Research on the Boundary Identification Method of Concealed Geological Bodies Based on Non-seismic Geophysical Data—— Taking Benxi-Xiuyan Area as an Example

In the process of deep geological structure research, gravity and aeromagnetic anomaly data are often used to invert the distribution law of concealed geological bodies. In order to reveal the boundaries of concealed geological bodies and divide structural divisions, this paper adopts fuzzy control simulated annealing genetic algorithm () to superimpose the gravity and aeromagnetic anomaly data in Benxi-Xiuyan area and classify the patterns, and identify the main geological body boundaries and fractures. Comparing the relationship with known geological maps, the two have a higher degree of matching. Experiments show that the application of SAGAFcm algorithm can quickly identify the boundaries of concealed geological bodies and provide a new means for field geological mapping.

Magnetic and gravity views of crust and lithosphere heterogeneity in the Wilkes Subglacial Basin of East Antarctica

<p>The Wilkes Subglacial Basin (WSB) is a major intraplate tectonic feature in East Antarctica. It stretches for ca 1400 km from the edge of the Southern Ocean, where it is up to 600 km wide towards South Pole, where it is less than 100 km wide. Recent modelling of its subice topography (Paxman et al., 2019, JGR) lends support to a long-standing hypothesis predicting that the wide basin is linked to flexure of more rigid and mostly Precambrian cratonic lithosphere induced by the Cenozoic uplift of the adjacent Trasantarctic Mountains,. However, there is also mounting evidence from potential field and radar exploration that its narrower structurally controlled sub-basins may have formed in response to more localised Mesozoic to Cenozoic extension and transtension that preferentially steered glacial erosion (Paxman et al., 2018, GRL).  </p><p>Here we exploit recent advancements in regional aerogeophysical data compilations and continental scale satellite gravity gradient imaging with the overarching aim of helping unveil the degree of 4D heterogeneity in the crust and lithosphere beneath the WSB. New views of crustal and lithosphere thickness stem from 3D satellite gravity modelling (Pappa et al., 2019, JGR) and these can be compared with predictions from previous flexural modelling and seismological results. By stripping out the computed effects of crustal and lithosphere thickness variations we then obtain residual intra-crustal gravity anomalies. These are in turn compared with a suite of enhanced aeromagnetic anomaly images. We then calculate depth to magnetic and gravity source estimates and use these results to help constrain the first combined 2D magnetic and gravity models for two selected regions within the WSB.</p><p>One first model reveals a major lithospheric scale boundary along the eastern margin of the northern WSB. It separates the Cambro-Ordovician Ross Orogen from a newly defined composite Precambrian Wilkes Terrane that forms the unexposed crustal basement buried beneath partially exposed early Cambrian metasediments and more recent Devonian to Jurassic sediments.</p><p>Our second model investigates a sector of the WSB further south, where the proposed Precambrian basement is modelled as being both shallower and of more felsic bulk composition. Although the lack of drilling precludes direct sampling of this cryptic basement, aeromagnetic anomaly patterns suggest that it may be akin to late Paleoproterozoic to Mesoproterozoic igneous basement exposed in part of the Gawler and Curnamona cratons in South Australia. We conclude that these first order differences in basement depth, bulk composition and thickness of metasediment/sediment cover are a key and previously un-appreciated intra-crustal boundary condition, which is likely to affect geothermal heat flux variability beneath different sectors of the WSB, with potential cascading effects on subglacial hydrology and the flow of the overlying East Antarctic Ice Sheet.</p>

Petrophysical constraints on magnetic anomalies associated with metamorphic reactions in northern Saskatchewan, Canada

The western Chipman domain of the east Athabasca mylonite triangle in northern Saskatchewan, Canada, displays a large positive aeromagnetic anomaly that is the result of retrograde magnetite production during exhumation. Petrologic, magnetic coercivity, and hysteresis analyses indicate that multidomain magnetite is the primary magnetic phase in rocks of the region. Measurements of magnetic susceptibility from the western Chipman domain document five orders of magnitude variation, while rocks from the eastern Chipman domain are paramagnetic. The distribution of Koenigsberger ratios is approximately a mixed bimodal lognormal distribution with peak ratios at 0.039 and 0.73, suggesting that magnetic susceptibility is more significant than remanent magnetization. However, remanent magnetization is an important contributor to total magnetization. Petrographic observations indicate that magnetite is primarily produced from the breakdown of hornblende. The consumption of hornblende is also texturally associated with the production of actinolite and the hydration-related breakdown of granulite facies mineral phases such as garnet and clinopyroxene. Based on the proximity of the positive aeromagnetic anomaly to the Cora Lake shear zone, late-stage deformation along the shear zone during exhumation of the east Athabasca mylonite triangle may have structurally controlled the infiltration of fluids resulting in the heterogeneous production of magnetite. These results document the utility of integrating aeromagnetic, petrologic, and rock magnetic data to transcend observational scales and better understand regional tectonometamorphic history.