the ultra-low-porosity (&lt;1%) crystalline host rocks. We present field observations, palaeostress analyses of fault slickenlines and fracture topology analyses demonstrating that these exceptionally well-connected fissure networks are related to rifting and penetrated to depths of at least 150 m below the main Miocene erosion surface