TY - JOUR

T1 - Morphotectonics, Volcanism and Hydrothermal Activity on the East Pacific Rise between 21°12′ S and 22°40′ S

AU - Krasnov, Sergey

AU - Poroshina, Irina

AU - Cherkashev, Georgiy

AU - Mikhalsky, Evgeniy

AU - Maslov, Mikhail

PY - 1997/1/1

Y1 - 1997/1/1

N2 - The morphotectonic setting of the East Pacific Rise (EPR) between 21°12′ and 22°40′ S and its recent and past hydrothermal activity were the focus of the Russian R/V Geolog Fersman's expedition in 1987-1988. The EPR axial zone in the study area is comprised of three segments separated by overlapping spreading centers (OSCs) near 21°44′ and 22°08′ S. The northern segment is the shallowest of three and has a distinct massive axial ridge, trapeziodal in cross-section, topped by a very wide flat summit surface and cut by a well-developed central graben. These features testify to intense magmatism and to a voluminous crustal magmatic chamber underlying the whole segment. Fine-scale segmentation is most clearly revealed in the structure of the central graben within which several 4th-order segments can be distinguished. This scale of segmentation is also reflected on flanks of the axis by variations in the character and intensity of faulting. According to structural and petrologic data, the magmatism is most intense in the central part of the segment which is probably located directly over a magmatic diapir supplying the melt to the whole segment. Magma migration at the subcrustal level from the center towards the ends of the segment with discrete injection into the crustal magmatic chamber is presumed. The central segment is broken into two morphologically distinct parts separated by a deval. In the subsided northern part, the wide summit of the axial ridge is cut by a well-developed, intensely fractured axial graben. In the southern part, the axial ridge is relatively elevated, but narrow with an ephemeral graben along its crest. The character and intensity of faulting on the axial flanks are also considerably different in the northern and southern parts of the segment. Thus, the magmatic supply to these two parts is thought to originate from two different sources. If so, then at present the magma chamber underlying the southern part of the segment is probably at the stage of replenishment, while in the north it is at the stage of deep cooling. The southern segment is structurally similar to the central one. However there is considerably less intensive magmatic activity in this region, especially south of 22°30′ S where the axial ridge is narrow, and triangular in cross-section. Both OSCs studied are marked by abrupt narrowing and sharp subsidence of the tips of axial ridges within the northern limbs. The southern OSC limbs are morphologically similar to normal sections of axial ridges. In both cases the flanks are structurally and morphologically disrupted adjacent to the OSCs and oblique structures can be traced far southward of the OSC flanks. Due to the spatial position of oblique structures on the the flanks it is presumed that the OSC near 22°07′ S is migrating northward. The 21°44′ S OSC zone has apparently undergone small spatial oscillations. In spite of the small amplitude of lateral displacement, this zone is marked by prominent bathymetric anomalies. Numerous massive sulfide deposits were discovered atop the axial ridge along the entire length of the uplifted and hydrothermally active northern segment. Ore metal concentrations in near-bottom waters are maximum over the southern part of the northern segment, while maximum concentrations of the same metals in surficial sediments are confined to the central part of the same segment. We surmise that there has been a recent along-axis shift of the zone of maximum hydrothermal activity from the middle of the segment to its present position in the southern part of the segment. Considering sedimentation rates, the age of this shift can be approximately estimated to be 5 to 10 thousand years before the present. The relatively Mg-enriched basalts of the middle part of the northern segment represent a tike of a more primitive pattern, while the relatively Fe-rich rocks of its southern part probably reflect a large degree of fractionation at shallow crustal levels. Considering this trend, in addition to morphotectonic data we presume that subaxial magma flow from the middle to the southern part of the segment is responsible for the along-axis shift of hydrothermal activity. In the central segment of the study area, massive sulfides have only been discovered south of the 21°55′ S deval, where the axial ridge shoals and where the existence of a subjacent magma chamber is presumed. The very weak manifestations of recent volcanism within the southern segment explain the absence of hydrothermal activity and sulfide deposits within this segment.

