Palaeomagnetism of igneous rocks of the Lake District (Caledonian) terrane, Northern England: palaeozoic motions and deformation at a leading edge of Avalonia

Piper, J. D. A.

doi:10.1002/(sici)1099-1034(199709)32:3<211::aid-gj740>3.0.co;2-o

Cited by 10 publications

(11 citation statements)

References 62 publications

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“…7) and this remanence is evidently a long term acquisition. This dipolar axis is comparable to magnetizations determined from late early Devonian parts of the batholith (Piper, 1997b) and probably records sporadic late Acadian thermal overprinting.…”

Section: C Palaeomagnetic Results From the Borrowdale Volcanic Grosupporting

confidence: 78%

“…The last magmatic pulse in this region comprises an east-west trending dyke swarm (Eastwood et al 1968). Magnetic remanence in these dykes does not permit a precise age assignment although it is approximately reversed with respect to the result from the (Cautleyan) Yarlside Volcanic Member of this study (Piper, 1997b). The dykes preserve a near-vertical orientation.…”

Section: E the Eycott Volcanic Group Northern Lake Districtcontrasting

confidence: 80%

“…Folds such as the Wrynose and Nan Bield anticlines are regarded as Acadian by Branney & Soper (1988) and Kneller et al (1994); they form the northern hinge of an Acadian mountain front structure known as the Westmorland Monocline (Kneller & Bell, 1993). North of the BVG outcrop, however, the major deformation occurred during successive emplacement of members of the Carrock Fell Complex (Piper, 1997b) and is therefore late Ordovician in age and contemporaneous with large scale buckle folding in the Welsh Caledonides.…”

Section: Geological Outlinementioning

confidence: 86%

“…4). Minor contrasts in demagnetization behaviour Piper, 1997b). Directions of the characteristic remanence (ChRM) components are accordant within clasts and dispersed between clasts ( Fig.…”

Section: B Palaeomagnetic Field Testsmentioning

confidence: 86%

“…The major F2 structure in the northern Lake District was forming during emplacement of the Carrock Fell Complex, where the layered gabbros have a remanence deformed by F2 folding (Harris & Dagger, 1987;Piper, 1997b) were magnetized close to their present near-vertical orientations after deformation was essentially complete (Piper, 1997b). The last magmatic pulse in this region comprises an east-west trending dyke swarm (Eastwood et al 1968).…”

Section: E the Eycott Volcanic Group Northern Lake Districtmentioning

confidence: 99%

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

1997

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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.

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Section: C Palaeomagnetic Results From the Borrowdale Volcanic Grosupporting

confidence: 78%

Section: E the Eycott Volcanic Group Northern Lake Districtcontrasting

confidence: 80%