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1.
A submersible study has been conducted in February–March 1978 at the axis of the East Pacific Rise near 21°N. The expedition CYAMEX, the first submersible program to be conducted on the East Pacific Rise, is part of the French-American-Mexican project RITA (Rivera-Tamayo), a 3-year study devoted to detailed geological and geophysical investigations of the East Pacific Rise Crest. On the basis of the 15 dives made by CYANA in the axial area of the Rise, a morphological and tectonic zonation can be established for this moderately-fast spreading center. A narrow, 0.6 to 1.2 km wide zone of extrusion (zone 1), dominated by young lava flows, is flanked by a highly fissured and faulted zone of extension (zone 2) with a width of 1 to 2 km. Further out, zone 3 is dominated by outward tilted blocks bounded by inward-facing fault scarps. Active or recent faults extend up to 12 km from the axis of extrusion of the East Pacific Rise. This represents the first determination from direct field evidence of the width of active tectonism associated with an accreting plate boundary. Massive sulfide deposits, made principally of zinc, copper and iron, were found close to the axis of the Rise. Other signs of the intense hydrothermal activity included the discovery of benthic fauna of gian size similar to that found at the axis of the Galapagos Rift. We emphasize the cyclic character of the volcanicity. The main characteristics of the geology of this segment of the East Pacific Rise can be explained by the thermal structure at depth below this moderately-fast spreading center. The geological observations are compatible with the existence of a shallow magma reservoir centered at the axis of the Rise with a half-width of the order of 10 km.  相似文献   

2.
A Seabeam reconnaissance of the 400 km-long fast-slipping (88 mm yr-1) Heezen transform fault zone and the 55 km-long spreading center that links it to Tharp transform defined and bathymetrically described several types of ridges built by tectonic uplift and volcanic construction. Most prominent is an asymmetric transverse ridge, at which abyssal hills adjacent to the fault zone have been raised 2–3 km above normal rise-flank depths. Topographic and petrologic evidence suggests that this uplift, which has produced a 5400 m scarp from the crest of the ridge to the floor of a 10 km-wide transform valley, is caused by rapid serpentinization of upper mantle which has been exposed to hydrothermal circulation by fault-zone fracturing of an unusually thin crust. Transverse ridges have been thought atypical of fast-slipping transforms. One class of volcanic ridge more common at these sites is the overshot ridge, formed by prolongation of spreading-center rift zones obliquely across the transform. Overshot ridges are well developed at Heezen transform, especially at the eastern end where an eruptive rift zone extending 60 km from the southern tip of the East Pacific Rise has built a transform-parallel ridge that fills the eastern transform valley. Obliteration of fault-zone structure by ridges overshooting from the spreading center intersections means that the topography of the aseismic fracture zones is not just inherited from that of the active transform fault zone. The latter has several en echelon and overlapping fault traces, linked by short oblique spreading axes that generally form pull-apart basins rather than volcanic ridges. Interpretation of the origin and pattern of the fault zone's tectonic and volcanic relief requires refinement of the plate geography and history of this part of the Pacific-Antarctic boundary, using new Seabeam and magnetic traverses to supplement and adjust the existing geophysical data base.  相似文献   

3.
Gallo  D. G.  Kidd  W. S. F.  Fox  P. J.  Karson  J. A.  Macdonald  K.  Crane  K.  Choukroune  P.  Seguret  M.  Moody  R.  Kastens  K. 《Marine Geophysical Researches》1984,6(2):159-185
During the Fall of 1979, a manned submersible program, utilizing DSRV ALVIN, was carried out at the intersection of the East Pacific Rise (EPR) with the Tamayo Transform boundary. A total of seven dives were completed in the vicinity of the EPR/Tamayo intersection depression and documented the geologic relationships that characterize the juxtaposition of these types of plate boundaries. The young volcanic terrain of the EPR axis can be traced into and across the Tamayo Transform valley but becomes buried by sedimentary talus that is being shed from sediment scarps along the unstable sediment slope that defines the north side of the intersection depression. Within 4 km of the transform boundary, the dominant trend (000°) of the fissures and faults that disrupt the rise-generated volcanics is markedly oblique to the regional direction of sea floor spreading (120°). Since no evidence was found to suggest that these structures accommodate significant amounts of strike-slip displacement, they are taken to reflect a distortion of the EPR extensional tectonic regime by a transform generated shear couple. The floor of the Tamayo Transform valley in this area is inundated by mass-wasted sediment, and the principal transform displacement zone is characterized at the surface by a narrow (<1.5 km) interval of fault scarps in sediment that trends parallel with the transform valley. Extrapolated to the west, this zone links with zones of transform deformation investigated during earlier submersible studies (CYAMEX and Pastouret, 1981). Evidence of low-level hydrothermal discharge was seen at one locality on the EPR axis and at another 8 km west of the axis at the edge of the zone of transform deformation.  相似文献   

