G. I. Alsop

Overburden deformation patterns and mechanisms of salt diapir penetration in the Central Graben, North Sea

Abstract Active and passive diapirism control the deformation and geometry of hydrocarbon traps in the overburden, and a more detailed understanding of this process will help reservoir prediction and hydrocarbon recovery. Cores studies of seven Central Graben diapirs indicate Zechstein salt penetrated Late Cretaceous chalk by extreme tectonic thinning with high-angle (>70° to bedding) normal faulting, tensile fracturing and pressure solution. Attenuation of the chalk significantly weakens the overburden, allowing buoyancy forces to dome up the overburden. Doming created enough topography for downslope sliding of chalk slabs on slip planes parallel to bedding or, in the case of the Kyle diapir, for chaotic debris flows of lithified chalk. Significant extensional bedding-parallel faults and slump folds are developed within Palaeocene shale on the diapiric flanks. Inter-granular slip in unconsolidated clastic material was probably the dominant deformation mechanism. Diapirs have penetrated the Palaeocene clastic sediments by maintaining topographic relief, so that unlithified sediment continually slid off the crest, producing translated intact rafts up to several tens of metres in thickness.

Vergence and facing patterns in large-scale sheath folds

The careful geometric analysis of minor structural detail elucidates the relationships and evolution of associated large-scale curvilinear hinge geometries, developed during WNW-directed Caledonian thrusting exposed in Neoproterozoic Moine psammites of the Moine Nappe. Reversals in the polarity of structural facing associated with minor folding, mark the position of major sheath folds which parallel transport. Upwardly convex sheaths (closing in the direction of thrust transport) cored by older gneissose basement inliers are termed culminations, whilst those opening in the transport direction (and cored by Moine psammites) are termed depressions. Sheath folds are bisected by transport parallel and foliation normal (culmination/depression) surfaces which separate not only the reversals in facing, but also delineate zones of minor fold hinge obliquity into clockwise and anticlockwise domains relative to the transport direction. The sense of obliquity of minor Z and S folds is thus dependent on position with respect to the surfaces of culmination and depression and not the fold axial surfaces. Surfaces of culmination and depression may be superimposed on original overturned antiformal and synformal folds to produce a variety of dome (culmination on antiform), saddle (depression on antiform), inverted saddle (culmination on synform) and basin (depression on synform) configurations. The curvilinear hinges of minor folds may also be asymmetrical about the transport direction and within the plane of the regional foliation to define patterns of fold hinge-line vergence. Classical concepts of fold limb vergence may thus relate to larger antiformal and synformal hinges, whilst the fold hinge-line vergence defines major curvilinear hinges associated with culminations and depressions. Major sheath folds may therefore be interpreted in terms of both minor fold hinge-line and limb vergence, coupled with fold axis obliquity and reversals in the polarity of structural facing. The ability to recognise consistent and reliable structural relationships between facing and hinge obliquity at small scales indicates that the regional deformation process forms a linked and coherent system through several orders of magnitude. # 1999 Elsevier Science Ltd. All rights reserved.

Glacial trinity: Neoproterozoic Earth history within the British-Irish Caledonides

Two distinct Neoproterozoic glacial episodes are known for the Dalradian Supergroup in the British-Irish Caledonides, the Port Askaig Formation and the Inishowen–Loch na Cille ice-rafted debris (IRD) beds. Here we describe a third, intermediate between those two, the Stralinchy-Reelan formations, composed of diamictite and IRD. Developed directly above these rocks is the Cranford Limestone, which consists of a basal, 1–6-m-thick, tan-gray dolostone overlain by a variably developed, but as much as 340-m-thick, sequence of thin-bedded limestone and dolostone. This unit exhibits a C isotopic trend that begins negative in the basal dolostone, reaches a nadir of −7‰, and then rises to 0‰–2‰. These characteristics match strikingly those of Marinoan-style cap carbonates. Consequently, we interpret the Port Askaig Formation, the Stralinchy-Reelan units, and the Inishowan–Loch na Cille beds as equivalents of the ca. 700 Ma Sturtian, the 635 Ma Marinoan, and the ca. 580 Ma Gaskiers glacials, respectively. Two additional observations are noteworthy. Carbonate rocks below the Port Askaig Formation record a δ13C decline to −6‰ that implies that such downturns may occur in both pre-Sturtian and pre-Marinoan strata. In addition, the Bonahaven Dolomite is not a cap carbonate to the Port Askaig Formation, but exhibits a δ13C rise to 12‰, which we correlate with the inferred global Keele peak. These data further document the utility of Neoproterozoic glacial–cap carbonate sequences in global correlations and denote the base of the Cranford Linestone as the Cryogenian-Ediacaran boundary.

