Abstract
SUMMARYWe propose a tectonic interpretation for the outer-SDRs (SDRs: Seaward-Dipping Reflectors) and Pannikar central ridge in the aborted Laxmi Basin west of India from wide-angle seismic reflection data. The outer-SDRs comprise syn-tectonic extrusives (lavas and/or volcaniclastics) emplaced above passively exhumed mid-to-lower mafic crust of continental origin. They erupted following sudden lithosphere weakening associated with isolation of a continental block (a ‘C-Block’). Continuous magmatic addition during crustal extension allowed stretching of the lower crust whilst maintaining constant or even increasing thickness. A similar process occurred at both conjugate margins allowing bulk, pure-shear plate separation and formation of linear magnetic anomalies. The Laxmi example can explain enigmatic features observed in mature oceans such as presence of distal buoyant plateaus of thick continental crust away from the margins.
Highlights
Volcanic passive margins (VPMs) form when continental extension is coeval with extensive mantle melting (e.g. Skogseid 2001)
In accordance with the interpretation of Nemcok & Rybar (2016) we find that the Laxmi Basin and bordering areas are an aborted continental extensional system with conjugate volcanic passive margins which probably developed from extended continental crust (Guan et al 2019, Figs 3b and c)
The Laxmi Basin case example is important in that it illustrates the early stage of continental breakup in a magma-rich environment
Summary
Volcanic passive margins (VPMs) form when continental extension is coeval with extensive mantle melting (e.g. Skogseid 2001). Many dykes crosscut inner-SDRs during their development at any location, most of them feeding the upper lavas at considerable distances from the edges of SDR wedges and related major faults (Klausen & Larsen 2002; Lenoir et al 2003; Abdelmalak et al 2015) This indicates that the magma is not all injected from a stable, permanent axial zone, a fundamental starting point in the model of Buck (2017) for a single. Our knowledge on the origin of outer-SDRs as well as on the type of middle/lower crust (oceanic or continental) underlying them, remains incomplete We tentatively address this topic below by considering the mode of continental breaking-up at VPMs. It is observed that syn-magmatic detachment faults bounding inner-SDRs dip continent-ward at conjugate VPMs (Fig. 2a). The relationships between this C-Block and nearby outer-SDRs bring into question the nature of outer-SDR lower crust and, by extension, the definition of the continent–ocean transition across VPMs we propose a tectonic model for outer-SDRs in the light of our findings
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