ANALYSIS OF FINITE STRAIN DISTRIBUTION WITHIN BASEMENT ROCKS OF THE KOREAN COLLISION BELT, SOUTH KOREA: IMPLICATIONS FOR EMPLACEMENT MECHANICS AND TECTONIC SHORTENING

The Korean Collision Belt formed during the Permo-Triassic collision of the North and South China blocks. This orogen exposes a ~100 km wide belt of Precambrian mid-crustal basement rocks, that largely consist of a metasedimentary cover group and a basal gneiss group that is migmatized in some areas. The basement rocks contain steeply dipping thrust faults that define at least 10 major thrust sheets. There are two end member models for how these rocks were brought up from mid-crustal depths: ductile thrusting or ductile mid-crustal flow. One way to distinguish between these two models is to determine the strain distributions in the transport direction within the rocks. Within a ductile thrust sheet, the largest finite strains and the lowest angles between the strain ellipse long axis and the fault should be closest to the fault. Moving structurally up through the thrust sheet the strain should systematically decrease, and the strain ellipse long axes should progressively rotate towards being perpendicular to the fault. Ductile flow should have the largest finite strains and the lowest angles between the strain ellipse long axes and faults at both the lower and upper bounding faults. The strain should decrease from both faults towards the middle of the flow zone where it should be smallest, and the ellipse long axis should change from low angles to the bounding faults to progressively higher angles towards the middle. To test these models, we conducted 2D (in the XZ plane) and 3D finite strain analysis along two hinterland to foreland transects, corresponding to two different depths, through the orogen. The Fry method was used in samples with quartz matrix and the R_f-phi technique was used on feldspar clasts to determine finite strain. Preliminary results from within a typical sheet show that strain is smallest (1.44) farthest away from the fault and increases towards the fault (2.44). The angle between the long axis of the strain ellipse and the fault is highest (86˚) farthest away from the fault and decreases towards it (4˚). This suggests that ductile thrusting may be the dominant process for emplacement of the mid-crustal basement rocks. The strain gives a minimum estimate for penetrative shortening which, combined with a balanced cross section, gives a total shortening estimate of over 100 km for the Korean Collision Belt.