Numerical modeling of metamorphic core complex formation: Implications for the destruction of the North China Craton

Abstract

Widespread magmatism, metamorphic core complexes (MCCs), and significant lithospheric thinning occurred during the Mesozoic in the North China Craton (NCC). It has been suggested that the coeval exhumation of MCCs with uniform NW-SE shear senses and magmatism probably result from the decratonization event during the paleo-Pacific Plate retreat. Here we use 2-D finite element thermo-mechanical numerical models to investigate critical parameters controlling the formation of MCCs under far-field extensional stress. We observe three endmember deformation modes: MCC mode, symmetric-dome mode, and pure-shear mode. The MCC mode requires Moho temperature ≥ 700 °C and extensional strain rate ≥ 5×10 -16 s -1 , implying that the lithosphere had already been thinned when the MCC was formed in the Mesozoic. Combining with the widespread MCCs with the same NW-SE extension direction in the NCC, we suggest that MCCs are surface expressions of both large-scale extension and craton destruction and that rollback of the paleo-Pacific slab might be the common driving force.

The role of gravitational body forces in the development of metamorphic core complexes

“…Through time, with further extension, topographic collapse, and erosion of the paleo-highlands, the model shows the formation of syntectonic sedimentary basins that cause an increase in concentration of strain rate into the high-angle conjugate shear zones. With further necking of the upper crust, the strain is more concentrated on the left limb of the conjugate shear zones, and the weak middle–lower crust slowly advects upward resulting in formation of symmetric domes, consistent with findings of Ma et al 13 (Fig. 8b, e ).…”

“…Furthermore, it is unknown how this setting of a thickened crust prior to extension can explain the evolution history of the heavily mylonitized detachment fault surface, which is typically exposed at low angles in the field. While some previous modeling studies showed that MCCs can form without a thickened crust 13 , 28 , in this paper we show that the formation of a MCC is attributed to post-orogenic collapse of a thickened crust by investigating the kinematics, timing, and mechanism responsible for formation of the MCCs found in the Cordillera of NA 41 (e.g., the Ruby 53 and Snake Range 54 MCCs). While in the central-southern section of the Cordillera of NA the Pacific-North America motion generated a trans-tensional setting and lithospheric extension following collision of the EPR with the trench at ~30 Ma 41 , in the northern section of the Cordillera of NA, the lithospheric extension and formation of MCCs coincided with the timing of an active convergent plate boundary zone (subduction) 41 (Fig.…”

“…While the Anderson fault theory 15 suggests that normal faults should form at high angles (>45°) under extension, the origin and mechanics of the low-angle detachment zone that forms a MCC, whether it originates with a high or low dip angle, have been highly debated 6 , 10 , 15 – 20 . Previous numerical simulations of MCC formation and evolution 13 , 21 – 30 have tried to explore the mechanisms and conditions of MCC formation. These models show that factors that can produce a low-angle detachment fault through an extensional boundary condition include (1) a viscosity or density contrast between the brittle upper crust and ductile middle–lower crust 26 , (2) localization of strain at a single pre-defined weak fault zone 1 , 10 , 11 , 23 , 26 , 31 , 32 or a single fault that forms spontaneously as the result of shear localization and strain-induced weakening 13 , 21 , 25 , 26 , 33 , 34 , (3) partial melting associated with an increase in temperature 23 , 24 , 26 , 28 , or (4) a thickened crust 26 , 31 .…”