Adjacent Spreading Center Research Articles

STRUCTURE AND DYNAMICS OF ENDOGENIC RELIEF OF THE TJÖRNES TRANSFORM ZONE (NORTHERN ICELAND)

The Tjörnes Transform Zone connects the Kolbeinsey spreading ridge and the Northern rift zone of Iceland. It includes two overlapping rift segments resulting from the rift zones propagating. Nowadays it includes several morphostructures that vary in their organization and dynamics, such as magmatic and amagmatic rifts, oblique fault zones, or tectonic-volcanic rises. They often overlap with each other promoting the development of block morphostructures of different scale. Using the morphometric analysis of present-day fault scraps we revealed the areas of different recent tectonic activity and faulting intensity, and explained geodynamic reasons of these differences. For instance, the most active morphostructures are spreading and rift segments of the western branch of the transform zone. The eastern branch has significantly weaker tectonic activity which is mainly due to the intensive volcanic processes. Nowadays the eastern branch of the transform zone continues its development that is expressed through the formation of block rises between the overlapping rift segments. The western branch has less complicated structure and gradually fading tectonic-magmatic activity. We also showed the functional changes of amagmatic rifts: at the early stages of transform zone development, they probably evolved as a continuation of adjacent spreading centers. Meanwhile now they have their own independent extension centers. All explored morphostructures influence each other in the process of overlapping that significantly impacts the endogenic relief formation, especially faulting, at different sections of the morphostructures.

Slip‐rate‐dependent melt extraction at oceanic transform faults

AbstractCrustal thickness differences between oceanic transform faults and associated mid‐ocean ridges may be explained by melt migration and extraction processes. Slow‐slipping transform faults exhibit more positive gravity anomalies than the adjacent spreading centers, indicating relative thin crust in the transform domain, whereas at intermediate‐spreading and fast‐spreading ridges transform faults are characterized by more negative gravity anomalies than the adjacent spreading centers, indicating thick crust in the transform domain. We present numerical models reproducing these observations and infer that melt can be extracted at fast‐slipping transforms, but not at slow‐slipping ones. Melt extraction is modeled as a three‐step process. (1) Melt moves vertically through buoyancy‐driven porous flow enhanced by subvertical dissolution channels. (2) Melt accumulates in and travels along a decompaction channel lining a low‐permeability barrier at the base of the thermal boundary layer. (3) Melt is extracted to the surface when it enters a melt extraction zone. A melt extraction width of 2–4 km and a melt extraction depth of 15–20 km are needed to fit the tectonic damages associated with oceanic plate boundaries that reach into the upper mantle. Our conclusions are supported by the different degrees of magmatic activities exhibited at fast‐slipping and slow‐slipping transforms as reflected in geological features, geochemical signals, and seismic behaviors. We also constrain that the maximum lateral distance of crust‐level dike propagation is about 50–70 km.

Short wavelength heterogeneity in the Galápagos plume: Evidence from compositionally diverse basalts on Isla Santiago

Analyses of basalts from the Galápagos archipelago and adjacent spreading center have shown that the underlying mantle plume is isotopically heterogeneous over length scales of 10s to 100s of km. We show that the convecting mantle is also compositionally heterogeneous on smaller length scales (kms). Our evidence comes from recent small‐volume mildly alkaline and tholeiitic basalts on Isla Santiago, central Galápagos. Flows from volcanic vents <5 km apart are homogeneous in terms of incompatible‐trace element and isotopic ratios but inter‐vent variations in these ratios are large, such that Santiago basalts display some of the most extensive ranges known for any Galápagos island. Geochemical indexes of depth of melting correlate with an eastward decrease in geophysical estimates of lithospheric thickness–from 55 to 43 km over a ∼10 km horizontal distance beneath Isla Santiago–suggesting that melts have not undergone significant lateral transport in the underlying crust. This lithospheric ‘step’ below the center of the island generally results in a greater proportion of enriched mantle melts contributing to basalts from west Santiago, than to those in the east, due to less melting of more depleted mantle as the plume upwells beneath thicker lithosphere. Nevertheless, the sporadic occurrence of isotopically enriched flows with low [Sm/Yb]n, and more isotopically depleted compositions with elevated [Sm/Yb]n, across Santiago suggests that portions of the underlying Galápagos plume are compositionally heterogeneous in terms of enriched and depleted reservoirs over short length scales (kilometers). In this respect the Galápagos mantle plume is similar to plumes beneath Hawaii and Canary Islands.

