Spreading-rate Dependent Mid-ocean Ridge Processes Expressed in Western Atlantic Lithosphere

Mid-ocean ridge processes;Plate tectonics;Seismic experiment

Kim, Sangmyung David ; Earth and Atmospheric Sciences

University:Georgia Institute of Technology

Department:Earth and Atmospheric Sciences

关键词: Mid-ocean ridge processes;    Plate tectonics;    Seismic experiment;   

Others  :  https://smartech.gatech.edu/bitstream/1853/11472/1/kim_sangmyung_d_200608_phd.pdf

美国|英语

来源: SMARTech Repository

【 摘 要 】

The Far-Offset Active-Source Imaging of Mantle (FAIM) experiment was conducted along an 800-km-long transect in the Western Atlantic to study the evolution of 108-157 m.y. lithosphere.The main transect (Line 1) crosses a transition from slow (13-14 mm/yr in half rate) to ultra-slow (~8 mm/yr) paleo spreading rates, and thus represents an ideal setting to study spreading-rate dependent processes as expressed in preserved lithospheric structure.This thesis presents results of four analysis efforts along this transect.We present a crustal model based on seismic refraction and wide-angle traveltime modeling, we extend the crustal model to an upper lithosphere density model using gravity constraints, we constrain Poissons ratio in oceanic Layer 3 using converted shear-wave phases, and we consider regional lithospheric structure by analysis of geoid/topography ratios. The crustal model indicates that a transition in crustal thickness accompanies the spreading-rate change, with the crust produced at slow rates being 1.0-1.5 km thinner.The gravity modeling shows that a density model can be constructed that simultaneously satisfies observed gravity, seismic constraints on crustal thickness, and our expectation of isostacy if ~1.3 km of low-density material is distributed into the upper 30-60 km of the mantle.This amount of material (~1.3 km) roughly equals the difference in thickness between slow and ultra-slow spreading crust, suggesting that that the thinner crust formed during very slow spreading arises due to melt retention in the mantle rather than decreased mantle melting.Modeling of mode-converted S-wave phases reveals a uniform of Poissons ratio (~0.27) in the lower crust.Along with the observation of sharp crust/mantle boundary, this result suggests that crust along the FAIM transect is primarily melt-derived igneous crust.Geoid versus topography relationships along Line 1 and nearby parallel tracks show abrupt changes that may originate from lateral changes in mantle density, possibly in response to the transition from slow to ultra-slow spreading.This type of observation may enable us to extend our inferences to a more regional scale.