Intra-transform Spreading Centers Research Articles

The formation of the 8˚20’ N seamount chain, east pacific rise

Near-axis seamounts provide a unique setting to investigate three-dimensional mantle processes associated with the formation of new oceanic crust and lithosphere. Here, we investigate the characteristics and evolution of the 8˚20’N Seamount Chain, a lineament of seamounts that extends ~ 175 km west of the East Pacific Rise (EPR) axis, just north of the fracture zone of the Siqueiros Transform Fault. Shipboard gravity, magnetic, and bathymetric data acquired in 2016 are utilized to constrain models of seamount emplacement and evolution. Geophysical observations indicate that these seamounts formed during four distinct episodes of volcanism coinciding with changes in regional plate motion that are also reflected in the development of intra-transform spreading centers (ITSCs) along the Siqueiros transform fault (Fornari et al. 1989; Pockalny et al. 1997). Although volcanism is divided into distinct segments, the magnetic data indicate continuous volcanic construction over long portions of the chain. Crustal thickness variations along the chain up to 0.75 km increase eastward, inferred from gravity measurements, suggest that plate reorganization has considerably impacted melt distribution in the area surrounding the Siqueiros-EPR ridge transform intersection. This appears to have resulted in increased volcanism and the formation of the 8˚20’N Seamounts. These findings indicate that melting processes in the mantle and subsequently the formation of new oceanic crust and lithosphere are highly sensitive to tectonic stress changes in the vicinity of fast-spreading transform fault offsets.

Microseismicity Indicates Atypical Small‐Scale Plate Rotation at the Quebrada Transform Fault System, East Pacific Rise

AbstractClosely spaced, multi‐strand ridge transform faults (RTFs) accommodate relative motions along fast spreading mid‐ocean ridges. However, the relations between RTFs and plate spreading dynamics are poorly understood. The Quebrada system is one of the most unique RTF systems at the East Pacific Rise, consisting of four transform faults connected by three short intra‐transform spreading centers (ITSCs). We use seven‐months of ocean bottom seismograph data to study the Quebrada system, and find abundant earthquakes unevenly distributed among three active faults. We identify two deep, diffuse seismicity clouds at the inside corners of the ITSC‐transform fault intersections, and one seismically active fracture zone. The observations suggest a complex regional plate‐motion pattern, including possible heterogeneous rotations within the Quebrada system. Evolution of multi‐strand RTFs may have resulted from a strong three‐dimensional local thermal and fluid effects, while the RTFs may have also regulated regional tectonics, forming an intricate feedback system.

Predominantly recycled carbon in Earth's upper mantle revealed by He-CO2-Ba systematics in ultradepleted ocean ridge basalts

