Geophysical framework and the Appalachian-Caledonide connection

Abstract

Summary Gravity and magnetic data from the Appalachian-Caledonide area demonstrate the overall continuity of the orogen while identifying its segmentation into areas of contrasting structural style. The extension of this segmentation into ‘drift’-covered areas (e.g. the southern U.S.A.) and offshore areas (e.g. around Newfoundland and the British Isles) provides an orogen-wide framework into which structural detail established at outcrop can be accommodated. Linear gravity and magnetic anomalies often reflect contrasts within the Precambrian basement that have controlled tectonic events to the present. Such basement structure has been investigated by deep seismic profiling, which has also identified large-scale thrusting throughout the Appalachians and the Caledonides with some thrusts even extending into the upper mantle. Speculation to greater depths based on conductivity and P-wave travel-time residuals suggests that traces of the early Palaeozoic collision zone may still exist in the lower crust and upper mantle beneath the northern Appalachians. The loading imposed by thrust sheets during that collision produced foreland basins in the eastern USA whose form and sedimentary record indicate the magnitude and duration of thrusting. Palaeomagnetic results suggest transcurrent movement in Devonian-Carboniferous time, but the early Palaeozoic collisional choreography has not yet been uniquely defined.

Relative sea-level changes and the development of a Cambrian transgression

“…The overall architecture of the Fucoid Beds, as indicated in Figure 4, suggests an overall transgressive regime punctuated by episodic shallowing events. The relative abundance of sandstones in the northernmost outcrops may relate to localized clastic input during shallowing events from the Shetland Promontory (Haworth et al 1988). The setting therefore was of a storm-dominated, oxygen depleted, onlapping shelf, which was anoxic below wave base, possibly in response to a loss of tidal circulation (McKie, 1990a) and upwelling of anoxic, P-rich ocean water (cf.…”

Brittle stars looking like starfish: the first fossil record of the Astrophiuridae and a remarkable case of convergent evolution

“…Deposition of the chalk sediments at both sampled localities took place in an open shelf setting well below storm-wave base, equivalent to a modern deep shelf to shallow slope environment, at the Isle of Rügen (Neumann, 2012; Thuy, 2013) and a deep shelf environment at Cringleford (Wood, 1988). Thus, thee paleo-depth at both localities is reconstructed as shallow bathyal and deep sublittoral, respectively (Neumann, 2012).…”

A Mechanism for Tectonic Inheritance at Transform Faults of the Iapetan Margin of Laurentia

“…The interpreted original trace of the continental margin, as well as the location and identity of transform faults, reflects assembly of a variety of data, which have been detailed in previous publications and will not be repeated here (e.g. Thomas 1977Thomas , 1991Thomas , 1993Thomas , 2006Thomas , 2011Goetz and Dickerson 1985;Haworth et al 1988;Keller et al 1989;Cawood and Botsford 1991;Mickus and Keller 1992;Astini 1996, 1999;Cherichetti et al 1998;Cousineau and Longuépée 2003;Harry et al 2003;Harry and Londono 2004;Tull and Holm 2005;Allen et al 2009Allen et al , 2010. The reconstruction of the rifted margin shown in Figure 1 integrates restorations of Appalachian shortening to palinspastic locations of pre-Iapetan basement rocks (now in external basement massifs), synrift sedimentary and igneous rocks, and sedimentary rocks of the passive-margin shelf and shelf edge, as well as geophysical resolution of the margin of continental crust.…”