Kulshan caldera: A Quaternary subglacial caldera in the North Cascades, Washington
Kulshan caldera: A Quaternary subglacial caldera in the North Cascades, Washington
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库尔尚火山口：华盛顿州北喀斯喀特的第四纪冰下火山口
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发表时间：
1996
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影响因子：
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通讯作者：
W. Hildreth
中科院分区：
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作者：
W. Hildreth
在大型火山碎屑喷发期间塌陷的火山口在喀斯喀特弧区异常罕见。在贝克山东北麓发现了更新世早期的 4.5 × 8 公里的库尔山破火山口，其中充满了流纹英安岩熔结岩，使得喀斯喀特地区仅发现了三个第四纪破火山口。北喀斯喀特基底岩石中的一条近乎垂直的环形断层包围了 30 km 2 的火山口内火燧岩（和混合塌陷角砾岩），厚度 > 1 km，但没有暴露底板。普吉特低地的塔普斯湖火山灰是相关的放射性尘埃。距离源头200公里，厚度达30厘米。远端火山灰降落和火山口内凝灰岩的特征表明，可能在冰下开始的喷发期间发生了大规模的岩浆作用。随后，科迪勒拉冰盖的几次前进消除了地形边缘，清除了火山口外的所有火山灰凝灰岩和近端沉降物的痕迹，并剥离了任何火山口前的喷出岩——只有一些切割火山口周围基底的硅质侵入物表明了这些岩石以前的存在。尽管火山口在结构上并未复苏，但一些早期的火山口内流纹英安岩熔岩侵入并直接沉积在火山灰岩或由喷发产物改造而成的火山口湖沉积物上。库尔山、罗克兰和火山口湖 (Mazama) 事件的沉降区域、浮石成分和喷发岩浆量 (>50 km 3 ) 相似，这三处第四纪破火山口形成喷发现已在喀斯喀特发现。
Calderas that collapse during large pyroclastic eruptions are anomalously rare in the Cascade arc. Recognition of the early Pleistocene 4.5 × 8 km Kulshan caldera, filled with rhyodacite ignimbrite at the northeast foot of Mount Baker, brings to only three the Quaternary calderas identified in the Cascades. A near-vertical ring fault cut in basement rocks of the North Cascades encloses 30 km 2 of intracaldera ignimbrite (and intermixed collapse breccia) >1 km thick but with no floor exposed. The Lake Tapps tephra in the Puget lowland is the correlative fallout; 200 km from the source, it is as thick as 30 cm. Features of the distal ash fall and the intracaldera tuff suggest large-scale phreatomagmatism during an eruption that may have started subglacially. Several advances of the Cordilleran ice sheet subsequently obliterated the topographic rim, removed every vestige of extracaldera ignimbrite and proximal fallout, and stripped any precaldera extrusive rocks-the former existence of which is suggested only by a few silicic intrusions that cut the circumcaldera basement. Although the caldera is not structurally resurgent, several early intracaldera rhyodacite lavas intrude and rest directly on ignimbrite or on ashy caldera-lake sediments reworked from the eruption products. Subsidence areas, pumice compositions, and volumes of magma erupted (>50 km 3 ) are similar for the Kulshan, Rockland, and Crater Lake (Mazama) events, the three Quaternary caldera-forming eruptions now recognized in the Cascades.