東北地方中部地域 (酒田-古川間) の新生代地質構造発達史 (第II部)

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Stratigraphy and geologic structures of the Late Cenozoic System in the area from Sakata to Furukawa which is across the central part of Northeast Honshu Arc are investigated to elucidate the tectonic evolution of convergent type island arc. Major geologic structures of this area are characterized by two broad anticrinoria : the Dewa Hills in the west and the Ou Backbone Ranges in the east. These anticrinoria are bounded to both sides by north-south trending reverse faults. Doming structures accompanied with cauldrons are remarkable in the Ou Backbone Ranges. Late Cenozoic tectonic evolution in this area is summarized as follows : In the Early Miocene, pyroclastics deposited on land or under shallow marine conditions accompanying with fault controlled subsidence. The regional stress field estimated from dyke swarm is east-west trending tension. In the early Middle Miocene (15Ma), the paleoenvironment of the western half of this area was upper to middle bathyal. At that time a large amount of basalt erupted on the sea floor in western part of the area. The feeder dykes of the basalt lava show NE-SW trending σ_<Hmax>? This tectonic movement is estimated to have close relationship with back arc spreading of the Sea of Japan. During late Middle Miocene time (14-9Ma), western part of the area continued to subside and mud deposited with weakening volcanisms. In the Ou Backbone Ranges, subsidence stopped and the basin was filled with andesitic to acidic pyroclastics in shallow seas. In the Late Miocene time, subsidence of the western area still continued. On the contrary, the central part of the Ou Backbone Ranges began to uplift and some cauldrons were formed accompanied with doming structure. Veins formed in this time show NE-SW trending σ_<Hmax>-The state of stress is characterized by extension or weak compression (σ_<Hmax> =σ_1orσ_2, σ_<Hmin> =σ_3 andσ_v =σ_1 orσ_z). Horizontal tectonic stress did not play an important role in the vertical tectonic movement in 15-5Ma. Regional stress field, weakened volcanism and low sedimentation rate suggest that the gentle subsidence in the western region was, caused by thermal subsidence of the Japan Sea bottom. In the Pliocene land area of the Ou Backbone Ranges spread gradually and the subsidence of the western area stopped. Minor faults probably formed in this time show extension or weak compression, σ_<Hmax> trending NW-SE. In the Late Pliocene (Matuyama Epoch) the uplifting of the Dewa Hills and subsidence of the intramountain basin in the E- W trending horizontal compressive stress field (σ_<Hmax>=σ_1, σ_<Hmin>=σ_2 andσ_v=σ_3). In the Pleistocene the present area was suffered shortening deformation ; reverse faulting and folding. Major geologic structures have been formed during this period.
東北大学の論文
1986-12-29

東北地方中部地域 (酒田-古川間) の新生代地質構造発達史 (第II部)

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