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The Longford-Down Massif

Thus the Longford-Down Massif is the accretionary prism (obducted sediments) of continental slope, trench and ocean floor sediments generated by NNW subduction.  The north-west volcanic arc is represented by lower Ordovician volcanics stretching from Mayo to Tyrone (Tyrone Igneous Complex). 

In the main Longford-Down outcrop, both the Ordovician (the area shown in green on the north side of the fault line marked in brown in the map below) and Silurian rocks (The area shown in purple and olive green on the south side of the fault line marked in brown) are largely of greywacke facies formed in deep water by submarine slumps and turbidity currents.  These turbidites are of sandstone grain size and individual beds vary from a few centimetres to a few metres thick with a large proportion of rock fragments within a fine-grained matrix.  They are interbedded with siltstones and argillites. The oldest rocks are of Caradoc-Ashgill age which form a NE-SW trending strip on the northern boundary of the Longford-Down Massif.  North of the Orlock Bridge Fault (shown with a brown line), the Ordovician rocks are comprised of greywackes and shales resting on agglomerates and spilites. South of the Orlock Bridge Fault, Silurian (up to Upper Llandovery) greywackes and siltstones dominate, with a few inliers of Upper Ordovician black mudstones and shales. 

Longford-Down Massif main exploration targets. The Company's land position is indicated on the inserted map of Ireland in red.

Four phases of deformation are recognised in the Ordovician and Silurian rocks of the Longford-Down Massif. The first phase of deformation, D1 is represented by tight F1 folds having varied and steep plunges and associated strong S1 cleavage.  Vertical shortening with lateral extension is shown by small southerly verging F2 folds with associated gently dipping crenulation cleavage S2, and north-west verging F3 monoclines, with associated S3 crenulation cleavage.  The flat-lying limbs of these folds extend up to 1 km across strike producing a widespread recumbent attitude of F1 folds. The fourth phase of deformation, D4 is represented by an orthorhombic system of steeply inclined kink bands, which effect a shortening along the strike of S1.

Wrench faults, with movements of up to 1 km, displace all fold structures and appear to result from a late renewal of NNW compression.

Regional metamorphism does not appear to have reached more than lower Greenschist facies in the Longford-Down Massif.

Although structural evidence shows younging to the north-west, palaeontological evidence proves the presence of progressively younger sediments to the south.  This opposing evidence is explained by invoking large faults consistently throwing down to the south; several major faults each parallel to the regional strike and each with a large stratigraphic displacement have been recognised, segmenting the area south of the Orlock Bridge Fault into a series of fault blocks. This structural profile is similar to that of a modern subduction complex, diagnostic of trench environments where continuous underthrusting of the inner trench wall by descending oceanic lithosphere produces a sequence of sedimentary slices.  In each slice, submarine fan facies (turbidites) overlie abyssal plain pelagic muds with the age of the slices increasing upwards.  In an idealised trench slope model, underthrusting produces these blocks of sediment bounded by low angle thrusts with folds facing oceanwards, but with continued underthrusting the thrust blocks are rotated through 40-50⁰ to become high angle thrusts.

The fault blocks in the Longford-Down Massif appear to be sedimentary slices, progressively added to from the south suggesting that the deformation itself was diachronous.