AB - The morphotectonic setting of the East Pacific Rise (EPR) between 21°12′ and 22°40′ S and its recent and past hydrothermal activity were the focus of the Russian R/V Geolog Fersman's expedition in 1987-1988. The EPR axial zone in the study area is comprised of three segments separated by overlapping spreading centers (OSCs) near 21°44′ and 22°08′ S. The northern segment is the shallowest of three and has a distinct massive axial ridge, trapeziodal in cross-section, topped by a very wide flat summit surface and cut by a well-developed central graben. These features testify to intense magmatism and to a voluminous crustal magmatic chamber underlying the whole segment. Fine-scale segmentation is most clearly revealed in the structure of the central graben within which several 4th-order segments can be distinguished. This scale of segmentation is also reflected on flanks of the axis by variations in the character and intensity of faulting. According to structural and petrologic data, the magmatism is most intense in the central part of the segment which is probably located directly over a magmatic diapir supplying the melt to the whole segment. Magma migration at the subcrustal level from the center towards the ends of the segment with discrete injection into the crustal magmatic chamber is presumed. The central segment is broken into two morphologically distinct parts separated by a deval. In the subsided northern part, the wide summit of the axial ridge is cut by a well-developed, intensely fractured axial graben. In the southern part, the axial ridge is relatively elevated, but narrow with an ephemeral graben along its crest. The character and intensity of faulting on the axial flanks are also considerably different in the northern and southern parts of the segment. Thus, the magmatic supply to these two parts is thought to originate from two different sources. If so, then at present the magma chamber underlying the southern part of the segment is probably at the stage of replenishment, while in the north it is at the stage of deep cooling. The southern segment is structurally similar to the central one. However there is considerably less intensive magmatic activity in this region, especially south of 22°30′ S where the axial ridge is narrow, and triangular in cross-section. Both OSCs studied are marked by abrupt narrowing and sharp subsidence of the tips of axial ridges within the northern limbs. The southern OSC limbs are morphologically similar to normal sections of axial ridges. In both cases the flanks are structurally and morphologically disrupted adjacent to the OSCs and oblique structures can be traced far southward of the OSC flanks. Due to the spatial position of oblique structures on the the flanks it is presumed that the OSC near 22°07′ S is migrating northward. The 21°44′ S OSC zone has apparently undergone small spatial oscillations. In spite of the small amplitude of lateral displacement, this zone is marked by prominent bathymetric anomalies. Numerous massive sulfide deposits were discovered atop the axial ridge along the entire length of the uplifted and hydrothermally active northern segment. Ore metal concentrations in near-bottom waters are maximum over the southern part of the northern segment, while maximum concentrations of the same metals in surficial sediments are confined to the central part of the same segment. We surmise that there has been a recent along-axis shift of the zone of maximum hydrothermal activity from the middle of the segment to its present position in the southern part of the segment. Considering sedimentation rates, the age of this shift can be approximately estimated to be 5 to 10 thousand years before the present. The relatively Mg-enriched basalts of the middle part of the northern segment represent a tike of a more primitive pattern, while the relatively Fe-rich rocks of its southern part probably reflect a large degree of fractionation at shallow crustal levels. Considering this trend, in addition to morphotectonic data we presume that subaxial magma flow from the middle to the southern part of the segment is responsible for the along-axis shift of hydrothermal activity. In the central segment of the study area, massive sulfides have only been discovered south of the 21°55′ S deval, where the axial ridge shoals and where the existence of a subjacent magma chamber is presumed. The very weak manifestations of recent volcanism within the southern segment explain the absence of hydrothermal activity and sulfide deposits within this segment.

KW - Axial morphology

KW - East Pacific Rise

KW - Hydrothermal activity

KW - Segmentation

KW - Side-scan sonar data

KW - Sulfide deposits

KW - Volcanism

UR - http://www.scopus.com/inward/record.url?scp=0031400732&partnerID=8YFLogxK

U2 - 10.1023/A:1004237304068

DO - 10.1023/A:1004237304068

M3 - Article

AN - SCOPUS:0031400732

VL - 19

SP - 287

EP - 317

JO - Marine Geophysical Research

JF - Marine Geophysical Research

SN - 0025-3235

IS - 4

ER -