The origin of the Charcot Anomaly, Antarctic Peninsula, and implications for the tectonic evolution of the mid-Cretaceous Pacific Gondwana margin.

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Understanding of Cretaceous plate motions, and closure of the global plate circuit, is hampered by the "Cretaceous Normal Superchron” (CNS) between 120 and 84 Ma, an interval of very stable geomagnetic field that produced no magnetic striping over the ocean floor. This coincided with eruption of several oceanic large igneous provinces (LIPs), huge outpourings of magma (> 100,000 km3) that created vast regions of volcanic and related rocks (Coffin and Eldholm, 1993), concentrated in time in what is called a superplume event, and associated with a global episode of plate reorganisation (Matthews et al., 2012). Lack of sea floor magnetic striping during the CNS has complicated the task of reconstructing plate motions in detail, leaving large uncertainties in the relative positions of continents, oceans, and the LIPs they bear. Perversely, prolonged stability of the geomagnetic field at these times means that sea floor magnetic striping, which is normally used to reconstruct plate motions, does not form when changes in plate motions can be at their greatest. 123 million years ago, one of the largest LIPs in Earth history was erupted in the palaeo-Pacific Ocean (Chandler et al., 2012; Taylor, 2006) at the beginning of the CNS, the 5 million km2 Ontong Java-Nui super-plateau (OJN; 2/3 the area of Australia). Shortly afterwards, as a result of the changes in palaeo- Pacific plate configuration associated with its formation, it is thought to have split into at least three component parts that are recognised today (e.g. Fig. 1) (Taylor, 2006), the Ontong-Java, Manihiki and Hikurangi plateaus (Fig. 1). 105 million years ago, still within the CNS, a global reorganisation of plate motions, associated with mountain building events on continental margins, was triggered by changes in subduction along the Gondwana margin in the vicinity of West Antarctica (Matthews et al., 2012). Matthews et al. (2012) presented two hypotheses for this to account for global plate reorganisation: 1) increased ridge crest–trench interaction at subduction zones; or 2) collision of the Hikurangi Plateau fragment of the OJN with the Gondwana margin (Fig. 1). The authors favoured their first hypothesis because Hikurangi Plateau collision, they argued, does not affect a sufficient length of the subducting margin to trigger plate motion changes. However, it has been observed (Taylor, 2006) that a further large piece of the OJN may have rifted-off the unusually straight Manihiki Scarp (Fig. 1) and been removed to the southeast. Preliminary plate motion modelling suggests that a LIP fragment derived from the Manihiki Scarp would have collided with the Gondwana margin in the vicinity of the Antarctic Peninsula. A candidate feature for evaluation has been identified near the southern end of the peninsula. Here, a high amplitude rectilinear aeromagnetic anomaly off the west coast, the Charcot Anomaly (CA) (Johnson and Ferris, 1997), is over 1000 km long (Golynsky et al., 2002) and represents one of the longest unexplained linear continental margin anomalies on Earth. Inboard of this anomaly lies the latitudinally restricted and enigmatic collisional-accretionary Palmer Land Event orogeny, the origin of whose second phase at 103–100 Ma remains unexplained (Vaughan et al., 2012). These data hint that the CA may be the geophysical expression of the trailing edge of this fragment of the OJN (Labelled “?CAP” on Fig. 1) and that the second phase of the Palmer Land Event orogeny may be the on-land expression of its collision with, and partial subduction beneath, the Gondwana margin 100 million years ago.

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The Scotia Arc: Geodynamic Evolution and Global Implications, 01 Jan 2013 - 01 Jan 1970, IACT Granada, Spain.
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Vaughan, A. P. M. , Eagles, G. , Ferraccioli, F. , Leat, P. , Jordan, T. and Coffin, M. (2013): The origin of the Charcot Anomaly, Antarctic Peninsula, and implications for the tectonic evolution of the mid-Cretaceous Pacific Gondwana margin. , The Scotia Arc: Geodynamic Evolution and Global Implications, IACT Granada, Spain, 2013 - unspecified .

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