Variations in magmatic processes along the East Greenland volcanic margin

Abstract

S U M M A R YSeismic velocities and the associated thicknesses of rifted and igneous crust provide key constraints on the rifting history, the differentiation between non-volcanic and volcanic rifted margins, the driving force of magmatism at volcanic margins, that is, active or passive upwelling and the temperature anomaly in the lithosphere. This paper presents two new wideangle seismic transects of the East Greenland margin and combines the velocity models with a compilation of 30-wide-angle seismic velocity models from several publications along the entire East Greenland margin. Compiled maps show the depth to basement, depth to Moho, crustal thickness and thickness of high velocity lower crust (HVLC; with velocities above 7.0 km s −1 ). First, we present two new wide-angle seismic transects, which contribute to the compilation at the northeast Greenland margin and over the oceanic crust between Shannon Island and the Greenland Fracture Zone. Velocity models, produced by ray tracing result in total traveltime rms-misfits of 100-120 milliseconds and χ 2 values of 3.7 and 2.3 for the northern and southern profiles with respect to the data quality and structural complexity. 2-D gravity modelling is used to verify the structural and lithologic constraints. The northernmost profile, AWI-20030200, reveals a magma starved break-up and a rapidly thinning oceanic crust until magnetic anomaly C21 (47.1 Ma). The southern seismic transect, AWI-20030300, exhibits a positive velocity anomaly associated with the Shannon High, and a basin of up to 15 km depth beneath flood basalts between Shannon Island and the continent-ocean boundary. Breakup is associated with minor crustal thickening and a rapidly decreasing thickness of oceanic crust out to anomaly C21. The continental region is proposed to be only sparsely penetrated by volcanism and not underplated by magmatic material at all compared to the vast amount of magmatism further south. Break-up is proposed to have occurred at the seaward boundaries of the continent-ocean transition zones at between ∼50 and ∼54 Ma, propagating from north to south based on a joint analysis incorporating transects from the Kejser Franz Joseph Fjord and Godthåb Gulf. Secondly, the variation of the HVLC along the East Greenland margin from 60 • to 77 • N and from transects of its conjugate margin shows inverted emplacement of prominent landward and seaward HVLC thickness portions from north to south in a distribution chart. The differences in the HVLC distribution are attributed to one or more of the following three models. In the first model it is inferred that a transfer zone/detachment acts as a barrier to northward magma flow. In the second model, underplating results in thicker and highly intruded lower crust with several small-scale feeder dykes that locally increase the lower crustal velocities. In the third model, a second magmatic event associated with the separation of the Jan Mayen microcontinent is considered. Lithosphericscale inhomogeneities might be responsible for the heterogene...

Asymmetry of high-velocity lower crust on the South Atlantic rifted margins and implications for the interplay of magmatism and tectonics in continental breakup

“…HVLC can make up a large part of the total magmatic output along volcanic rifted margins, and as studies in the North Atlantic have shown, variations in size and physical properties of the HVLC in these settings hold important clues to mantle melting scenarios (Fernàndez et al, 2010;Kelemen and Holbrook, 1995;Korenaga et al, 2002;Ridley and Richards, 2010;Voss et al, 2009;White et al, 2008). Alternatively, it might be speculated that portions of the HVLC form postrift (Franke, 2013).…”

The origin of the asymmetry in the Iceland hotspot along the Mid-Atlantic Ridge from continental breakup to present-day

“…Average half-spreading rates (29-33 km/Myr) and crustal thicknesses at this time were at their highest observed values (Fig. 3, Breivik et al, 2006Smallwood and White, 2002;Voss et al, 2009). For example, oceanic crustal thickness was ∼8 km along the early AR (Breivik et al, 2006), as thick or thicker along much of the RR, and significantly thicker (>30 km) along the Iceland-Greenland and IcelandFaeroe volcanic ridges (e.g.…”

“…3). During 43-28 Ma, seafloor spreading at the AR was probably slower than at the RR and MR by as much as 30% (Breivik et al, 2006;Mosar et al, 2002;Smallwood and White, 2002;Voss et al, 2009), likely related to lithospheric stretching or the very earliest stages of rifting at the KR. Crustal thickness generated from 43 to 28 Ma along the middle and northern portions of the AR was only 3.5-5.5 km (Breivik et al, 2006), similar to that of normal (not hotspot influenced) oceanic crust at the same ultra-slow spreading rate of ∼7 km/Myr (Dick et al, 2003;White et al, 2001).…”

“…White and McKenzie, 1989). Seismic studies document igneous crustal thicknesses of up to ∼35 km along both continental margins near the center of the Iceland hotspot track, and thicknesses 15 km extending 1000 km along the margins to the north and south (Breivik et al, 2006;Holbrook et al, 2001;Mjelde et al, 2008;Voss et al, 2009). Shortly after breakup (∼54-52 Ma), oceanic crust began forming along three main spreading centers, the RR, AR, and Mohns Ridge (MR) (Fig.…”

“…Modeled spreading rate evolution for the North Atlantic. Geological estimates of the spreading rates at MR (Breivik et al, 2009;Voss et al, 2009), RR (Smallwood and White, 2002), AR (red, Breivik et al, 2006), and KR (yellow, Mosar et al, 2002) were averaged to create a mean North Atlantic spreading rate through time (black). Since 33 Ma, spreading rates by Mosar et al (2002) for all four ridges are incorporated.…”