Factors controlling the stratified emplacement of mantle-derived magma within the crust:2-D thermomechanical modelling
The emplacement and ascent process of mantle-derived magma in the crust depends not only on the physical properities of the magma itself,but also on the strength and thermal state of the host rocks.The previous numerical or analogue simulations mainly focused on how physical properities of the magma influence its ascent and emplacement process in the crust,however,little attention is paid on the thermal state and rheology of the host rocks.In particular,what factors control the stratified emplacement of the periodic mantle-derived magma in the crust remains enigmatic.In this study,we employed two-dimensional thermal-mechanical models with newly-developed multiple magma pulses and dike generation algorithms to investigate the magma dynamics from melt ascent in the deep mantle to magma emplacement in the crust.We systematically examined the influences of crustal thermal gradient(the thermal gradients for the upper and lower crust are denoted as Guc and GLC,respectively)and crustal rheological strength on the process of magma ascent and style of magma emplacement.The following two conclusions can be withdrawn from our modelling results:(1)The crustal thermal gradient controls the depth of magma emplacement.A cold host crust(Guc=GLC=12.5K/km)facilitates the accumulation of magma in the depth of the lithosphere,and the surface uplift above the volcano relative to the surrounding region is less than 140m.A warm host crust(GUC=GLC=15K/km)facilitates the accumulation of magma in the lower crust and final emplacement in the interface of upper and lower crust,forming a valley at the center and peaks at the flanks on the surface,with the elevation up to 3 km.When the host crust is hot enough(GUC=GLC=17.5K/km),the magma chamber is only formed in the lower crust.The topographic evolution is similar to that in the warm crust,but has smaller magnitude(～1.5km).(2)Under the same thermal regime,the relative strength of the upper and lower crust determines the style of magma emplacement.A weak lower crust favors the formation of a magma chamber at the lowermost crust,while a weak upper crust favors the eruption of the magma at the surface.In summary,our modelling results show that the combination of the host rocks thermal gradient and rheological stratification controls the modes of magma emplacement in the crust.The hotter and the bigger the rheological contrast pronounced of the host crust,the easier it is to form stratified magma emplacement in the crust.Every magma pulse replenishing always causes an abrupt increase of the overpressure in magma chamber.Our modelling results have broad implications for understanding not only the modes of magma storage in the crust prior to eruption,but also the topography evolution above the region with magmatic activities.
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