Posted on 26/04/2011
Martin Menzies plenary lecture opens the International Craton Conference in China.
This conference helped solve some long standing issues in Earth Sciences that relate to craton stability/instability and the role of active margins.
| Crustal thickness beneath China - reactivated craton in the east (after Li et al 2006) |
How do we know when a craton has been reactivated?
Normally stable Archaean cratonic cores are defined by a thick seismic lid (high velocity) and very low heat flow but the North China Craton is renowned for its “ridge-like” heat flow and thin lithosphere akin to that found in ocean basins. This can only have happened by loss of the cratonic keel (>120 km) due to thermo-tectonic processes.
So when did it happen?
Palaeozoic kimberlites entrain pieces of a jigsaw (i.e. xenoliths) which when put back together reveal a cold, thick (200 km) and melt-depleted keel at 480 Ma. In contrast, volcanic eruptions in the Cenozoic (<60Ma) bring up pieces of a different jigsaw which when put back together show a thin oceanic-like lithosphere (hot, thin (<80km) and fertile which agrees very well with the geophysical constraints (high heat flow =hot, shallow low velocity zone =thin). So changes happened in the last 400 Ma.
Can it be more tightly constrained?
Extrusive and intrusive activity in the Mesozoic (120–130 Ma) shows that by that time the lithosphere was hot and reactivated, allowing the transfer of magmas to the surface, and this is validated by the evolving character of the source of volcanic rocks which changes from enriched (keel) to depleted (oceanic ) around 80–100 Ma. So change took place in a 350 Ma window (480–130 Ma)
What process was responsible?
Recent seismic tomographic models show that slabs remain coherent to the 660 km discontinuity and beyond to D”. Such processes transfer OH (water) into the Earth with its consequent accumulation in the transition zone (TZ) at 660 km depth. However the coherent slabs on the 660 km discontinuity beneath eastern China may only be <80Ma old so we must appeal to palaeo-Pacific subduction occurring in the early Phanerozoic. Cold wet slabs need 100–200 Ma to warm up so they can rise through the upper mantle. If we can assume such processes operated during the Phanerozoic a solution may exist for the loss of the keel beneath the North China craton.
How did this affect the craton ?
During the construction of the North China Craton, plate tectonic processes were multi-directional with the closure of many ocean basins and the convergence of five subduction zones over the last 400 Ma. It has been speculated that this may have transferred considerable volumes of water to the TZ – “equal to all of the world’s oceans”. This is the first piece of bad news for the North China Craton which has most probably been subjected to “bottom-up” hydration during the last 400 Ma. This decreases the viscosity and buoyancy of the keel. The second piece of bad news is the lack of protective mobile belts that normally shield the cratonic cores from post-Archaean destructive margin processes. Should there have been any separation of the crust from the mantle by low angle faults then delamination may have assisted in the keel removal. The result is that although we have exposures of Archaean rocks at the surface (i.e., a craton) the lower lithosphere has the chemical and physical attributes of an ocean basin (i.e., thin, fertile and hot) so that we do not have a cratonic core/keel in geophysical terms (i.e., a high velocity anomaly).
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