Basalts erupted within intra-transform spreading centers can be valuable probes of geochemical components in Earth's upper mantle, and provide constraints on the proportions of mantle carbon that are juvenile (primordial) vs. tectonically recycled. Here we present new results for submarine basalt glasses, erupted within the Garrett Transform Fault (GTF) and to its north and south along the East Pacific Rise (EPR). Analyses of 3He/4He, and He and CO2 concentrations in vesicles and glass were performed, through a series of crushing and melting experiments plus FTIR spectroscopy. Trace elements were analyzed by laser ablation ICP-MS.The GTF basalts provide further tests for the origin of volatile-undersaturated basalts using CO2/Ba and CO2/Nb systematics. CO2 is highly correlated with Ba and Nb in basalts erupted in the transform domain (n = 13, including 5 undersaturated basalts) and does not show the variability expected from mixing between undegassed and variably degassed melts. Rather, the melts appear to originate from a heterogeneous mantle source that was variably depleted through partial melting, and limited mixing of melts is involved in their generation. The CO2/Ba and CO2/Nb weight ratios of 106 ± 8 and 308 ± 27, respectively, are similar to values determined previously for a global suite of undersaturated mid-ocean ridge basalts (MORBs).The ridge and transform domains have distinct 3He/4He ratios. Along the nearby EPR, 3He/4He = 8.5 - 9.1 RA, while within the Garrett Transform Fault 3He/4He = 9.2 - 10.1 RA. These two basalt populations are also distinct in their Pb-Sr-Nd isotope compositions based on earlier regional studies. The distinct populations result from partial melting of two different mantle source compositions. Melting of depleted mantle containing a small amount (∼1 to 5%) of enriched, ancient heterogeneities occurs beneath the EPR. Melting of ultradepleted mantle (in which the heterogeneities have been removed by earlier melting beneath the EPR) occurs beneath the GTF. This explains the distinction between intra-transform and spreading ridge domains for 3He/4He, if the heterogeneities were enriched in U, Th and He and had low 3He/4He as would be found in tectonically recycled material. The enriched mantle component sampled by the EPR basalts has molar CO2/3He =2×109, and it dominates the CO2/3He ratio generally ascribed to the upper mantle source for mid-ocean ridge basalts. In contrast, the ultradepleted MORB mantle component sampled by the GTF basalts has CO2/3He =3×108 or less. This indicates that most of the carbon in Earth's upper mantle originates from tectonic recycling.

Ultra-depleted melt refertilization of mantle peridotites in a large intra-transform domain (Doldrums Fracture Zone; 7–8°N, Mid Atlantic Ridge)

The Doldrums transform system offsets the Equatorial Mid Atlantic Ridge by ~630 km at 7–8° N. This transform system consists of four intra-transform spreading centers (ITRs) bounded by five transform faults. The northernmost ITR is linked to the MAR axis by a ~ 180 km-long transform. Here, during two R/V A. N. Strakhov expeditions (S06 and S09), mantle peridotites were dredged along the transverse and median ridge of the transform, across the western flank of the ITR valley. Residual harzburgites were mainly sampled along the northern Doldrums transform valley, whereas plagioclase-bearing peridotites showing evidence for melt-rock interaction characterize the ITR domain. Petrological and geochemical observations reinforced by geochemical modelling are used to define the behaviour of trace elements during melt extraction and melt-rock reaction in our rocks. Results suggest that residual peridotites derive from mantle rocks that have undergone a degree of partial melting up to 12%, with melting likely starting at the transition of garnet-spinel stability fields, whereas peridotites which suffered melt-rock reactions have been divided into two types: (i) pl-impregnated peridotites, formed by migration of melts at high porosity and high melt-rock ratio; and (ii) refertilized peridotites, generated at reduced porosity, when small fractions of the same percolating melt crystallized clinopyroxene and minor plagioclase. We suggest that the refertilizing agent was a melt highly depleted in incompatible trace elements, in turn produced by an ultra-depleted mantle source. This mantle experienced previous degrees of melt extraction at the ridge axis, before being transposed laterally along the transform where it melted a second time during the opening of the intra-transform spreading segment.

Large-scale structure of the Doldrums multi-fault transform system (7-8ºN Equatorial Atlantic): preliminary results from the 45th expedition of the R/V A.N. Strakhov

