Emplacement of giant radial dikes in the northern Tharsis region of Mars

[1] Three distinct sets of graben are associated with the volcano Alba Patera on Mars. One set, approximately circumferential to the edifice, has long been accepted to have formed as a tectonic response to an extensional stress regime associated with the evolution of the Alba Patera edifice. A second set includes mainly linear structures interpreted by many workers to have formed in response to very large-scale regional stresses. We infer that the third set of graben, all of which are relatively linear, none of which are strictly parallel to members of the second set, and many of which contain numerous pit craters, formed above long (∼1000 km), laterally propagating regional dikes emanating from a volcanic center located to the south within the Tharsis region. The expected geometries of such dikes (several hundred meters depth to dike top, ∼20 km depth to dike base, mean dike width ∼30–90 m) are modeled on the assumption that they were fed from a shallow magma reservoir centered on a neutral buoyancy horizon, expected to be present at a depth of ∼10 km on Mars. The volumes of magma in the dikes are consistent with a reservoir similar in size to those inferred to be present under the Tharsis shield volcanoes provided that the dikes were emplaced during caldera collapse episodes. The sizes of the graben associated with these dikes are consistent with the relaxation, during or immediately after dike emplacement, of preexisting regional extensional stresses of a few tens of MPa.

火星上的阿尔巴·帕特拉火山与三组不同的地堑有关。其中一组大致环绕山体，长期以来人们认为它是作为对与阿尔巴·帕特拉山体演化相关的拉张应力状态的构造响应而形成的。第二组主要包括一些线性构造，许多研究者认为它们是对非常大规模的区域应力的响应而形成的。我们推断第三组地堑都相对呈线性，它们都不与第二组中的地堑严格平行，而且其中许多包含大量坑状火山口，它们形成于从塔尔西斯地区南部的一个火山中心发出的长（约1000千米）且横向延伸的区域岩墙之上。这类岩墙预期的几何形状（岩墙顶部深度几百米，岩墙底部深度约20千米，平均岩墙宽度约30 - 90米）是基于以下假设建模的：它们由一个以中性浮力层为中心的浅岩浆库供给，预计在火星上该中性浮力层位于约10千米深处。岩墙中的岩浆体积与一个大小类似于据推测存在于塔尔西斯盾状火山之下的岩浆库相符，前提是这些岩墙是在破火山口塌陷阶段就位的。与这些岩墙相关的地堑的大小与在岩墙就位期间或之后不久，几十兆帕的既有区域拉张应力的松弛情况相符。