4.
Morphology and tectonics of the Galapagos Triple Junction   总被引：1，自引：0，他引：1  
We describe the results of GLORIA and SEABEAM surveys, supplemented by other marine geophysical data, of the Galapagos Triple Junction where the Pacific, Cocos and Nazca plates meet. The data allowed detailed topographic and tectonic maps of the area to be produced. We located each spreading axis with a precision of about 1 km. All three plate boundaries change character as the triple junction is approached to take on morphologies typical of slower spreading axes: the fast-spreading East Pacific Rise develops the morphology of a medium-spreading rise, and the medium-spreading Cocos-Nazca Rise takes on the appearance of a slow-spreading ridge. The axis of the East Pacific Rise was found to be completely continuous throughout the survey area, where it runs along the 102°05 W meridian. The Cocos-Nazca axis, however, fails to meet it, leaving a 20-km-wide band of apparently normal East Pacific Rise crust between its tip and the East Pacific Rise axis. As a consequence there must be considerable intra-plate deformation within the Cocos and Nazca plates. A further 40 km of the Cocos-Nazca axis is characterised by oblique faulting that we interpret to be a sign of rifting of pre-existing East Pacific Rise crust. We infer that true sea-floor spreading on the Cocos-Nazca axis does not begin until 60 km east of the East Pacific Rise axis. Other areas of similar oblique faulting occur on the Pacific plate west of the triple junction and along the rough-smooth boundaries of the Galapagos Gore. We present a model involving intermittent rifting, rift propagation, and sea-floor spreading, to explain these observations.  相似文献   

5.
A wealth of geologic information has been collected during studies of the Matuyama/Brunhes magnetic reversal boundary on the East Pacific Rise at 21°N. Five ALVIN and two CYANA dives, and a series of deep-tow traverses show that abyssal hills in this region of the Pacific are created near the spreading axis by inward dipping normal faulting and by back-tilting of these fault blocks. Outward dipping faults occur but are of less importance in the creation of relief. Tectonic disruption of the crust, particularly through tilting, is less pronounced than in the Atlantic. Small volcanoes approximately 50 m high and 400 m wide are common on the abyssal hills. A significant number of the volcanoes may have split apart at the spreading axis attesting to the narrowness of the crustal accretion zone on the East Pacific Rise. Active faulting is restricted to less than 10 to 12 km off-axis, although minor recent faulting may have been detected 23 km off-axis. Crustal sections exposed by faulting reveal that massive lava flows and sheet flows are common in the upper portion of oceanic layer two, but are less abundant than pillow lavas.  相似文献   

6.
The axis of the East Pacific Rise is defined by a topographic block about 15 km wide and 300 to 350 m high which is flanked by abyssal hills 100 to 200 m high and 3 to 5 km wide. These hills often are tilted such that their steep slopes face the axis. An empirical model explaining these features combines axial extrusion to form the central block and rotational faulting to lower the shoulders of the axial block to the regional depth and tilt them outward.The axial block is offset about 10 km left-laterally at 10.0°S and a similar amount right-laterally at 11.5°S. Offsets (or lack of offsets) of young magnetic anomalies indicate that these axial displacements occurred between 1.7 and 0.9 m.y. ago and 0.7 m.y. ago and the present in the north and south. respectively. These small axial offsets are interpreted to be the result of either brief episodes of asymmetric see-floor spreading or discrete jumps in the site of spreading activity. Both axial shifts were to the west; a unidirectional sequence of such shifts occurring at the above rate of one per million years would be difficult to differentiate from true regional asymmetric spreading and might explain that phenomenon on other medium-to fast-spreading rises.Reconnaissance data from the east flank of the East Pacific Rise indicate that spreading activity began on that part of the rise between the 9°S and 13.5°S fracture zones approximately 8.2 m.y. ago when the site of crustal accretion jumped westward from the now dormant Galapagos Rise. Slope change in crust approximately 2 and 6 m.y. old imply faster spreading rates between about 6 and 2 m.y. ago than either before or after that time. Identification and correlation of anomaly 3 allows an estimate of about 90 mm/y for this higher east flank spreading rate. Since 1.7 m.y. ago spreading rates have averaged about 80 mm/y to the west and 77 mm/y to the east.  相似文献   

