Deep explosive volcanism on the Gakkel Ridge and seismological constraints on shallow recharge at TAG active mound

Abstract

Seafloor digital imagery and bathymetric data are used to evaluate the volcanic characteristics of the 85°E segment of the ultraslow spreading Gakkel Ridge (9 mm yr -1 ). Imagery reveals that ridges and volcanic cones in the axial valley are covered by numerous, small-volume lava flows, including a few flows fresh enough to have potentially erupted during the 1999 seismic swarm at the site. The morphology and distribution of volcaniclastic deposits observed on the seafloor at depths of ~3800 m, greater than the critical point for steam generation, are consistent with having formed by explosive discharge of magma and CO 2 from source vents.Microearthquakes recorded on a 200 m aperture seismometer network deployed on the Trans-Atlantic Geotraverse active mound, a seafloor massive sulfide on the Mid-Atlantic Ridge at 26°N, are used to image subsurface processes at the hydrothermal system. Over nine-months, 32,078 local microearthquakes (M L = -1) with single-phase arrivals cluster on the southwest flank of the deposit at depths <125 m. Microearthquakes characteristics are consistent with reaction-driven cracking driven by anhydrite deposition in the shallow secondary circulation system. Exit fluid temperatures recorded at diffuse vents on the mound during the microearthquake study are used to explore linkages between seismicity and venting. AcknowledgmentsI owe my deepest gratitude to my advisers, Rob Sohn and Adam Soule, for their invaluable guidance, continual patience and support throughout the course of my graduate work. They contributed enormous energy and enthusiasm to my research and taught me how to question, pursue knowledge, and take chances. I would like to express great appreciation to all the members of my thesis committee for their insights during the preparation of this thesis: Andy Solow, Jeff McGuire, Nafi Toksöz, and Meg Tivey. I would especially like to thank Andy Solow for guidance on statistical methods; Jeff McGuire for sharing his expertise in seismology and for the field experience of working on an active source survey; Nafi Toksöz and the MIT Earth Resources Laboratory, including Fuxian Song and Junlun Li, for sharing their work on small-scale seismics; and Meg Tivey for her knowledge of the geochemical processes and structure of the TAG active mound. I would also like to thank my Thesis Defense Chair, Susan Humphris, for her feedback and patient explanations of Gakkel Ridge geochemistry. It is an honor for me to benefit from the expertise of these scientists. Assistance provided by the WHOI Academic Programs Office, especially Julia Westwater, is greatly appreciated. I would like to extend thanks to Jim Yoder for allowing me a voice about the direction of the program, and the JCMG&G, especially Rob Evans, for encouraging my progress. I wish to acknowledge my professors, especially Ralph Stephen, Jack Whitehead, and Lauren Mullineaux, for their engaging teaching. Thanks to my administrator, Kelly Servant, for her encouragement, and to my fellow students in the Joint Program and WHO...