Galapagos Microplate Research Articles

A new species of the bathyal yeti crab genus Kiwa Macpherson, Jones & Segonzac, 2005 (Decapoda: Anomura: Kiwaidae) from a hydrothermal vent field in the Eastern Pacific Ocean, with a key to the genus

Abstract A new species of yeti crab is described from a hydrothermal vent field near the equatorial Eastern Pacific Rise (EPR)-Galapagos Microplate at a depth of 1,628 m. Kiwa gemman. sp. is the fifth species of the family Kiwaidae Macpherson, Jones & Segonzac, 2005 to be described thus far. The new species can be distinguished from other congeners by having the lateral margin of chelipeds without spines, slender and mostly straight chelar fingers, and the spinose mesial margin of the endopod of the uropod. Also notable are the unique short, stiff, scaly-tipped setae on the antennal peduncles and chelipeds, which have not been reported to occur in congeners. A key to the species of the genus KiwaMacpherson, Jones & Segonzac, 2005 is provided.

Opening of Hess Deep Rift at the Galapagos Triple Junction

AbstractAt the Galapagos triple junction, the westward propagating Cocos‐Nazca (C‐N) Rift breaks into ~0.5 Ma crust accreted at the East Pacific Rise. Rifting transitions to full magmatic seafloor spreading in the wake of the propagating tip. The 25‐km‐long Hess Deep rift is the transitional segment from rifting to spreading. Intrarift ridge (IRR), located within Hess Deep rift, is interpreted as a detachment fault, which exhumes deep‐seated rocks to the seafloor. Although transitional segments must have occurred throughout the westward propagation of C‐N Rift, IRR is the only obvious detachment fault along the base of the Rift scarps in the last ~5 Ma of its propagation. IRR formation may be in response to a decrease in spreading rate (~40 to <20 mm/yr) and presumed lower melt supply, resulting from the formation of the Galapagos microplate ~1.4 Ma, which now controls the opening at the C‐N Rift tip.

A data processing method for MAPR hydrothermal plume turbidity data and its application in the Precious Stone Mountain hydrothermal field

Hydrothermal plume is an important constituent of seabed hydrothermal circulation and is also one of the characteristics of active hydrothermal vents. Portable Miniature Autonomous Plume Recorders (MAPR) attached to a towed deep-sea instrument was used to search for hydrothermal plumes and hydrothermal vents. We introduced the basic principle of MAPR based on deep towing technology to detect plumes, then analyzed the factors affecting the quality of the MAPR data and presented a data correction method for MAPR, including instrument location correction, noise reduction processing, system error elimination and seawater background reduction. Finally we applied the method to analyze MAPR data obtained during the Chinese DY115-21 cruise on R/V Dayang I in the “Precious Stone Mountain” hydrothermal field on the Galapagos Microplate. The results provided a better understanding of the distribution of the hydrothermal activity in this field, indicating the presence of a new hydrothermal vent.

The recent history of the Galapagos triple junction preserved on the Pacific plate

At the Galapagos triple junction, the Cocos and Nazca plates are broken by a succession of transient rifts north and south of the Cocos–Nazca (C–N) Rift. Modeling has suggested that each rift initiated at the East Pacific Rise (EPR), its location controlled by the distance of the C–N Rift tip from the EPR. Evidence on the Pacific plate confirms that each transient rift formed a true RRR triple junction with the EPR and clarifies the history of the region. At ∼1.5Ma the triple junctions began jumping rapidly toward the C–N Rift suggesting that the C–N Rift tip moved closer to the EPR. Pacific abyssal hills became broad and shallow indicating enhanced magma supply to the region. At ∼1.4Ma, the Galapagos microplate developed when extension became fixed on the southern transient rift to form the South scarp of the future Dietz rift basin. Lavas flooded the area and a Galapagos–Nazca magmatic spreading center initiated at the EPR. We suggest that a hotspot was approaching the southern triple junction from the west. The hotspot crossed to the Nazca plate ∼1.25Ma. Dietz seamount formed within the young spreading center, dikes intruded Dietz rift basin, and eruptions built volcanic ridges. Since ∼0.8Ma magmatic spreading has jumped northward twice, most recently to Dietz volcanic ridge. Amagmatic extension to the east has formed the large North scarp of Dietz rift basin. Northward jumping of the southern triple junction has maintained the microplate boundary close to the proposed hotspot.