7.
This paper is a report of geological observations made using the submersible ALVIN on the crest of the East Pacific Rise near 21°N. The profile is 6 km long and crosses a 5–10 km wide plateau which rises 100 m±above the rise flanks. At the axis are exposed fresh glassy pillow lavas with no sediment accumulation in a region termed the neovolcanic zone. This zone is about one kilometer wide and includes elongate ridges of pillow lavas and seventeen hydrothermal vent fields in the study area. Outside the neovolcanic zone the seafloor is extensively fissured in another zone which is up to two kilometers wide. The neovolcanic zone and the fissured zone are included within a rift valley or graben about 3 to 5 km wide and 50 m±deep. This rift valley is asymmetrically located on the west side of the axial plateau; the neovolcanic zone in the study area is asymmetrically located on the east side of the rift graben. Fissured crust is not common outside the rift graben or in the neovolcanic zone; similarly, large throw faults such as those which form the edges of the graben are not found outside of it. These observations can be interpreted according to a volcanic-tectonic cycle in which volcanic eruptions and hydrothermal circulation are followed by a tectonic phase which includes fissuring and vertical movements. When a new cycle starts it may involve a lateral shift of the spreading axis. Lavas along the dive profile are suggested to be no older than a few thousand years based on sediment accumulation. In contrast, seafloor spreading rates here predict crust up to 105 yr old. This observation suggests that lavas from the neovolcanic zone can spread laterally about a kilometer or more and overlap on older crust.  相似文献   

8.
A deeply-towed instrument package was used in a detailed survey of the crest of the East Pacific Rise (EPR) near 3°25S, where the Pacific and Nazca plates are separating at 152 mm/yr. A single 90 km-long traverse of the rise crest extends near-bottom observations onto the rise flanks. A ridge at the spreading axis is defined by its steep regional slopes, probably caused by rapid crustal contraction as the spreading magma chamber freezes, rather than by outward-facing fault scarps. It can be divided into a marginal horst-and-graben zone with low (<50 m), symmetric fault blocks, and a 2 km-wide elongate axial shield volcano that is unfaulted except for a narrow crestal rift zone. This has a summit graben (10–35 m deep) probably formed by caldera collapse, and narrow pillow basalt walls built over vent fissures. Small, low (<50 m) volcanic peaks occur on the shield volcano and the horst-and-graben zone, and some may have been built away from the spreading axis. Major plate-building lava flows issue from the crestal rift zone and flow an average of 500 m down the sides of the volcano. The marginal horst-and-graben zone results from tensional faulting of a thin crust of lava, and evolves by progressive shearing on inclined fault planes. Crustal extension continues at least as far as 20 km from the axis, and the large, long fault blocks formed in thicker crust beyond the subaxial magma chamber develop into abyssal hills. Pelagic sedimentation, at a maximum rate of 22 m/106 years, gradually infills open fissures and smooths the small-scale roughness of the fault blocks. Off-axis volcanism has also resulted in smoother crust, and built seamounts.Comparison of the EPR at 3°25S with the Famous Rift and Galapagos Rift reveals a similarity in the processes and small-scale landforms at fast, medium and slow-spreading ridges. There are significant differences in the medium-scale landforms, probably because plate-boundary volcanic and tectonic processes act on crust of very different strength, thickness, and age.Contribution of the Scripps Institution of Oceanography, new series.  相似文献   

