Thermal evolution of the North Atlantic lithosphere: New constraints from magnetic anomaly inversion with a fractal magnetization model

Abstract

[1] Using recently published global magnetic models, we present the first independent constraint on North Atlantic geothermal state and mantle dynamics from magnetic anomaly inversion with a fractal magnetization model. Two theoretical models of radial amplitude spectrum of magnetic anomalies are found almost identical, and both are applicable to detecting Curie depths in using the centroid method based on spectral linearization at certain wave number bands. Theoretical and numerical studies confirm the robustness of this inversion scheme. A fractal exponent of 3.0 in the magnetic susceptibility is found suitable, and Curie depths are well constrained by their known depths near the mid-Atlantic ridge. While generally increasing with growing ages, North Atlantic Curie depths show large oscillating and heterogeneous patterns related most likely to small-scale sublithospheric convections, which are found to have an onset time around 40 Ma and a scale of about 500 km, and are in preferred transverse rolls. Hotspots in North Atlantic also contribute to large geothermal and Curie-depth variations, but they appear to connect more closely to geochemical anomalies or small-scale convection than to mantle plumes. Curie depths can be correlated to heat flow gridded in a constant 1 interval, which reveals decreasing effective thermal conductivity with depths within the magnetic layer. North Atlantic Curie points are mostly beneath the Moho, suggesting that the uppermost mantle is magnetized from serpentinization and induces long-wavelength magnetic anomalies. Small-scale convection and serpentinization together may cause apparent flattening and deviations in heat flow and bathymetry from theoretical cooling models in old oceanic lithosphere.

A global reference model of Curie-point depths based on EMAG2

“…This reflects the active magmatism and upwelling of the hot asthenosphere. GCDM confirms the early results in the north Atlantic11, that in the background of increasing Curie depths with oceanic crustal ages22, anomalous off-ridge upwelling and oscillations in Curie isotherm are observed in all major sea basins, and in most cases, they are associated with small-scale convection or hotspots. For example, the Hawaii seamount trail, the Bermuda Rise, and the Cape Verde Islands all show small Curie depths.…”

Estimating depth to the bottom of magnetic sources at Wikki Warm Spring region, northeastern Nigeria, using fractal distribution of sources approach

“…Both z t and z o of magnetic sources were evaluated from the same power spectrum instead of adherence to established theoretical guidelines as proposed by Tanaka et al (1999) and explained earlier in this manuscript. The introduction of fractal exponents in the inversion effectively suppresses lateral correlations of magnetic sources Todoeschuck, 1993, 1995;Maus et al, 1997;Ravat et al, 2007;Bouligand et al, 2009) and leads to z b conformable to known geological constraints (Li et al, 2013).…”

“…The application of Eq. (3) results in larger centroid depth values, even if small block sizes are used (Bansal et al, 2011;Li et al, 2013). Eqs.…”

“…However, the absence of a peak at low wavenumbers may indicate fractal behavior of the magnetization (Bouligand et al, 2009). Larger wavenumbers were not considered because they are likely from noises and they are not needed, according to numerical and theoretical tests (Li et al, 2013). Figure 7 shows the power spectrum calculation plot for the main grid (110 × 110 km 2 ).…”

“…In this study, we compute the DBMS using aeromagnetic data assuming a fractal distribution of magnetic sources to identify the geothermal system of the WWS region. Crustal magnetization is not completely uncorrelated but follows a fractal spatial distribution (Gregotski et al, 1991;Pilkington and Todoeschuck, 1993;Fedi et al, 1997;Maus et al, 1997;Li et al, 2013). The ultimate aim is to determine the DBMS with minimal uncertainty, to make geological and tectonic interpretations using the relation between DBMS and CPD, and to estimate the average surface heat flow of the WWS region.…”