長野県神城盆地の局地的な地形変化に対する完新世の花粉化石群集の応答

摘要

We examined the relationship between pollen assemblage changes and geomorphic development in Kamishiro Basin, Nagano Prefecture. The data for pollen analysis were collected on the walls of the Hakuba trench across Kamishiro fault excavated in 1996 by Geological Survey of Japan. The trench exposed four discrete faulting events in the past 7,000 years. Some of these events are correlated with simultaneous rising of the local base of erosion. In particular, the emergence of L 2 terrace coincides with Ev. 3. We examined geomorphological and paleohy-drological evidence associated with base-level change and earthquakes and tried to correlate the change with innovation of pollen assemblage. Five local pollen zones and two sub-zones were recognized from the result of the analysis. I : 105,000-9,000 cal yrs BP, II : 9,000-7,700 cal yrsBP, III : 6,000-4,300 cal yrs BP, IV (sub-zone IV-a and IV-b) : 4,300-1,700 cal yrs BP, V : 1,200 calyrsBP~. The pollen assemblage changes from zone III show the change of the moisture in Kamishiro Basin very well. The result of this analysis revealed the correlation that pollen assemblages such as Alnus and Osmunda have changed simultaneously with river bed morphology. These assemblages are presumed to respond to the moisture in the basin. Moreover, the pollen assemblages form the Riparian forest also increase in phase with the emergence, especially in zone IV (about 4,000-3,000 cal yrs BP). These allow us to presume that the moisture in Kamishiro Basin decreased. Then, we can conclude that the changes observed on the pollen assemblages reflect the coseismic subsidence of the hanging wall side of the Kamishiro fault. Considering that land-form changes were caused by faulting, the pollen assemblages make it possible to record past fault activity based on the alternations verified.%糸魚川一静岡構造線活断層系北部に位置する長野県神城断層において，1996年に行われたトレrnンチ調査時に採取された堆積物を用いて花粉分析を行い，断層活動に伴う地形変化に対する完新rn世の花粉化石群集の応答を検討した．花粉化石には，断層運動に伴う地形変化による環境変化をrn堆積物中に記録している可能性がある．そのため，断層の活動履歴および盆地内に分布する河成rn段丘の離水年代と花粉分析結果の対比を行うことにより，断層運動と花粉化石群集の対応を検討rnした．rn神城断層は，過去7,000年間に4回のイベントをくり返しており，イベントと一部の河床高度rn低下が同時期に起こったことが明らかになった．花粉ダイアグラムは，河床高度の低下と同期的rnに，生育が水分状態に支配されるような分類群，および盆地の上流域に分布すると考えられる分rn類群の変動を示している．rnしたがって，神城断層の白馬トレンチに認められる花粉分類群の変動は，地形変化を反映してrnいると考えられる．このことば，断層運動が地形変化を伴う場合，花粉組成の変動から過去の断rn層運動を推定できる可能性を示している．