Palaeomagnetism of the Borrowdale and Eycott volcanic groups, English Lake District: primary and secondary magnetization during a single late Ordovician polarity chron

Abstract

Late Ordovician volcanic rocks of the English Lake District typically have a magnetic remanence dominated by a single characteristic component. Previous investigations have interpreted this remanence as both of primary (pre-folding) and secondary origin. Palaeomagnetic field tests have been conducted on (a) andesite blocks from an autobrecciated lava top, (b) andesite blocks in mass-flow breccias, and (c) fault-blocks tilted during Ordovician caldera collapse to establish the time of remanence acquisition. All three tests show that the lavas retain a magnetization acquired during initial cooling: magnetizations of the breccias are coherent within clasts and random between clasts, whilst magnetizations of the tilted fault blocks converge with better that 95 % confidence when corrected for the effects of caldera collapse. In contrast, the volcaniclastic sedimentary and pyroclastic rocks possess an Ordovician secondary remanence acquired after strata had been tilted by volcano-tectonic subsidence. A distributed sample of 65 andesite and basalt sheets through the Borrowdale Volcanic Group has a mean remanence direction D/I = 341.9/-48.9°( α 95 = 4.0°) yielding a positive fold test and a palaeomagnetic pole at 12.7°E, 4.3°S (dp/dm = 3.5/5.3°). A progressive steepening of the palaeofield direction is recorded during emplacement of the Borrowdale Volcanic Group (~I = -39°to I = -51°) which continued into the interval of volcanotectonic overprinting (I = -62°); the equivalent motion of Eastern Avalonia is ~20°into higher southerly latitudes.Both the Eycott and Borrowdale volcanic groups exhibit uniform normal polarity throughout. Correlation with the geomagnetic time scale for the Ordovician restores the broad correlation between the two groups by constraining their emplacement and partial overprinting to a single long normal polarity chron occupying the Nemagraptus gracilis and earlier part of the Diplograptus multidens biozones (late Llandeilo and early Caradoc). All the volcanism, therefore, occurred within a period of no more than ~5 Ma. The palaeomagnetic evidence confirms that the Borrowdale Volcanic Group was affected by both syn-volcanic deformation (caldera collapse) and regional compressive deformation prior to deposition of the (late Ordovician-Silurian) Windermere Supergroup. The succession of primary and secondary Ordovician palaeomagnetic poles from the Lake District inlier defines an anticlockwise apparent polar wander (APW) loop with the apex correlating with 'soft' closure of the Iapetus Ocean and late Ordovician deformation. The APW paths from Avalonia and Baltica converge at this point as subduction ceased and the arc subsided beneath the sea after mid-Caradoc times.

Palaeomagnetically defined rotations of fault-bounded continental blocks in the North Anatolian Shear Zone, North Central Anatolia

“…The deformation is considered to be ductile and distributed over a wide area in the continuum models, while in discrete models the deformation causes rotation of internally undeformed continental blocks. As stated before by McKenzie and Jackson (1983) and Piper et al (1997b), the deformation of continental lithosphere adjacent to intracontinental transform faults does not generally occur on a single fault, but is distributed over a zone. This zone of deformation is generally segmented into rigid continental blocks displaying simple to complex rotations.…”

“…Two different models have been proposed for the kinematics of vertical axis rotation in the upper crust, the continuum and discrete deformation models (Ron et al, 1984;Nelson and Jones, 1987;King et al, 1994;Piper et al, 1997b). The deformation is considered to be ductile and distributed over a wide area in the continuum models, while in discrete models the deformation causes rotation of internally undeformed continental blocks.…”

Correlation of welded ignimbrites on Pantelleria (Strait of Sicily) using paleomagnetism

“…In the past, regional tilting associated with caldera collapse has been constrained by paleomagnetic studies of ignimbrites from the English Lake District (Channell and McCabe 1992;Piper et al 1997). …”

Incursion of a large-volume, spatter-bearing pyroclastic density current into a caldera lake: Pavey Ark ignimbrite, Scafell caldera, England

“…3). The intracaldera ignimbrites and associated mesobreccias reveal a complex pattern of caldera-floor fragmentation and differential subsidence along numerous intersecting faults; downsag (inward-tilting) during this piecemeal collapse is evident from thickness variations of the ignimbrites (Branney and Kokelaar 1994) and from palaeomagnetic studies (Channell and McCabe 1992;Piper et al 1997). Caldera collapse was followed by deposition of talus breccias along active and degrading fault scarps, and by subaerial extrusion of two lava domes: Scafell Dacite and Rosthwaite Rhyolite (Figs.…”