The Equatorial portion of the Mid Atlantic Ridge is displaced by a series of large offset oceanic transforms, also called “megatransforms”. These transformdomains are characterized by a wide zone of deformation that may include different conjugated fault systems and intra-transform spreading centers (ITRs).Among these megatransforms, the Doldrums system (7-8oN) is arguably the less studied, although it may be considered the most magmatically active. Newgeophysical data and rock samples were recently collected during the 45th expedition of the R/V Akademik Nikolaj Strakhov. Preliminary cruise results allowto reconstruct the large-scale structure and the tectonic evolution of this poorly-known feature of the Equatorial Atlantic. Swath bathymetry data, coupled withextensive dredging, were collected along the entire megatransform domain, covering an area of approximately 29,000 km2. The new data clearly indicate thatthe Doldrums is an extremely complex transform system that includes 4 active ITRs bounded by 5 fracture zones. Although the axial depth decreases toward thecentral part of the system, recent volcanism is significantly more abundant in the central ITRs when compared to that of the peripheral ITRs. Our preliminaryinterpretation is that a region of intense mantle melting is located in the central part of the Doldrums system as consequence of either a general transtensive regimeor the occurrence of a more fertile mantle domain. Large regions of basement exposure characterize the transform valleys and the ridge-transform intersections.We speculate that different mechanisms may be responsible for the exposure of basement rocks. These include the uplift of slivers of oceanic lithosphereby tectonic tilting (median and transverse ridges formation), the denudation of deformed gabbro and peridotite by detachment faulting at inner corner highs, andthe exposure of deep-seated rocks at the footwall of high-angle normal faults at the intersection of mid-ocean ridges with transform valleys

Thermal segmentation of mid‐ocean ridge‐transform faults

Abstract3‐D finite element simulations are used to calculate thermal structures and mantle flow fields underlying mid‐ocean ridge‐transform faults (RTFs) composed of two fault segments separated by an orthogonal step over. Using fault lengths and slip rates, we derive an empirical scaling relation for the critical step over length ( ), which marks the transition from predominantly horizontal to predominantly vertical mantle flow at the base of the lithosphere under a step over. Using the ratio of step over length (LS) to , we define three degrees of segmentation: first‐degree, corresponding to type I step overs ( ≥ 3); second‐degree, corresponding to type II step overs (1 ≤ < 3); and third‐degree, corresponding to type III step overs ( <1). In first‐degree segmentation, thermal structures and mantle upwelling patterns under a step over are similar to those of mature ridges, where normal mid‐ocean ridge basalts (MORBs) form. The seismogenic area under first‐degree segmentation is characteristic of two, isolated faults. Second‐degree segmentation creates pull‐apart basins with subdued melt generation, and intratransform spreading centers with enriched MORBs. The seismogenic area of RTFs under second‐degree segmentation is greater than that of two isolated faults, but less than that of an unsegmented RTF. Under third‐degree segmentation, mantle flow is predominantly horizontal, resulting in little lithospheric thinning and little to no melt generation. The total seismogenic area under third‐degree segmentation approaches that of an unsegmented RTF. Our scaling relations characterize the degree of segmentation due to step overs along transform faults and provide insight into RTF frictional processes, seismogenic behavior, and melt transport.

Melt extraction pathways at segmented oceanic ridges: Application to the East Pacific Rise at the Siqueiros transform

[1] Throughout the global mid-ocean ridge system, transform faults offset spreading centers. Conductive cooling may be more efficient beneath transform faults, producing a thickened lithosphere that directs melt away from the transform. However, recent observations of thickened crust along transform faults at fast ridges suggest melt redistribution toward transforms, intra-crustal melt production, or efficient extraction of melt. We apply a 3-D model of melt migration and extraction along an oceanic transform domain bounded by ridge segments. Melt is assumed to travel vertically before collecting and migrating beneath a low-permeability boundary inclined towards the ridge axis. A melt extraction zone, which may be geologically interpreted as the presence of faults and/or dikes leading to rapid lateral and vertical melt migration toward plate boundaries, affects the pattern of crustal accretion at segmented ridges. First, we examine a generic ridge-transform-ridge geometry and then a model that represents the Siqueiros transform on the East Pacific Rise. On the basis of crustal thickness variations within the intra-transform spreading centers along the fast-slipping Siqueiros fault, we constrain the presence of a melt extraction zone within 10 km of the transform zone.

