This study analyzes up‐to‐date gravity data in the Galapagos triple junction region to understand crustal structure and melt distribution beneath the propagating Cocos‐Nazca spreading center (CNSC). ...Application of a standard thermal model to the mantle Bouguer gravity anomaly (MBA) does not appear to result in a realistic crustal thickness in this region. The cross‐CNSC MBA profiles flatten and axial values increase from east toward the western end of the CNSC. A simple smoothing filter applied to the standard thermal model with different filter widths can explain the progressive flattening of the MBA and is interpreted as different distribution widths (concentrations) of partial melt in the mantle. The east‐west residual MBA gradient along the CNSC is similar to the east flank of the East Pacific Rise (EPR), suggesting that the along‐CNSC gradient could partly reflect the shallow mantle properties associated with the EPR.
Plain Language Summary
This study investigates changes in crustal thickness and shallow mantle properties beneath the westward propagating Cocos‐Nazca spreading center (CNSC) by analyzing shipboard gravity data combined with satellite gravity data in the Galapagos triple junction region. By assuming a general 1‐D plate cooling model, we correct the age‐induced thermal effect of the lithosphere with a standard thermal model, which suggest thinner crust near the western tip of the CNSC increasing in thickness toward the east. However, results also indicate that the crust continues to thicken from the CNSC axis to the north and south edges of the gore, which does not seem realistic. Instead, these gravity signals most likely reflect properties within the shallow mantle, specifically a eastward decrease in the width (and hence increase in concentration) of melt within the shallow mantle beneath the CNSC. The residual gravity gradient along the CNSC axis is similar to that on the east flank of the East Pacific Rise (EPR), suggesting that the CNSC axis gravity gradient could in part reflect variation in the mantle density beneath the EPR.
Key Points
Shipboard and satellite gravity data were used to explore the crustal structure in the Galapagos triple junction region
The gravity‐derived crustal thickness with a standard thermal model does not appear to be realistic in this region
A simple smoothing filter applied to the standard thermal model could explain gravity variation along the Cocos‐Nazca spreading center
New investigations of the Southwest Indian and Arctic ridges reveal an ultraslow-spreading class of ocean ridge that is characterized by intermittent volcanism and a lack of transform faults. We find ...that the mantle beneath such ridges is emplaced continuously to the seafloor over large regions. The differences between ultraslow- and slow-spreading ridges are as great as those between slow- and fast-spreading ridges. The ultraslow-spreading ridges usually form at full spreading rates less than about 12 mm yr(-1), though their characteristics are commonly found at rates up to approximately 20 mm yr(-1). The ultraslow-spreading ridges consist of linked magmatic and amagmatic accretionary ridge segments. The amagmatic segments are a previously unrecognized class of accretionary plate boundary structure and can assume any orientation, with angles relative to the spreading direction ranging from orthogonal to acute. These amagmatic segments sometimes coexist with magmatic ridge segments for millions of years to form stable plate boundaries, or may displace or be displaced by transforms and magmatic ridge segments as spreading rate, mantle thermal structure and ridge geometry change.
The region of the Mid‐Atlantic Ridge (MAR) between the Fifteen‐Twenty and Marathon fracture zones displays the topographic characteristics of prevalent and vigorous tectonic extension. Normal faults ...show large amounts of rotation, dome‐shaped corrugated detachment surfaces (core complexes) intersect the seafloor at the edge of the inner valley floor, and extinct core complexes cover the seafloor off‐axis. We have identified 45 potential core complexes in this region whose locations are scattered everywhere along two segments (13° and 15°N segments). Steep outward‐facing slopes suggest that the footwalls of many of the normal faults in these two segments have rotated by more than 30°. The rotation occurs very close to the ridge axis (as much as 20° within 5 km of the volcanic axis) and is complete by ∼1 My, producing distinctive linear ridges with roughly symmetrical slopes. This morphology is very different from linear abyssal hill faults formed at the 14°N magmatic segment, which display a smaller amount of rotation (typically <15°). We suggest that the severe rotation of faults is diagnostic of a region undergoing large amounts of tectonic extension on single faults. If faults are long‐lived, a dome‐shaped corrugated surface develops in front of the ridges and lower crustal and upper mantle rocks are exposed to form a core complex. A single ridge segment can have several active core complexes, some less than 25 km apart that are separated by swales. We present two models for multiple core complex formation: a continuous model in which a single detachment surface extends along axis to include all of the core complexes and swales, and a discontinuous model in which local detachment faults form the core complexes and magmatic spreading forms the intervening swales. Either model can explain the observed morphology.
