Ito Garrett T.

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Ito

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Garrett T.

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Constructing the crust along the Galapagos Spreading Center 91.3°–95.5°W : correlation of seismic layer 2A with axial magma lens and topographic characteristics

(American Geophysical Union, 2004-10-21) Blacic, Tanya M. ; Ito, Garrett T. ; Canales, J. Pablo ; Detrick, Robert S. ; Sinton, John M.

Multichannel seismic reflection data are used to infer crustal accretion processes along the intermediate spreading Galapagos Spreading Center. East of 92.5°W, we image a magma lens beneath the ridge axis that is relatively shallow (1.0–2.5 km below the seafloor) and narrow (∼0.5–1.5 km, cross-axis width). We also image a thin seismic layer 2A (0.24–0.42 km) that thickens away from the ridge axis by as much as 150%. West of 92.7°W, the magma lens is deeper (2.5–4.5 km) and wider (0.7–2.4 km), and layer 2A is thicker (0.36–0.66 km) and thickens off axis by <40%. The positive correlation between layer 2A thickness and magma lens depth supports the interpretation of layer 2A as the extrusive volcanic layer with thickness controlled by the pressure on the magma lens and its ability to push magma to the surface. Our findings also suggest that narrower magma lenses focus diking close the ridge axis such that lava flowing away from the ridge axis will blanket older flows and thicken the extrusive crust off axis. Flow of lava away from the ridge axis is probably promoted by the slope of the axial bathymetric high, which is largest east of 92.5°W. West of ∼94°W the “transitional” axial morphology lacks a prominent bathymetric high and layer 2A no longer thickens off axis. We detect no magma lens west of 94.7°W where a small axial valley appears. The above changes can be linked to the westward decrease in the magma and heat flux associated with the fading influence of the Galapagos hot spot on the Galapagos Spreading Center.
The role of elasticity in simulating long-term tectonic extension

(Oxford University Press, 2016-01-27) Olive, Jean-Arthur ; Behn, Mark D. ; Mittelstaedt, Eric ; Ito, Garrett T. ; Klein, Benjamin Z.

While elasticity is a defining characteristic of the Earth's lithosphere, it is often ignored in numerical models of long-term tectonic processes in favour of a simpler viscoplastic description. Here we assess the consequences of this assumption on a well-studied geodynamic problem: the growth of normal faults at an extensional plate boundary. We conduct 2-D numerical simulations of extension in elastoplastic and viscoplastic layers using a finite difference, particle-in-cell numerical approach. Our models simulate a range of faulted layer thicknesses and extension rates, allowing us to quantify the role of elasticity on three key observables: fault-induced topography, fault rotation, and fault life span. In agreement with earlier studies, simulations carried out in elastoplastic layers produce rate-independent lithospheric flexure accompanied by rapid fault rotation and an inverse relationship between fault life span and faulted layer thickness. By contrast, models carried out with a viscoplastic lithosphere produce results that may qualitatively resemble the elastoplastic case, but depend strongly on the product of extension rate and layer viscosity U × ηL. When this product is high, fault growth initially generates little deformation of the footwall and hanging wall blocks, resulting in unrealistic, rigid block-offset in topography across the fault. This configuration progressively transitions into a regime where topographic decay associated with flexure is fully accommodated within the numerical domain. In addition, high U × ηL favours the sequential growth of multiple short-offset faults as opposed to a large-offset detachment. We interpret these results by comparing them to an analytical model for the fault-induced flexure of a thin viscous plate. The key to understanding the viscoplastic model results lies in the rate-dependence of the flexural wavelength of a viscous plate, and the strain rate dependence of the force increase associated with footwall and hanging wall bending. This behaviour produces unrealistic deformation patterns that can hinder the geological relevance of long-term rifting models that assume a viscoplastic rheology.
Observational and theoretical studies of the dynamics of mantle plume–mid-ocean ridge interaction

(American Geophysical Union, 2003-11-20) Ito, Garrett T. ; Lin, Jian ; Graham, David W.

Hot spot–mid-ocean ridge interactions cause many of the largest structural and chemical anomalies in Earth's ocean basins. Correlated geophysical and geochemical anomalies are widely explained by mantle plumes that deliver hot and compositionally distinct material toward and along mid-ocean ridges. Compositional anomalies are seen in trace element and isotope ratios, while elevated mantle temperatures are suggested by anomalously thick crust, low-density mantle, low mantle seismic velocities, and elevated degrees and pressures of melting. Several geodynamic laboratory and modeling studies predict that the width over which plumes expand along the ridge axis increases with plume flux and excess buoyancy and decreases with plate spreading rate, plume viscosity, and plume-ridge separation. Key aspects of the theoretical predictions are supported by observations at several prominent hot spot–ridge systems. Still, many basic aspects of plume-ridge interaction remain enigmatic. Outstanding problems pertain to whether plumes flow toward and along mid-ocean ridges in narrow pipe-like channels or as broad expanding gravity currents, the origin of geochemical mixing trends observed along ridges, and how mantle plumes alter the geometry of the mid-ocean ridge plate boundary, as well as the origin of other ridge axis anomalies not obviously related to mantle plumes.
Morphology and segmentation of the western Galápagos Spreading Center, 90.5°–98°W : plume-ridge interaction at an intermediate spreading ridge

