Evolution of oceanic margins : rifting in the Gulf of California and sediment diapirism and mantle hydration during subduction

Abstract

This thesis investigates three processes that control the evolution of oceanic margins. Chapter 2 presents seismic images of a ~2-km-thick evaporite body in Guaymas Basin, central Gulf of California. In rifts, evaporites form under conditions unique to the latest stages of continental rupture, and the presence, age, thickness, and shape place new constraints on the history of early rifting there. Chapter 3 presents numerical experiments that show that diapirs can form in sediments on the down-going plate in subduction zones and rise into the mantle wedge, delivering the sedimentary component widely observed in arc magmas. Chapter 4 presents measurements of seismic anisotropy from wide-angle, active-source data from the Middle America Trench that address the hypothesis that the upper mantle is hydrated by seawater flowing along outer-rise normal faults. These measurements indicate that the upper mantle is ~1.57 to 6.89% anisotropic, and this anisotropy can be attributed to bendingrelated faulting and an inherited mantle fabric. Accounting for anisotropy reduces previous estimates for the amount of water stored in the upper mantle of the downgoing plate from ~2.5 to 1.5 wt%, a significant change in subduction zone water budgets. AcknowledgementsThis thesis punctuates my own transformation from student to new scientist, but it also marks a change from many other states of incipiency to nascence. In any transformation and in any new thing there is woe. This woe can be wisdom, but it can also be madness. Dan Lizarralde has a knack for sorting wisdom from madness, and he showed me how to see the difference and navigate the space in between. I am grateful that he took a chance on me, first as a summer student at WHOI and then as a graduate student, and he never stopped holding the proverbial carrot at just the right distance.Mark Behn and John Collins also understand wisdom and madness. They took an early interest in my work and have offered ceaseless encouragement since I first arrived at WHOI. They also provided the critical eyes that are essential to good science, and to the development of good scientists, and I am truly grateful for time I shared with these guys. Donna Shillington and Alison Malcolm had no hesitations about joining my thesis committee, and they have been nothing but supportive the whole time.I am lucky to have spent 5 months at sea with the fine officers and crews of the R/V Langseth, R/V Endeavor, and F/V Tiki. At WHOI, talented people like Jeff Dusenberry and Jonathan Murry kept our computers and code running. I am grateful that the folks in the education offices MIT and WHOI work hard to make the Joint Program the amazing experience it is. I am also thankful that the U.S. values scientists and students and supports us by giving grants like National Science Foundation (NSF) Division of Ocean Science (OCE) #824497 and NSF OCE-MARGINS #825178 and #841063 to Dan Lizarralde, as well as NSF Division of Earth Sciences #0652707 to Mark Behn.Camilo Ponton can talk science, but he was also there ...