Geology of the Llanidloes area: Geological description - Structure and metamorphism
|This page is part of a category of pages providing a summary of the geology of the Llanidloes district (British Geological Survey Sheet 164).
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Authors: Wilson, D, Burt, C E, Davies, J R, Hall, M, Jones, N S, Leslie, A B, Lusty, P A J, Wilby, P R, and Aspden, J A.
The main structural elements within the district are shown in Figure P930913. The tectonic history of the Welsh Basin records the sequence of events associated with the separation of the Neoproterozoic basement of Avalonia from Gondwana during the Early Ordovician, and the subduction of oceanic crust (the Iapetus Ocean) beneath the Avalonian palaeoplate as it converged with Baltica and Laurentia (e.g. Howells, 2007). The eventual closure of Iapetus and the collision of Avalonia with Baltica, and then Laurentia, formed part of the Caledonian orogenic cycle (Cambrian to early Mid Devonian), and occurred from the mid to late Silurian onwards. The structural features of the Llanidloes district are largely the result of tectonic movements that occurred within these later stages of the orogenic cycle, during an episode widely known as the Acadian Orogeny (McKerrow et al., 2000; Soper and Woodcock, 2003; Woodcock et al., 1988). Although the orogeny is traditionally thought to have resulted from the impact of Avalonia with the more northern Laurentian supercontinent (part of modern-day North America), recent studies suggest instead that a collision to the south may have been responsible (Woodcock et al., 2007).
The Acadian Orogeny largely overprinted evidence of previous tectonism. Synsedimentary movements deduced from profound thickness and facies variations within the Telychian turbidite lobe systems (see above) occurred across structures which were reactivated during the orogeny. Within the district these include the Central Wales Lineament, an important structure which extends northwards from the River Wye valley west of Llangurig [SN 9104 7982] to Pennant [SN 8788 9783], and is marked by a complex zone of later folding and faulting that continues into adjacent districts (Figure P930913). The Acadian deformation of the Tywi and Pontesford lineaments, structures viewed as influential during Wenlock and Ludlow deposition in the east of the district, are well seen in the adjacent Rhayader and Montgomery districts (Davies et al., 1997; Cave and Hains, 2001).
The Acadian Orogeny ended sedimentation and resulted in uplift of the entire Welsh Basin. The basinal succession was folded by several orders of magnitude and faulted; a cleavage was widely developed as the rocks underwent low-grade (anchizone to epizone) metamorphism (Merriman, 2006). One of the largest regional folds is the Central Wales Syncline, a first-order structure in the western part of the district that developed above or adjacent to the Central Wales Lineament (Davies et al., 1997; Smith, 1987b). The complementary Teifi Anticlinorium (Davies et al., 2006; Jones, 1912) lies to the west of the syncline, its trace coincident with the core of the second-order Plynlimon Inlier; additional second-order periclinal structures which expose Ordovician strata include the Mynydd-y-groes, Bryn Mawr and Van inliers, in the central part of the district, and the Carno Inlier in the extreme north. Further second-order folds include the Llanidloes Syncline and Tylwch Anticline south-east of the Van Inlier, and the Trannon Syncline to the north. The broad disposition of Ordovician and Silurian strata is largely controlled by the second-order structures and, to a lesser extent, by lower-order folds (Plate P775119). The majority of second-order folds are open structures with axial planes that dip at steep angles to the west or north-west; lower-order structures are open to tight, their profiles usually controlled by the host lithology, particularly in the thinly interbedded turbidites.
A closely spaced cleavage is apparent in the muddier lithologies across much of the district, but it becomes weaker and patchily developed eastwards and is mostly absent in the extreme east; it is generally inclined at steep angles towards the west-north-west. Cleavage is commonly axial planar to the folds or transects their axial traces in a clockwise sense by a few degrees, and there is little evidence of the strong cleavage transection that has been reported from elsewhere within the basin (Woodcock et al., 1988, and references therein). It has been suggested that the clockwise transected folds reflect a noncoaxial (sinistral) transpressive strain history that resulted from oblique plate collision, although the reasons for this remain unclear (Soper, 1986; Soper et al., 1987; Woodcock et al., 1988).
Acadian faults include a number of north–south aligned structures along the Central Wales Lineament that can be traced, as individual faults or anastomosing splays (such as the Dylife Fault Zone), from Pennant in the north of the district to the Wye Valley in the south. They are associated with a belt of tight folding and probably record displacements forced above a deep basement fault or shear zone during the orogeny. Comparable north to north-north-east-trending faults and tight folds also occur within the Plynlimon and Van inliers, and define the Carno Inlier in the north of the district.
The dominant fault trend was imposed across the district during a late to post-Acadian event as a series of predominantly north-east to east-north-east structures along which mineralisation has occurred in places (Davies et al., 1997; Fletcher et al., 1993, Figure P930913). They typically show variable vertical offsets, which may be interpreted in terms of oblique or strike-slip displacements, and possibly involving the reactivation of pre-existing structures. They include the Llanerchyraur fault plexus in the north of the district, and the Dylife-Esgairgaled-Llechwedd and Dyfyngwm structures which may represent splays of the Camdwr Fault of the adjacent Aberystwyth district. In the central part of the Llanidloes district, a plexus of faults along which the Van, Nantyricket and Nantygwrdy lodes are situated, may be an eastward continuation of the Castell Fault which is recognised as a single structure to the west. Major fractures to the south and east of the Van Inlier include the Clywedog, Cefn Carnedd and the contiguous Bryn Posteg and Wenallt-Highgate structures, the latter locally defining the base of the Penstrowed Grits escarpment. It is likely that some faults within the district were reactivated during the subsequent Variscan (late Carboniferous) and Alpine (Cainozoic) orogenic cycles. Although there is no direct evidence for this, there are indications elsewhere that some mineral veins were active as late as the early Carboniferous (Fletcher et al., 1993).
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