We reconstruct the history of the mode of accretion of an area of the Mid‐Atlantic Ridge south of the Kane fracture zone using bathymetric morphology. The area includes 200 km of the spreading axis ...and reaches to 10 Ma on either side. We distinguish three tectonic styles: (1) volcanic construction with eruption and intrusion of magma coupled with minor faulting, (2) extended terrain with abundant large‐offset faults, (3) detachment faulting marked by extension on single long‐lived faults. Over 40% of the seafloor is made of extended terrain and detachment faults. The area includes products of seven spreading segments. The spreading axis has had detachment faulting or extended terrain on one or both sides for 70% of the last 10 Ma. In some parts of the area, regions of detachment faulting and extended terrain lie close to segment boundaries. Regions of detachment faulting initiated at 10 Ma close to the adjacent fracture zones to the north and south, and then expanded away from them. We discuss the complex evidence from gravity, seismic surveys, and bathymetry for the role of magma supply in generating tectonic style. Overall, we conclude that input of magma at the spreading axis has a general control on the development of detachment faulting, but the relationship is not strong. Other factors may include a positive feedback that stabilizes detachment faulting at the expense of volcanic extension, perhaps through the lubrication of active detachment faults by the formation of low friction materials (talc, serpentine) on detachment fault surfaces.
Key Points:
10 Ma tectonic history of spreading along 200 km of the Mid‐Atlantic Ridge
At any one time 70% of the axis has detachment faulting on one or both sides
Relationship between detachment faulting and axial magmatism is not strong
At the Galapagos triple junction in the equatorial Pacific Ocean, the Cocos‐Nazca spreading center does not meet the East Pacific Rise (EPR) but, instead, rifts into 0.4 Myr‐old lithosphere on the ...EPR flank. Westward propagation of Cocos‐Nazca spreading forms the V‐shaped Galapagos gore. Since ~1.4 Ma, opening at the active gore tip has been within the Cocos‐Galapagos microplate spreading regime. In this paper, bathymetry, magnetic, and gravity data collected over the first 400 km east of the gore tip are used to examine rifting of young lithosphere and transition to magmatic spreading segments. From inception, the axis shows structural segmentation consisting of rifted basins whose bounding faults eventually mark the gore edges. Rifting progresses to magmatic spreading over the first three segments (s1–s3), which open between Cocos‐Galapagos microplate at the presently slow rates of ~19–29 mm/year. Segments s4–s9 originated in the faster‐spreading (~48 mm/year) Cocos‐Nazca regime, and well‐defined magnetic anomalies and abyssal hill fabric close to the gore edges show the transition to full magmatic spreading was more rapid than at present time. Magnetic lineations show a 20% increase in the Cocos‐Nazca spreading rate after 1.1 Ma. The near‐axis Mantle Bouguer gravity anomaly decreases eastward and becomes more circular, suggesting mantle upwelling, increasing temperatures, and perhaps progression to a developed melt supply beneath segments. Westward propagation of individual Cocos‐Nazca segments is common with rates ranging between 12 and 54 mm/year. Segment lengths and lateral offsets between segments increase, in general, with distance from the tip of the gore.
