智利阿塔加馬盆地區域之活動構造特性

Abstract

中安地斯山脈位於納茲卡板塊與南美板塊的碰撞造山帶，是世界上最活躍的造山帶之一。納茲卡板塊以每年約7-8公分的速率隱沒至南美板塊之下，其中大部分的聚合速率被消耗在海溝及碰撞造山帶前緣。然而由於缺乏定年資訊，位於中安地斯山脈西側的弧前區域，其構造活動速率仍是未知的。為此，我們利用構造地形學的概念分析位於弧前區域的阿塔加馬盆地的構造活動特性，而該處有大量的火山沉積物可提供年代資訊。 我們利用衛星影像描繪當地的構造線，發現此區域的構造線主要為南北走向，且主要集中在盆地的北部邊緣及南部邊緣。由於當地侵蝕速率相當低，因此可利用地表的高程起伏來代表岩層位態的變化，並利用具剪切滑移之斷層彎曲褶皺模型(pure-shear fault-bend fold model)模擬並計算縮短量。另外搭配年代資訊可獲得此區域構造之縮短速率約0.2 mm/yr，顯示板塊擠壓的應力未在此累積太多而可能轉移至其他地方。 此外，於盆地北部區域有一斷層系統，此斷層系統在聖帕德羅德阿塔加馬(San Pedro de Atacama)城鎮中分叉出兩分支斷層，並在城鎮裡形成斷層崖。另外，在城鎮北側河道旁發現一系列河階，顯示此斷層系統在近期仍有活動，且可能會對該城鎮造成危害。

The central Andes in South America is formed as the Nazca plate subducts northeastward beneath the South American plate along the Peru-Chile trench with a velocity of ~70-80 mm/yr. Most of the convergence is thought to be absorbed in the trench and in the eastern mountain front. However, the convergence at present in the forearc region is still not well understood since the age constraint of deformation is limited. In order to know the partitioning of plate convergence in the forearc region, we analyzed the active tectonic characteristics of the Atacama Basin, since there are volcanic sediments covering the surface and they can provide age constraints. With the help of various satellite images, we identified many N-S trending compressional structures around the Atacama Basin. The active structures are found mainly in the northern and southern part of the basin. Because the erosion rate is extremely low, the relief of land surface there can be used as a deformation marker, and we can obtain the shortening amounts by pure-shear fold-bend fault model. With the age constraints, we obtained the shortening rate at about 0.2 mm/yr, much lower than that in the other parts of the central Andes. This indicates the forearc region may act like a rigid block at least at this latitude. Furthermore, a series of fault system was found at the northern edge of the basin. The fault system has two branches in the main settlement, San Pedro de Atacama, and produces tectonic scarps inside the town. In the hanging-wall of the fault, uplifted river terraces are present. This suggests the fault system is still active and may pose seismic hazard for this area.