Magnetic fabric (anisotropy of magnetic susceptibility) constraints on emplacement mechanism of clastic dikes: an example from the Cretaceous Dadaepo Basin in SE Korea

Abstract:

Emplacement mechanisms of clastic dikes, which are discordant and tabular bodies comprised of weakly to strongly lithified clastic detritus, have been a matter of considerable interest over the last 20 years. Clastic dikes are generally classified into neptunian and injected dikes. Using the magnetic fabrics (AMS), we attempt to classify the clastic dikes in the late Cretaceous Dadaepo Basin, SE Korea, and interpret their emplacement mechanisms. The neptunian dikes exhibit a typical oblate sedimentary fabric which makes a sharp contrast with the injected dikes. The fabrics of the injected dikes are greatly influenced by current conditions (flow directions, rheological properties, and rates) and transportation types (imbrication or rolling) of filling materials. Based on the AMS fabrics, they are classified into four types. (1) Type-VP is formed by grain imbrication in low- to moderate-energy vertical flow of a Newtonian fluid and characterized by a bilateral symmetry of fabrics across the dike. (2) Type-VT results from grain rolling in vertical high-energy flow and is characterized by subvertical k₂ and subhorizontal k₁ axes on the dike plane. (3) Type-HP is formed by grain imbrication in horizontal low- to moderate-energy flow, resulting in subvertical k₃ and subhorizontal k₁ and k₂ axes. (4) Type-HT is formed by grain rolling in horizontal high-energy flow, resulting in streaked k₂-k₃ on the dike plane and horizontally clustered k₁ axes. The AMS fabrics of each type can be a significant indicator for flow direction. The observed AMS fabric of low-energy current immediately above the source layer indicates that fluidized clastic materials in the lower part of injected dike can flow laterally by lateral propagation of new or pre-existing fractures due to a dominant horizontal pressure gradient. Based on abundant AMS fabrics of high-energy current, coexistence of paleoseismic structures, and tectonic setting of the basin, earthquake-induced liquefaction is the most plausible trigger for the dike formation. Systematic attitudes of the injected dikes also indicate their potential as a powerful tool for reconstructing paleostress field.