40Ar/39Ar and K-Ar chronology of Pleistocene glaciations in Patagonia
40Ar/39Ar and K-Ar chronology of Pleistocene glaciations in Patagonia
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DOI：
10.1130/b25177.1
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发表时间：
2004-03
影响因子：
4.9
通讯作者：
B. Singer;R. P. Ackert;H. Guillou
中科院分区：
文献类型：
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作者：
B. Singer;R. P. Ackert;H. Guillou
在更新世期间，阿根廷布宜诺斯艾利斯湖以东，南纬46.5 °，至少有19个终碛被沉积，因为来自巴塔哥尼亚冰帽的皮埃蒙特冰川推进到邻近南安第斯山脉的半干旱高平原上。这些沉积物的特殊保存提供了一个难得的机会来记录过去120万年来冰盖的波动。4 0 Ar/ 3 9 Ar增量加热和未加标的K-Ar实验的玄武岩熔岩流与冰碛物互层提供了一个年代学框架的整个冰川序列。布宜诺斯艾利斯湖科迪勒拉以东90 km处和安第斯山脉沿着Rio Gallegos以南120 km处的3个熔岩的4 0 Ar/ 3 9 Ar等时线年龄强烈表明，冰帽最大东延时间约为2000年。一千一百万年在256 k. y内至少沉积了6个冰碛。1016 ′ 10 ka和760 ′ 14 ka玄武岩喷发。同样，六个年轻的，更近端的冰碛层沉积在1651千年。这一时期被下面的760 ′ 14 ka玄武岩和109 ′ 3 ka Cerro Volcan玄武岩流包围，这些玄武岩流掩埋了所有六个冰碛。再加上在原地宇宙成因的表面暴露年龄的冰碛石巨石，109万年的年龄塞罗火山意味着冰碛石沉积在倒数第二个当地冰川对应于海洋氧同位素阶段6。再向西到布宜诺斯艾利斯湖，还有六个比火山玄武岩流更年轻的冰碛。这些冰碛上的巨砾的表面暴露年龄，结合上覆的湖相沉积物的14 C年龄，表明沉积在末次盛冰期（LGM，23-16 ka）。虽然南极的尘埃记录表明，重要的巴塔哥尼亚冰川在60-40 ka，冰碛对应于海洋氧同位素阶段4没有保存在布宜诺斯艾利斯湖;显然，这些被超越，并在23 ka的年轻冰前进抹杀。尽管如此，巴塔哥尼亚冰川作用的总体模式是在过去100万年的连续冰川推进过程中，冰向东的范围逐渐缩小。我们假设，巴塔哥尼亚安第斯山脉的构造驱动隆起始于上新世，但一直持续到第四纪，部分原因是过去200万年智利海隆扩张中心的俯冲，最大化了1.1Ma的积冰面积和冰度。随后的深冰川侵蚀减少了堆积面积，随着时间的推移，冰川面积减少。
During the Pleistocene, east of Lago Buenos Aires, Argentina, at 46.5 °S, at least 19 terminal moraines were deposited as piedmont glaciers from the Patagonian ice cap advanced onto the semi-arid high plains adjacent to the southern Andes. Exceptional preservation of these deposits offers a rare opportunity to document ice-cap fluctuations during the last 1.2 m.y. 4 0 Ar/ 3 9 Ar incremental-heating and unspiked K-Ar experiments on four basaltic lava flows interbedded with the moraines provide a chronologic framework for the entire glacial sequence. The 4 0 Ar/ 3 9 Ar isochron ages of three lavas that overlie till 90 km east of the Cordillera at Lago Buenos Aires, and another 120 km from the Andes along Rio Gallegos at 51.8 °S that underlies till, strongly suggest that the ice cap reached its greatest eastward extent ca. 1100 ka. At least six moraines were deposited within the 256 k.y. period bracketed by basaltic eruptions at 1016 ′ 10 ka and 760 ′ 14 ka. Similarly, six younger, more proximal moraines were deposited during an ∼651 k.y. period bracketed by an underlying 760 ′ 14 ka basalt and the 109 ′ 3 ka Cerro Volcan basalt flow that buried all six moraines. Coupled with in situ cosmogenic surface exposure ages of moraine boulders, the 109 ka age of Cerro Volcan implies that moraines deposited during the penultimate local glaciation correspond to marine oxygen isotope stage 6. Further westward toward Lago Buenos Aires, six additional moraines younger than the Cerro Volcan basalt flow occur. Surface exposure dating of boulders on these moraines, combined with the 1 4 C age of overlying varved lacustrine sediment, indicates deposition during the Last Glacial Maximum (LGM, 23-16 ka). Although Antarctic dust records signal an important Patagonian glaciation at 60-40 ka, moraines corresponding to marine oxygen isotope stage 4 are not preserved at Lago Buenos Aires; apparently, these were overrun and obliterated by the younger ice advance at 23 ka. Notwithstanding, the overall pattern of glaciation in Patagonia is one of diminishing eastward extent of ice during successive glacial advances over the past 1 m.y. We hypothesize that tectonically driven uplift of the Patagonian Andes, which began in the Pliocene, yet continued into the Quaternary, in part due to subduction of the Chile rise spreading center during the past 2 m.y., maximized the ice accumulation area and ice extent by 1.1 Ma. Subsequent deep glacial erosion has reduced the accumulation area, resulting in less extensive glaciers over time.