A link between deglaciation and rifting of Cimmeria

J. Gregory Shellnutt & Meng-Wan Yeh

Department of Earth Sciences, National Taiwan Normal University, 88 Tingzhou Road Section 4, Taipei 116, Taiwan; jgshelln@ntnu.edu.tw; marywyeh@gmail.com

This webpage is a summary of: Yeh, M.-Y. and J. G. Shellnutt (2016), The initial break-up of Pangæa elicited by Late Palæozoic deglaciation. Sci. Rep. 6:31442, doi:10.1038/srep31442

Rifting of Gondwanan terranes from the southern margin of the Palæotethys Ocean began during the Late Palæozoic and was accompanied by the eruption of continental flood basalts (Lapierre et al., 2004; Chauvet et al., 2009; Zhai et al., 2013; Shellnutt et al., 2014, 2015). The rifting occurred from the Arabian Peninsula in the west, across India, to Australia in the east and led to the creation of the Neotethys Ocean and the semi-contiguous ribbon continent known as Cimmeria (Sengör et al., 1988).

Continent break-up/rifting requires significant input of energy. Therefore, in order to minimize energy input, continental rifting sites are often spatially associated with pre-existing crustal structures such as suture zones of ancient orogens or regions of the crust with differential thickness (Vauchez et al., 1997; Courtillot et al., 1999; Buiter & Torsvik, 2014; Keppie, 2015). A given ‘structural heterogeneity’ may be exploited by either a mantle-plume (active extension) or a regional stress field (passive extension) associated with plate boundary forces that become a focal point for flood basalt volcanism (Buiter, 2014). Moreover, icosahedral structures with triple-junction configuration form quasi-hexagonal fractures that minimize the total boundary length and produce polyhedral plate configurations rather than lenticular shapes (Anderson, 2002; Sears, 2007).

Some of the Cimmerian rift-related basalts are thought to be related to a regional-scale mantle plume or a series of smaller mantle plumes but there are five main issues that make such a model untenable (Lapierre et al., 2004; Chauvet et al., 2009; Zhai et al., 2013). They are:

1. Plate separation did not occur along suture zones that would later be exploited by mantle plumes during the Mesozoic break-up of East Gondwana (Figure 1a),
2. a clearly defined aulacogen associated with the Cimmerian rifts has not been identified,
3. the Cimmerian rift boundary follows the stress tessellation instead of following the fracture tessellation that produces polyhedral plates (Figure 1f),
4. ultramafic volcanic rocks and a large radial dyke swarm appear to be absent, and
5. the mantle potential temperature estimates of the Early Permian basalts (T_P = 1360°C to 1460°C) are closer to ambient (T_P = 1300°C to 1400°C) mantle conditions than anomalously hot (T_P>1550°C) mantle conditions.

Figure 1: Early Permian paleo-geographic map reconstructions of Pangæa via open-source software GPlates 1.5 and the database provided within: http://www.gplates.org/index.html. Please refer to “method” section in Yeh & Shellnutt, (2016) for detailed settings and list of references. Further geological features such as pre-existing suture (Stampfli & Borel, 2002), orogenic belts, and glacial deposits are superimposed on the paleo-geographic locations. (a) Gondwana during the Gzhelian stage (299 – 303 Ma) showing the distribution of pre-existing Precambrian (thick green lines) and Cambrian (thick purple lines) suture zones, and orogenic belts (dark brown region). The pink region marks the distribution of Pan-African (ca. 500 Ma) magmatism (Lin et al., 2013). The continent rift margin that separated the Cimmerian terranes is marked by dark green dash line and does not follow pre-existing suture zones but cuts through Pan-African magmatic rocks. (b) Glacial cover before the Gzhelian stage (299 – 303 Ma) indicated by the distribution of glacial deposits on land (grey cross). (c) Maximum glacial extent through Asselian to Sakmarian stage (290 – 299 Ma) indicated by the distribution of glacial deposits on land (grey cross) and glacial marine diamictite (grey diamond). (d) The beginning of Neotethys rifting and breakoff of Cimmerian terranes around the Artinskian stage (280 - 290 Ma). The blue square (Oman), red circle (Kashmir), and orange cross (Tibet) mark the location of Early Permian basalts. The red region marks the continent rift margin. Both temporal and geographical correlation can be observed for the glacial retreat, continent rift margin, and the distribution of Early Permian basalts. (e) Continued spreading of the Neotethys Ocean and further rifting between Africa, India and Australia (red region) during the Rodian stage (269 - 272 Ma). (f) Fracture tessellation defined by major rift margins around Jurassic time (~200 Ma). Such configuration is similar to an icosahedron arrangement composed of numerous pentagons with hexagons as marked by the white dashed lines (Sears, 2007).

