Magnetic Fabric as a Marker of Thermal Maturity in Sedimentary Basins: A New Approach for Reconstructing the Tectono-Thermal Evolution of Fold-and-Thrust-Belts

Abstract

Organic and inorganic paleothermal indicators (vitrinite reflectance- Ro%; illite content in mixed-layer illite-smectite- I% in I-S) are routinely used to unravel the burial and tectonic evolution of fold-and-thrust belts by determining levels of thermal maturity of sedimentary successions and the amount of tectonic overburden currently eroded. However, the reliability of these reconstructions depends on the availability and quality of thermal indicators. We introduce the use of the anisotropy of magnetic susceptibility (AMS) as an additional proxy for thermal maturity estimations in weakly deformed sedimentary successions. Using original and formerly published data from the Northern Apennines, we calibrate a quantitative relationship between the degree of AMS Foliation (ranging from 1.011 to 1.087) and the respective I% in I-S and Ro% values. For the computed I% values from the I-S versus F correlation, we recognize that F values ranging from 1 to 1.050 define the early diagenetic zone (I% in I-S < 60), F values between 1.050 and 1.108 mark the late diagenetic zone (60 < I% in I-S < 95), while F values > 1.108 indicate anchizone conditions (I% in I-S > 95). The computed Ro% versus F correlation provides boundaries for the immature and mature stage of hydrocarbon (HC) generation (0.2 < Ro% < 0.8) with F = 1.055 indicating the transition from diagenesis (immature stage of HC generation) to catagenesis (mature stage of HC generation). The proposed model might serve as a tool for reconstructing the burial and thermal evolution of sedimentary units devoid of organic matter or in siliciclastic sediments consisting of a mixture of detrital and diagenetic minerals.