The geometry of drag zones adjacent to salt diapirs

Drag zones are highly strained regions developed adjacent to the flanks of salt diapirs, and are produced when the sedimentary overburden is folded or rotated into steeply dipping attitudes sub-parallel to the diapiric walls. This case study focuses on a diapiric province on Cape Breton Island, Nova Scotia, where five Visean-age salt diapirs penetrate Upper Carboniferous (Namurian–Stephanian) conglomerates, sandstones, shales and coal seams. 2D marine seismic coverage extends over several diapirs thus allowing the drag zones studied onshore to be located relative to the adjacent diapir. The width of diapiric drag zones within the case studies varies from 70u2009m up to 500u2009m, with narrow drag zones reflecting the low mean competence of shales and siltstones which may have been shallowly buried and poorly lithified at the onset of diapirism. Broader drag zones are dissected by two sets of extensional fractures together with major faults, suggesting that the overburden was semi-lithified and displayed greater flexural rigidity. Overburden displaying high competency contrasts results in strain localization and partitioning, with pervasive granulation seams and minor faults developed in sandstones and bedding-parallel shears in coal and shale horizons. Drag zones are segmented by steeply outward-dipping faults associated with decametric, asymmetric drag profiles. These faults facilitate the upward movement of material and effectively expand the diapiric process into adjacent overburden via a newly termed process of lateral diapiric accretion.

The geometry and kinematics of flow perturbation folds

Abstract Minor folds formed synchronous with ductile deformation in high strain zones can preserve a record of the scale and kinematics of heterogeneous flow. Using structures associated with WNW-directed Caledonian thrusting in N Scotland, we show that localised perturbations in flow resulted in the generation of predominantly cylindrical minor folds with hinges lying at low angles to the transport direction. These define a series of larger-scale fold culminations (reflecting ‘surging flow’) or depressions (reflecting ‘slackening flow’) that are bisected by transport-parallel culmination and depression surfaces. The fold patterns suggest a dominance of layer-normal differential shearing due to gradients in shear strain normal to transport. Culmination surfaces are marked by along-strike reversals in the polarity of structural facing and vergence of minor folds which, contrary to classic fold patterns, define reverse asymmetric relationships. Culmination surfaces separate folding in to clockwise (Z folds) and anticlockwise (S folds) domains relative to the transport lineation. The dip of fold axial planes systematically increases as their strike becomes sub-parallel to transport resulting in a 3D statistical fanning arrangement centred about the transport direction. Thus, mean S- and Z-fold axial planes intersect precisely parallel to the transport lineation and potentially provide a means of determining transport directions in cases where lineations are poorly preserved. Culminations display convergent fold patterns with fold hinges becoming sub-parallel to transport towards the culmination surface and underlying detachment, whilst axial planes define overall concave up listric geometries which are bisected by the culmination surface. Thus, around culminations and depressions there are ordered, scale-independent relationships between transport direction, shear sense, fold facing, vergence and hinge/axial plane orientations. The techniques described here can be applied and used predictively within any kinematically coherent system of ductile flow.