Melt generation, crystallization, and extraction beneath segmented oceanic transform faults

We examine mantle melting, fractional crystallization, and melt extraction beneath fast slipping, segmented oceanic transform fault systems. Three‐dimensional mantle flow and thermal structures are calculated using a temperature‐dependent rheology that incorporates a viscoplastic approximation for brittle deformation in the lithosphere. Thermal solutions are combined with the near‐fractional, polybaric melting model of Kinzler and Grove (1992a, 1992b, 1993) to determine extents of melting, the shape of the melting regime, and major element melt composition. We investigate the mantle source region of intratransform spreading centers (ITSCs) using the melt migration approach of Sparks and Parmentier (1991) for two end‐member pooling models: (1) a wide pooling region that incorporates all of the melt focused to the ITSC and (2) a narrow pooling region that assumes melt will not migrate across a transform fault or fracture zone. Assuming wide melt pooling, our model predictions can explain both the systematic crustal thickness excesses observed at intermediate and fast slipping transform faults as well as the deeper and lower extents of melting observed in the vicinity of several transform systems. Applying these techniques to the Siqueiros transform on the East Pacific Rise we find that both the viscoplastic rheology and wide melt pooling are required to explain the observed variations in gravity inferred crustal thickness. Finally, we show that mantle potential temperature Tp = 1350°C and fractional crystallization at depths of 9–15.5 km fit the majority of the major element geochemical data from the Siqueiros transform fault system.

Thermo-mechanical control of axial topography of intra-transform spreading centers

The Quebrada/Discovery/GoFar fracture zone region on the East Pacific Rise at ~ 4°S spreads at a “fast” half-rate of ~ 70 mm/yr and includes 8 intra-transform spreading centers with lengths ranging from 5 to 70 km. The longer spreading centers have uniform axial highs typically associated with fast spreading ridges. However, the Quebrada and GoFar fracture zones each contain 2–3 short spreading centers with axial depths as great as 4800 m, reminiscent of axial valleys at “slow” spreading centers such as the Mid-Atlantic ridge. Residual mantle Bouguer gravity anomalies (RMBA) indicate that these anomalously deep spreading centers are not isostatically compensated by thin crust. Instead, there must be a dynamic component to the axial topography similar to that invoked to explain the uncompensated topography at axial highs and median valleys for normal fast and slow spreading ridges. We show that a simple “rheological” parameter combining inferred crustal thickness and predicted depth of the 1000 °C isotherm correlates well with the axial depth at all spreading centers in the Quebrada/Discovery/GoFar region. Spreading centers with cold or thin crust tend to be deeper than those with hot and/or thick crust, with relief apparently amplified by stresses in the lithosphere. In addition, we note that the Quebrada and GoFar fracture zones are regions of normal to positive RMBA, contrary to a recent suggestion that negative RMBA and thicker crust is typical of fracture zones on fast spreading ridges.

Reconciling late Neogene Pacific absolute and relative plate motion changes

New models of Pacific absolute plate motion relative to hot spots and models of relative plate motion involving the Pacific plate all agree there was a significant change in the late Neogene (Chron 3A, ∼5.89 Ma), reflecting a more northerly absolute motion than previously determined. As Pacific absolute plate motion became slightly more northerly, left‐stepping transform segments came under compression. Some left‐stepping segments became microplates with clockwise rotation; others show clear evidence of compressional deformation. Conversely, right‐stepping transforms came under tension, and many developed intratransform spreading centers or show similar evidence for transform magmatism. Several large left‐stepping transform offsets represent portions of the Nazca plate protruding into the Pacific and as such act as obstacles to the more northerly Pacific absolute plate motion. We suggest these obstructions act to enhance the generally tensile equatorial Pacific stress regime caused by distant slab pull. As a consequence, the greater French Polynesia region has experienced diffuse volcanism that increased considerably following the Chron 3A plate motion change. We propose that since the Hawaii‐Emperor Bend time, the distant Pacific slab pull and the friction between the large, buoyant Ontong Java plateau and the northern margin of the Australia plate have produced episodes of increased tensile stresses in the equatorial Pacific. We believe these stress excursions are responsible for much of the intraplate volcanism observed in a wide triangular region from Samoa to the East Pacific and Pacific‐Antarctic Rises.

