幔源岩浆在地壳中分层侵位的控制因素:二维热-力学模拟

Factors controlling the stratified emplacement of mantle-derived magma within the crust: 2-D thermomechanical modelling

幔源岩浆在地壳内的上升和聚集样式不仅依赖于岩浆自身的性质,还取决于围岩的强度和热状态。已有数值和物理模型大多关注岩浆自身物性对其上升过程的影响,而对围岩流变强度或热状态如何影响岩浆的上升和聚集过程的研究相对薄弱,尤其是周期性的幔源岩浆在壳内分层侵位的受控因素仍然不清楚。本文利用二维热-力学数值模拟方法,通过发展多期岩浆脉冲和岩墙生成算法,研究了岩浆从深部地幔上升至地壳内部侵位的动力学过程,系统测试了地壳(围岩)地温梯度(上、下地壳的地温梯度分别以GUC和GLC表示)和地壳强度对岩浆上升过程和聚集样式的影响。模拟结果表明:(1)地壳地温梯度对岩浆的侵位深度有重要影响,岩浆侵入冷地壳(GUC=GLC=12.5K/km),岩浆主体在岩石圈深度聚集,地表的相对高差小于140m;岩浆侵入温地壳(GUC=GLC=15K/km)在下地壳底部聚集形成岩浆房,上升至上、下地壳界面,岩浆房上方的地表地形呈现中心拗陷两翼隆起的形态,地表最大高程可达3km;岩浆侵入热地壳(GUC=GLC=17.5K/km)仅在下地壳底部聚集形成岩浆房,地形演化特征与温地壳背景的情况类似,但最大高程小于1.5km。(2)在同等地壳地温梯度条件下,上、下地壳的相对强度决定了岩浆的聚集样式:下地壳的强度越弱,岩浆更易在下地壳聚集形成岩浆房;上地壳的强度越弱,岩浆更易在地表喷发。进一步分析表明:岩浆的聚集样式受地壳地温梯度与地壳流变分层性的共同控制,地壳越热且流变强度分层性越显著则越有利于岩浆在地壳中的多层级侵位;每一期岩浆脉冲的供给均会导致岩浆房内部的超压值骤增。我们的模拟结果对理解火山喷发前壳内岩浆的赋存状态及岩浆活动区的地形演化具有启示意义。

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 The following two conclusions can be withdrawn from our modelling results: (1) The crustal thermal gradient controls the depth of magma emplacement.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 3km. 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.