Rise Flanks Research Articles

Upslope flow of turbidity currents on the northwest flank of the Cearà Rise: western Equatorial Atlantic*

ABSTRACTA piston core (RC16‐57) raised from the northwestern flank of the Ceará Rise contained several turbidites up to 62 cm thick with grain sizes ranging from clay to coarse sand. These turbidites were similar in composition to terrigenous turbidites found throughout the Amazon Cone, continental rise and abyssal plains of the western Equatorial Atlantic. The core site (RC16‐57) on the Ceará Rise, however, was 156 m above the level of the adjacent Amazon Cone (the source of the turbidites). Thus the turbidity currents which deposited these beds apparently had to flow upslope for 17 km to reach the core site. Sub‐bottom reflectors observed on a 3.5 kHz echogram that extended from the Amazon Cone upslope past the core site suggested that these and deeper turbidites extended from the cone up the rise flank to distances of up to 40 km from the cone/rise boundary and to elevations up to 400 m above the level of the cone at the base of the rise. An equally plausible explanation could be that the turbidity currents that deposited these sediments were in excess of 400 m in thickness and thus would not require uphill flow to reach their observed location on the rise flank. The absence of terrigenous turbidites from the bases of topographic knolls on the continental rise and abyssal plains throughout the western Equatorial Atlantic indicated, however, that turbidity currents were normally less than 100 m thick and hence would seem to rule out this explanation. The average gradient of the rise flank in this region was about 1 : 1000 (\sim 0.5°).

Structural geomorphology of a fast-spreading rise crest: The East Pacific Rise near 3�25?S

A deeply-towed instrument package was used in a detailed survey of the crest of the East Pacific Rise (EPR) near 3°25′S, 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°25′S 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.

Regional shape and tectonics of the equatorial East Pacific Rise

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°10′N 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.

Deep-tow observations at the mounds abyssal hydrothermal field, Galapagos Rift

Small 5–20 m high mounds of lithified sediment were discovered in 1972 during a deep-tow survey 20–30 km south of the spreading axis of the Galapagos Rift, at a depth of 2.7 km. They were reexamined with an improved deep-tow instrument in 1976. The features are 20–50 m wide, conical in section and generally elongate in plan, and arranged in parallel chains that are commonly 1–2 km long, and aligned parallel to the rise flank's fault-block relief. Their rough sides, sloping at 35–45° from sharp crests, are encrusted with thick deposits of manganese oxide. Manganese crusts also occur on the sea floor in 10–20 m wide stripes of high acoustic reflectivity which extend discontinuously for several kilometers, generally on the crests of low rises (sometimes linking the cones and ridges of a chain). Although they are structures of the 30 m thick sediment blanket rather than of the volcanic bedrock, many chains and stripes overlie minor bedrock faults, and their plan patterns mimic those of minor faults and fissures exposed and mapped at the present spreading axis. These landforms and bands of mineralization are interpreted as the result of precipitation around the discharge vents of an active hydrothermal circulation in the oceanic crust, though no plumes of modified bottom water were found above them. They seem to form only after the basaltic crust has been completely buried with sediment, even along the scarps at fault-block boundaries which are the likely sites of discharge on younger crust. Their existence demonstrates the ability of localized discharges to vent through a thin blanket of pelagic sediment, and their shapes and distribution patterns emphasize the importance of zones of fracturing for controlling the site and scale of circulation in the bedrock.

The interrelationships of crustal and upper mantle parameter values in the Pacific

The data from some 400‐odd seismic refraction measurements of crustal structure in the Pacific Ocean basin are examined from the standpoint of possible systematic interrelationships of crustal and upper mantle parameter values as a function of crustal age, changes in heat flow alone and in combination with crustal age, changes in crustal spreading rate, changes in free air gravity anomaly values, and possible changes in the petrology of newly formed crust on the rise crests. It is found that there are systematic interrelationships between depth of water, thickness of the crystalline rock crust, mean seismic velocity of the crust, and seismic velocity of the mantle and also that these interrelationships change with crustal age. The age periods that appear to have significance in terms of the duration for a given set of interrelationships are 0–25, 25–36, 36–63, 63–90, and 90–≈110 m.y. Although it is likely that this pattern of changes in the interrelationships of crustal and upper mantle parameter values at intervals of between 20 and 30 m.y. continues for ages greater than 110 m.y., the data available for older age areas free of volcanic and tectonic disturbance are too few to permit the establishment of meaningful results. In the first age zone (0–25 m.y.) all parameter values for the crust and upper mantle vary directly with each other, so that on an overall basis the decrease in heat flow encountered in moving away from the rise crest axis is accompanied by an increase in depth of water, a thickening of the crust, and an increase in mean crustal velocity and mantle velocity. The second age zone (25–36 m.y.) marks a zone of transition in which a new pattern is established, the mean velocity of the crust bearing an inverse relationship to the velocity of the mantle and the thickness of the crust. This zone of transition appears to be related to the rise flank zone of low heat flow. The change from a direct to an inverse relationship between mean crustal velocity and the velocity of the mantle appears to be related to an anhydrous change as a consequence of temperatures building up beneath the Moho in association with an underlying phase transformation boundary in the upper mantle. The parameter values of the crust and upper mantle found beyond the zone of low heat flow appear to represent frozen characteristics resulting from the anhydrous transformation's not being able to reverse because the water released escaped. There is a correlation of crustal and upper mantle parameter values with past crustal spreading rate and also free air anomaly values for crust having an age of &gt;36 m.y. The correlation with spreading rate is related to the time available for modification as the crust passed through zones of crustal change such as that associated with the low heat flow zone. The correlation with gravity is related to the original composition of the crust formed beneath the rise crest spreading center and its modified compositional thickness. All the evidence suggests that the original Moho is a chemical discontinuity and that the original composition and structure of the crust and upper mantle is related to density segregation. Once established, this structure is modified by hydrothermal metamorphism, the gabbro‐garnet granulite‐eclogite transformation, and olivine‐serpentine transformation. These transformations appear to go on concurrently where heat flow exceeds 1.7 µcal/cm² s, but in the zone of low heat flow, each appears to become progressively inactive and terminates. Beyond the zone of low heat flow and for ages of &gt;36 m.y. the only active process is densification and subsidence of the crust as a consequence of continued cooling with increasing distance from the rise crest, the subsidence being in part due to the weight of the attendant increase in depth of water. Otherwise, crustal relations are as they were developed in the zone of low heat flow.