Large-scale recumbent folds in the Gaick "sea of psammite": implications for Caledonian structural architecture in the central Grampian Highlands, Scotland

Graham Leslie & Maarten Krabbendam

British Geological Survey, Murchison House, Edinburgh EH9 3LA

agle@bgs.ac.uk

BGS resurvey of the Gaick plateau region (1:50k Sheet 64W, Newtonmore) provides a missing link when constraining divergent regional facing in the central Grampian Highlands. This change had previously been related to fanning of early folds across the upright and late ‘Drumochter Dome’; more recent (1990s) BGS mapping suggested a change to southerly regional facing in Strathspey across the Glen Banchor ‘high’. Our new mapping (2002) shows that southerly facing is ubiquitous south of Strathspey and continues thus into Perthshire.

The new research reveals a series of previously undocumented km-scale folds. Grampian Group psammite and micaceous psammite dominate an area of c. 400 km 2 – the "sea of psammite" of the title. The psammites comprise tabular-bedded, rather monotonous lithologies with no discernible lithostratigraphical sub-units, and only rarely preserved way-up criteria (cross-bedding), to aid structural analysis. Vergence of the main regional (S2) biotite foliation on bedding reveals N-S trending km-scale recumbent F2 fold traces; folds which have gently-plunging to subhorizontal E-W fold axes and gently east-dipping axial planes. The available younging criteria show that these folds are always south facing. Thus, there is i) no regional change in facing across the Drumochter region from north to south and ii) no necessity to invoke large-scale D1 or D3 folds linked to regional facing change. In fact, the Gaick region lacks any significant post-D2 folding, and the only D1 folds observed are the relatively small-scale localised structures which occur on the A9 section at Crubenmore.

The overall structure in D2 in the Grampian Highlands tract between Strathspey and the Highland Boundary Fault comprises panels in which the constituent recumbent folds become more asymmetrical (indicating a larger non-coaxial shear component) up towards the structurally highest recumbent fold, i.e. the Tay Nappe (see box above). The Gaick recumbent folds (the Gaick Nappe) are broadly symmetrical and never develop the pronounced L-fabric observed in places in the Tay Nappe folds farther south. The change to more symmetrical form deeper into the structural pile appears progressive and may possibly reflect buttressing against basement blocks at depth versus translation and non-coaxial shearing at the higher structural levels. The separate panels are now delineated by younger ‘Steep Belts’ (i.e. the Geal Charn – Ossian Steep Belt to the north of the Gaick and the Cairnwell – Tummel Steep Belt to the south) which rotate the recumbent D2 structures into more upright geometries. The Highland Border Downbend has a similar role at the southern margin of this domain. Interestingly, there is no indication of a NE-SW ‘Caledonian’ regional structural trend prior to Steep belt formation (D3). Recumbent D2 structures may result from a different kinematic framework to that which produced the younger Steep Belts.