Early Cretaceous Tectonics Across The North Pacific: New Insights From Multiphase Tectonic Extension In Eastern Eurasia

How deep mantle processes affected plate interactions and the dynamics of deformation on both sides of the Paleo-Pacific has been a first order scientific challenge. The ubiquitous Early Cretaceous multiphase tectonic extensional structures in eastern Eurasia (EE) show marked contrasts to the episodic compressional structures in western North America (WNA), which provides convincing arguments linking deep and shallow tectonic processes.Recent studies on Early Cretaceous tectonics in EE have shown that the continent is characterized by multiple phase of tectonic extension and weak compression, forming extensional structures in several major provinces in a vast area of ca. 3000 km x 3000 km. The peak tectonic extension occurred at 135-120 Ma, in addition to extensional episodes at pre- (160-145 Ma) and post-peak (120 Ma-) stages. Kinematic analysis reveals an identical NW-SE-oriented tectonic extension field for their formation. In addition, synextensional magmatism sourced from ancient and juvenile crust or lithospheric mantle was episodically active and peaked at ca. 160, 125 and 100-80 Ma, respectively. Particularly, a magmatic flare-up of extensional origin occurred at 125 x 5 Ma in eastern China. In contrast, the WNA Cordillera displays a prolonged and episodic tectonic compression beginning -170 Ma (Jurassic). Tectonic deformation involved the Nevadan, Sevier and Laramide orogenies from ca. 175 Ma, 125 Ma and 80 Ma, respectively, to form the Cordillera orogenic system. During the Nevadan orogeny intensive plate convergence from 154 to 150 Ma contributed to continental arc magmatism. The Sevier orogen is characterized by thin-skinned thrust sheets while the Laramide is dominated by shallow slab dip oceanic subduction and basement-core uplifts of Archean crust in the foreland. Significant sinistral strike-slip shearing at -140-125 Ma is documented in the Early Cretaceous. Widespread crustal shortening and emplacement of major batholiths (magmatic flare-up) were contemporaneous with accretion of the high pressure-low temperature (blueschist) Franciscan Complex at ca. 125-100 Ma during the Sevier orogeny.The EE extensional provinces constitute part of the retreating (Paleo-) Pacific-Eurasia subduction system, while crustal shortening along the WNA was resulted from the advancing Farallon-North America subduction system. Stratified mantle convection is needed, however, when taking the tectonic evolution of the continental margins and the mid-ocean range as an integral system of the evolving globe. Shallow-mantle convection contributed to subduction of oceanic plates, which resulted in Andean-type subduction zones on both sides of the Ocean at the early stage of subduction. Possibly from 160 Ma on, eastward deep mantle flow occurred, which induced migration of the shallow mantle convection systems. As a result, the west-dipping Paleo-Pacific slabs became steepened, stagnated and subsequently folded in the mantle transition zone, while the shallow part of the east-dipping Farallon slab became flattened, and its deep part subsequently penetrated the transition zone. The resultant eastward migration of shallow mantle convection systems continuously drove the retreating subduction of the Paleo-Pacific (or Izanagi) plate, advancing subduction of the Farallon plate and eastward migration of the Paleo-Pacific-Farallon mid-ocean ridge.As a consequence, multiple phases of tectonic extension dominated the deformation of the continental interior in EE and episodic compressional tectono-magmatic activities occurred along the continental margin of WNA.