Investigating the distribution of magmatism at the onset of Gondwana breakup with novel strapdown gravity and aeromagnetic data

Abstract

Massive volumes of mafic magmatism forming the Karoo-Ferrar Large Igneous Province (LIP) in Southern Africa and Antarctica preceded Jurassic breakup of the Gondwana Supercontinent. This widespread LIP magmatism is attributed to a major mantle plume, or plumes, impacting an area thousands of kilometres across. Magmas in lava flows and shallow sills, which flowed laterally hundreds to thousands of kilometres, form most of the exposed LIP. Hence, the distribution of shallow level mafic rocks may not reflect the location of mantle melting. In contrast, large deepseated mafic intrusions such as gabbros likely more directly overlie areas of mantle melting. Antarctic exposures of such intrusions are limited to the Dufek Intrusion and outcrops >1000 km to the north, hence the true pattern of mantle melting is poorly constrained. Regional aeromagnetic and aerogravity data suggest other Jurassic mafic intrusions are present, but detailed analysis of these bodies is lacking. To define more precisely the distribution of mafic intrusions we use data from the first stand-alone strapdown gravity survey in Antarctica. This innovative technique allows collection of aerogravity data during draped flight, resolving anomalies with a wavelength of ~6 km and a root-mean-square error of 1.8 mGal. Combining this new gravity data with coincident aeromagnetic data we investigate a ~50 mGal gravity and associated >1000 nT magnetic high in Coats Land, East Antarctica. Our interpretation is that the so called 'Halley High' reflects a large gabbroic body ~80 km long, 30 km wide and ~6 km thick, equivalent to the inferred total size to the better known Stillwater layered mafic intrusion in the US. Our interpretation of a large mafic intrusion supports the suggestion from reconnaissance aeromagnetic data that this and other similar anomalies are Jurassic mafic intrusions. These large mafic intrusions, and hence underlying mantle melting, appear restricted to a linear band parallel to the continental margin. This structured pattern of mantle melting is consistent with shallow mantle convection and lithospheric extension playing a significant role in the later stages of the Karoo-Ferrar magmatism. An apparent 650 km gap in mafic intrusions adjacent to the continental Weddell Sea Rift System suggests different mantle processes were occurring here. Future joint magnetic and strapdown gravity surveying presents a key opportunity to fully constrain the location and extent of mantle melting during Gondwana breakup.

Past ice sheet-seabed interactions in the northeastern Weddell Sea Embayment, Antarctica

“…The smoothness of the seafloor at and around most of the ramps suggests that it consists of sedimentary material. In contrast, the rugged terrain described inland of ramp 6 may represent a bedrock substrate, which geographically coincides with the inferred location of a large mafic intrusion (Jordan and Becker, 2018). When drifting icebergs run aground on a sedimentary seafloor substrate with sufficient force, they produce ploughmarks with berms on either side (see also Class A bedforms).…”

New gravity-derived bathymetry for the Thwaites, Crosson and Dotson ice shelves revealing two ice shelf populations

“…CC BY 4.0 License. feature distorting the gravity signature (Jordan and Becker, 2018), but is not seen on profiles across the offshore regions (Fig. 3).…”

“…Other geological factors such as dense gabbroic intrusions, or local sedimentary basins could further distort the recovered bathymetry if they are away from the direct bathymetric observations which would mitigate the impact of such features on the final bathymetric model. Underlying geological factors can, in some cases, be revealed by 190 coincident aeromagnetic data, as in the case of the Brunt Ice Shelf (Jordan and Becker, 2018;Hodgson et al, 2019) and Ross Ice Shelf (Tinto et al, 2019). In our study, tight correlation between high amplitude magnetic and gravity anomalies is only seen beneath the grounded part of Thwaites Glacier (Fig.…”

Past and future dynamics of the Brunt Ice Shelf from seabed bathymetry and ice shelf geometry

“…By integrating gravity and aeromagnetic data to constrain the subsurface geology, we have developed a procedure to provide the most reasonable estimate of the sub-ice shelf bathymetry in otherwise un-surveyed areas. Gravity data are from a "strapdown" type of sensor which provides a resolution of ∼ 6 km and root-mean-squared error of ∼ 1.8 mGal (Jordan and Becker, 2018). This was combined with regional data from previous surveys and compilations (Aleshkova et al, 2000;Forsberg et al, 2017;Jordan et al, 2017).…”