9.
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10.
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11.
Sea Beam and Deep-Tow were used in a tectonic investigation of the fast-spreading (151 mm yr-1) East Pacific Rise (EPR) at 19°30 S. Detailed surveys were conducted at the EPR axis and at the Brunhes/Matuyama magnetic reversal boundary, while four long traverses (the longest 96 km) surveyed the rise flanks. Faulting accounts for the vast majority of the relief. Both inward and outward facing fault scarps appear in almost equal numbers, and they form the horsts and grabens which compose the abyssal hills. This mechanism for abyssal hill formation differs from that observed at slow and intermediate spreading rates where abyssal hills are formed by back-tilted inward facing normal faults or by volcanic bow-forms. At 19°30 S, systematic back tilting of fault blocks is not observed, and volcanic constructional relief is a short wavelength signal (less than a few hundred meters) superimposed upon the dominant faulted structure (wavelength 2–8 km). Active faulting is confined to within approximately 5–8 km of the rise axis. In terms of frequency, more faulting occurs at fast spreading rates than at slow. The half extension rate due to faulting is 4.1 mm yr-1 at 19°30 S versus 1.6 mm yr-1 in the FAMOUS area on the Mid-Atlantic Ridge (MAR). Both spreading and horizontal extension are asymmetric at 19°30 S, and both are greater on the east flank of the rise axis. The fault density observed at 19°30 S is not constant, and zones with very high fault density follow zones with very little faulting. Three mechanisms are proposed which might account for these observations. In the first, faults are buried episodically by massive eruptions which flow more than 5–8 km from the spreading axis, beyond the outer boundary of the active fault zone. This is the least favored mechanism as there is no evidence that lavas which flow that far off axis are sufficiently thick to bury 50–150 m high fault scarps. In the second mechanism, the rate of faulting is reduced during major episodes of volcanism due to changes in the near axis thermal structure associated with swelling of the axial magma chamber. Thus the variation in fault spacing is caused by alternate episodes of faulting and volcanism. In the third mechanism, the rate of faulting may be constant (down to a time scale of decades), but the locus of faulting shifts relative to the axis. A master fault forms near the axis and takes up most of the strain release until the fault or fault set is transported into lithosphere which is sufficiently thick so that the faults become locked. At this point, the locus of faulting shifts to the thinnest, weakest lithosphere near the axis, and the cycle repeats.  相似文献   

12.
Analysis of new multibeam bathymetry and all available magnetic data shows that the 340 km-long crest of the East Pacific Rise between Rivera and Tamayo transforms contains segments of both the Pacific-Rivera and the Pacific-North America plate boundaries. Another Pacific-North America spreading segment (Alarcon Rise) extends 60 km further north to the Mexican continental margin. The Pacific-North America-Rivera triple junction is now of the RRR type, located on the risecrest 60 km south of Tamayo transform. Slow North America-Rivera rifting has ruptured the young lithosphere accreted to the east flank of the rise, and extends across the adjacent turbidite plain to the vicinity of the North America-Rivera Euler pole, which is located on the plate boundary. The present absolute motion of the Rivera microplate is an anticlockwise spin at 4° m.y.–1 around a pole located near its southeast corner; its motion has recently changed as the driving forces applied to its margins have changed, especially with the evolution of the southern margin from a broad shear zone between Rivera and Mathematician microplates to a long Pacific-Rivera transform. Pleistocene rotations in spreading direction, by as much as 15° on the Pacific-Rivera boundary, have segmented the East Pacific Rise into a staircase of en echelon spreading axes, which overlap at lengthening and migrating nontransform offsets. The spreading segments vary greatly in risecrest geomorphology, including the full range of structural types found on other rises with intermediate spreading rates: axial rift valleys, split shield volcanoes, and axial ridges. Most offsets between the segments have migrated southward, but within the past 1 m.y. the largest of them (with 14–27 km of lateral displacement) have shown dueling behavior, with short-lived reversals in migration direction. Migration involves propagation of a spreading axis into abyssal hill terrain, which is deformed and uplifted while it occupies the broad shear zones between overlapping spreading axes. Tectonic rotation of the deformed crust occurs by bookshelf faulting, which generates teleseismically recorded strike-slip earthquakes. When reversals of migration direction occur, plateaus of rotated crust are shed onto the rise flanks.  相似文献   

