The continent-ocean (Seve-Köli) boundary in the Sarek-Padjelanta Mts. revisited: Swedish Caledonides

In the Swedish Caledonides, the continent-ocean boundary is represented by the basal thrust of the Iapetus-derived Köli Nappe Complex (KNC) which overlies the Seve Nappe Complex (SNC) derived from the outer rifted margin of Baltica and the continent-ocean transition (COT). Current maps and literature subdivide the Seve teconostratigraphy in northern Sweden (Sarek-Padjelanta region) into three mega-lenses, the lowest of which (Vaimok lens) is composed of quartz-feldspathic gneisses and subordinate marble, all hosting eclogites. It is overlain by the Sarek lens composed of a COT dyke complex and, along its western margin, a narrow belt of phyllites, schists, quartzite, marbles and greenschists. Overlying this belt is Tsäkkok lens of the same composition as the Vaimok lens. The situation with an intermediate-pressure and often excellently preserved sheeted-dyke complex intercalated between two lenses of complexly deformed high-pressure rocks invited to intricate models of accretion, including out-of-sequence stacking of slices of the continental margin during early Ordovician subduction. Based on a detailed study of a select area and examination of the evolution of previous concepts, this study reinterprets the narrow belt along the western margin of the Sarek Lens as typical of the lower KNC. The favoured explanation of the presence of intermediate-pressure Köli rocks below high-pressure Seve rocks is recumbent folding and/or mega-sheath folding of the Seve-Köli nappe sequence during Scandian (Siluro-Devonian) accretion, by analogy with the situation in places in the central Swedish Caledonides. Previous models of early out-of-sequence stacking during subduction of the continental margin need reconsideration.

在瑞典加里东造山带（Swedish Caledonides）中，大陆-大洋边界以源自伊阿珀托斯洋的科利推覆体（Köli Nappe Complex, KNC）的基底逆冲断层为代表，该推覆体覆于源自波罗的古大陆外缘裂谷边缘及大陆-大洋转换带（continent-ocean transition, COT）的塞夫推覆体（Seve Nappe Complex, SNC）之上。当前的地质图件与文献将瑞典北部萨雷克-帕耶兰塔地区的塞夫构造地层单元划分为3个巨型岩席，其中最低位的瓦伊莫克岩席（Vaimok lens）由长英质片麻岩及少量大理岩组成，所有岩性均产有榴辉岩。该岩席之上覆有由大陆-大洋转换带脉岩组合构成的萨雷克岩席，且沿其西缘分布有一条由千枚岩、片岩、石英岩、大理岩及绿片岩组成的狭窄岩带。该岩带之上覆有与瓦伊莫克岩席岩性组成一致的恰科克岩席（Tsäkkok lens）。这套保存极佳的中压脉岩组合夹于两套变形复杂的高压岩席之间，催生了诸多复杂的增生演化模型，其中包括早奥陶世俯冲过程中大陆边缘岩片的逆序堆叠模式。本研究通过对选定研究区的详细解剖及对既有学术认知演化历程的梳理，将萨雷克岩席西缘的狭窄岩带重新解释为下伏科利推覆体的典型特征。结合瑞典中部加里东造山带的已知实例类比，针对高压塞夫岩系之下产出中压科利岩系这一现象，当前最受支持的解释为：在斯堪的亚期（志留纪-泥盆纪）增生过程中，塞夫-科利推覆体序列发生了平卧褶皱及/或巨型鞘褶皱。此前提出的大陆边缘俯冲早期发生逆序堆叠的相关模型亟需重新审视。