(American Geophysical Union, 2003-12-13) Sinton, John M. ; Detrick, Robert S. ; Canales, J. Pablo ; Ito, Garrett T. ; Behn, Mark D.

Complete multibeam bathymetric coverage of the western Galápagos Spreading Center (GSC) between 90.5°W and 98°W reveals the fine-scale morphology, segmentation and influence of the Galápagos hot spot on this intermediate spreading ridge. The western GSC comprises three morphologically defined provinces: A Western Province, located farthest from the Galápagos hot spot west of 95°30′W, is characterized by an axial deep, rift valley morphology with individual, overlapping, E-W striking segments separated by non-transform offsets; A Middle Province, between the propagating rift tips at 93°15′W and 95°30′W, with transitional axial morphology strikes ∼276°; An Eastern Province, closest to the Galápagos hot spot between the ∼90°50′W Galápagos Transform and 93°15′W, with an axial high morphology generally less than 1800 m deep, strikes ∼280°. At a finer scale, the axial region consists of 32 individual segments defined on the basis of smaller, mainly <2 km, offsets. These offsets mainly step left in the Western and Middle Provinces, and right in the Eastern Province. Glass compositions indicate that the GSC is segmented magmatically into 8 broad regions, with Mg # generally decreasing to the west within each region. Striking differences in bathymetric and lava fractionation patterns between the propagating rifts with tips at 93°15′W and 95°30′W reflect lower overall magma supply and larger offset distance at the latter. The structure of the Eastern Province is complicated by the intersection of a series of volcanic lineaments that appear to radiate away from a point located on the northern edge of the Galápagos platform, close to the southern limit of the Galápagos Fracture Zone. Where these lineaments intersect the GSC, the ridge axis is displaced to the south through a series of overlapping spreading centers (OSCs); abandoned OSC limbs lie even farther south. We propose that southward displacement of the axis is promoted during intermittent times of increased plume activity, when lithospheric zones of weakness become volcanically active. Following cessation of the increased plume activity, the axis straightens by decapitating southernmost OSC limbs during short-lived propagation events. This process contributes to the number of right stepping offsets in the Eastern Province.
Magmatic and tectonic extension at mid-ocean ridges : 2. Origin of axial morphology

(American Geophysical Union, 2008-09-30) Ito, Garrett T. ; Behn, Mark D.

We investigate the origin of mid-ocean ridge morphology with numerical models that successfully predict axial topographic highs, axial valleys, and the transition between the two. The models are time-dependent, simulating alternating tectonic and magmatic periods where far-field extension is accommodated by faulting and by magmatism, respectively. During tectonic phases, models predict faults to grow on either side of the ridge axis and axial height to decrease. During magmatic phases, models simulate magmatic extension by allowing the axial lithosphere to open freely in response to extension. Results show that fault size and spacing decreases with increasing time fraction spent in the magmatic phase F M . Magmatic phases also simulate the growth of topography in response to local buoyancy forces. The fundamental variable that controls the transition between axial highs and valleys is the “rise-sink ratio,” (F M /F T )(τ T /τ M ), where F M /F T is the ratio of the time spent in the magmatic and tectonic periods and τ T /τ M is the ratio of the characteristic rates for growing topography during magmatic phases (1/τ M ) and for reducing topography during tectonic phases (1/τ T ). Models predict the tallest axial highs when (F M /F T )(τ T /τ M ) ≫ 1, faulted topography without a high or valley when (F M /F T )(τ T /τ M ) ∼ 1, and the deepest median valleys when (F M /F T )(τ M /τ T ) < 1. New scaling laws explain a global negative correlation between axial topography and lithosphere thickness as measured by the depths of axial magma lenses and microearthquakes. Exceptions to this trend reveal the importance of other behaviors such as a predicted inverse relation between axial topography and spreading rate as evident along the Lau Spreading Center. Still other factors related to the frequency and spatial pervasiveness of magmatic intrusions and eruptions, as evident at the Mid-Atlantic and Juan de Fuca ridges, influence the rise-sink-ratio (F M /F T )(τ T /τ M ) and thus axial morphology.
Axial high topography and partial melt in the crust and mantle beneath the western Galapagos Spreading Center

