Emplacement and 3D geometry of crustal-scale saucer-shaped intrusions in the Fennoscandian Shield

Abstract

Saucer-shaped intrusions of tens of meters to tens of kilometres across have been observed both from surface geological mapping and geophysical observations. However, there is only one location where they have been reported to extend c. 100 km laterally, and emplaced both in a sedimentary basin and the crystalline basement down to 12 km depth. The legacy BABEL offshore seismic data, acquired over the central Fennoscandian Shield in 1989, have been recovered and reprocessed with the main goal of focusing on this series of globally unique crustal-scale saucer-shaped intrusions present onshore and offshore below the Bothnian Sea. The intrusions (c. 1.25 Ga), emplaced in an extensional setting, are observed within both sedimentary rocks (<1.5 Ga) and in the crystalline basement (>1.5 Ga). They have oval shapes with diameters ranging 30–100 km. The reprocessed seismic data provide evidence of up-doming of the lower crust (representing the melt reservoir) below the intrusions that, in turn, are observed at different depths in addition to a steep seismically transparent zone interpreted to be a discordant feeder dyke system. Relative age constraints and correlation with onshore saucer-shaped intrusions of different size suggest that they are internally connected and fed by each other from deeper to shallower levels. We argue for a nested emplacement mechanism and against a controlling role by the overlying sedimentary basin as the saucer-shaped intrusions are emplaced in both the sedimentary rocks as well as in the underlying crystalline basement. The interplay between magma pressure and overburden pressure, as well as the, at the time, ambient stress regime, are responsible for their extensive extent and rather constant thicknesses (c. 100–300 m). Saucer-shaped intrusions may therefore be present elsewhere in the crystalline basement to the same extent as observed in this study some of which are a significant source of raw materials.

Origin of the Baltic Sea basin by Pleistocene glacial erosion

“…1). Crustal thinning and aborted rifting during the Mesoproterozoic was accompanied by rapakivi granite intrusion and the deposition of Jotnian arkosic sandstones of km thickness (Buntin et al 2019). Only the southern Baltic basin was affected subsequently by Caledonian orogenesis and Mesozoic rifting (Van Balen & Heeremans 1998).…”

Transgressive Sills and Lateral Lava Flows: On the Visual Observation of Igneous Sheets in Rugged Mountainous Terrains and the Optical Illusion Factor

“…Lateral sill propagation in response to differing host-rock lithologies in layered strata is indeed an intrusion mechanism that is widely accepted amongst geologists worldwide for sill emplacement in volcanic and sedimentary environments alike [5,13,20]. Sill deflection in response to pre-existing subvertical structures in both volcanic and sedimentary settings is a well documented mechanism too [3,5,15].…”

“…Sills may occur in a wide variety of forms, extent and thicknesses. When it comes to sills in the upper crust, these are commonly thought of as either rather simple magma storage champers or simple conduits of magmas ascending through the crust, or they may form part of complex and intricate sill complexes capable of lateral magma transport as well as transport of magmas to storage chambers higher in the crust or to surface magmatism [1][2][3][4][5][6][7]. Although sill systems have most commonly been reported in rift-related or passive sedimentary settings, they are not uncommon in volcanic settings, e.g.…”

“…areas affected by basaltic magmatism such as in flood basalt provinces [3]. When it comes to sills in rifted or passive sedimentary settings in offshore areas, seismic imaging associated with prospective activity has revealed the wide variety of sills in such settings, including sills as isolated bodies and internally connected sills in entire sill complexes, thus enabling geologists to estimate/interpret intrusion mechanisms prevailing in such settings [5]. The natural limitations associated with detecting small-scale structures using seismic imaging (sub-seismic structures) in offshore sedimentary settings can to some degree be compensated for by visual observations and measurements of exposed sills in onshore sedimentary and/or volcanic settings.…”

Structural controls on the location, geometry and longevity of an intraplate volcanic system: the Tuatara Volcanic Field, Great South Basin, New Zealand

“…Constraining the 3D geometry of magma plumbing systems and assessing how intraplate volcanism relates to and/or may be influenced by pre-existing structures is difficult because (see Magee et al, 2018 and references therein): (i) geophysical and geodetic data typically provide only a relatively low resolution view of subsurface magma or igneous rock distribution, and capture little information on host rock structure; (ii) outcrop analyses of ancient plumbing systems allow detailed analyses of intrusion geometry and host rock structure, but limitations in exposure at Earth's surface mean we cannot often place these observations within a 3D context; and (iii) petrological and chemical data, whilst providing crucial insights into melt and magma evolution, are often interpreted within a poorly defined structural framework. Reflection seismology provides a powerful tool for imaging the 3D geometry of volcanoes and magma plumbing systems in the subsurface (e.g., Bischoff et al, 2017;Buntin et al, 2019;Magee et al, 2019;Magee et al, 2016;McLean et al, 2017;Morley, 2018;Quirie et al, 2019;Reynolds et al, 2017;Sun et al, 2019a).…”