Craton Destruction 1: Cratonic Keel Delamination Along a Weak Midlithospheric Discontinuity Layer

Abstract

Cratons are generally observed to retain thick (>180 km) conductive keels for billions of years. However, some cratons have undergone keel removal, with well‐documented examples being the eastern North China Craton (NCC) and the Wyoming Craton (WC). These keelless subregions appear to have kept a lithospheric bottom at ~80–100‐km depths. This is also the depth range where modern cratons, including the remaining portions of the NCC and the WC, have seismically visible midlithospheric discontinuity layers (MLDLs). MLDLs are proposed to be regions of preferential accumulation of metasomatic minerals and/or anomalously wet (>1,000 ppm) peridotites, both of which would lead to a relatively weak rheology. We propose that the cratonic keels of the eastern NCC (ENCC) and the western WC (WWC) utilized this weak MLDL layer to delaminate from overlying lithosphere. We first explore this hypothesis with a lubrication‐theory‐based analytical model. This model suggests a close relationship between a cratonic keel's long‐term stability and the strength of the MLDL's edge. We further test this prediction with less idealized 2‐D numerical experiments which reveal that (a) dense lower keels beneath MLDL‐bearing cratons can persist for billions of years as long as the MLDL's edges abut relatively cold and strong lithosphere; (b) MLDL edge failure can induce rapid intramantle lower keel delamination; and (c) the predicted rates of keel delamination along a ~10‐km‐thick MLDL with a hydrous olivine or metasomatic mineral‐dominated rheology are consistent with observations for the removal speeds of the WWC and the ENCC.

Structural inheritance in the North Atlantic

“…Subsequently, extensive magmatism would inhibit thermal re-equilibration of the lithosphere and allow rejuvenation to continue for a long time. Liu et al (2018) and Wang et al (2018) propose models where a "Mid-Lithospheric Discontinuity" or the lower crust can act as a sub-horizontal weakness zone along which the lithosphere may delaminate.…”

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

“…Moresi et al, 2003). More details on the numerical approach are given in Andrés-Martínez et al (2015), Hasenclever (2010), Hasenclever et al (2011), the attached appendices, and those in Part 1 (Liu et al, 2018). Table 1 shows the parameters used in numerical experiments.…”

“…We use the method in Part 1 (Liu et al, 2018) to determine melting regions. Parameters of crustal melting, for example, the solidus at room temperature and pressure T m 0 , the solidus's dependence on degree of melting (∂T m /∂f) P , and the solidus's dependence on pressure (∂T m /∂P) f , are crudely estimated by numerically fitting experimental results for melting of amphibolite ( Figure A1a; López & Castro, 2001).…”

“…This appendix describes the additional algorithms used here for numerical experiments that were not presented in the appendix of Part 1 (Liu et al, 2018).…”

“…Because the lower keel is denser than its adjacent asthenosphere and materials adjacent to the edge of metasome are soft enough, the denser lower keel starts to delaminate along the metasome. (The conditions for and evolution of this delamination process are the focus of Part 1; Liu et al, 2018.) (b) Delamination stops where the metasome MLDL disappears.…”