Control of the symmetry of plume‐ridge interaction by spreading ridge geometry

The Iceland, Galápagos, and Azores plumes have previously been identified as interacting asymmetrically with adjacent spreading centers. We present evidence that the flow fields in these plume heads are radially symmetric, but the geometry of the mid‐ocean ridge systems imparts an asymmetric compositional structure on outflowing plume material. First, we quantify the degree of symmetry in geophysical and geochemical observables as a function of plume center location. For each plume, we find that bathymetry and crustal thickness observations can be explained using a single center of symmetry, with these calculated centers coinciding with independently inferred plume center locations. The existence of these centers of symmetry suggests that the flow fields and temperature structure of the three plume heads are radially symmetric. However, no centers of symmetry can be found for the incompatible trace element and isotopic observations. To explain this, we develop a simple kinematic model to predict the effect of mid‐ocean ridge geometry on the chemical composition of outflowing plume material. The model assumes radially symmetric outflow from a compositionally heterogeneous plume source, consisting of a depleted mantle component and enriched blebs. These blebs progressively melt out during flow through the melting regions under spreading centers. Asymmetry in trace element and isotopic profiles develops when ridges on either side of the plume center receive material that has been variably depleted according to the length of flow path under the ridge. This model can successfully explain compositional asymmetry around Iceland and Galápagos in terms of an axisymmetric plume interacting with an asymmetric ridge system.

Petrology of Baby Bare and Mama Bare lavas

The Three Bares are small volcanoes that host the only‐known hydrothermal vents active in oceanic crust older than 2 Ma. Major element data for Baby and Mama Bare lavas indicate formation by low extents of partial melting of a depleted mantle source, with limited cooling and crystallization prior to eruption. These lavas are compositionally unlike all lavas produced at the adjacent spreading center, but resemble a subset of lavas from small seamounts nearby. We conclude that the Three Bares represent off‐axis volcanism rather than normal crustal accretion. Such off‐axis volcanism increases basement roughness locally, and thus plays an important role in hydrothermal circulation in sedimented ridge flank settings.

East Pacific Rise from Siqueiros to Orozco Fracture Zones: Along‐strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers

We have used the Sea Beam multibeam echo‐sounding system to survey the East Pacific Rise (EPR) from 8°20′N to 18°30′N, obtaining complete coverage of the EPR axial neovolcanic zone and all intervening transform faults. Here we focus on the EPR neovolcanic zone and the definition of overlapping spreading centers (OCS's) between the Orozco and Siqueiros transform faults. The neovolcanic zone is narrow (0.5–2.0 km) and continuous along strike and occurs within and near a strike‐continuous axial summit graben. The neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2–10 km wide and which continues uninterrupted along strike for 40–140 km. Within 20–50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens, and within only 3–6 km of the intersection the neovolcanic zone turns sharply into the transform valley. At seven locations between the Siqueiros and Orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance (1–15 km). In contrast with a classic ridge/transform/ridge plate boundary, however, offset rise terminations overlap each other by a distance approximately 3 times greater than their offset. They curve toward each other, and often one merges into the other along strike. Separating the OSC's is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers. The overlap basin is characterized by volcanic constructional edifices. A key difference between these offsets and small transform faults in the Atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes. A continuous axial depth profile reveals a long‐wavelength (20–60 km) undulation of the rise axis which may be associated with variations in the magmatic budget along strike. The lowest points in the axial depth profile occur near transform faults and at OSC's, suggesting that these features are associated with along‐strike minima in the magmatic budget. We suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that OSC's develop where magmatic pulses fail to meet due to misalignment of fracture systems, sinuous bends in the rise axis, or other heterogeneities. Transform faults fail to develop on fast spreading centers where the lateral offsets are small (<15 km) because the lithosphere is too thin and weak to maintain a classic rigid plate ridge/transform fault pattern. The OSC geometry is unstable and evolves rapidly, and the overlapping rises may propagate at rates faster than the local spreading rate. One of the two OSC's prevails, while the other is abandoned. While the locations of OSC's may not be fixed relative to the rise axis, they may tend to recur near the same place, creating scars in older lithosphere which resemble fracture zones, but whose fine‐scale structure is quite different. There is excellent agreement between the observed structure of OSC's and laboratory studies of OSC's using wax models, photoelastic studies of overlapping cracks, and boundary element modeling of overlapping cracks. Seismic refraction and reflection near two OSC's suggests that large offset OSC's have separate magma chambers beneath the offset rise axes, while small offset OSC's may share a single axial magma chamber. Three‐dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some OSC tips. Waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated FeTi basalts and associated magnetic anomalies.

The memory of the accreting plate boundary and the continuity of fracture zones

A detailed aeromagnetic anomaly map of the Mesozoic seafloor-spreading lineations southwest of Bermuda reveals the dominant magnetic grain of the oceanic crust and the character of the accreting boundary at the time of crustal formation. The magnetic anomaly pattern is that of a series of elongate lobes perpendicular to the fracture zone (flowline) trends. The linear sets of magnetic anomaly peaks and troughs have narrow regions of reduced amplitude anomalies associated with the fracture zones. During the period of Mesozoic geomagnetic polarity reversals (when 1200 km of central North Atlantic seafloor formed), the Atlantic accreting boundary consisted of stationary, elongate, spreading center cells that maintained their independence even though sometimes only minor spatial offsets existed between cells. Normal oceanic crustal structure was formed in the spreading center cells, but structural anomalies and discontinuities characteristic of fracture zones were formed at their boundaries, which parallel flowlines of Mesozoic relative plate motion in the central North Atlantic. We suggest that the memory for a stationary pattern of independent spreading center cells resides in the young brittle lithosphere at the accreting boundary where the lithosphere is weakest; here, each spreading center cell independently goes through its cylce of stress buildup, stress release, and crustal accretion, after which its memory is refreshed. The temporal offset between the peaks of the accretionary activity that takes place within each cell may provide the mechanism for maintaining the independence of adjacent spreading center cells through times when no spatial offset between the cells exists.

