Fault-Propagation Fold vs. Fault-Bend Fold: Two Different Modes of Deformation in Accretionary Prisms

Abstract:

Well exposed on land accretionary prisms of Neogene age in the Miura and Boso Peninsulas in central Japan offer excellent examples for consideration of the significance and implication of two different modes of layer-parallel shortening: fault-propagation folds and fault-bend folds. Some folds in the beds in the Misaki and Shiramazu Formations of middle Miocene to early Pliocene and early Pleistocene age respectively explain long distant layer-parallel slip made of many small scale (tens of cm order) duplex structures of antiformal stack or hinterland-dipping type, or both. The duplexed layer parts are unconformably overlain by liquefied or fluidized turbidite layers on top, suggesting very shallow burial depths during deformation or duplication as slide or slump deposits. Some blocks of hinterland-dipping type duplexes are involved in breccia beds that are formed by liquefaction or mud diapiric intrusion. In addition, in the Misaki Formations, large scale duplex structures of hinterland-dipping type with bi-divergent thrust system on orders of hundreds of meters are mapped in stratified layers. Those duplex systems of various scales are a type of fault-bend fold that is characterized by layer parallel slip within a single, specific bed, that then propagated upward to slip again within the other specific bed. Many of the examples of fault-bend folds formed at shallow burial depths, probably just after the deposition. Another type of layer parallel shortening in the Misaki and Shiramazu Formations is fault-propagation folds that are characterized by propagation of thrust faults by forming synclines and anticlines, resulting in an upward concave thrust system that splays out from the basal slip to develop. These two different types of folds are developed within layers in one side, and in another side cutting through the layers, and could be applied to the submarine Nankai prism of the same ages.