Craton Destruction 2: Evolution of Cratonic Lithosphere After a Rapid Keel Delamination Event

摘要

Cratonic lithosphere beneath the eastern North China Craton has undergone extensive destruction since early Jurassic times (approximately 190Ma). This is recorded in its episodic tectonic and magmatic history. In this time, its lithosphere changed thickness from approximately 200km to 60km. This change was associated with a peak time (approximately 120Ma) of lithospheric thinning and magmatism that was linked with high surface heat flow recorded in rift basins. We believe that these records are best explained by a two-stage evolutionary process. First, approximately 100km of cratonic keel underlying a weak midlithospheric discontinuity layer (approximately 80-100km) was rapidly removed in 10-20Ma. This keel delamination stage was followed by a protracted (approximately 50-100Ma) period of convective erosion and/or lithospheric extension that thinned the remaining lithosphere and continuously reworked the former cratonic lithospheric mantle. This study focuses on numerical exploration of the well-recorded second stage of the eastern North China Craton's lithospheric evolution. We find that (1) lithospheric mantle capped by thick crust can be locally replaced by deeper mantle material in 100Ma due to small-scale convective erosion; (2) asthenospheric upwelling and related extension can replace lithospheric mantle over horizontal length scales of 50-150km, and account for observed mushroom-shaped low-velocity structures; (3) modeling shows conditions that could lead to the multiple eastern North China Craton magmatic pulses between 190 and 115Ma that are associated with temporal and spatial changes in magma source petrology and a magmatic hiatus; and (4) a wet midlithospheric discontinuity layer provides a potential source material for on-craton magmatism.