Thermal structure of oceanic transform faults

We use three-dimensional fi nite element simulations to investigate the temperature structure beneath oceanic transform faults. We show that using a rheology that incorporates brittle weakening of the lithosphere generates a region of enhanced mantle upwelling and elevated temperatures along the transform; the warmest temperatures and thinnest lithosphere are predicted to be near the center of the transform. Previous studies predicted that the mantle beneath oceanic transform faults is anomalously cold relative to adjacent intraplate regions, with the thickest lithosphere located at the center of the transform. These earlier studies used simplifi ed rheologic laws to simulate the behavior of the lithosphere and underlying asthenosphere. We show that the warmer thermal structure predicted by our calculations is directly attributed to the inclusion of a more realistic brittle rheology. This temperature structure is consistent with a wide range of observations from ridge-transform environments, including the depth of seismicity, geochemical anomalies along adjacent ridge segments, and the tendency for long transforms to break into small intratransform spreading centers during changes in plate motion.

Segmentation of transform systems on the East Pacific Rise: Implications for earthquake processes at fast-slipping oceanic transform faults

Seven of the eight transform systems along the equatorial East Pacific Rise from 12° N to 15° S have undergone extension due to reorientation of plate motions and have been segmented into two or more strike-slip fault strands offset by intratransform spreading centers (ITSCs). Earthquakes recorded along these transform systems both teleseismically and hydroacoustically suggest that segmentation geometry plays an important role in how slip is accommodated at oceanic transforms. Results of thermal calculations suggest that the thickness of the brittle layer of a segmented transform fault could be significantly reduced by the thermal effect of ITSCs. Consequently, the potential rupture area, and thus maximum seismic moment, is decreased. Using Coulomb static stress models, we illustrate that long ITSCs will prohibit static stress interaction between transform segments and limit the maximum possible magnitude of earthquakes on a given transform system. Furthermore, transform earthquakes may have the potential to trigger seismicity on normal faults flanking ITSCs.

Tectonic evolution of the Tehuantepec Ridge

The Tehuantepec Ridge is one of the most prominent lithospheric structures of the Cocos Plate, yet its tectonic evolution has remained poorly constrained until now. Calculated ocean floor ages and spreading rates, morphostructural analysis of the ridge and the surrounding ocean floor are used to infer the tectonic evolution and pattern of the Tehuantepec Ridge and associated structures. The ocean floor age estimates south of the Tehuantepec Ridge suggest an age of ∼26 Ma at the Middle America trench. A mean age difference across the Tehuantepec Ridge of ∼7 Ma suggests that the Tehuantepec Ridge would be formed as a long transform fault on the Guadalupe plate, prior to 15 Ma. The full-spreading rate estimates show that there might be a differential full-spreading rate across the transform fault between 15 and 5 Ma, when the oceanic plate just south of Clipperton Fracture Zone was decelerating between 15 and 10 Ma, then accelerating for the next 5 Ma. We propose a model in which between 13 and 8 Ma temporarily transpressional intratransform-spreading centers would have been existing as a consequence of the East Pacific Rise onset with an angle of ∼10° clockwise with respect to the ceased Mathematician ridge. Our model estimates indicate that between 21 and 15 Ma a prominent pseudofault was formed south of the Tehuantepec Ridge due to an unstable offset on the Mathematician ridge that migrated northward. The Tehuantepec Ridge and the pseudofault trace bound a deeper oceanic basin than the surrounding area, with no corresponding anomalous depth on the western side of the East Pacific Rise. The complete absence of a pseudofault trace and a deeper oceanic basin on the western side of the East Pacific Rise, as well as the spreading rate changes, suggest the existence of a microplate embedded into the Cocos Plate just south of the Tehuantepec Ridge. Also, the asymmetric and sharp morphology of the Tehuantepec Ridge suggests that it may be the expression of a major transpressional structure along the former transform fault on the Guadalupe plate 15–20 Ma ago.

