A Triple Complex of Low-Angle Oblique Impact Structures in the Midland Valley of Scotland

Synopsis: The Loch Leven basin (56° 12' N, 3° 23' W) in the Midland Valley of Scotland has been identified as the site of the primary impact of a low-angle oblique impact event dating from the end of the Carboniferous. Together with two further downrange structures, it forms a chain of craters which appear to have been produced by fragments of a large asteroid which disintegrated on impact. Topography: The Loch Leven crater of Kinross-shire, East-Central Scotland is an elongated structure (18 x 8 km) which lies 30 km N of Edinburgh and 40 km SW of St. Andrews (Fig. 1). It has been preserved by a quartz-dolerite sill which was emplaced around the edges of the crater infill and dates from 290 Ma. The sill was intruded at a depth of 1.8 km, so that the structure which survives represents the eroded base of the crater. The structure has a central ridge and a series of lateral terraces (Fig 2). By analogy with the features of the lunar crater Schiller [1] and other elliptical craters [2], Loch Leven is believed to be a low-angle oblique impact structure. These structures result from impact of objects which disintegrate into multiple fragments before burial. The fragments ricochet downrange, forming multiple secondary impact structures [2]. Petrology: Impactites of the structure include impact melts, suevites, lithic breccias and friction-melt rocks. The country rocks of the Loch Leven area are mostly lower Carboniferous sandstones and some of these have been affected by the impact. At the eastern (proximal) end of the structure, thermal effects dominate, with formation of indurated quartzitic rocks containing tridymite (Fig 3, SEM) and devitrified glassy textures (Fig 4, XPL) but no Planar deformation features (PDFs). At the western (distal) end of the structure, however, target rocks do contain PDFs (Fig 5, XPL), supporting previous theoretical predictions of an asymmetric " canoe-shaped " distribution of shock pressures in these oblique structures [1]. Interstitial melting textures are common in these rocks, and new-growth micas are seen between grain boundaries in otherwise " normal " sandstones (Fig 6, SEM). The most altered lithologies are found in the central ridge of the structure which contains some highly vesicular rocks one of which is mainly composed of vesiculated lechatelierite (Fig 7, SEM). Unaltered quartz grains in this rock contain planar fractures and PDFs (Fig 8, oil mount, XPL). The structure also contains a distinctive suite of …