13.
The geography of the East Pacific Rise (EPR) between 10°N and 6°S, redetermined by new surface ship surveys, is characterized by long spreading axes orthogonal to infrequent transform faults. Near 2°10N the EPR is intersected by the Cocos-Nazca spreading center at the Galapagos triple junction. The present pattern was established 27-5.5 m.y.b.p. by a complex sequence of rise-crest jumps and reorientations from a section of the Pacific-Farallon plate boundary. Transverse profiles of the rise flanks can be matched by thermal contraction curves for aging lithosphere, except between the triple junction and 4°S, where the east flank is anomalously shallow and almost horizontal. Most sections of spreading axis have the 10–30 km wide, 100–400 m high, axial ridge that is characteristic of fast spreading centers. However, within 60 km of the triple junction the rise crest structure is atypical, with an axial rift valley and elevated rift mountains, despite a spreading rate of 140 mm/yr. With the exception of this atypical section, the bathymetric profile along the spreading axis is remarkably even, with continuous, gentle slopes for hundreds of kilometers between major transform faults, where step-like offsets in axial depths occur. Most of the observations can be accommodated by a model in which the long spreading axes are underlain by continuous crustal magma chambers that allow easy longitudinal flow of magma, and whose size controls the style and dimensions of EPR crestal topography.Contribution of the Scripps Institution of Oceanography, new series.  相似文献   

14.
西太平洋中部地区是西太平洋板块边缘沟-弧-盆体系构造演化的关键区域,其地质特征与构造演化一直是地学家关注的焦点问题之一。开展岩石圈有效弹性厚度的研究对于认识该区域的形成演化具有重要的科学意义。本文采用滑动窗口导纳技术,并在挠曲模型中考虑了表面荷载和内部荷载同时存在的情况,计算得到该区域的岩石圈有效弹性厚度(Te)。计算结果显示,研究区的Te值整体上为0~50 km,其变化基本上与构造单元相吻合,且与主要的构造边界密切相关。除海底火山具有相对较小的Te值(15~20 km)外,太平洋板块整体上具有较强的岩石圈强度(25~30 km)。马里亚纳海沟和菲律宾海沟的岩石圈强度从外隆起到海沟方向表现为明显的减弱,表明岩石圈由外隆起向海沟发生了弱化。帕里西维拉海盆西部相较于东部具有较弱的岩石圈强度,这可能与海盆的非对称扩张有关。卡罗琳板块的岩石圈整体上表现为相对均一的低Te值特征(<15 km)。欧里皮克海隆、卡罗琳海岭和索罗尔海槽的Te值为3 km,这可能是强烈的火山作用所导致的结果。  相似文献   

15.
Overlapping spreading centers (OSCs) are a type of ridge axis discontinuity found along intermediate and fast spreading centers. The ridges at these locations overlap and curve towards each other. and are separated by an elongate overlap basin. A high resolution Deep-Tow survey was conducted over the 12°54 N OSC (offset 1.6 km) on the East Pacific Rise in order to study the structure of a small OSC on a fine scale. A detailed tectonic study and Deep-Tow 3-D magnetic inversion were performed on the data. Towards the tips of both limbs, the apparent age of lava flows increases, the density of exposed faults and fissures increases, and the axial graben loses definition and disappears. No active hydrothermal vents were detected in the area. These observations suggest that the magmatic budget steadily decreases along axis approaching and OSC, even where the offset is small. In contrast with OSCs which have a large offset (>5 km), the 3-D magnetic inversion solution for this OSC produced no evidence for highly magnetized areas near the tip of either spreading center.  相似文献   

16.
The right-lateral Blanco Transform Fault Zone (BTFZ) offsets the Gorda and the Juan de Fuca Ridges along a 350 km long complex zone of ridges and right-stepping depressions. The overall geometry of the BTFZ is similar to several other oceanic transform fault zones located along the East Pacific Rise (e.g., Siquieros) and to divergent wrench faults on continents; i.e., long strike-slip master faults offset by extensional basins. These depressions have formed over the past 5 Ma as the result of continual reorientation of the BTFZ in response to changes in plate motion. The central depression (Cascadia Depression) is flanked by symmetrically distributed, inward-facing back-tilted fault blocks. It is probably a short seafloor spreading center that has been operating since about 5 Ma, when a southward propagating rift failed to kill the last remnant of a ridge segment. The Gorda Depression on the eastern end of the BTFZ may have initially formed as the result of a similar occurrence involving a northward propagating rift on the Gorda ridge system. Several of the smaller basins (East Blanco, Surveyor and Gorda) morphologically appear to be oceanic analogues of continental pull-apart basins. This would imply diffuse extension rather than the discrete neovolcanic zone associated with a typical seafloor spreading center. The basins along the western half of the BTFZ have probably formed within the last few hundred thousands years, possibly as the result of a minor change in the Juan de Fuca/Pacific relative motion.  相似文献   