Distributed deformation ahead of the Cocos-Nazca Rift at the Galapagos triple junction

The Galapagos triple junction is not a simple ridge-ridge-ridge (RRR) triple junction. The Cocos-Nazca Rift (C-N Rift) tip does not meet the East Pacific Rise (EPR). Instead, two secondary rifts form the link: Incipient Rift at 2°40′N and Dietz Deep volcanic ridge, the southern boundary of the Galapagos microplate (GMP), at 1°10′N. Recently collected bathymetry data are used to investigate the regional tectonics prior to the establishment of the GMP (∼1.5 Ma). South of C-N Rift a band of northeast-trending cracks cuts EPR-generated abyssal hills. It is a mirror image of a band of cracks previously identified north of C-N Rift on the same age crust. In both areas, the western ends of the cracks terminate against intact abyssal hills suggesting that each crack initiated at the EPR spreading center and cut eastward into pre-existing topography. Each crack formed a short-lived triple junction until it was abandoned and a new crack and triple junction initiated nearby. Between 2.5 and 1.5 Ma, the pattern of cracking is remarkably symmetric about C-N Rift providing support for a crack interaction model in which crack initiation at the EPR axis is controlled by stresses associated with the tip of the westward-propagating C-N Rift. The model also shows that offsets of the EPR axis may explain times when cracking is not symmetric. South of C-N Rift, cracks are observed on seafloor as old as 10.5 Ma suggesting that this triple junction has not been a simple RRR triple junction during that time.

Cracking of lithosphere north of the Galapagos triple junction

The Galapagos triple junction is a ridge-ridge-ridge triple junction where the Cocos, Nazca, and Pacifi c plates meet around the Galapagos microplate. Directly north of the large scarps of the Cocos-Nazca Rift, a 250-km-long and 50-km-wide band of northwest-southeast‐trending cracks with volcanics at their western ends crosscuts and blankets the north-south‐trending abyssal hills of the East Pacifi c Rise. It appears that the roughly northeast-southwest extension of East Pacifi c Rise‐generated seafl oor has been accommodated by a succession of minor rifts that, during at least the past 4 m.y., had their triple junctions with the East Pacifi c Rise at distances of 50‐100 km north of the tip of the propagating Cocos-Nazca Rift. We propose that the rift locations are controlled by stresses associated with the dominant Cocos-Nazca Rift, and scaled by the distance of its tip to the East Pacifi c Rise. We speculate that similar ephemeral rifts occurred south of the Cocos-Nazca Rift and were instrumental in the origin of the rotating Galapagos microplate ca. 1.5 Ma.

Counter-rotating microplates at the Galapagos triple junction

An 'incipient' spreading centre east of (and orthogonal to) the East Pacific Rise at 2 degrees 40' N has been identified as forming a portion of the northern boundary of the Galapagos microplate. This spreading centre was described as a slowly diverging, westward propagating rift, tapering towards the East Pacific Rise. Here we present evidence that the 'incipient rift' has also rifted towards the east and opens anticlockwise about a pivot at its eastern end. The 'incipient rift' then bounds a second microplate, north of the clockwise-rotating Galapagos microplate. The Galapagos triple junction region, in the eastern equatorial Pacific Ocean, thus consists of two counter-rotating microplates partly separated by the Hess Deep rift. Our kinematic solution for microplate motion relative to the major plates indicates that the two counter-rotating microplates may be treated as rigid blocks driven by drag on the microplates' edges3.

Pb, Sr, and Nd isotopic systematics of rocks from the Galapagos Microplate

Pb, Sr, and Nd isotopic analyses were performed on a suite of compositionally variable rock samples representing an approximately 2500 m thickness of oceanic crustal lithologies from the deep rift valleys and spreading centers along the periphery of the Galapagos Microplate at the junction of the Pacific, Cocos, and Nazca plates. Samples were obtained from lower peridotite and gabbro layers, Fe–Ti basalts, very recent neovolcanic flows, and the thin sedimentary cover. The samples were dredged, cored, or video-grabbed during the research cruise Sonne 60. For unaltered igneous rocks, 206Pb/204Pb ratios range from 17.70 to 18.90, 87Sr/86Sr ratios from 0.7024 to 0.7035, clustering around 0.7027, while 143Nd/144Nd ratios range from 0.51307 to 0.51324. Based on these data, little systematic isotopic variation can be associated with any specific area or lithology of the microplate. One exception is a series of very thin volcanic flows from the Summit Graben area, where Pb and Nd isotope ratios change gradually, with each successive flow. The extent of the observed Pb isotopic variation covers almost the entire range of ratios for the East Pacific Rise mid-oceanic ridge basalts published to date, suggesting that the part of the mantle responsible for formation of the Galapagos Microplate rocks is no more isotopically homogeneous than the East Pacific Rise mantle as a whole, taken over a length of several thousands of kilometres.

