Figure 3
From: The initial break-up of Pangæa elicited by Late Palæozoic deglaciation
Schematic diagrams illustrating crustal evolution and the development of rifting under passive extensional tectonic setting.
N-S trending cross sections showing the relative thickness of the brittle crust (grey), ductile crust (orange) and mantle (brown). The thickness of ice sheet (pale blue) is assumed to be 3–4 km that induces ~1 km of lithospheric depression53. (a) Hypothetical crustal profile with ice cover during the Gzhelian stage (299 Ma–303 Ma). The region under ice cover is affected by the vertical stress (grey arrow) from the weight of ice sheet. The estimated brittle strength for the upper crust is (ơ1 − ơ3) equal to 30 + 18.6 MPa km−1 and would induce crustal depression (black arrow) and ductile flow (white arrow) of the mantle away from the center of the ice sheet50. (b) Northward advancement of ice sheet around Asselian to Sakmarian stage (290 Ma–299 Ma) further depressed the crust and increased mantle flow. Brittle deformation is suppressed as the increase of vertical stress moved the stress circle (brown - original state; black – finite state) further away from the Mohr failure envelop (red line). (c) Early stage of glacial retreat around the Artinskian stage (280 Ma–290 Ma). The deglaciated region experiences isostatic rebound (pale blue arrow) with estimated brittle strength for the upper crust to be (ơ1 − ơ3) equal to 18.6 MPa km−1 50. The melted glacier water penetrates the upper crust and increases the fluid pressure (blue arrow) and reduces the brittle strength significantly (ơ1 − ơ3 = 5.58 MPa km−1)55. The combination of isostatic rebound (brown to dashed black circle) and increased fluid pressure (dashed black to black circle) moves the stress circle towards the Mohr failure envelop and triggers brittle failure and fault swarm development. (d) The stress heterogeneity between deglaciated and ice covered regions further weaken the crust. Mantle backflow and crustal rebound induced decompressional melting. Mantle-derived melts upwelled along pre-developed fault zones forming regional flood basalts around the later Artinskian stage (280 Ma–290 Ma). (e) Further extension of the crust and ascending of asthenosphere (green) around Rodian stage (269 Ma–272 Ma) completed the break off of Cimmerian terranes.
