Evidence for a major Neoproterozoic orogenic unconformity within the Dalradian Supergroup of NW Ireland
Evidence for a major Neoproterozoic orogenic unconformity within the Dalradian Supergroup of NW Ireland
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爱尔兰西北部达尔拉德超群内新元古代造山带不整合面的证据
DOI：
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发表时间：
2004
影响因子：
2.7
通讯作者：
G. Alsop
中科院分区：
文献类型：
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作者：
D. Hutton;G. Alsop
最近，在不列颠群岛的加里东地区，关于新元古代达尔拉德超群的沉积和变形年龄，出现了一种有趣而矛盾的情况。同位素证据，虽然有限，表明该序列的下部可能已经沉积，并经历了前寒武纪变形（即前600马）之前，沉积的上部序列（即后600马）。鉴于这些现有的限制，很明显，在插入序列中需要一个（或多个）主要的断裂，以保持连贯的构造地层。我们目前的证据，这样的不整合面保存在基地的伊斯代尔亚群在西北爱尔兰。在沿着这一区域公认的边界的砾岩中发现了含有既存构造变质史的改造碎屑。下伏层序还显示出预先存在的变形组构，显示砾岩底部的侵蚀截短。这些关系，再加上显著的侵蚀和切除的下盘序列，广泛的厚度和相变化的上盘单元，意味着一个主要的构造不整合存在于这个序列。由于砾岩在地层上位于前寒武纪（c。600 Ma）熔岩，碎屑中所含的结构组构和下伏层序也必须是前寒武纪时代的，与在层序较年轻部分观察到的已确立的早古生代（加里东期）造山变形完全无关。根据定义，达尔拉德超群实际上可能包括（至少）两个不同的构造地层序列。
An intriguing and contradictory scenario has recently developed in the Caledonides of the British Isles concerning the ages of deposition and deformation of the Neoproterozoic Dalradian Supergroup. Isotopic evidence, although limited, suggests that the lower parts of the sequence may have been deposited and undergone Precambrian deformation (i.e. pre-600 Ma) prior to deposition of the upper parts of the sequence (i.e. post-600 Ma). Given these existing constraints, it is clear that a major break (or breaks) would be required in the intervening sequence to maintain a coherent tectonostratigraphy. We present evidence for such an unconformity preserved at the base of the Easdale Subgroup in NW Ireland. Reworked clasts that contain a pre-existing tectonometamophic history are identified within a conglomerate that lies along this regionally recognized boundary. The underlying sequence also exhibits pre-existing deformational fabrics that display erosional truncation at the base of the conglomerates. These relationships, together with significant erosion and excision of the footwall sequence, and extensive thickness and facies variations in the hanging-wall units, imply that a major tectonic unconformity exists within this succession. As the conglomerate lies stratigraphically below Precambrian (c. 600 Ma) lavas, the structural fabrics contained within the clasts and the underlying sequence must also be of Precambrian age and totally unrelated to the well-established Early Palaeozoic (Caledonian) orogenic deformation observed in the younger parts of the sequence. The Dalradian Supergroup, as defined, may actually comprise (at least) two distinct tectonostratigraphic sequences.