Plain Language Summary
A fundamental question in the study of mid‐ocean ridges is how spreading centers initiate and change through time. At the Galapagos triple junction in the equatorial Pacific, the oceanic crust is being broken apart to form rift basins, which develop into individual mid‐ocean ridge spreading segments. Bathymetry, magnetic, and gravity data were collected to help understand the stages in the transition from rift basins to a magmatic seafloor spreading center. From inception, the developing spreading center is composed of individual segments that are offset from each other. Currently, rifting progresses to magmatic spreading over three segments, which opened at slow spreading rates. Older spreading segments originated at faster‐spreading rates. Data indicate that the change from rifting to full magmatic spreading was more rapid in these older segments. Near the spreading axis, gravity data show that temperatures increase beneath segments as they mature, suggesting that magma supply is developing beneath the segments. Propagation of the western ends of segments is common. Understanding how seafloor spreading initiates and forms a segmented mid‐ocean ridge provides information on how mantle melting is established beneath a rift zone and how tectonic plate boundaries evolve.
Key Points
At the Galapagos triple junction rifting changes to full magmatic spreading in stages within the Cocos‐Galapagos microplate spreading regime
Magnetic anomalies and linear off‐axis topography show magmatic spreading developed rapidly when opening was between Cocos and Nazca plates
Initial segment lengths at the tip of the Galapagos gore appear constant at ~20 km, but segment lengths are larger farther from the tip
The Early Ordovician Grampian Orogeny in the British Isles represents a classic example of collision between an oceanic island arc and a passive continental margin, starting around 480 Ma. The South ...Mayo Trough in western Ireland preserves a complete and well-dated sedimentary record of arc collision. We sampled sandstones and conglomerates from the Rosroe, Maumtrasna and Derryveeny Formations in order to assess erosion rates and patterns during and after arc collision. U–Pb dating of zircons reveals a provenance dominated by erosion from the upper levels of the Dalradian Supergroup (Southern Highland and Argyll Groups), with up to 20% influx from the colliding arc into the Rosroe Formation, but only 6% in the Maumtrasna Formation (~
465 Ma). The dominant source regions lay to the northeast (e.g. in the vicinity of the Ox Mountains, 50 km distant, along strike). The older portions of the North Mayo Dalradian and its depositional basement (the Annagh Gneiss Complex) do not appear to have been important sources, while the Connemara Dalradian only plays a part after 460 Ma, when it supplies the Derryveeny Formation. By this time all erosion from the arc had effectively ceased and exhumation rates had slowed greatly. The Irish Grampian Orogeny parallels the modern Taiwan collision in showing little role for the colliding arc in the production of sediment. Negligible volumes of arc crust are lost because of erosion during accretion to the continental margin.
In the classic mid-ocean ridge model, new seafloor is generated through a combination of magmatic diking feeding lava flows at the spreading axis, and the formation of short-offset, high-angle normal ...faults that dip toward the axis. These processes lead to the formation of a layered magmatic crust and linear, ridge-parallel abyssal hills on both ridge flanks. This model of ocean crust generation applies well to fast-spreading mid-ocean ridges (i.e., > 80 mm yr super(-1)), but it is not always valid at slower-spreading ridges. Instead, at slow-spreading ridges such as the Mid-Atlantic Ridge (MAR), which is opening at about 25 mm yr super(-1), the formation of long-lived faults (called detachments) on one flank of the ridge axis is an important process in seafloor formation (Cann et al., 1997; Karson, 1999; MacLeod et al., 2009; Schroeder et al., 2007; Smith et al., 2008; Tucholke et al., 1998). In fact, active detachment faults have been identified along nearly half of the MAR axis between 12 degree and 35 degree N (Escartin et al., 2008).
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.
Key Points
Distributed deformation is observed at the Galapagos TJ
Secondary cracks form north and south of the C‐N Rift at the EPR
Crack locations are controlled by stresses from the tip of the C‐N Rift
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