Rifting of the oceanic Azores Plateau with episodic volcanic activity

B Storch,A A P Koppers,R H W Romer,C Beier,K M Haase

doi:10.1038/s41598-020-76691-1

B Storch, A A P Koppers + Show 3 more

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https://doi.org/10.1038/s41598-020-76691-1

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Abstract

Extension of the Azores Plateau along the Terceira Rift exposes a lava sequence on the steep northern flank of the Hirondelle Basin. Unlike typical tholeiitic basalts of oceanic plateaus, the 1.2 km vertical submarine stratigraphic profile reveals two successive compositionally distinct basanitic to alkali basaltic eruptive units. The lower unit is volumetrically more extensive with ~ 1060 m of the crustal profile forming between ~ 2.02 and ~ 1.66 Ma, followed by a second unit erupting the uppermost ~ 30 m of lavas in ~ 100 kyrs. The age of ~ 1.56 Ma of the youngest in-situ sample at the top of the profile implies that the 35 km-wide Hirondelle Basin opened after this time along normal faults. This rifting phase was followed by alkaline volcanism at D. João de Castro seamount in the basin center indicating episodic volcanic activity along the Terceira Rift. The mantle source compositions of the two lava units change towards less radiogenic Nd, Hf, and Pb isotope ratios. A change to less SiO2-undersaturated magmas may indicate increasing degrees of partial melting beneath D. João de Castro seamount, possibly caused by lithospheric thinning within the past 1.5 million years. Our results suggest that rifting of oceanic lithosphere alternates between magmatically and tectonically dominated phases.

Highlights

Oceanic plateaus with a crustal thickness to 30 km cover large areas in the oceans and these bathymetric swells affect oceanic currents and marine life[1,2]
The upper ~ 1.2 km of the Azores Plateau crust along the Hirondelle Basin formed within ~ 500 kyrs with the lower 1000 m-thick portion erupting within 350 kyrs
The formation of volcanoes with heights of > 1 km is followed by tectonic extension with normal faulting but there is no evidence for magmatic spreading with production of new basaltic crust

Summary

Introduction

Oceanic plateaus with a crustal thickness to 30 km cover large areas in the oceans and these bathymetric swells affect oceanic currents and marine life[1,2]. The Azores Plateau formed 10 to 4 million years a go[10] and is rifted by the NW–SE striking ultraslow Terceira R ift[11,12,13]. Large portions of the Azores Plateau probably formed by enhanced melt production close to the Mid-Atlantic Ridge (MAR) between 10 and 4 Ma ago, possibly with the abundant eruption of tholeiitic basalts from large melt volumes in the head of a deep mantle p lume[10,30]. The extension may have occurred in two phases with the first by normal faulting of existing crust of the entire Azores plateau, and the second very recent phase with magmatic intrusions along the Terceira Rift[35]. The Hirondelle Basin is seismically active implying ongoing tectonic extension in this a rea[38]

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