17.
SeaMARC II and Sea Beam bathymetric data are combined to create a chart of the East Pacific Rise (EPR) from 8°N to 18°N reaching at least 1 Ma onto the rise flanks in most places. Based on these data as well as SeaMARC II side scan sonar mosaics we offer the following observations and conclusions. The EPR is segmented by ridge axis discontinuities such that the average segment lengths in the area are 360 km for first-order segments, 140 km for second-order segments, 52 km for third-order segments, and 13 km for fourth-order segments. All three first-order discontinuities are transform faults. Where the rise axis is a bathymetric high, second-order discontinuities are overlapping spreading centers (OSCs), usually with a distinctive 3:1 overlap to offset ratio. The off-axis discordant zones created by the OSCs are V-shaped in plan view indicating along axis migration at rates of 40–100 mm yr–1. The discordant zones consist of discrete abandoned ridge tips and overlap basins within a broad wake of anomalously deep bathymetry and high crustal magnetization. The discordant zones indicate that OSCs have commenced at different times and have migrated in different directions. This rules out any linkage between OSCs and a hot spot reference frame. The spacing of abandoned ridges indicates a recurrence interval for ridge abandonment of 20,000–200,000 yrs for OSCs with an average interval of approximately 100,000 yrs. Where the rise axis is a bathymetric low, the only second-order discontinuity mapped is a right-stepping jog in the axial rift valley. The discordant zone consists of a V-shaped wake of elongated deeps and interlocking ridges, similar to the wakes of second-order discontinuities on slow-spreading ridges. At the second-order segment level, long segments tend to lengthen at the expense of neighboring shorter segments. This can be understood if segments can be approximated by cracks, because the propagation force at a crack tip is directly proportional to crack length.There has been a counter-clockwise change in the direction of spreading on the EPR between 8 and 18° N during the last 1 Ma. The cumulative change has been 3°–6°, producing opening across the Orozco and Siqueiros transform faults and closing across the Clipperton transform. The instantaneous present-day Cocos-Pacific pole is located at approximately 38.4° N, 109.5° W with an angular rotation rate of 2.10° m.y.–1 This change in spreading direction explains the predominance of right-stepping discontinuities of orders 2–4 along the Siqueiros-Clipperton and Orozco-Rivera segments, but does not explain other aspects of segmentation which are thought to be linked to patterns of melt supply to the ridge axis.There are 23 significant seamount chains in the mapped area and most are created very near the spreading axis. Nearly all of the seamount chains have trends which fall between the absolute and relative plate motion vectors.  相似文献   

18.
High-resolution Sea Beam bathymetry and Sea MARC I side scan sonar data have been obtained in the MARK area, a 100-km-long portion of the Mid-Atlantic Ridge rift valley south of the Kane Fracture Zone. These data reveal a surprisingly complex rift valley structure that is composed of two distinct spreading cells which overlap to create a small, zero-offset transform or discordant zone. The northern spreading cell consists of a magmatically robust, active ridge segment 40–50 km in length that extends from the eastern Kane ridge-transform intersection south to about 23°12′ N. The rift valley in this area is dominated by a large constructional volcanic ridge that creates 200–500 m of relief and is associated with high-temperature hydrothermal activity. The southern spreading cell is characterized by a NNE-trending band of small (50–200 m high), conical volcanos that are built upon relatively old, fissured and sediment-covered lavas, and which in some cases are themselves fissured and faulted. This cell appears to be in a predominantly extensional phase with only small, isolated eruptions. These two spreading cells overlap in an anomalous zone between 23°05′ N and 23°17′ N that lacks a well-developed rift valley or neovolcanic zone, and may represent a slow-spreading ridge analogue to the overlapping spreading centers found at the East Pacific Rise. Despite the complexity of the MARK area, volcanic and tectonic activity appears to be confined to the 10–17 km wide rift valley floor. Block faulting along near-vertical, small-offset normal faults, accompanied by minor amounts of back-tilting (generally less than 5°), begins within a few km of the ridge axis and is largely completed by the time the crust is transported up into the rift valley walls. Features that appear to be constructional volcanic ridges formed in the median valley are preserved largely intact in the rift mountains. Mass-wasting and gullying of scarp faces, and sedimentation which buries low-relief seafloor features, are the major geological processes occurring outside of the rift valley. The morphological and structural heterogeneity within the MARK rift valley and in the flanking rift mountains documented in this study are largely the product of two spreading cells that evolve independently to the interplay between extensional tectonism and episodic variations in magma production rates.  相似文献   

