Morphotectonics, Volcanism and Hydrothermal Activity on the East Pacific Rise between 21°12′ S and 22°40′ S
Morphotectonics, Volcanism and Hydrothermal Activity on the East Pacific Rise between 21°12′ S and 22°40′ S
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南纬 21°12′ 至 22°40′ 之间东太平洋海隆的地貌构造、火山活动和热液活动
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发表时间：
1997
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通讯作者：
M. Maslov
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作者：
S. Krasnov;I. Poroshina;G. Cherkashev;E. Mikhalsky;M. Maslov
21 °12′ ~ 22°40′ S东太平洋海隆（EPR）的地貌构造背景及其近期和过去的热液活动是1987- 1988年俄罗斯Fersman考察的重点。北方段是三个明显的块状轴向脊中最浅的一个，横截面呈锯齿状，顶部有一个非常宽的平坦山顶表面，并被一个发育良好的中央地堑切割。这些特征证明了整个地堑段下存在着强烈的岩浆活动和巨大的地壳岩浆房，在中央地堑的构造中，精细的分段显示得最为清楚，在地堑内部可以区分出几个四级地堑段。断层的性质和强度的变化也反映了这种分割的规模。根据构造和岩石学数据，岩浆活动在中段最强烈，可能直接位于岩浆底辟之上，为整个段提供熔体，岩浆在亚地壳水平上从中段向两端迁移，并离散注入地壳中段断裂成两个形态上不同的部分，中间被一个凹陷分开。在沉降的北方部分，轴向脊的宽顶被一个发育良好的、强烈破碎的轴向地堑切割。在南部，轴脊相对较高，但狭窄，沿其顶部有一个短暂的地堑沿着。断层的性质和强度也有很大的不同，在该段的南部和南部的轴向侧翼。因此，这两个部分的岩浆供应被认为来自两个不同的来源。如果是这样的话，那么，目前南段下伏的岩浆房可能处于补充阶段，而北段则处于深部冷却阶段，南段与中段的构造相似。但在22°30′ S以南，岩浆活动相对较弱，轴向脊较窄，剖面呈三角形，两个OSC的特征都是轴向脊尖在北方翼内突然变窄和急剧下沉。南部的OSC limbsare形态相似的轴向脊的正常截面。在这两种情况下，侧翼在结构和形态上与OSC相邻，并且倾斜结构可以追溯到OSC侧翼的远南。根据斜向构造的空间位置，推测22°07′ S附近的OSC正向北迁移，而21°44′ S附近的OSC带明显经历了小的空间振荡。尽管侧向位移幅度很小，但该带具有显著的测深异常。在轴向脊顶沿整个隆起和热液活跃的北段发现了许多块状硫化物矿床。近底沃茨中的矿石金属浓度在北方段的南部最高，而表层沉积物中的相同金属的最高浓度仅限于同一段的中部。我们推测，最近热液活动最大带从中段沿长轴向移动到了现在的南段位置。考虑到沉积速率，这一转变的年龄可近似估算为距今5 ~ 1万年前，其中中段相对富镁的玄武岩代表了一种较原始的样式，而南段相对富铁的岩石可能反映了地壳浅部的高度分馏。根据这一趋势，除了地貌构造资料外，我们推测，从中段向南段的近轴岩浆流动是热液活动沿轴向转移的原因，在研究区中段，块状硫化物仅在21°55′ S底凹以南发现，在轴向山脊和假设存在的subjunctional岩浆房。非常弱的表现，最近的火山活动在southsegment解释了没有热液活动和硫化物沉积在这段。
The morphotectonic setting of the East Pacific Rise (EPR) between21°12′ and 22°40′ S and its recent and past hydrothermalactivity were the focus of the Russian R/V Geolog Fersman’s expeditionin 1987–1988.The EPR axial zone in the study area is comprised of three segmentsseparated by overlapping spreading centers (OSCs) near 21°44′ and22°08′ S. The northern segment is the shallowest of three and hasa distinct massive axial ridge, trapeziodal in cross-section, toppedby a very wide flat summit surface and cut by a well-developedcentral graben. These features testify to intense magmatism and to avoluminous crustal magmatic chamber underlying the whole segment.Fine-scale segmentation is most clearly revealed in the structure ofthe central graben within which several 4th-order segments can bedistinguished. This scale of segmentation is also reflected on flanks of theaxis by variations in the character and intensity of faulting.According to structural and petrologic data, the magmatism is mostintense in the central part of the segment which is probably locateddirectly over a magmatic diapir supplying the melt to the whole segment.Magma migration at the subcrustal level from the center towards the ends ofthe segment with discrete injection into the crustal magmatic chamber ispresumed.The central segment is broken into two morphologically distinct partsseparated by a deval. In the subsided northern part, the wide summit of theaxial ridge is cut by a well-developed, intensely fractured axialgraben. In the southern part, the axial ridge is relatively elevated, butnarrow with an ephemeral graben along its crest. The character and intensityof faulting on the axial flanks are also considerably different in thenorthern and southern parts of the segment. Thus, the magmatic supply tothese two parts is thought to originate from two different sources. If so,then at present the magma chamber underlying the southern part of thesegment is probably at the stage of replenishment, while in the north it isat the stage of deep cooling.The southern segment is structurally similar to the central one. Howeverthere is considerably less intensive magmatic activity in this region,especially south of 22°30′ S where the axial ridge is narrow, andtriangular in cross-section.Both OSCs studied are marked by abrupt narrowing and sharp subsidence ofthe tips of axial ridges within the northern limbs. The southern OSC limbsare morphologically similar to normal sections of axial ridges. In bothcases the flanks are structurally and morphologically disrupted adjacent tothe OSCs and oblique structures can be traced far southward of the OSCflanks. Due to the spatial position of oblique structures on the the flanksit is presumed that the OSC near 22°07′ S is migrating northward.The 21°44′ S OSC zone has apparently undergone small spatialoscillations. In spite of the small amplitude of lateral displacement, thiszone is marked by prominent bathymetric anomalies.Numerous massive sulfide deposits were discovered atop the axial ridgealong the entire length of the uplifted and hydrothermally active northernsegment. Ore metal concentrations in near-bottom waters are maximumover the southern part of the northern segment, while maximum concentrationsof the same metals in surficial sediments are confined to the central partof the same segment. We surmise that there has been a recentalong-axis shift of the zone of maximum hydrothermal activity fromthe middle of the segment to its present position in the southern part ofthe segment. Considering sedimentation rates, the age of this shift can beapproximately estimated to be 5 to 10 thousand years before the present.The relatively Mg-enriched basalts of the middle part of thenorthern segment represent a tike of a more primitive pattern, while therelatively Fe-rich rocks of its southern part probably reflect alarge degree of fractionation at shallow crustal levels. Considering thistrend, in addition to morphotectonic data we presume that subaxial magmaflow from the middle to the southern part of the segment is responsible forthe along-axis shift of hydrothermal activity.In the central segment of the study area, massive sulfides have only beendiscovered south of the 21°55′ S deval, where the axial ridgeshoals and where the existence of a subjacent magma chamber is presumed.The very weak manifestations of recent volcanism within the southernsegment explain the absence of hydrothermal activity and sulfide depositswithin this segment.