Heilogjiang Complex – Fragment of the Jurassic Accretionary Wedge in the Tectonic Windows of the Overlying Plate: Flat-Slab Subduction Model

Abstract

The Circum-Pacific Late Albian-Cenomanian orogenic belts (including the Sikhote-Alin-Western-Sakhalin belt) were formed as a result of the deformation of mainly epioceanic terranes – fragments of Jurassic-Early Cretaceous accretionary wedges with ophiolites and other fragments of oceanic crust, turbidite basins, and island-arc systems. To the west of the Sikhote-Alin - Northern-Sakhalin belt and orthogonally, there are structures of earlier consolidation, which include the Bureya-Jiamusi-Khanka fragment of the orogenic belt of the Late Cambrian-Early Ordovician consolidation of the Late Proterozoic - Cambrian complexes. Within this belt, four isolated outcrops of the Heilongjiang complex are mapped, combining metamorphites of the epidote-amphibolite and glaucophane-schist facies and representing a fragment of an accretionary wedge of the Jurassic age. It was assumed that these outcrops marked the suture, i.e., represented the remains of the closed Mudanjiang paleoocean, separating the original Jiamusi terrane (and the Bureya-Jiamusi-Khanka belt) located to the west of Central Asia structures. The proposed article provides data that the Heilongjiang complex does not mark a suture, but is an underground near-horizontal continuation of the marginal continental accretionary wedge of the Nadanhad - Bikin terrane (flat subduction model) brought to the surface at the site of the antiform bending. The unity of the compared parts of the accretionary wedge is emphasized by the close matrix age, the similarity of detritus zircon populations, and the similarity of the composition and age of allochthonous inclusions (limestone, chert, Late Paleozoic and Early Mesozoic basalt). An important common feature is that Late Paleozoic and Early Mesozoic basalts from allochthonous inclusions in both cases are represented by the N-MORB and OIB types, with no traces of suprasubduction volcanism in the matrix. The Heilongjiang complex forms, according to this interpretation, a tectonic window among the more ancient formations of the Jiamusi terrane. There is no need to assume the existence of the Mudanjiang Ocean to explain the formation of the Heilongjiang complex. The features of the structure of this complex and the conditions of its occurrence can be explained by the processes of flat subduction of the Pacific slab in the Jurassic and its deformation in the Early Cretaceous.