19.
The Nereus Deep (23°N) lies in the central portion of the Red Sea, in a region which marks a transition between the nearly continuous axial rift valley of the southern Red Sea and the northern Red Sea, where a well defined axial rift is absent. The deep-tow survey and associated heat flow measurements reported here show that the Nereus Deep is a short segment of axial rift, and it is the northernmost deep where petrology, heat flow, magnetics, and morphology all indicate classic seafloor spreading. Heat flow measured in the Nereus Deep is characterized by non-linear gradients and closely-spaced variability indicative of active hydrothermal circulation associated with seafloor spreading. The two axial highs which we have mapped in Nereus differ markedly in that the southernmost appears younger or at least has had a more recent phase of volcanism. The two axial highs are offset left laterally approximately 2 km. This small offset or bend in the axial course has been labelled the Nereus shear zone, and, despite its small extent, it mimics many of the major features of small offset, slow-slipping transform faults. This shear zone may result from shear stresses associated with misalignments in succeeding volcanic episodes. The Nereus Deep appears to represent one of the earliest phases of seafloor spreading. The Red Sea seems to be opening towards the north, and the Nereus Deep is near the tip of propagation, but it is clear from this study that rift propagation in a site of initial rifting differs greatly from that observed along a well developed, fast spreading center like the East Pacific Rise.  相似文献   

20.
We present results from a SeaMARC II bathymetry, gravity, and magnetics survey of the northern end of the large-offset propagating East Rift of the Easter microplate. The East Rift is offset by more than 300 km from the East Pacific Rise and its northern end has rifted into approximately 3 Ma lithosphere of the Nazca Plate forming a broad (70–100 km) zone of high (up to 4 km) relief referred to as the Pito Rift. This region appears to have undergone distributed and asymmetric extension that has been primarily accommodated tectonically, by block faulting and tilting, and to a lesser degree by seafloor spreading on a more recently developed magmatic accretionary axis. The larger fault blocks have dimensions of 10–15 km and have up to several km of throw between adjacent blocks suggesting that isostatic adjustments occur on the scale of the individual blocks. Three-dimensional terrain corrected Bouguer anomalies, a three-dimensional magnetic inversion, and SeaMARC II backscatter data locate the recently developed magmatic axis in an asymmetric position in the western part of the rift. The zone of magmatic accretion is characterized by an axis of negative Bouguer gravity anomalies, a band of positive magnetizations, and a high amplitude magnetization zone locating its tip approximately 10 km south of the Pito Deep, the deepest point in the rift area. Positive Bouguer gravity anomalies and negative magnetizations characterize the faulted area to the east of the spreading axis supporting the interpretation that this area consists primarily of pre-existing Nazca plate that has been block faulted and stretched, and that no substantial new accretion has occurred there. The wide zone of deformation in the Pito Rift area and the changing trend of the fault blocks from nearly N-S in the east to NW-SE in the west may be a result of the rapidly changing kinematics of the Easter microplate and/or may result from ridge-transform like shear stresses developed at the termination of the East Rift against the Nazca plate. The broad zone of deformation developed at the Pito Rift and its apparent continuation some distance south along the East Rift has important implications for microplate mechanics and kinematic reconstructions since it suggests that initial microplate boundaries may consist in part of broad zones of deformation characterized by the formation of lithospheric scale fault blocks, and that what appear to be pseudofaults may actually be the outer boundaries of tectonized zones enclosing significant amounts of stretched pre-existing lithosphere.  相似文献   

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