Ryan A. Portner

Eruptive and tectonic history of the Endeavour Segment, Juan de Fuca Ridge, based on AUV mapping data and lava flow ages

High-resolution bathymetric surveys from autonomous underwater vehicles ABE and D. Allan B. were merged to create a coregistered map of 71.7 km2 of the Endeavour Segment of the Juan de Fuca Ridge. Radiocarbon dating of foraminifera in cores from three dives of remotely operated vehicle Doc Ricketts provide minimum eruption ages for 40 lava flows that are combined with the bathymetric data to outline the eruptive and tectonic history. The ages range from Modern to 10,700 marine-calibrated years before present (yr BP). During a robust magmatic phase from >10,700 yr BP to ∼4300 yr BP, flows erupted from an axial high and many flowed >5 km down the flanks; some partly buried adjacent valleys. Axial magma chambers (AMCs) may have been wider than today to supply dike intrusions over a 2 km wide axial zone. Summit Seamount formed by ∼4770 yr BP and was subsequently dismembered during a period of extension with little volcanism starting ∼4300 yr BP. This tectonic phase with only rare volcanic eruptions lasted until ∼2300 yr BP and may have resulted in near-solidification of the AMCs. The axial graben formed by crustal extension during this period of low magmatic activity. Infrequent eruptions occurred on the flanks between 2620–1760 yr BP and within the axial graben since ∼1750 yr BP. This most recent phase of limited volcanic and intense hydrothermal activity that began ∼2300 yr BP defines a hydrothermal phase of ridge development that coincides with the present-day 1 km wide AMCs and overlying hydrothermal vent fields.

A detrital record of lower oceanic crust exhumation within a Miocene slow-spreading ridge: Macquarie Island, Southern Ocean

Uplift, exhumation, and denudation of the lower oceanic crust are recorded by sedimentary rocks of Macquarie Island (54°30′S, 158°54′E), which were deposited within the slow-spreading proto–Macquarie spreading ridge between ca. 9 and 12 Ma. Measured stratigraphic sections typically contain basal basaltic breccia lithofacies that are overlain by a thick sequence of enriched mid-ocean-ridge basalt (E-MORB) with thin intercalations of gabbroic sedimentary lithofacies. Basaltic detritus has zeolite to lower-greenschist metamorphic grades typical of the upper oceanic crust, and gabbroic detritus has upper-greenschist to amphibolite metamorphic grades typical of the lower oceanic crust. Breccia clast counts and sedimentary structures indicate that basaltic lithofacies were locally derived from the footwalls of adjacent spreading-related faults. Sedimentary structures, detrital clinopyroxene major- and trace-element geochemistry, and 206 Pb/ 238 U zircon geochronology indicate that the gabbroic lithofacies were more distally derived from a Paleogene-aged tholeiitic MORB source. Detrital zircon populations of ca. 27 and ca. 33 Ma correspond to oceanic magnetic anomalies 8o and 13o, respectively, and exclude ca. 8.5 Ma gabbroic rocks of Macquarie Island as a potential source. Geodynamic reconstructions show that anomaly 8o crust from the Southeast Indian Ridge was juxtaposed against the active proto–Macquarie spreading ridge when sedimentary rocks of Macquarie Island were deposited and was a likely source for the gabbroic lithofacies. The proto–Macquarie spreading ridge and Southeast Indian Ridge were connected by the Jurru long-offset transform, which has undergone significant transpression since 27 Ma. This transpression formed a bathymetric transverse ridge that was composed of structurally isolated blocks of heterogeneously aged Paleogene source crust, which provided the source for Macquarie Island9s gabbroic sedimentary lithofacies.

Geochemical fingerprint of hyaloclasts in glassy fragmental rocks of Macquarie Island (Southern Ocean): implications for volcanogenic sedimentary processes at a waning mid-ocean ridge

Two main types of glassy fragmental rocks formed along the Proto-Macquarie Spreading Ridge: (i) hyaloclastite breccia; and (ii) pillow-fragment breccia. Examples now exposed on Macquarie Island, Southern Ocean, were largely sourced from proximal pillow lavas. In each of seven samples examined, hyaloclasts (basaltic glass grains) have a narrow major- and trace-element geochemical range, consistent with derivation of each sample from a single volcanic eruption event. Moreover, every sample analysed within the one stratigraphic section (at three sites) displays distinctive major- and trace-element geochemistry compared with the other two sections. This suggests that hyaloclasts at each site represent discrete magma batches. A single source for these glassy fragmental rocks contrasts with the dominant fault-scarp-derived polymict sedimentary rocks on Macquarie Island. We suggest that the hyaloclasts analysed in this study were deposited in small basins between the slopes of growing pillow cones along the mid-ocean ridge. The geochemical analyses presented here encompass (weakly) fractionated (e.g. Bauer Bay) to near-primitive (e.g. Pyramid Peak) compositions. All samples presented here lie within the range of the enriched- to normal-MORB suites previously reported for the island that include the least fractionated MORB melts known globally. The interpretation of geochemically distinct magma batches over the small area of the island suggests very limited magma mixing consistent with an immature or waning magmatic system. We relate these geochemical characteristics to: (i) volcanism near a very long offset transform; and (ii) genesis of magmas during the waning stages of slow seafloor spreading within a very narrow (<50 km-wide) spreading corridor.

Geology of the Alarcon Rise, Southern Gulf of California

Author(s): Clague, DA; Caress, DW; Dreyer, BM; Lundsten, L; Paduan, JB; Portner, RA; Spelz-Madero, R; Bowles, JA; Castillo, PR; Guardado-France, R; Le Saout, M; Martin, JF; Santa Rosa-del Rio, MA; Zierenberg, RA | Abstract: