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Department of Earth Sciences Structural and tectonic history of the Lower Carboniferous, Scremerston, Northumberland CHARLIE KENZIE Department of Earth Sciences, Durham University Introduction Geological & Tectonic Setting It is proposed that limestones were deposited as a result of a long period of Yoredale-type deltaic sedimentation [1] during the Carboniferous . Throughout this epoch, northern England was subject to continued deformation typified by crustal extension and basin subsidence [2] , especially during the Dinantian, when continental welding took place prior to the formation of Pangea [3] . Scremerston is located on the North-East coast of England (fig.1), where the well exposed cliffs and tidal outcrops provide an ideal locality for the study of structural formations. Analysis of the micro-structures in the area are used to investigate the tectonic history of the lower-carboniferous strata present at Scremerston. Fig. 1 (a)Map showing location of Scremerston. (b) Geological map showing the study area. The main stratiagraphic sequences from west to east are; Scremerston Coal Member (SCG), Tyne Limestone Formation (TYLS), Alston Formation (AG) and Oxford Limeston (OXL). Typical Fold Style & Fold Mechanism Structural data was collected along a profile trending in an east-west direction labeled P (fig.1). Several readings were taken at a large fish- tail fold, seen in fig.2 opposite, and the data was used to produce a stereonet displaying the structure of a typical fold of the area. P Fig. 3. Photo of large fish tail fold, which shows flexural-slip folding in a multilayer. Layers on the convex side of a surface slip toward the hinge line f relative to those on the concave side. The shear sense reverses across the hinge line line f. The dotted blue lines show converging dip isogon geometry indicative of 1B fold geometry and signifying flexural-slip folding. A decent outcrop showing the profile plane of a typical fold in the area is seen in fig.3 opposite. The fold can be described as periclinal in form and shows tight dishrmonic buckle folding [5] . The beds in the area are upright (fig.2iii) so we can be sure that this structure shows a clockwise asymmetric fold, or z-fold and we can therefore deduce that folding was caused by non-coaxial deformation. Additional evidence for noncoaxial deformation is found in en echelon gash fractures (fig.2ii), where curved fibers indicate the existence of a component of rotation during deformation relative to the incremental strain axes and thereby imply a noncoaxial deformation [4] . Additionally, we can be sure that the folds in the area or non-cylindrical since hinge exposures (fig.2i) show deformed plunging hinges. Folding across the survey area show unequivocally that strain in the area was heterogeneous. Analyses of the the fold shown in fig.3 suggests that most deformation is caused by heteorgenous simple shear since the beds remain a constant thickness. This also suggests that a folding is most likely accommodated by flexural-slip. We have to assume that all the layers have a mean high competence and that friction between the layers remained low throughout folding. The layers on the convex side slip toward the fold hinge relative to the layer on the concave side. Relative slip is greatest at the limbs and decreases to zero at the hinge line [4] where the shear sense changes. Analysis of the fold in fig.3 below reveals a class 1B multilayer geometry since the dip isogons are convergent, the orthogonal thickness from hinge to limb is constant and the axial trace thickness increases. Class 1B geometry is typical of flexural-sip folds, which further indicates that folding was accommodated by this mechanism. The sliding of the layers past each other causes slikenlines , which develop perpendicular to the fold hinge β on the bedding surface. The orientation of the lineations in the bedding surface lie on a great circle normal to the fold hinge β (fig. 4i). β References [1] FIELDING, C. R., & Johnson, G. A. (1987). Sedimentary structures associated with extensional fault movement from the Westphalian of NE England . Geological Society, Special Publications , 28, 511-516. [2] FRANK, M. C., & Tyson, R. V. (1995). Parasequence-scaleorganic facies variations through an Early Carboniferous Yoredale cyclothem, Middle Limestone Group, Scremerston, Northumberland . Journal of the Geological Society, London , 41-50. [3] TWISS, R. J., & Moores, E. M. (1992). Structural Geology & Tectonics. California: W. H. Freeman and Company. [4] WARR, L. N. (2000). The Variscan Orogeny: the welding of Pangea. In N. W. Strachan, Geological History of Britain and Ireland (p. 271). Oxford: Blackwell Science Ltd. [5] DePAOLA, N., Holdsworth, R. E., McCaffrey K. J. W., Barchi M. R. (2005) Partitoned transtension: an alternative to basin inversion models [6] FRASER, A. J., Gawthorpe, R. L., (2003). An atlas of carboniferous basin evolution in Northern England. Geological Society, London, Memoir 28, 79pp. Tectonic Regime 1m 3cm Fig. 2i Fold hinge shown in outcrop shown plunging approximately towards the east and indicates non-cylindrical folding. 2ii en echelon gash fractures showing rotated fibers indicating shearing as shown by arrows and noncoaxial deformation. Possibly formed as extension fractures during shearing, and they are orientated approximately perpendicular to the minimum compressive stress σ 3 . 2iii Cross bedding indicating that the beds are the correct way up and have not been overturned. i ii iii Fig. 4i Stereonet showing the orinetation of the slickenslide lineations on the bedding surface. The lineations lie on a great circle normal to the fold hinge β indicating that folding was accommodated by flexural slip. Fig. 4ii Stereonet showing the orientation of the bedding on the fold (fig.3). Bedding dips to the east and west. Sctrutural features provide evidence for both compressional and extensional deformation at Scremerston. Jointing (fig.5) and normal faulting indicate extensional forces while folding, conjugate faulting and thrust faults indicate compressional deformation. Thus, a progression of extensional followed by compressional deformation seems most likely. A period of regional extension is widely established in northern Britain during the Dinantian epoch [6] , which is followed by a period of crustal shortening and compression due to the Variscan orogeny [5] and in turn is superseded by further extension related to he emplacement of the Whin Sill suite (fig. 5). Periods of changing deformation regimes may explain why the structures at Scremerston display such heterogeneity. Slickenline orientations and offset bedding planes inidcate that faulting in the area is mainly stirke-slip [5] . The coal and sandstones are cross-cut by two conjugate fault systems (fig. 6), which sees to accommodate the change in vergence in the folds (fig. 7). Due to its higher competence, the limestone strata accommodates all of the folds in the area and allows for both brittle and ductile deformation to occur simultaneously. Analysis of the fold in fig.3 shows that the z-fold structure indicates a large scale antiform to the west (fig.7) σ 3 σ 1 10cm Fig. 5. Two sets of jointing that intersects the mineral veins. Possible further extensional deformation post intrusion of nearby Whin Sill suite. W E Fig.6 Block diagram of conjugate fault system with principle stresses. Conjugate fault accommodates the vergence of the folds shown on fig.7 Fig.7 Cross-section showing folding through profile P (fig.2). Z-folds show that large antimform lies to the west. Conjugate fault system is shown and allows for vergence of the folds.
