Upper-mantle structure of the Borborema Province, NE Brazil, from P-wave tomography: Implications for rheology and volcanism

Abstract

We have performed a tomographic study of the upper mantle under the Borborema Province by inverting relative P-wave traveltime residuals (including the PKPdf phase). The events were recorded by more than 50 broad-band and short-period stations deployed across the region, resulting in a total data set of 1912 relative residuals. A multichannel phase cross-correlation technique was utilized to develop the measurements, which were then inverted through an iterative, non-linear inversion scheme based on the subspace inversion method. The Fast Marching Method, a grid-based numerical algorithm that traces an interface evolving over a narrow band of nodes that are updated solving the eikonal equation by finite differences using upwind entropy, was considered to solve the forward problem. Traveltimes from outside of the 3-D model were calculated using a global reference model. The resulting tomographic images reveal structure down to depths of ~600 km, under an area extending approximately 800 km in the EW direction and 900 km in the NS direction. The most important features revealed by the tomographic images include: (i) a relatively fast lithospheric mantle under the southern Borborema Plateau, when compared to the lithospheric mantle north of the Patos Lineament; (ii) a marked, shallow ( < 150 km) low-velocity anomaly under the northeastern most corner of the Borborema Province; and (iii) a low-velocity channel bordering the Borborema Plateau at asthenospheric (250 - 400 km) depths. The lithospheric velocity contrast is interpreted as arising from a colder, stronger lithosphere south of the lineament, while the asthenospheric low-velocity channel is interpreted as resulting from lateral flow from a distant mantle plume (located in SE Brazil). We argue that the rheological contrast validates a stretching model recently proposed to explain the Plateau's elevated topography, and that the postulated lateral flow represents the source of magmas feeding intraplate volcanism in NE Brazil for the past ~80 Ma. © The Author(s) 2018.Data were collected with support of the Instituto Nacional de Ciência e Tecnologia em Estudos Tectônicos (INCT-ET) of the Centro Nacional de Desenvolvimento Científico e Tecnológico (CNPq, grant number 57.3713/2008-01) and the BOrborema Deep Electromagnetic and Seismic experiment (BODES) (CNPq, grant number 400743/2014-0). FLSN acknowledges support from the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) through a 4-yr scholarship to complete his PhD at the Universidade Federal do Rio Grande do Norte (UFRN). JJ thanks CNPq for his research fellowship (CNPq, grant number 304421/2015-4). MS acknowledges support by Brazilian Science Without Border Program, grant number 40.2174/2012-7.Peer reviewe