Tectonic stress and magma chamber size as controls on dike propagation:: Constraints from the 1975-1984 Krafla rifting episode
Tectonic stress and magma chamber size as controls on dike propagation:: Constraints from the 1975-1984 Krafla rifting episode
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DOI：
10.1029/2005jb003879
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发表时间：
2006-12-09
影响因子：
3.9
通讯作者：
Brandsdottir, Bryndis
中科院分区：
文献类型：
--
作者：
Buck, W. Roger;Einarsson, Pall;Brandsdottir, Bryndis
[ 1] 1975年至1984年，在冰岛北方裂谷带的克拉夫拉段沿着，对分散板块边界上岩墙侵入事件的研究最为深入。地震和大地测量表明，中央岩浆房为横向扩展的岩石圈厚度的许多倍的岩脉提供了补给。岩脉长度，岩脉宽度，破火山口沉降，熔岩挤出的模式强烈表明，岩脉传播的影响，构造应力的变化与每个岩脉侵入事件和岩浆压力与岩脉开口。这些观测结果启发我们建立一个定量模型，描述玄武质岩脉远离岩浆房的侧向扩展。我们假设只要有足够的驱动压力（定义为岩脉尖端的岩浆压力和构造应力之差），岩脉就会扩展。对于一个特定的岩墙侵入事件，开放岩墙和岩浆房被视为一个封闭系统。在一个事件中，岩浆压力随着岩浆从岩浆房中抽出的体积而线性降低。岩石圈的相对构造张力随岩墙宽度的增加而线性减小。当驱动压力等于迫使岩浆流出岩浆室所需的“爆发”压力时，岩脉开始扩张。当驱动压力达到最小值时停止。通常，堤坝宽度与该“停止”压力成比例，合理的值为1 m。传播距离除取决于破裂压力和终止压力外，还取决于构造应力的初始分布和岩墙切割岩石圈的厚度。岩脉的侵入改变了构造应力分布，使得后续岩脉的传播距离和方向可能与第一个岩脉不同。在岩浆房再填充一段时间后，如果达到爆发压力，新的岩墙就会形成。对于一个理想的扩张段，构造应力场演化产生一个沿一个方向传播的岩脉序列，然后是沿相反方向传播的岩脉序列。每个序列中的第一个岩脉应该是最长的，随后是连续较短的岩脉。当岩浆房附近的构造应力大部分被释放时，岩浆就可能开始喷出。岩墙传播和挤压的模型模式与Krafla事件的数据一致。岩浆房的大小应该有一个主要的影响岩浆系统在其他构造环境与较大的岩浆房生产较长的特征岩脉。
[ 1] The best-studied dike intrusion events on a divergent plate boundary occurred along the Krafla segment of the northern rift zone in Iceland from 1975 - 1984. Seismic and geodetic measurements there showed that a central magma chamber fed dikes that propagated laterally many times the thickness of the lithosphere. The patterns of dike length, dike width, caldera subsidence, and lava extrusion strongly suggest that dike propagation is affected by tectonic stresses that change with each dike intrusion event and that magma pressures are linked to the dike opening. These observations have inspired us to develop a quantitative model for the lateral propagation of basaltic dikes away from a magma chamber. We assume dikes propagate as long as there is sufficient driving pressure, defined as the difference between magma pressure and tectonic stress at the dike tip. The opening dike and the magma chamber are treated as a closed system for a given dike intrusion event. During an event, magma pressure is reduced linearly with the magma volume withdrawn from the chamber. Relative tectonic tension in the lithosphere is reduced linearly as the dike width increases. A dike begins propagation when the driving pressure equals the "breakout'' pressure needed to force the magma out of the chamber. It stops when the driving pressure reaches a minimum value. Generally, the dike width is proportional to this "stopping'' pressure, and a reasonable value gives a width of 1 m. Besides the breakout and stopping pressures, the propagation distance depends on the initial distribution of tectonic stress and the thickness of the lithosphere cut by a dike. The intrusion of a dike changes the tectonic stress distribution so that subsequent dikes may propagate different distances and directions than the first dike. After a period of magma chamber refilling, a new dike can initiate if the breakout pressure is reached. For an idealized spreading segment the tectonic stress field evolves to produce a sequence of dikes propagating in one direction followed by a sequence of dikes propagating in the opposite direction. The first dike in each sequence should be the longest followed by successively shorter dikes. When tectonic stresses close to a magma chamber have been largely relieved, then extrusion of magma may start. The model pattern of dike propagation and extrusion is consistent with data from the Krafla episode. Magma chamber size should have a major effect on magmatic systems in other tectonic settings with larger magma chambers producing longer characteristic dikes.