KINEMATICS OF MID-CRUSTAL ROCKS IN CONTINENT-CONTINENT COLLISION ZONES: KOREAN COLLISION BELT, SOUTH KOREA

Continent-continent collisional orogenies have been well studied in many areas. However, many important questions regarding the deeper mid-crustal rocks at the bases of these belts still remain. Sheets emplaced from mid-crustal depths contain copious amounts of information regarding the kinematic, mechanics, and deformation history of mountain belts, but they are rarely exposed at the surface. The Korean collision belt is an ancient, plunging fold-thrust belt that is eroded deeply enough to expose mid-crustal rocks at the surface. By sampling along a transport-parallel transect from northwest to southeast through the Korean collision belt, we address two basic questions about mid-crustal deformation in continent-continent collision zones. First, how do kinematic indicators vary through the fold-thrust belt? Second, how does the strain distribution vary from hinterland to foreland?

Using microstructural and mesoscopic structural analysis, as well as strain analyses (Fry and Rf-phi methods) the kinematic and deformation history along the transect was determined. Compositional analysis of samples taken along the transect yielded mineral assemblages implying that deformation conditions ranged from lower amphibolite to granulite facies. Using statistical analyses of outcrops and thin sections the shear sense was determined at various locations along the transect. Generally, the percentage of kinematic indicators consistent with hinterlandward shearing is greater closer to the hinterland (NW) and the percentage of kinematic indicators consistent with forelandward shearing is higher towards the foreland (SE). Despite this general trend, there are some locations that differ from this pattern; this is likely due to some local inhomogeneity. The Fry method was used to determine strain ellipses within the transport plane in predominantly quartzitic rocks. The less quartz-rich rocks were typically gneissic with feldspar clasts. The Rf-phi technique was used by fitting ellipses to the feldspar clasts at the micro- and meso-scopic scales. From these analyses, we observed smaller axial ratios towards the central portion of the transect with systematically larger strains towards the two bounding faults of the mid-crustal rock package.