Mongol-Kazakh

Figure A63. Mongol-Kazakh anomaly, interpreted as the Mongol-Kazakh slab, with (horizontal) [vertical] cross sections through (A)[D] the UUP07 p-wave) and (B)[D] the combined SL2013 and S40RTS s-wave models at 2330 km; C) the location of the modern geological record that we interpret to have formed during the subduction of the slab.

Figure A63. Mongol-Kazakh anomaly, interpreted as the Mongol-Kazakh slab, with (horizontal) [vertical] cross sections through (A)[D] the UUP07 p-wave) and (B)[D] the combined SL2013 and S40RTS s-wave models at 2330 km; C) the location of the modern geological record that we interpret to have formed during the subduction of the slab.


The Mongol-Kazakh anomaly (Figure A63) is located below northern Siberia from the core-mantle boundary up to the mid-mantle. The anomaly is widely known in the literature as the Mongol-Okhotsk anomaly (van der Voo et al., 1999a) and is interpreted to represent lithosphere of the Mongol-Okhotsk ocean, which has been inferred to have subducted until the latest Jurassic-earliest Cretaceous, based on paleomagnetic and geological data (Van der Voo et al., 1999a; 2015). We renamed the slab after its present-day location, whereby the center point lies below the borders between Mongolia, Kazakhstan, China, and Russia, because we consistently name slabs after their present location rather than their interpreted geological history. The base of the slab merges with the graveyard of slabs under Asia at the base of the mantle (Van der Voo et al., 1999a). The southern part of the Z-shaped Mongol-Kazakh slab sensu (Van der Voo et al., 1999a; 2015) was redefined by van der Meer et al. (2010) as the Central China slab and may (also) consist of Paleotethyan lithosphere, as suggested by Stampfli and Borel (2004). Subduction of and within the Mongol-Okhotsk ocean started well before the Mesozoic (Tomurtogoo et al., 2005; Donskaya et al., 2013) and pre-Mesozoic lithosphere may not be visible any longer in the slab graveyard above the core-mantle boundary. Van der Voo et al. (2015) noted that the deepest part of the Mongol-Kazakh slab is trending N-S, consistent with Triassic (~250-220 Ma) orientations of the Mongol-Kazakh subduction zone, whereas upwards the slab kinks and eventually becomes oriented ~W-E, similar to the trend of the Mongol-Okhotsk suture zone. These authors argued that the shape of the slab is consistent with an oroclinal closure of the Mongol-Okhotsk Ocean suggested by the geological structure of Mongolia and southern Siberia. The shape of the slab may thus suggest a ~250-220 Ma age of the deepest, N-S striking part of the slab (below ~2000 km). Closure of the Mongol-Kazakh ocean follows from paleomagnetic constraints from the North China and Amurian blocks and Siberia, and is slightly younger than we previously assumed: 140±10 Ma (Cogne et al., 2005; Van der Voo et al., 2015).
Recently Shephard et al. (2014) performed mantle convection modelling, suggesting that the Mongol-Kazakh slab underwent as much as 60 longitude degrees westward motion through the mantle during its descent to the core-mantle boundary, and should thus be found not further east than 35°E, instead of 60-100°E preferred here, and in Van der Voo et al. (1999a; 2015). Our correlation philosophy, also used in van der Meer et al. (2010; 2012) in linking slabs to their geological record assumes that slabs do not significantly move laterally relative to each other which, when viewed globally, implies a preference for slab remnants to sink vertically as was recently corroborated by Domeier et al. (2016). In other words, the modern distribution of slab remnants in the deep mantle can be associated with the paleo-subduction zone configuration, which was found to lead to coherent correlations of plate reconstructions with mantle structure for the last 250-300 Ma (van der Meer et al., 2010; 2012; 2014). The mantle convection model of Shephard et al. (2014), and similarly Fritzell et al. (2016), predicts that the Mongol-Kazakh slab has moved >4000 km westward through the lower mantle since it detached at ~140 Ma (i.e., at ~3 cm/year, about twice as fast as the sinking rate) relative to the Aegean slab. What is driving large lateral mantle “winds” in the mantle flow modelling is not known, however, so far there has been no supporting evidence that such large mantle winds actually exist. Moreover, one would equally expect that strong mantle winds would deflect or destroy mantle plumes, which then would question the basic premise of hotspot-based absolute plate motion models that are used to drive such “slab-prediction” modelling. There is compelling evidence, however, provided by Domeier et al. (2016), that any lateral mantle flow in the past ~130 Myr did not appreciably perturb the overall radial sinking of slab and also not appreciably perturb the rising of plumes as is implied by their use of the Seton et al. (2012) absolute plate motion model. Although we believe that mantle flow modeling is on the longer term the only viable avenue to quantitatively and dynamically link plate-tectonic evolution to mantle dynamics, at present the modeling of mantle flow itself is based on uncertain data and large assumptions, e.g. mantle rheology, and thereby introduces large uncertainty and complexity to the slab identification problem it tries to solve.


Comments

Mongol-Kazakh — 2 Comments

    • Hi Nadja,

      Nice blog, thanks for mentioning us! Check out the Balkan slab: we interpreted that slab to represent the lithosphere that broke off after the formation of the Urals. It is, like the Mongol-Kazakh slab, located at the core-mantle boundary, but farther to the west. We (and many before us) interpreted the Mongol-Kazakh slab to reflect the slab associated with closure of the Mongol-Okhotsk ocean.

      DJJvH

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