U-series disequilibria in MORB from the Garrett Transform and implications for mantle melting

Abstract Here, we report 238U–230Th–231Pa–226Ra disequilibria and 87Sr/86Sr measurements in 11 mid-ocean ridge basalt (MORB) glasses from the Garrett Transform (∼13°30′S latitude on the East Pacific Rise [EPR]) whose compositions range from primitive, depleted high-MgO basalts to evolved basalts. U and Th concentrations of samples range between 3 and 75 ppb and between 6 and 220 ppb, respectively, with a corresponding large variation in Th/U (1.5–2.9). (230Th)/(232Th) varies from 1.2 to 1.6 such that (230Th)/(238U) range from ∼15% excess 230Th in a high-Th/U evolved sample to ∼25% excess 238U in a high-MgO sample with low Th/U. Out of 11 samples, 7 have 238U excess, an unusual feature for MORB. All samples have 226Ra excesses, with (226Ra)/(230Th) varying between 1.3 and 3.8 constraining ages since eruption to Comparing the Garrett Transform to the Siqueiros Transform shows a remarkable correspondence between sample setting, composition and disequilibria systematics. Both settings produce linear trends of (230Th)/(238U) as a function of Th/U, consistent with mixing between two melts derived from different depths in the melting column. The mixing relationships are identical in both locations: The most incompatible rich samples with the highest Th/U and 230Th excess come from the ridge–transform intersection (RTI), whereas the most incompatible element poor basalts with the lowest (230Th)/(238U) and Th/U are erupted along leaky transform faults. Samples with intermediate Th/U and (230Th)/(238U) all come from within intra-transform spreading centers, consistent with the spreading centers acting to homogenize these diverse magmas. The cause of variation in Th/U could reflect either melting processes or different long-lived sources. No clear indication exists within these data.

Aftershock sequences in the mid-ocean ridge environment: an analysis using hydroacoustic data

Hydroacoustic data from autonomous arrays and the U.S. Navy's Sound Surveillance System (SOSUS) provide an opportunity to examine the temporal and spatial properties of seismicity along portions of the slow-spreading Mid-Atlantic Ridge (MAR), intermediate-spreading Juan de Fuca Ridge (JdFR) and fast-spreading East Pacific Rise (EPR). Aftershock and foreshock events are selected from the hydroacoustic earthquake catalog using single-link cluster (SLC) analysis, with a combined space–time metric. In the regions examined, hydroacoustic data improve the completeness level of the earthquake catalog by ∼1.5–2.0 orders of magnitude, allowing the decay constant, p, of the modified Omori law (MOL) to be determined for individual sequences. A non-parametric goodness-of-fit test indicates six of the seven sequences examined are described well by a MOL model. The p-values obtained for individual ridge and transform sequences using hydroacoustic data are larger than that previously estimated from the analysis of a stacked sequence generated from teleseismic data. For three sequences along the Siqueiros, Discovery and western Blanco Transforms, p-values are estimated to be ∼0.94–1.29. The spatial distribution of aftershocks suggests that the mainshock rupture is constrained by intra-transform spreading centers at these locations. An aftershock sequence following a 7.1 M s thrust event near the northern edge of the Easter Microplate exhibits p=1.02±0.11. Within the sequence, aftershocks are located to the north of a large topographic ridge, which may represent the surface expression of the shallow-dipping fault that ruptured during the mainshock. Two aftershock sequences near 24°25′N and 16°35′N on the MAR exhibit higher p-values, 1.74±0.23 and 2.37±1.65, although the latter estimate is not well constrained because of the small number of aftershocks. Larger p-values along the ridge crest might reflect a hotter thermal regime in this setting. Additional monitoring, however, will be needed to determine if p-value differences between the ridge and transform sequences are robust. A 1999 sequence on the Endeavour segment of the JdFR, which has been correlated with changes in the hydrothermal system, is described poorly by the MOL model. The failure of the MOL model, the anomalously large number of earthquakes within the sequence and absence of a clearly dominant mainshock are inconsistent with aftershock activity and the simple tectonic origin that has been proposed previously for this sequence.

