Deep‐tow and sea beam studies of dueling propagating ridges on the East Pacific Rise near 20°40′S

Abstract

Nontransform ridge axis discontinuities, such as propagating rifts, overlapping spreading centers (OSCs) and saddle points, occur frequently along the mid-ocean ridge system. We used Sea Beam and deep-tow to study an example of a relatively large nontransform O • offset near 20 40 S on the East Pacific Rise (EPR). The two spreading center tips are 15 km apart and separated by a diffuse shear zone. A broad V-shaped wake of disturbed topography and magnetics occurs on the flanks of the EPR near the offset. On a fine scale, the offset is tectonically much more complex than either the typical OSC or propagating rift structures for two reasons. First, it is a "dueling propagator" in which structures produced by successive short-term episodes of northward and southward propagation have overprinted each other, with migration toward the south prevailing over the long term. Second, its offset distance is such that its tectonic behavior may be transitional from that of a propagating rift to that of an OSC system. The offset's rate of migration to the south has been '20 mm/yr over the last 2 m.y., far less than total opening rate of 162 mm/yr. Distinct propagation events may have been much more rapid, however, surging north and south at rates of 160-230 mm/yr. In the process, several fossil spreading ridges 30-50 km long have been abandoned and rafted to the side. We propose a fine scale tectonic evolution in which a propagating spreading center either curves inside or runs outside of a soon to be abandoned, propagating rift tip. We call this process 'self-decapitation.' Dueling propagation at 20ø40'S m•y be caused b• competing magmatic sources at shallow points along the EPR: one to the north near 20øS and one to the south near 21"30'S. The 20"40'S dueling propagator occurs at a long-wavelength low in the axial depth profile between these competing sources. In general, the division of spreading centers into segments separated by ridge axis discontinuities occurs at low points along the axis and may be caused by the pattern of upper mantle convection and the factors governing flow of magma along the strike of the ridge. overlap basin striking parallel to the overlapping spreading centers, whose depth is typically 5-10% of the Copyright 1988 by the American Geophysical Union. width of the offset. The temporal offset at OSCs is Paper number 6B6256 generally less than 0.2 m.y., so that the offset distance is 0148-0227/88/006B-6256505.00 generally less than 10-15 km, depending on the spreading 2875 2876 MACDONALD ET AL..' DUELING PROPAGATING RIDGES, EAST PACIFIC RISE 120 ø W 110 ø 1 O0 ø 90 ø RIVERA 84 m__•_m • yr OROZCO 104 mm • CLIPPERTON 120 m__•m • yr SIQUEIROS COCOS

Magnetization of axial lavas from the southern East Pacific Rise (14°–23°S): Geochemical controls on magnetic properties

“…The ridge crest extends uninterrupted by a major transform for approximately 1100 km between the Garrett transform and the Easter Microplate (Figure 1), although the ridge crest is punctuated by small, predominantly left-stepping offsets over much of this distance [Lonsdale, 1989;Scheirer and MacdonaM, 1993]. A prominent right-stepping offset occurs near 20.7øS [Macdonald et al, 1988a] (Figure 1 inset). Abundant hydrothermal plumes [Urabe et al, 1995], a nearly continuous axial magma chamber [Detrick et al, 1993], and the inflated cross-sectional area of the ridge crest [Scheirer and Macdonald, 1993] all suggest that the portion of the EPR north of the "Hump" area at 18.5øS [Sinton eta!., 1991] is magmatically robust.…”

Temporal and petrogenetic constraints on volcanic accretionary processes at 9-10 degrees north East Pacific Rise

“…Ridge segment scale observations have linked AMC depth with magma supply (e.g., Macdonald et al, 1988), and there may be a direct connection between volcanic activity and melt lens depth even at a local scale. Along these same lines, we note that the shallowest portion of the melt lens is located beneath the ridge axis, which is clearly the most active part of the OSC.…”

“…However, the amount of sedimentation covering several of the pillow ridge samples, including samples , is quite substantial and greater than what might be expected from an eruption age of ~2 kyrs. Qualitative sedimentation metrics (e.g., Macdonald et al, 1988; suggest ages of 5-20 kyrs when sediment has accumulated enough to connect between pillows, as is the case for these samples. However, sedimentation rates along the EPR vary widely, ranging from 0.3-2.6 cm/yr (Dekov and Kupstov, 1992;Lonsdale and Spiess, 1980;Marchig et al, 1986;McMurtry et al, 1981) as do age estimates based on sediment cover (Ballard et al, 1981;Macdonald et al, 1988), and such estimates may be different than actual eruption ages by more than an order of magnitude .…”

“…Ridge discontinuities also appear to separate regions of geochemically distinct melt supply, as observed by significant changes in petrology, lava geochemistry, and volcanic and hydrothermal activity across discontinuities (e.g., Langmuir et al, 1986;Thompson et al, 1985). However, although magmatic discontinuities have been inferred from correlations among bathymetry (particularly variations in the axial geometry observed in cross-sectional profiles), tectonic structure (e.g., presence or absence of an axial graben), and magmatic structure (e.g., the presence or absence of an axial magma chamber (AMC)) (e.g., Kent et al, 2000;Macdonald et al, 1988), direct evidence of intrinsic relationships between magmatic and tectonic segmentation has typically been obfuscated by a lack of geophysical constraints on crustal melt distribution in places where geochemical data are also available.…”

“…Ridge discontinuities exist on many scales, ranging from 1 st order transform faults that offset ridge axes by 100s of kms, to 4 th order discontinuities such as small (<0.5 km), non-overlapping, lateral offsets or kinks or bends in the strike of the ridge axis (Batiza and Margolis, 1986;Langmuir et al, 1986;Macdonald et al, 1988;1998;White et al, 2001;2002;. Ridge discontinuities also appear to separate regions of geochemically distinct melt supply, as observed by significant changes in petrology, lava geochemistry, and volcanic and hydrothermal activity across discontinuities (e.g., Langmuir et al, 1986;Thompson et al, 1985).…”