Zero-offset fracture zones

Normal oceanic crust in the North Atlantic is typically formed in strips 50 to 80 km wide separated by fracture zones. Anomalous seismic crustal structure (less than 10 km wide) is found beneath fracture zones regardless of the amount of offset. Although many of these fracture zones exhibit minor and variable offsets in the seafloor-spreading magnetic lineations, they separate crust with distinctively different basement morphology and magnetic signatures even when there is no offset apparent in the magnetic lineations. The pattern of both the basement relief and the magnetic anomalies provides the evidence for the persistence of minor and variable offset fracture zones formed by the decoupling of adjacent spreading centers over a period of at least 15 m.y. of seafloor spreading.

Moderate and large earthquakes in the South Sandwich Arc: Indicators of tectonic variation along a subduction zone

Moderate and large earthquakes of the South Sandwich region that occurred between 1929 and 1963 were relocated. Variations in the long‐term seismicity along the length of the South Sandwich arc are observed to correlate with changes in the age of the subducted lithosphere and with the position of a fracture zone inferred to be on the incoming plate. The northern portion of the arc generated several events of moderate size (6 ≤ Ms < 7) but no large events (Ms ≥ 7) in the zone of underthrusting during this century. In this region, relatively old lithosphere (∼70 m.y. old) is being subducted underneath a young plate (∼7 m.y. old). The central portion of the arc produced almost no events of magnitude 6 or larger during the period studied. This section of the arc is located across from the position of an inferred fracture zone on the plate that is being subducted. The southern portion of the South Sandwich arc is being uplifted in response to the subduction of buoyant lithosphere generated at an adjacent spreading center. This is consistent with the model of ridge‐trench convergence proposed by DeLong et al. Several events of moderate and large size occurred in both the zone of underthrusting and the area seaward of the trench in this region. The low level of seismicity of the South Sandwich arc, when compared to other island arcs of similar convergence rates (7–9 cm/yr), indicates that aseismic slip may account for a substantial portion of the convergence in this subduction zone. This paper also reports fault plane solutions for three earthquakes of the region.

Late Paleogene-Neogene Planktonic Biostratigraphy and Its Geologic Implications, California: ABSTRACT

Planktonic microfossils indicate that at least in some typical sections, the Narizian Stage (Kreyenhagen Shale) is middle Eocene, spanning part of the range for the P10-P14 zones (about 49-45 m.y. ago). The Refugian Stage is essentially late Eocene, representing Paleogene zones P15-P17 (about 45-37.5 m.y. ago). Oligocene time is represented approximately by the Zemorrian Stage, defined by Paleogene zones P18-P22 (about 37.5-22.5 m.y. ago). Miocene subdivisions are: early Miocene, Saucesian Stage (Neogene zones N4-N8), radiometric age span about 23-15 m.y. ago; middle Miocene, Relizian, and Luisian Stages (Neogene zones N9-N12), radiometric age span about 15-11 m.y. ago; and the late Miocene Mohnian Stage (Neogene zones N13-N18), radiometric age span about 11-3 m.y. ago. T e Pliocene is represented by the Repettian, Venturian, and the lower part of the Wheelerian Stage (Neogene zones N19-N21) with a radiometric age span of about 3-1.79 m.y. ago. The Quaternary is younger than 1.79 m.y., embracing the upper part of the Wheelerian Stage, the Hallian, and younger units, representing Neogene zones N22 and N23. Employing planktonic criteria, boundaries of stages based on benthic species are clearly time-transgressive. A major structural change occurred in the Zemorrian Stage, the Oligocene, introducing a new set of basin patterns with the destruction of the Farallon plate and the adjacent spreading center, about 29-24 m.y. ago. A second important structural change occurred about 4 m.y. ago, in the late Miocene, with the development of a different pattern of basins and the formation of the Gulf of California. This new tectonic pattern resulted from, or was related to, the development of a westerly motion of the Pacific plate. The major volcanism of the California Tertiary occurred following the destruction of the Farallon plate, during the early and middle Miocene. End_Page 429------------------------------ Maximum sedimentation rates for the middle Eocene Narizian Stage (Kreyenhagen Shale) are less than 100 m/m.y., for the late Eocene (Refugian Stage) about 100 m/m.y., for the Oligocene Zemorrian Stage about 166 m/m.y., for the early Miocene about 140 m/m.y., for the middle Miocene about 500 m/m.y., for the late Miocene about 250 m/m.y., and for the Pliocene-Pleistocene they are almost 2,000 m/m.y. The very rapid sedimentation rates, following the second major change in tectonic patterns about 4 m.y. ago, are similar to those today in Santa Barbara basin; they are less than the much higher rates recognized for the late Pleistocene in basins of the continental borderland. End_of_Article - Last_Page 430------------