Reconstructing Ocean‐Plate Stratigraphy (OPS) to Understand Accretionary Style and Mélange Fabric: Insights From the Bangong‐Nujiang Suture (Tibet, China)

Abstract

Accretionary wedges are tectonic settings overriding subduction zones and generally show broad structural diversity, which, in first order, is reflected in tectonic accretion or erosion (Clift & Vannucchi, 2004). Therein, the accreted materials can involve variable amounts of oceanic basement or trench-fill sediments, which occur as either coherent units or mélanges (Meneghini et al., 2009). Wedge diversity is further complicated by the fact that mélanges may result from either sedimentary, tectonic or diapiric mechanisms, or a combination thereof (Festa et al., 2012). The nature of the lower plate, including oceanic-basement topography and volume of sediments, exerts a fundamental control on the wedge diversity (Clift & Vannucchi, 2004).

Source and Migration of Fluids in a Meso-Tethyan Subduction Zone: Fluid Inclusion Study of Syn-Mélange Veins from the Mugagangri Accretionary Complex

“…These matrixes have a typical scaly cleavage and are pervasively overprinted by post-phyllitic cleavage. These features indicate that the broken formations reach the maximum lithification at ~5 km [64].…”

“…Moreover, the types of fluid inclusions were Based on the field geological observation and section measurement, the MG (Gaize) is subdivided into three deformation domains (Figure 3A-C): (1) Domains I and III consist mainly of thrust sheets with coherent bedding that have not undergone mélange formation. These domains exhibit numerous paleo-flow markers, such as inclined bedding and groove casts, indicating a north to south sediment source from the Qiangtang terrane [64]; (2) Domain II is predominantly mélange and underlies Domains I and III. It exhibits a block-in-matrix fabric, with the blocks often wrapped within the matrix in a lens-like type [32].…”

“…The mélanges can be classified into three types based on the lithology and genesis: (1) Broken formation, which constitutes a major portion of the mélanges, primarily composed of "native blocks" and consisting of sandstone, symbolizing tectonically disrupted trench fills [64]. (2) Tectonic mélanges, containing oceanic crust materials (exotic blocks) [65], which are mainly volcanic rocks, such as siliceous rock and basalt below the broken formation, only occurring in Domains II-1 and -2 (Figure 3A,B) [32].…”

“…The mélanges mentioned above, were initially developed in the subduction and can be used to restore the ocean-plate stratigraphy. ( 3) Mélanges (olistostrome), which have a sedimentary origin and are differentiated by poorly developed shear fabric [66], blocks with low aspect ratios, and local transition to conglomerate, and they cannot be restored for the olistostromes [64].…”

“…Although these limestones often have the potential for decarbonization, the reactions are not expected to occur until depths of 85~100 km [85]. Herein, the crack-seal veins with low temperature (151-178 • C) only developed in the sandstone blocks, which were dragged down by low-relief seamount to the updip limit of the seismogenic depth (>5 km) [64] (Figure 9B). These features suggest that CH 4 was not significantly expelled, because the thermal cracking had just begun.…”