Proximal stratigraphy and syn-eruptive faulting in rhyolitic Grants Ridge Tuff, New Mexico, USA
Proximal stratigraphy and syn-eruptive faulting in rhyolitic Grants Ridge Tuff, New Mexico, USA
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DOI：
10.1016/s0377-0273(97)00075-9
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发表时间：
1998-04-01
影响因子：
2.9
通讯作者：
Valentine, GA
中科院分区：
文献类型：
--
作者：
Keating, GN;Valentine, GA
3.3 Ma格兰茨山脊凝灰岩是一个5公里(3)长的黄玉流纹岩序列的一部分，其近端沉积由底部火山碎屑流、涌流和散落沉积、厚的中央褐辉岩和上部的涌流和散落沉积组成。下伏的沉积和花岗岩基岩的大岩块(小于或等于2米)存在于较低的火山碎屑流和散落沉积物中，这表明喷发序列始于爆炸性的、挖掘管道的喷发。块状、未焊接的中央点火沸石显示了侵位后变形的证据。上部火山碎屑潮沉积以细火山灰为主，一些岩层含有增生的火山岩、软沉积物变形特征和泥质包裹的石灰岩，表明这些后期喷发具有爆炸性的热液成分。上部的瀑布和涌浪沉积被河流改造的火山碎屑沉积所覆盖，这些火山碎屑沉积以相对平坦的表面截断了主要部分。近端、上部火山碎屑涌流和普林尼瀑布沉积仅保存在深5-8m、宽5-8m的小地堑中，在那里它们下沉到褐沸石中，并受到保护，不会被改造。地堑内的火山碎屑涌流和崩塌沉积被许多小的正断层抵消。有些断层的位移量在剖面上向上减小，表明断裂作用可能是同喷发的。几个地块边界的断层向下延伸到褐沸石中，但最上面的、经过河流改造的盖夫拉层并没有被这些断层切割。断裂机制可能与60m厚的充填山谷的点火沸石沿古山谷轴线的沉降和压实有关。上部火山碎屑涌浪层的脆性和软性破裂与地堑断裂的褶皱接触，表明上部喷发序列侵位过程中发生了下沉。伴随着晚期热液岩浆爆发的地震活动可能对褐沸石的突然沉降和压实起到了作用。(C)1998年爱思唯尔科学公司。
Proximal deposits of the 3.3 Ma Grants Ridge Tuff, part of a 5-km(3) topaz rhyolite sequence, are composed of basal pyroclastic flow, surge, and fallout deposits, a thick central ignimbrite, and upper surge and fallout deposits. Large lithic blocks (less than or equal to 2 m) of underlying sedimentary and granitic bedrock that are present in lower pyroclastic flow and fallout deposits indicate that the eruptive sequence began with explosive, conduit-excavating eruptions. The massive, nonwelded central ignimbrite displays evidence for postemplacement deformation. The upper pyroclastic surge deposits are dominated by fine ash, some beds containing accretionary lapilli, soft-sediment deformation features, and mud-coated Lithic lapilli, indicating an explosive, hydromagmatic component to these later eruptions. The upper fall and surge deposits are overlain by fluvially reworked volcaniclastic deposits that truncate the primary section with a relatively planar surface. The proximal, upper pyroclastic surge and Plinian fall deposits are preserved only in small grabens (5-8 m deep and wide), where they subsided into the ignimbrite and were protected from reworking. The pyroclastic surge and fall deposits within the grabens are offset by numerous small normal faults. The offset on some faults decreases upward through the section, indicating that the faulting process may have been syn-eruptive. Several graben-bounding faults extend downward into the ignimbrite, but the uppermost, fluvially reworked tephra layers are not cut by these faults. The faulting mechanism may have been related to settling and compaction of the 60 m thick, valley-filling ignimbrite along the axis of the paleovalley. Draping surge contacts against the graben faults and brittle and soft-style disruption of the upper pyroclastic surge beds indicate that subsidence was ongoing during the emplacement of the upper eruptive sequence. Seismicity accompanying the late-stage hydromagmatic explosions may have contributed to the abrupt settling and compaction of the ignimbrite. (C) 1998 Elsevier Science B.V.