Regardless of the influence of a putative mantle plume, northward slab pull under E-W trending subduction zones along the northern margin of the Palæotethys is considered to be the driving mechanism that generated the tensional stress for the ‘Cimmerian rifts’ (Figure 1c). Yet without pre-existing ‘structural heterogeneities’, no generic relationship can be established to demonstrate the occurrence criteria for rifting sites but the linear extent of the rifts is thousands of kilometers (Stampfli et al., 1991; Gutierrez-Alonso et al., 2008). Therefore, it is very likely that ‘structural heterogeneities’ existed and developed prior to rifting.

The Late Palæozoic is characterized as a time of large continental and alpine glaciers that covered the Polar regions of southern Gondwana (López-Gamundi, 1997; Fielding et al., 2008). Glacial retreat occurred just prior to Cimmerian rifting and volcanism suggesting a possible link between them. Incorporating surface processes such as glaciation and weathering to internal lithosphere dynamics was established with the glacio-isostasy concept. Furthermore, seismo-tectonic activity and large boundary fault movements occur at the deglaciation front, and after glaciation due to glacial isostatic adjustment (Stewart et al., 2000; Steffen et al., 2014).

Numerical modeling has demonstrated a significant decrease of both normal and shear stress and fault stability for regions with an increase of glacial unloading. Rheological laws indicate the yield strength of the dry upper crust is governed by Byerlee’s Law and three times weaker under an extensional regime than under a compressional regime (Figure 2a) (Burov, 2015). The maximum brittle strength relative to depth of the passive rifted Cimmerian upper crust without the influence of glacial loading can be estimated (σ₁- σ₃) to be 18.6 MPa km^-1 (Figure 2a) (Burov, 2015). Based on the numerical modeling results (Steffen et al., 2014) glacial loading would increase the vertical stress to ~30 MPa and consequently depress the lithosphere by ~1 km. Glacial unloading not only reduces the vertical loading stress, but the horizontal rebound stress is also reduced up to 7 MPa approximately 10 ka after deglaciation regardless of the background tectonic stress.

Figure 2: 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 depression (Steffen et al., 2014). (a) Hypothetical crustal profile with ice cover during the Gzhelian stage (299 – 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 (σ₁- σ₃) 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 sheet (Burov, 2015). (b) Northward advancement of ice sheet around Asselian to Sakmarian stage (290 – 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 - 290 Ma). The deglaciated region experiences isostatic rebound (pale blue arrow) with estimated brittle strength for the upper crust (σ₁- σ₃) equal to 18.6 MPa km^-1 (Burov, 2015). The melted glacier water penetrates the upper crust and increases the fluid pressure (blue arrow) and reduces the brittle strength significantly (σ₁- σ₃ = 5.58 MPa km^-1) (Sibson, 1974). 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 envelope and triggers brittle failure and fault swarm development. (d) The stress heterogeneity between deglaciated and ice-covered regions further weakens 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 - 290 Ma). (e) Further extension of the crust and ascending of asthenosphere (green) around Rodian stage (269 - 272 Ma) completed the break off of Cimmerian terranes.

The 30 MPa vertical loading stress does not significantly increase the brittle strength of the upper crust. However, large differential stress environments can be expected for the deglaciation front as the covered region experiences higher normal and horizontal shear stresses (σ₁- σ₃ = 30 + 18.6 MPa km^-1, Figure 2) whereas the deglaciated region experiences strong postglacial rebound stress (lower normal and horizontal shear stresses) (Burov, 2015). Repeated advance and retreat cycles of major ice sheets can cause major changes in vertical load, fluid pressures and crustal strain that can trigger elastic flexing of the lithosphere and viscous flow in the mantle and thus generate focused regions of structural heterogeneities on which tectonic stress can act (Figure 2) (Steffen et al., 2014; Stewart et al., 2000). By taking the increased hydrostatic pore pressure (λ = 0.7) due to deglaciation into account, the estimated brittle strength of the upper crust after deglaciation is (σ₁- σ₃) 5.58 MPa km^-1 (Figure 2) (Sibson, 1974). Furthermore, glacial melt water may fill pore spaces and newly developed fractures that further weaken the upper crust and allow for massive concentrated fault swarm development. Mantle backflow associated with glacial isostatic rebound may induce decompressional melting and thus mantle-derived melts could percolate along previously developed fault zones and produce the regional flood basalts as terranes rift away (Figure 2).

The marine glacial deposits around the Kashmir Valley and Salt Range (Pakistan), Southern Qiangtang (Tibet), and Australia are evidence that the southern Tethys margin experienced glaciation during the Late Palæozoic (Wopfner & Jin, 2009; Fan et al., 2015). The reconstruction based upon Asselian and Sakmarian glacial deposits shows temporal and geographical coincidence between the glacial retreat margins and Cimmerian rifting sites (Figure 1d). Due to the effects of glacial loading on the lithosphere, we suggest that the southern hemisphere ice sheet contributed to the development of crustal weak zones along the Tethyan southern margin. Consequently, the formation of the Tethyan weak zones contributed to the formation of a linear passive rift margin that extended for thousands of kilometers (Figure 1).

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