David F. Naar

Variations in axial morphology along the Galápagos spreading center and the influence of the Galápagos hotspot

The Galapagos Spreading Center (GSC) is marked by systematic changes in axial morphology between the Inca Fracture Zone (FZ) at 85.5°W and the 95.5°W propagator. We analyze these changes using new swath bathymetry and magnetic data acquired aboard the B/O Hesperides during the Galapagos96 experiment. Within ∼350 km of the Galapagos hotspot the ridge axis is associated with an East Pacific Rise (EPR)-like axial high. At increasing distance from the hotspot the axial high broadens and deepens forming a distinctive transitional axial morphology (TAM). The axis in this transitional region is typically a broad zone (∼20 km wide) consisting of very rough volcanic and fault-generated topography. West of 95°W, this TAM evolves into a 20–40 km wide, 400–1500 m deep axial valley typical of the slow spreading Mid-Atlantic Ridge (MAR). There is not an abrupt change from axial high to rift valley along the GSC, but a distinct TAM occurs over a distance of ∼200–300 km along-axis and is accompanied by a gravity-estimated crustal thickening of >1–2 km. The boundary between an axial high and this TAM is quite abrupt and occurs along a segment that is less than 9 km long. These changes in axial morphology are primarily caused by variations in magma supply along the GSC due to the entrainment and dispersal of plume mantle from the Galapagos hotspot. However, the changes in morphology are not symmetric about the Galapagos FZ at 91°W. The axial high topography extends farther east of the 91°W FZ than to the west, and the rift valley which develops west of 94°W is not found at comparable distances along the GSC east of the hotspot. Axial depth variations are also asymmetric across the 91°W FZ. This asymmetry in both morphology and axial depth variation is attributed to a full spreading rate increase along the GSC from 46 mm/yr at 97°W to 64 mm/yr at 85°W. Off-axis depth changes are symmetric about the 91°W FZ and suggest that 15–40% of on-axis depth variation is dynamically supported.

Development of small carbonate banks on the south Florida platform margin: response to sea level and climate change

Abstract Geophysical and coring data from the Dry Tortugas, Tortugas Bank, and Riley’s Hump on the southwest Florida margin reveal the stratigraphic framework and growth history of these carbonate banks. The Holocene reefs of the Dry Tortugas and Tortugas Bank are approximately 14 and 10 m thick, respectively, and are situated upon Pleistocene reefal edifices. Tortugas Bank consists of the oldest Holocene corals in the Florida Keys with earliest coral recruitment occurring at ∼9.6 cal ka. Growth curves for the Tortugas Bank reveal slow growth (

Morphology and distribution of seamounts surrounding Easter Island

We investigate the morphology and distribution of a seamount population on a section of seafloor influenced by both superfast seafloor spreading and hotspot volcanism. The population under investigation is part of a broad chain of seamounts extending eastward from the East Pacific Rise, near Easter Island. In order to define the morphological variability of the seamounts, basal shape, cross-sectional area, volume, flatness, and flank slope are plotted against height for 383 seamounts with heights greater than 200 m, based on bathymetry data collected by GLORI-B and SeaBeam 2000, during three cruises onboard the R/V Melville in the spring of 1993. Nearly complete swath mapping coverage of the seamounts is available for the analysis of size and shape distribution. We quantitatively describe the seamount population of this active region, in which seamounts cover ∼27% of the seafloor, and account for ∼4.2% of the total crustal volume. Over 50% of the total volume (61,000 km3) of seamounts used in this study is made up by the 14 largest seamounts, and the remaining volume is made up by the 369 smaller seamounts (>200 m in height). Our analysis indicates there are at least two seamount populations in the Easter Island-Salas y Gomez Island (25°–29°S, 113°–104°W) study area. One population of seamounts is composed of short seamounts ( 1200 m), shield-like, pointy cones (flatness ∼1200 m) originate exclusively from a hotspot source, but only a portion of the smaller volcanoes (<∼1200 m) are formed from a hotspot source. The remainder would be presumably formed by a normal mantle or mixed source.

Morphology of San Antonio submarine canyon on the central Chile forearc

A multibeam survey was conducted over San Antonio submarine canyon, near Valparaiso, Chile, in April and May 1993 using the SeaBeam 2000 system on the R/V Melville. The bathymetric data from this survey reveal a canyon with an overall sinuosity of 1.25, a broad, roughly U-shaped cross-section along most of its length, and an almost constant channel slope above the forearc structural high. The course of the canyon is deflected to the north by a prominent structural high opposite the town of San Antonio. SeaBeam 2000 side-scan sonar data reveal high backscatter material on the floor of the canyon with a longitudinal fabric, reminiscent of stream braiding, and point bars formed on the inside of channel bends. We interpret this high backscatter material to be coarse sediment, transported down the canyon as turbidity currents. The source of these turbidity currents is probably the Rio Maipo, which enters the ocean near the head of the canyon.

Tectonic/volcanic segmentation and controls on hydrothermal venting along Earth's fastest seafloor spreading system, EPR 27°–32°S

[1]xa0We have collected 12 kHz SeaBeam bathymetry and 120 kHz DSL-120 side-scan sonar and bathymetry data to determine the tectonic and volcanic segmentation along the fastest spreading (∼150 km/Myr) part of the global mid-ocean ridge system, the southern East Pacific Rise between the Easter and Juan Fernandez microplates. This area is presently reorganizing by large-scale dueling rift propagation and possible protomicroplate tectonics. Fracture patterns observed in the side-scan data define structural segmentation scales along these ridge segments. These sometimes, but not always, correlate with linear volcanic systems defining segmentation in the SeaBeam data. Some of the subsegments behave cohesively, with in-phase tectonic activity, while fundamental discontinuities occur between other subsegments. We also collected hydrothermal plume data using sensors mounted on the DSL-120 instrument package, as well as CTDO tow-yos, to determine detailed structural and volcanic controls on the hydrothermal vent pattern observed along 600 km of the Pacific-Nazca axis. Here we report the first rigorous correlation between coregistered hydrothermal plume and high-resolution marine geophysical data on similar scales and over multisegment distances. Major plume concentrations were usually found where axial inflation was relatively high and fracture density was relatively low. These correlations suggest that hydrothermal venting is most active where the apparent magmatic budget is greatest, resulting in recent eruptions that have paved over the neovolcanic zone. Areas of voluminous acoustically dark young lava flows produced from recent fissure eruptions correlate with many of the major hydrothermal vent areas. Increased crustal permeability, as gauged by increased fracture density, does not enhance hydrothermal venting in this area. Axial summit troughs and graben are rare, probably because of frequent volcanic resurfacing in this superfast spreading environment, and are not good predictors of hydrothermal activity here. Many of the hydrothermal areas are found in inflated areas near the ends of segments, suggesting that abundant magma is being supplied to these areas.

Statistical Self-Similarity of Hotspot Seamount Volumes Modeled as Self-Similar Criticality

The processes responsible for hotspot seamount formation are complex, yet the cumulative frequency-volume distribution of hotspot seamounts in the Easter Island/Salas y Gomez Chain (ESC) is found to be well-described by an upper-truncated power law. We develop a model for hotspot seamount formation where uniform energy input produces events initiated on a self-similar distribution of critical cells. We call this model Self-Similar Criticality (SSC). By allowing the spatial distribution of magma migration to be self-similar, the SSC model recreates the observed ESC seamount volume distribution. The SSC model may have broad applicability to other natural systems.