Abstract
The Early to Middle Miocene Emi Group is well exposed along the coast line in the Emi area, southern part of the Boso Peninsula. It is composed mainly of tuffaceous siltstone with tuffaceous and volcaniclastic sandstones. The strata are highly faulted, folded and disrupted. The minor structures were analyzed by precise mapping and careful observation of various styles of deformations under semi-lithified conditions.
First, dish structures and other dewatering structures including web structures were formed. Next, bedding parallel or slighly oblique thrust faults occurred to make a possible duplex structures. Abundant thrust faults cut the previous structures to bring all the strata horizontally shortened. Assuming the thrusting occurred when the bedding was still nearly horizontal, NWSE horizontal compression was inferred. Strike slip faults of NE trend at last occurred.
Such series of deformation indicate that the Izu forearc tuffaceous sediments were accreted along the proto-Sagami trough during Early to Middle Miocene subduction of the Philippine Sea plate to Honshu.