(American Geophysical Union, 2008-12-04) Blacic, Tanya M. ; Ito, Garrett T. ; Shah, Anjana K. ; Canales, J. Pablo ; Lin, Jian

The hot spot-influenced western Galápagos Spreading Center (GSC) has an axial topographic high that reaches heights of ∼700 m relative to seafloor depth ∼25 km from the axis. We investigate the cause of the unusual size of the axial high using a model that determines the flexural response to loads resulting from the thermal and magmatic structure of the lithosphere. The thermal structure simulated is appropriate for large amounts of cooling by hydrothermal circulation, which tends to minimize the amount of partial melt needed to explain the axial topography. Nonetheless, results reveal that the large axial high near 92°W requires that either the crust below the magma lens contains >35% partial melt or that 20% melt is present in the lower crust and at least 3% in the mantle within a narrow column (<∼10 km wide) extending to depths of 45–65 km. Because melt fractions >35% in the crust are considered unreasonable, it is likely that much of the axial high region of the GSC is underlain by a narrow region of partially molten mantle of widths approaching those imaged seismically beneath the East Pacific Rise. A narrow zone of mantle upwelling and melting, driven largely by melt buoyancy, is a plausible explanation.
Magmatic and tectonic extension at the Chile Ridge : evidence for mantle controls on ridge segmentation

(John Wiley & Sons, 2016-06-24) Howell, Samuel M. ; Ito, Garrett T. ; Behn, Mark D. ; Martinez, Fernando ; Olive, Jean-Arthur ; Escartin, Javier E.

We use data from an extensive multibeam bathymetry survey of the Chile Ridge to study tectonomagmatic processes at the ridge axis. Specifically, we investigate how abyssal hills evolve from axial faults, how variations in magmatic extension influence morphology and faulting along the spreading axis, and how these variations correlate with ridge segmentation. The bathymetry data are used to estimate the fraction of plate separation accommodated by normal faulting, and the remaining fraction of extension, M, is attributed primarily to magmatic accretion. Results show that M ranges from 0.85 to 0.96, systematically increasing from first-order and second-order ridge segment offsets toward segment centers as the depth of ridge axis shoals relative to the flanking highs of the axial valley. Fault spacing, however, does not correlate with ridge geometry, morphology, or M along the Chile Ridge, which suggests the observed increase in tectonic strain toward segment ends is achieved through increased slip on approximately equally spaced faults. Variations in M along the segments follow variations in petrologic indicators of mantle melt fraction, both showing a preferred length scale of 50 ± 20 km that persists even along much longer ridge segments. In comparison, mean M and axial relief fail to show significant correlations with distance offsetting the segments. These two findings suggest a form of magmatic segmentation that is partially decoupled from the geometry of the plate boundary. We hypothesize this magmatic segmentation arises from cells of buoyantly upwelling mantle that influence tectonic segmentation from the mantle, up.
Magmatic and tectonic extension at mid-ocean ridges : 1. Controls on fault characteristics

(American Geophysical Union, 2008-08-02) Behn, Mark D. ; Ito, Garrett T.

We use 2-D numerical models to explore the thermal and mechanical effects of magma intrusion on fault initiation and growth at slow and intermediate spreading ridges. Magma intrusion is simulated by widening a vertical column of model elements located within the lithosphere at a rate equal to a fraction, M, of the total spreading rate (i.e., M = 1 for fully magmatic spreading). Heat is added in proportion to the rate of intrusion to simulate the thermal effects of magma crystallization and the injection of hot magma into the crust. We examine a range of intrusion rates and axial thermal structures by varying M, spreading rate, and the efficiency of crustal cooling by conduction and hydrothermal circulation. Fault development proceeds in a sequential manner, with deformation focused on a single active normal fault whose location alternates between the two sides of the ridge axis. Fault spacing and heave are primarily sensitive to M and secondarily sensitive to axial lithosphere thickness and the rate that the lithosphere thickens with distance from the axis. Contrary to what is often cited in the literature, but consistent with prior results of mechanical modeling, we find that thicker axial lithosphere tends to reduce fault spacing and heave. In addition, fault spacing and heave are predicted to increase with decreasing rates of off-axis lithospheric thickening. The combination of low M, particularly when M approaches 0.5, as well as a reduced rate of off-axis lithospheric thickening produces long-lived, large-offset faults, similar to oceanic core complexes. Such long-lived faults produce a highly asymmetric axial thermal structure, with thinner lithosphere on the side with the active fault. This across-axis variation in thermal structure may tend to stabilize the active fault for longer periods of time and could concentrate hydrothermal circulation in the footwall of oceanic core complexes.

Ito Garrett T.

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