Anomalous seafloor spreading of the Southeast Indian Ridge near the Amsterdam‐St. Paul Plateau

The Amsterdam‐St. Paul Plateau is bisected by the intermediate‐rate spreading Southeast Indian Ridge, and numerous geophysical and tectonic anomalies arise from the interactions of the Amsterdam‐St. Paul hotspot and the spreading center. The plate boundary geometry on the hotspot platform evolves rapidly (on timescales <1 Myr), off‐axis volcanism is abundant, the seafloor does not deepen away from the axis, and transform faults do not have fracture zone extensions. Away from the hotspot platform the ridge‐transform geometry is typical of mid‐ocean ridges globally. In contrast, the Amsterdam‐St. Paul Plateau spreading segments are shorter, they often overlap each other significantly, and the intervening discontinuities are smaller, more ephemeral, and more migratory. Abyssal hills are smaller and less uniform on the hotspot platform than on neighboring spreading segments. From gravity and isostasy analysis the average thickness of the platform crust is ∼10 km, approximately 50% thicker than that of typical oceanic crust. Most of the isostatic compensation of the hotspot plateau occurs at the Moho or within the lower crust, and the effective elastic thickness of the plateau lithosphere is ∼1.6 km, less than half that of adjacent spreading segments. Away from the platform some transform faults contain intratransform spreading centers; on the platform the two transform faults have valleys which may be depocenters for abundant axial or off‐axis volcanism and mass wasting. Although not wellconstrained by magnetics coverage, the Amsterdam‐St. Paul hotspot appears to have been “captured” by the Southeast Indian Ridge, enhancing crustal production at the ridge since about 3.5 Ma. Prior to this time the hotspot formed a line of smaller, isolated volcanoes on older Australian plate. The underlying cause for the present‐day crustal accretion anomalies is the effect of melt generation from separate sources of mantle upwelling (due to plate spreading and the hotspot) which has a consequent effect of weakening the lithosphere.

Tectonic reconstruction of the Clipperton and Siqueiros Fracture Zones: Evidence and consequences of plate motion change for the last 3 Myr

Bathymetry, side‐looking sonar and magnetics data from the northern East Pacific Rise have been analyzed to determine the tectonic history of the Clipperton Fracture Zone (CFZ) and the Siqueiros Fracture Zone (SFZ) over the last 2–3 m.y. Results of tectonic reconstructions indicate a series of counterclockwise changes in spreading direction at ∼2.5 Ma (1–2°), ∼1.5 Ma (1–2°), and ∼0.5 Ma (4–5°). Along the right‐stepping Clipperton Transform, the most recent change in spreading direction has resulted in fracture zone normal compression, which we propose has created a median ridge and transform‐parallel troughs along the active transform fault. Volcanic intersection highs located near the ridge‐transform intersections (RTI) of the CFZ are interpreted to be the result of fracture‐zone‐normal extension that has created pathways for magma emplacement into/onto the older lithosphère across from the RTI. Along the left‐stepping SFZ, we propose the changes in spreading direction have generated extension across the transform and have resulted in the formation of intratransform spreading centers and flexural transverse ridges. Tectonic reconstructions indicate a single Euler pole is unable to describe the motion of the Pacific‐Cocos plate boundary between the CFZ and SFZ during periods of spreading direction change since ∼3 Ma. Transtensional transforms will adjust to a new spreading direction almost immediately, while transpressional transforms will experience compression for a period of time comparable to half the age offset of the compressional transform.