Flow perturbation folding in shear zones

Abstract Variable patterns of displacement in shear zones may result in arcuate fold and fault traces as recognized by Mike Coward and co-workers in the early 1980s. Within such deformation zones, localized perturbations in flow may undergo acceleration (surging flow) or deceleration (slackening flow) with respect to the adjacent regions. Such flow cells may govern the orientation and geometries of folds and fabrics and thereby provide evidence of the scale and nature of deformation associated with heterogeneous flow in the high-strain zones. The length/width ratio of individual flow cells (measured in the direction of flow) may vary from <1 to >1 for situations when flow cells are, respectively, dominated by layer-parallel shear (LPS) or layer-normal shear (LNS). Folds initiating at high angles to transport are associated with LPS, whereas LNS may generate folds with slight clockwise (sinistral LNS) or anticlockwise (dextral LNS) trends relative to flow. Continued progressive deformation may subsequently modify and reduce angular relationships between folds and fabrics, but these geometric obliquities are generally preserved. Using examples from the Moine metasediments of northern Scotland, we show that folding displays predictable geometric patterns that can be related to the development of flow perturbation cells associated with Caledonian ductile thrusting under mid-crustal conditions. The differing relative timing of folds and individual ductile thrusts reflects the complexity of flow cells within ductile imbricates and additionally highlights the progressive foreland-directed propagation of ductile thrusting. These geometric relationships developed during contractional shear are compared with those generated in extensional systems, to provide an overall framework for the study of perturbation patterns. The geometric arrangement of mean fold axial planes about the flow direction results in their intersection forming parallel to the transport direction. This relationship permits transport directions to be calculated via the axial-planar intersection method (AIM), and also allows comparison with other techniques devised primarily for the study of soft-sediment deformation and palaeoslope analysis.

Shear zone folds: records of flow perturbation or structural inheritance?

Abstract Deformation within shear zones can be both temporally and spatially variable, resulting in multiple generations of folds which display a range of scales and overprinting relationships in mylonitic rocks associated with high strain zones. Despite such complexities, two main fold associations are broadly recognized in many shear zone settings: early tight to isoclinal sheath folds, often with mylonitic limbs that are post-dated by one or more local generations of synshearing folds which are preserved within, or root downwards into mylonitic high strain zones. These latter structures locally fold the mylonitic foliation and lineation whilst displaying geometric characteristics that are kinematically compatible with the movement regime of the major shear zone. Using examples related to ductile thrusting in Moine metasediments of north Scotland, we show that both types of fold display predictable geometric patterns on fabric topology plots. Fold axes and axial surfaces display consistent changes in asymmetry and sense of obliquity relative to local, transport-parallel mineral lineations that can be used to map out a series of culminations and depression zones. The sheath folds preserve more acute, but almost identical geometric patterns compared to the later synshearing folds, with culmination and depression zones often coinciding in location and scale. Detailed analysis also demonstrates that the distribution of finite strain is systematically linked to the architecture of all folds and that clear and predictable relationships exist between the fabric topologies of both the sheath folds and synshearing folds. These consistent topological relationships could be explained in terms of a fold evolution model, where sheath folds represent a more highly deformed and evolved variety of synshearing folds originally generated during perturbations in ductile flow. However, an alternative fold inheritance model predicts that the gross structural architecture generated during sheath folding may subsequently control the geometry and govern the orientation of the synshearing folds. Both models may be widely applicable in a broad range of shear zone environments.

Eye-to-eye with a mega–sheath fold: A case study from Wadi Mayh, northern Oman Mountains

Sheath folds are highly curvilinear folds, typically considered to develop by hinge rotation toward the transport direction during intense deformation. Most sheath folds are recognized on the centi meter to meter scale, with very few detailed studies of larger kilometer-scale structures. We here present evidence for a megascale sheath fold superbly exposed in the northern Oman Mountains. The Saih Hatat culmination consists of an exhumed highpressure subduction zone with deep-level eclogites and garnet blueschists (As Sifah unit), epidote blueschists (Hulw unit) and carpholite-bearing metasediments (part of the “Upper plate”). Sheath folds are present at a variety of scales throughout the As Sifah, Hulw, and Upper plate tectonic packages. Curvilinear fold hinges along a 15-km-long profi le outline the Wadi Mayh mega–sheath fold, which closes to the SSW and faces to the NNE. It consists of at least four secondary sheath folds with cross-sectional (y-z) eye folds measur ing 200 m stacked on top of one another within Permian and Triassic shelf car bonates. These eye folds display highly elliptical geometries (Ryz > 5) and cat’s-eye fold patterns consistent with simple/general shear. The sheath fold is enveloped by Ordovician and older rocks that do not show equal amounts of internal deformation. The upper bounding envelope is a detachment that has been subsequently sheath folded. The lower boundary of the sheath fold is the Upper plate–Lower plate detachment, which forms a major high-strain shear zone showing little vertical displacement but signifi cant horizontal displacement. The sheath fold displays a SSW elongation (x axis) of at least 15 km and possibly up to 25 km, coupled with signifi cant amounts of vertical fl attening and WNW-ESE shortening.

