Peter Lonsdale

Structure and tectonic history of the eastern Panama Basin

New marine geophysical data allow the preparation of revised bathymetric and magnetic anomaly charts of the Panama Basin and demonstrate that the eastern part of the basin, between the fracture zone at long 83°W and the Colombian continental margin, was formed by highly asymmetric sea-floor spreading along the boundary of the Nazca and Cocos plates 27 to 8 m.y. B.P. Lineated magnetic anomalies recording this history are oriented approximately east-west. The oldest set of north-flank anomalies overlaps in age with those adjacent to the Grijalva scarp, south of the western Panama Basin, where they are oriented 065°. Younger anomalies (5C to 5) in the eastern basin are approximately parallel to anomalies of this age identified on the Carnegie platform and the flanks of the Costa Rica rift. The eastern basin now contains a pattern of fossil spreading centers (including the Malpelo rift) and transform faults (including the Yaquina graben) that were abandoned 8 m.y. B.P. by a shift in plate boundaries that transferred a large section of the Cocos plate to the Nazca plate. Cessation of Nazca-Cocos spreading east of long 83°W was heralded by a 3-m.y. deceleration of spreading on the eastern segments, which created rough topography and axial rift valleys typical of slow-spreading ridges. Westward jumping of the Nazca-Cocos-Caribbean triple junction rejuvenated the northern segment of the fracture zone at long 83°W, causing uplift of the adjacent Coiba Ridge. Recently, active transform faulting has jumped farther west, from the foot of the Coiba Ridge to the Panama fracture zone. Apart from changes in plate boundaries, the main event in the tectonic evolution of the region was initiation about 22 to 20 m.y. B.P. of the hot spot that created the Malpelo, Cocos, and Carnegie Ridges. Precursors of effusive ridge-building volcanism included major fracturing of the oceanic crust to the north of the present Malpelo Ridge. Both processes hamper identification of magnetic anomalies in the vicinity of the ridges. Our interpretation of the tectonic history is also incomplete in the easternmost parts of the basin, where data are insufficient; this impairs our interpretation of the adjacent continental geology in terms of changing interaction between oceanic and continental plates. The geologic history of the Isthmus of Panama is compatible with our application of the plate-tectonic model.

Clustering of suspension-feeding macrobenthos near abyssal hydrothermal vents at oceanic spreading centers☆

Abstract A community of abundant suspension-feeding organisms was photographed around an active hydrothermal vent at the Galapagos Rift. A site on the crest of the East Pacific Rise where hydrothermal discharge is suspected also has a dense colony of sessile organisms. The high standing crop of macrobenthos in these patches probably results from local increases of deep-sea food supply near hydrothermal plumes in the bottom water.

Paleogene history of the Kula plate: Offshore evidence and onshore implications

Paleocene to middle Eocene magnetic anomalies were mapped over oceanic crust that accreted at the Kula-Pacific spreading center and is now obliquely entering the western Aleutian Trench between 179°E and 168°E. The strike of anomalies and the pattern of abyssal hills and fracture zones changed abruptly during 56-55 Ma, when north-south spreading veered to northwest-southeast (310°-130°). Kula-Pacific spreading ceased in 43 Ma. A 75-km-long section of the fossil Kula Rift axis has avoided subduction, although it now intersects the trench axis (almost orthogonally) near 171.5°E. A narrow remnant of the former Kula plate, northwest of this fossil spreading center, is bounded by a fossil Kula-Pacific transform with a high transverse ridge alongside a sediment-filled transform valley. Anomalies on this remnant show that Eocene Kula-Pacific spreading was highly asymmetric (2:1). The 56-55 Ma change in Kula plate rotation inferred from the change in spreading direction coincided with birth of the Aleutian subduction zone, and was probably a consequence of the resulting change in slab-pull stresses on the oceanic lithosphere. The change in direction of Kula-North American motion is a plausible explanation for the detachment of continental terranes from the Pacific Northwest and their migration around the Gulf of Alaska, and for the early Eocene demise of Alaska Range are volcanism. The cessation of Kula-Pacific spreading coincides with a major change in Pacific-plate rotation, and the subsequent direction of convergence of the Pacific plate with the Aleutian arc was similar to the 55-43 Ma direction of Kula-plate convergence.

