Deep crustal structure beneath large igneous provinces and the petrologic evolution of flood basalts

Abstract

[1] We present a review of seismological constraints on deep crustal structures underlying large igneous provinces (LIPs), largely from wide-angle seismic refraction surveys. The main purpose of this review is to ascertain whether this seismic evidence is consistent with, or contrary to, petrological models for the genesis of flood basalt lavas. Where high-quality data are available beneath continental flood basalt (CFB) provinces (Emeishan, Columbia River, Deccan, Siberia), high-velocity structures (V p ∼ 6.9-7.5 km/sec) are typically found immediately overlying the Moho in layers of order ∼5-15 km thick. Oceanic plateau (OP) LIPs exhibit similar layers, with a conspicuous layer of very high crustal velocity (V p ∼ 7.7 km/sec) beneath the enormous Ontong-Java plateau. These structures are similar to inferred ultramafic underplating structures seen beneath active hot spots such as Hawaii, the Marquesas, and La Reunion. Petrogenetic models for flood basalt volcanism based on hot plume melting beneath mature lithosphere suggest that these deep seismic structures may consist in large part of cumulate bodies of olivine and clinopyroxene which result from ponding and deep-crustal fractionation of ultramafic primary melts. Such fractionation is necessary to produce basalts with typical MgO contents of ∼6-8%, as observed for the vast bulk of observed flood basalts, from primary melts with MgO contents of order ∼15-18% (or greater) such as result from hot, deep melting beneath the lithosphere. The volumes of cumulate bodies and ultramafic intrusions in the lowermost crust, often described in the literature as "underplating," are comparable to those of the overlying basaltic formations, also consistent with petrological models. Further definition of the deep seismic structure beneath such prominent LIPs as the Ontong-Java Plateau could place better constraints on flood basalt petrogenesis by determining the relative volumes of ultramafic bodies and basaltic lavas, thereby better constraining the overall process of LIP emplacement.

Future accreted terranes: a compilation of island arcs, oceanic plateaus, submarine ridges, seamounts, and continental fragments

“…We caution against relating crustal units of this FAT to oceanic crust because oceanic plateaus and submarine ridges are formed differently from typical oceanic crust. Many oceanic plateaus and submarine ridges have a basal unit of high seismic velocities (7.0-7.9 km s −1 ), which is highlighted in a compilation by Ridley and Richards (2010). and Gupta et al (2010) suggest that this mafic basal unit is underplated material due to plume magmatism.…”

“…The basal cumulate layer may be a ductile layer that serves as a detachment to allow for underplating, an idea originally speculated by Schubert and Sandwell (1989) to develop in plateaus that exceed 15 km in thickness based on the rheological relationship of strength with depth. Even though this layer is not found in all LIPs and seamounts of great thicknesses (Ridley and Richards, 2010) (Fig. 6), the cumulate or underplated magma layer could definitely serve as a ductile layer to initiate detachment within the subduction zone.…”

“…Early studies have suggested that the high seismic velocity lower crustal layer was representative of a ductile layer that occurs in crust greater than 15 km thick (Schubert and Sandwell, 1989). However, the theory that all large oceanic igneous provinces will have a ultramafic layer was debunked by the compilation of Ridley and Richards (2010). We calculated an average crustal density from the Pwave velocities from 23 seismic refraction studies of oceanic plateaus and submarine ridges (Table 4).…”

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).…”

Triggering of the largest Deccan eruptions by the Chicxulub impact

“…5): Flood basalts at the surface (regime 1) are fed by crustal dike and sill systems (regime 2), which are in turn fed by partial melting of the sublithospheric mantle within the hot, rising mantle plume head (regime 5), with porous melt migration coalescing into channelized fl ow through the overlying intact lithosphere (regime 4). Petrologic considerations (Cox, 1980), seismic imaging (Ridley and Richards, 2010;Richards et al, 2013), and thermodynamic models for melt equilibrium (Farnetani et al, 1996;Karlstrom and Richards, 2011;Richards et al, 2013) suggest a mediating zone (regime 3) at the crust-mantle boundary (Moho). Here, ultramafi c mantle-derived melts collect in a density trap as laterally extensive magma chambers, undergoing crystal fractionation of Fe-and Mg-rich minerals (olivine and pyroxene) while lower-density, eruptible basaltic liquid is evolved.…”