Beryllium 10, thorium 230, and protactinium 231 in Galapagos microplate sediments: Implications of hydrothermal activity and paleoproductivity changes during the last 100,000 years

Biogenic particle fluxes from highly productive surface waters, boundary scavenging, and hydrothermal activity are the main factors influencing the deposition of radionuclides in the area of the Galapagos microplate, eastern Equatorial Pacific. In order to evaluate the importance of these three processes throughout the last 100 kyr, concentrations of the radionuclides 10Be, 230Th, and 231Pa, and of Mn and Fe were measured at high resolution in sediment samples from two gravity cores KLH 068 and KLH 093. High biological productivity in the surface waters overlying the investigated area has led to 10Be and 231Pa fluxes exceeding production during at least the last 30 kyr and probably the last 100 kyr. However, during periods of high productivity at the up welling centers off Peru and extension of the equatorial high‐productivity zone, a relative loss of 10Be and 231Pa may have occurred in these sediment cores because of boundary scavenging. The effects of hydrothermal activity were investigated by comparing the 230Thex concentrations to the Mn/Fe ratios and by comparing the fluxes of 230Th and 10Be which exceed production. The results suggest an enhanced hydrothermal influence during isotope stages 4 and 5 and to a lesser extent during isotope stage 1 in core KLH 093. During isotope stages 2 and 3, the hydrothermal supply of Mn was deposited elsewhere, probably because of changes in current regime or deep water oxygenation. A strong increase of the Mn/Fe ratio at the beginning of climatic stage 1 which is not accompanied by an increase of the 230Thex concentration is interpreted to be an effect of Mn remobilization and reprecipitation in the sediment.

Tectonic rotations of dikes in fast-spread oceanic crust exposed near Hess Deep

Oceanic crust that initially formed at the fast-spreading (130 mm/yr) East Pacific Rise at ∼1 Ma is exposed in the walls of Hess Deep rift at the northern edge of the Galapagos microplate. Steep fault scarps expose a dismembered vertical sequence of pillow basalts, sheeted diabase dikes and gabbroic rocks. Diabase dikes in these units are dominantly vertical; however, extensive outcrops of steeply to moderatley dipping dikes exist in some places. The nonvertical attitude of the dikes may be the result of tectonic rotations attending sea-floor spreading at the East Pacific Rise or the result of complex rotations of fault blocks during the opening of the Hess Deep rift at ∼0.65 Ma.

Structural Pattern of the Galapagos Microplate and Evolution of the Galapagos Triple Junctions

A 13,000‐km2 microplate intervenes to prevent the junction of the Nazca, Cocos, and Pacific plates in the eastern equatorial Pacific Ocean west of the Galapagos Islands. The microplate is rotating clockwise at 6°/m.y. about an “absolute” pole 400 km southeast of its center, so that although its northern margin has a similar motion to adjacent parts of the Cocos plate, its southern margin is 50% slower. The perimeter of the Galapagos microplate is best defined by bathymetry, and has been surveyed with Sea Beam and examined by Deep Tow. Two of the three triple junctions on its margins are ridge‐ridge‐ridge (RRR) junctions 160 km apart on the crest of the East Pacific Rise (EPR); the most like a classic RRR junction is the Pacific‐Nazca‐Galapagos intersection at 1°10′N, where the EPR axis is joined by a narrow volcanic ridge 70 km long and 1 km high. The Cocos‐Nazca‐Galapagos junction at the west end of the Cocos‐Nazca spreading center is inferred to be of the RFF type, but the “transform fault” branches are represented by large nontransform offsets. Lack of stable transform faults is related to rapid westward migration of the junction, caused by lengthening of the Cocos‐Nazca axis at 60–70 mm/yr. This rate, which was once required to maintain a stable Nazca‐Cocos‐Pacific RRR junction, has persisted even after birth of the microplate about 1 Ma. Continuing propagation of the Cocos‐Nazca axis has opened up the Hess Deep rift basin in the eastern EPR flank in advance of the eruptive propagator tip. This rift valley has a relief of more than 3 km from its uplifted rims to the floor, where severe attenuation exposes basal layers of the EPR crust. The similar Dietz Deep rift basin, at the southeastern margin of the microplate 70 km from Hess Deep, was formed when a lengthening Galapagos‐Nazca spreading axis rifted through another part of the EPR flank.