Structural geomorphology of a fast-spreading rise crest: The East Pacific Rise near 325?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.

Hydrothermal plumes, hot springs, and conductive heat flow in the Southern Trough of Guaymas Basin

Abstract A 9 km length of one of the axial rift valleys of a Gulf of California spreading center was mapped by Deep Tow and examined on ten “Alvin” dives. The Deep Tow CTD towed 10–100 m above the seafloor sensed 20 thermal plumes with temperature anomalies of up to +0.39°C. Most plumes overlie deposits of hydrothermal minerals (mainly anhydrite and sulfides) that form columns and mounds rising up to 25 m above the muddy rift floor. About 130 such sites were mapped by side-scan sonar, and about 20 of them were photographed and sampled with “Alvin”. Two of the sites that had been raised tectonically above the trough floor have only inactive residual mounds, but all the others examined by submersible had active hydrothermal vents. Fluids discharge diffusely through the porous deposits, as relatively low-temperature ( 2 W/m2) adjacent to the thermal springs. The patterns of hydrothermal deposits and measured heat flow, interpreted in light of structural information from the Deep Tow profiler, suggest that shallow sills, some extending laterally from more massive hill-forming intrusions, act as cap rocks for intense, shallow hydrothermal circulations above a magma chamber. Hot discharges from this system occur mainly through tectonic fractures in the sills and around their peripheries; recharge probably occurs on a regional scale, from cooler parts of the rift valley. Some warm discharges, inactive barite deposits, and nonlinear thermal gradients probably mark continuing and past cooling of underlying shallow intrusions, by both recharging and temporary closed-system circulations.

1 Ma East Pacific Rise oceanic crust and uppermost mantle exposed by rifting in Hess Deep (equatorial Pacific Ocean)

A series of dives with the French submersible Nautile has been conducted in the region of the Galapagos triple junction, at the tip of the Cocos-Nazca propagator. Most of the dives were devoted to the study of Hess Deep, a rift valley produced by rifting of oceanic crust created at the axis of the very fast-spreading East Pacific Rise. The dives have enabled the structures and lithologies to be assessed and confirmed earlier dredging evidence for abundant basal crust and mantle rocks on an intra-rift ridge located to the north of the axis of Hess Deep. This shows that this situation is not specific not slow-spreading ridges. A nearly complete and unique log of young (1 Ma) crust and uppermost mantle representative of very fast accretion can be reconstructed even though the section is discontinuous and highly dismembered. We propose two models that can explain the exposure on the ocean floor of the deep foundation of the oceanic crust. One model stresses symmetry and vertical tectonics through isostasy/diapirism of serpentinized mantle. Another model requires a larger amount of extension and emphasizes asymmetry with denudation of the upper crust through low-angle detachment faulting. In both models, which differ mainly in the structure at depth, amagmatic extension of East Pacific Rise crust by the Cocos-Nazca propagator tip results in uplift and denudation of lower crust and upper mantle.

Petrology of the axial ridge of the Mariana Trough backarc spreading center

Abstract The axial ridge of the Mariana Trough backarc basin, between 17°40′N and 18°30′N rises as much as 1 km above the floor of a 10–15 km wide rift valley. Physiographic segmentation, with minor ridge offsets and overlaps, coincides with a petrologic segmentation seen in trace element and isotope chemistry. Analyses of 239 glass and 40 aphyric basalt samples, collected with alvin and by dredging, show that the axial ridge is formed largely of (olivine) hypersthene-normative tholeiitic basalt. About half of these are enriched in both LIL elements and volatiles, but are depleted in HFS elements like other rocks found throughout much of the Mariana Trough. The LIL enrichments distinguish these rocks from N-MORB even though Nd and Sr isotope ratios indicate that much of the crust formed from a source similar to that for N-MORB. In addition to LIL-enriched basalt there is LIL depleted basalts even more closely resembling N-MORB in major and trace elements as well as Sr, Nd and Pb isotopes. Both basalt varieties have higher Al and lower total Fe than MORB at equivalent Mg level. Mg# ranges from relatively “primitive” (e.g. Mg# 65–70) to more highly fractionated (e.g. Mg# 45–50). Highest parts of the axial ridge are capped by pinnacles with elongated pillows of basaltic andesite (e.g. 52–56%) SiO 2 . These are due to extreme fractional crystallization of basalts forming the axial ridge. Active hydrothermal vents with chimneys and mats of opaline silica, barite, sphalerite and lesser amounts of pyrite, chalcopyrite and galena formed near these silicic rocks. The vents are surrounded by distinctive vent animals, polychaete worms, crabs and barnacles. Isotope data indicate that the Mariana Trough crust was derived from a heterogeneous source including mantle resembling the MORB-source and an “arc-source” component. The latter was depleted in HFS elements in previous melting events and later modified by addition of H 2 O and LIL elements.