Segmentation of the Pacific‐Nazca Spreading Center, 1°N–20°S

A continuous Sea Beam swath along 2500 km of the crest of the fast‐spreading East Pacific Rise (EPR) defines the location and morphology of seven systems of spreading centers. They are bounded by six transform fault systems, where reconnaissance swaths mapped additional short axes of plate accretion. The spreading center systems are subdivided by nontransform offsets, most of which step right on the central part of the plate boundary (12°S–4.5°S) and left on the northern and southern parts. These staircases of small lateral offsets were probably created by changes in spreading direction, the right steps by a counterclockwise rotation as the 12°S–4.5°S EPR matured after 6 Ma from the propagating western boundary of the Bauer microplate. Because the evolution of this microplate also eliminated transform offsets inherited from the Pacific‐Farallon boundary (the pre‐24‐Ma ancestor of the Pacific‐Nazca EPR), led to segmentation of the (fossil) Galapagos Rise, and created the present right‐offset transform fault systems on the EPR, it is identified as the principal event in the development of the mapped plan pattern. A recent small clockwise rotation in Pacific‐Nazca relative motion is inferred to be responsible for most of the small left steps along the boundary and for segmentation of the transforms into systems of en echelon fault zones linked by short intratransform spreading centers. The plan segmentation that results from the plate and microplate histories obstructs the shallow horizontal distribution of magma along the rise crest, allowing spatial variations in the rate of vertical magma delivery to be topographically expressed by the variable cross section of the axial ridge and by the along‐strike relief of the neovolcanic zone. The pattern of variations is much more complex than that predicted by models of transform‐bounded asthenosphere upwelling and regularly spaced melt diapirs. For example, the shallowest spreading segments, as well as the deepest, are short links between long fault zones in transform fault systems, and some long straight parts of the rise crest have humped long profiles consistent with a diapir‐fed model, whereas others have a remarkably flat crest line with no topographic evidence for spreading cells.

Structure and topography of the Siqueiros transform fault system: Evidence for the development of intra-transform spreading centers

The Siqueiros transform fault system, which offsets the East Pacific Rise between 8°20′N–8°30′N, has been mapped with the Sea MARC II sonar system and is found to consist of four intra-transform spreading centers and five strike-slip faults. The bathymetric and side-looking sonar data define the total width of the transform domain to be ≈20km. The transform domain includes prominent topographic features that are related to either seafloor spreading processes at the short spreading centers or shearing along the bounding faults. The spreading axes and the seafloor on the flanks of each small spreading center comprise morphological and structural features which suggest that the two western spreading centers are older than the eastern spreading centers. Structural data for the Clipperton, Orozco and Siqueiros transforms, indicate that the relative plate motion geometry of the Pacific-Cocos plate boundary has been stable for the past ≈1.5 Ma. Because the seafloor spreading fabric on the flanks of the western spreading centers is ≈500 000 years old and parallels the present EPR abyssal hill trend (350°) we conclude that a small change in plate motion was not the cause for intra-transform spreading center development in Siqueiros. We suggest that the impetus for the development of intra-transform spreading centers along the Siqueiros transform system was provided by the interaction of small melt anomalies in the mantle (SMAM) with deepseated, throughgoing lithospheric fractures within the shear zone. Initially, eruption sites may have been preferentially located along strike-slip faults and/or along cross-faults that eventually developed into pull-apart basins. Spreading centers C and D in the eastern portion of Siqueiros are in this initial pull-apart stage. Continued intrusion and volcanism along a short ridge within a pull-apart basin may lead to the formation of a stable, small intra-transform spreading center that creates a narrow swath of ridge-parallel structures within the transform domain. The morphology and structure of the axes and flanks of spreading centers A and B in the western and central portion of Siqueiros reflect this type of evolution and suggest that magmatism associated with these intra-transform spreading centers has been active for the past ≈0.5–1.0 Ma.