The Caledonian strike-swing and associated lineaments in NW Ireland and adjacent areas : sedimentation, deformation and igneous intrusion patterns

Part of the major swing in strike associated with the Greenland-Labrador Promontory of the North Atlantic Caledonides is exposed in considerable detail in the metasedimentary and meta-igneous rocks of the Neo-Proterozoic Dalradian Supergroup in northwest Ireland. We show that the strike swing controlled Dalradian stratigraphy and sedimentation patterns and is therefore at least c. 600 Ma in age. Our interpretation of the stratigraphy allows a reconstruction of this promontory in the Laurentian continental margin and shows that it separated a NE-trending. gently inclined and broad continental shelf in the north from an E-trending. more steeply inclined and narrower shelf to the south. Once established in the geometry of the Dalradian sequence, the strike swing and basement promontory had a major influence on Caledonian deformation patterns. It controlled: the nucleation of major folds: the geometry of the regionally developed primary cleavages; the facing direction of the early folds: and it also acted as a major buttress in perturbing and deflecting. on a regional scale, the orogenic transport vector. Accurate restoration of the sinistral displacement on a late Caledonian fault system shows that the apices of the strike swing at different stratigraphic levels, and their associated sedimentary facies changes, define a line trending approximately N12ºE. This line is coincident with (again, after fault restoration): six out of the eight Devonian Donegal granites: the vast majority of the mantle-related, ultrabasic ‘Appinite’ suite (with most of these bodies concentrated at the intersection of this line and the NE-trending Main Donegal Granite shear zone): and the highest density of the Neo-Proterozoic basic tholeiite suite. Thes e and other data are used to suggest that the Donegal Lineament is the expression of a major steeply inclined fault in the basement beneath the Dalradian which is intimately related to the morphology of the promontory. The fault, which might be as old as 1800 Ma, probably reached down into the lithospheric mantle. controlling igneous activity, including the ascent, emplacement site and perhaps sources of the magmas. Another complementary strike swing and parallel lineament is described from the Dalradian rocks of the adjacent part of Scotland. It is suggested that these and other major lineaments (i.e old pre-Caledonian faults) which have either NNE. NE, or ESE trends reflect not so much the location of the northern British Isles at a 120º triple junction during Iapetus rifting. but rather the exploitation of these pre-existing trends in the pre-Caledonian Atlantic borderlands during Iapetus opening.

Transport-parallel cross folds within a mid-crustal Caledonian thrust stack, northern Scotland

Abstract Cross folds are typically associated with zones of anomalous foliation trend that lie at high angles to orogenic strike. This case study concentrates on a region of transport-parallel cross folding developed by buckling during Caledonian ductile thrusting within the Moine and Naver Nappes of northern Scotland. Detailed structural analysis reveals a systematic angular relationship between the trend of tectonic transport, fold axes, and the vergence of minor folds. Large-scale cross folding is considered here to be related to wrench-dominated differential shearing during thrust-sense displacements along an important, possibly out-of-sequence structure, here termed the Ben Blandy Shear Zone. This suggests that patterns of folding within the internal parts of the Caledonian orogen in Scotland are principally controlled by the kinematic constraints imposed by low-angle thrusting. Thus, early, ‘main-phase’ folding is associated with the initiation and propagation of ductile thrusts, whilst later, secondary structures, including cross folds can be related to the development of flow perturbations during displacement along well-established, regionally important detachments.