A high-temperature hydrothermal deposit on the seabed at a gulf of California spreading center

Abstract A submersible dive on a turbidite-covered spreading axis in Guaymas Basin photographed and sampled extensive terraces and ledges of talc. The rock contains siliceous microfossils, smectite, and euhedral pyrrhotite as well as rather pure iron-rich talc. Sulfur and oxygen isotopes indicate precipitation around a hydrothermal vent, at about 280°C.

PETROLOGY OF THE EAST PACIFIC RISE CRUST AND UPPER MANTLE EXPOSED IN HESS DEEP (EASTERN EQUATORIAL PACIFIC)

The Hess Deep, a rifted oval-shaped depression located east of the Galapagos Triple Junction at the tip of the Cocos-Nazca ridge (about 101°W, 2°N), was explored in 1988 during 21 submersible dives. A total of 11 dives were devoted to the exploration of the E-W trending Intrarift ridge (15 km in length, 3000–5400 m in depth) north of the Hess Deep depression. The Intrarift ridge represents an outcrop of recent (1 m.y.) crustal and subcrustal material created at the axis of the East Pacific Rise (EPR), and emplaced during the lithospheric extention responsible for the westward propagation of the Cocos-Nazca rift (Francheteau et al., 1990). The lithospheric block has undergone cataclastic deformation and was dislocated by tectonic activity as attested to by the mixed and erratic distribution of rock types and by the occurrence of polygenic breccias and gabbroic mylonites. The samples are metamorphosed to varying degrees, but their protolith textures are generally well preserved. Their relic mineralogy indicates that they consist of harzburgites, dunites, gabbroic cumulates (gabbronorites and olivine gabbros), isotropic gabbros, dolerites, and basalts. Some samples of refractory harzburgites and most dunitic cumulates (with local accumulation of chromite) have been impregnated by wehrlitic and gabbroic primitive melts similar to those described from the mantle-crust transition zone of the Samail ophiolite complex (Oman). The mineral chemistry indicate that the ultramafics partly reequilibrated with the magmatic impregnations in the liquidus-solidus temperature range of 980–1100°C. The dolerites and basalts have been derived from mid-ocean ridge basalt primary melts presenting a broad range of incompatible element composition which suggests intermittent cycles of magmatic processes involving a progressive melting of a composite source with discontinous extraction of liquids as proposed for the EPR volcanics near 13°N (Hekinian et al., 1989). Most of the rocks underwent partial retrograde metamorphism and recorded several episodes of recrystallization from the upper greenschist facies (ultramafics and gabbros) to diagenetic alteration (volcanics). The cumulate gabbronorites, the isotropic gabbros, and some dolerites were partially albitized and amphibolitized during the penetration of seawater in the ocean crust prior to serpentinization. Several samples of unfoliated amphibolites are believed to be completely metamorphosed gabbroic rocks. The gabbroic cumulates and the plagioclase-rich melt impregnations were variably rodingitized (presence of various Ca-silicates such as epidote, prehnite, hydrogarnet, and zeolite) in relation to the serpentinization of the peridotites. One dive located on the scarps forming the northern wall of the Hess Deep to the east of the explored area, revealed the presence of in situ outcrops of isotropic gabbros, doleritic dikes, and extrusives and permitted to observe the contact between the sheeted dike complex and the high level isotropic gabbros.

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

Abstract 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 flanks 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.