智利北部阿塔加馬盆地之一慢速滑移褶皺逆衝構造系統

Abstract

南美大陸的西岸是地球上主要的板塊交界帶之一，Nazca板塊在此以每年63~80公釐的速度向東北隱沒至南美板塊之下。但儘管板塊聚合的速度如此之快，現今在中部安地斯山脈西側的弧前區域，並未發現太多因板塊聚合而產生的擠壓變形，或是構造活動現象。因此我們運用構造地形學的觀念，在智利北部，安地斯山脈西側的Atacama Basin東南隅，選定一處由地下構造活動所造成的地表變形作為研究區域，以期瞭解此弧前區域變形的機制與其在板塊聚合中所扮演的角色。 由於Atacama Basin處在氣候極為乾燥的區域，其侵蝕與沉積速率皆相當緩慢，因此，我們假設研究區域中現今的地表起伏都是受到新期構造活動所造成的。我們利用SRTM (Shuttle Radar Topographic Mission)所製作出的數值地形模型圖 (Digital Elevation Models)套疊上Quickbird高解析度衛星影像後，可看出六條南北向發育的山脊，彼此約略平行，各自高數公尺到十數公尺。接著我們使用RTK (Real Time Kinematic)-GPS至現地進行三條剖面量測，獲得了精確的座標與量化之地形起伏樣貌。可知這些山脊的東翼較陡且短，而西翼的坡度則較緩，延伸較長。推測這些山脊皆發育在一個較主要的背斜之西翼上；而此主要的背斜構造則是一個受到下部向西傾斜之逆斷層活動所形成的，具剪切滑移之斷層彎曲褶皺(shear fault-bend fold)。 利用Suppe等人(2004)所提出的模型與方程式，假設此構造於活動過程中深度、剪切滑移角度、斷坡角度等都不隨時間改變，我們便可將RTK量測出的地表剖面向下推衍，繪製出此構造於深部的發育形貌。同時我們也自受到褶皺拱彎的地層中採集不同的岩石樣本，例如：中酸性凝灰岩(ignimbrite)、湖相沉積岩(lake deposits)。藉由氬-氬定年與鈾-釷定年的結果輔助，我們知道表層的凝灰岩年代約在中更新世mid-Pliocene (~3Ma)，而局部覆蓋的湖相沉積岩則沉積於~440ka。如果上述觀察到的構造在此湖相沉積物覆蓋、成岩之時就已開始活動，那麼我們可將地表變形量歸因於此構造活動所致，便能藉此進一步推算出此構造系統長期的活動速率約為~ 0.1 mm/yr。 如此慢速的活動速率與其他同樣位於中部安地斯山之弧前區域的研究結果大略一致，顯示Nazca板塊向東推擠的應力似乎未累積在南美大陸上部地殼的弧前區域，而可能移轉到了其他地方。因此，不同於北部、南部安地斯山，在此區域可能仍有其他因素扮演了控制應變機制的重要角色。

Atacama Basin (~23°S, ~68°W) lies on the western edge of the Central Andes. It is a unique compressional depression in the forearc region of the South American plate boundary, which was formed by the oblique convergence between the Nazca and the South American plates. The Nazca plate is subducting northeastward beneath the South American plate at a rate of about 68~80 mm/yr based on different methods. However, this forearc region does not seem to absorb a lot of deformation at present. In order to understand the characteristics and mechanisms of active forearc deformation related to the plate convergence, we chose to investigate the SE margin of the Atacama Basin, where active structures have been described previously. Since the hyper-aridity of the Atacama Basin results in extremely low erosion and sedimentation rates, we believe the present relief of land surface there is mostly produced by neotectonic activity, and can be used as a deformation marker. Combining various remote-sensing data sets, such as an SRTM-DEM, Google Earth platform, and higher resolution QuickBird satellite images, several N-S trending ridges are mapped in this area. For further investigating tectono-geomorphic features, we performed detailed geomorphic surveys by real-time kinematic (RTK) GPS in the field to obtain high resolution topographic profiles across these features. These ridges are generally several tens of meters high, with their height decreasing northward. These ridges are interpreted to minor duplex fault-bend folds which grew on the backlimb of one larger asymmetric anticline. This major fold with steep forelimb facing east is likely formed by a shear fault-bend fold, and may be associated with an underlying west-dipping thrust fault. According to the principle proposed by Suppe (2004), the model without changes in depth, ramp angle, shear angle, and backlimb angle is selected to reconstruct the geometry of the structure beneath. We also suggest that this fault would merge at depth with the major active thrust system as a branch. We also performed 40Ar/39Ar and U-Th dating of deformed strata. The surface ignimbrites mostly deposited in late Pliocene (3.0~3.2 Ma), and part of them are covered by a thin layer of lake deposits during ~440ka. If the structures have been active since the deposition of the lake deposits, the total deformation would yield a very low long-term slip rate of the faults, in an order of 10-1 mm/yr. This result is similar to other researches in the forearc region of Central Andes, but is distinctly different from the Northern and Southern Andes. This very low slip rate of active structures may thus play important roles in the evolution of the forearc deformation belt, as well as the landscape development in this area.