Workshop Report - Ridge 2000 Program

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Consequently, the sin;J1e most important objective within this theme is to understan the processes that control the segmentation and episodicity of lithosphere accretion. Within our overall objective we recognize three major sets of problems: 1. the nature, generation and evolution of ridge segmentation; 2. the determination and explanation of the physical processes that operate along and between individual ridge segments; and, 3. the early thermo-mechanical evolution of the oceanic lithosphere. 1. The Nature, Generation and Evolution of <strong>Ridge</strong> Segmentation. Recently there has been a watershed of new ideas concerning the processes which control the accretion of oceanic crust and lithosphere at mid-ocean ridges. One important insight is that along-strike and temporal variations in tectonic and magmatic activity are at least as important as the well-documented variations in structure perpendicular to strike (i.e.. aging and deepening of the lithosphere). The rise axis undulates up and down hundreds of meters along strike, the deep regions of the rise occurring at transform faults and other ridge axis discontinuities such as the recently discovered overlapping spreading centers and "zero-offset" transform faults. Those discontinuities define a tectonic and volcanic segmentation of the ridge axis on the order of tens to several hundred kilometers which appears to be a global phenomenon. Segmentation defines a three-dimensional rather than two-dimensional process of volcanism, tectonism, and crustal accretion, systematically spaced along the present plate boundary. As this process has been discovered only recently, there are a number of outstanding questions. For example, what are the causes of ridge crest segmentation and how do the segments evolve in time and space? Is segmentation related to melting anomalies in the upper mantle which deliver basaltic magma to the ridge? Do the mid-sections of the segments overlie zones of high melt flux, while the ends of the segments represent null points of magma replenishment? What is the relationship between the axial magma chamber and segmentation? Do thermoelastic stresses in the lithosphere also contribute to segmentation of the ridge due to lateral thermal contraction of the plate as it cools? Are accretionary processes at spreading segments which terminate against major transform faults conditioned by these boundaries and thus more stable? Are other kinds of discontinuities the consequence of waxing and waning phases of melt generation? Do these phases migrate laterally? How stable are the segments with small offsets and does their persistence and/or temporal variability relate to upper mantle processes? What is the response of the architecture and segmentation of the ridge axis to changes in plate motion? 31
2. Characterization of Individual Segments and their Boundaries. Once the characteristics of segmentation are known in detail, it will be of great interest to determine the spatial and temporal stability of the segment. This includes possible cyclic or episodic interplay among volcanic and tectonic processes within a single segment as well as the dynamic interaction between individual segments focused at their boundaries. The nature of the lithosphere created by individual segments may vary along the strike of the segment and with time in a periodic or episodic manner as suggested by the diverse morphologies and petrologic characteristics of individual segments. Individual processes that must be understood are the nature of faulting and fissuring, the depth of the brittle to ductile transition, and the thermo-mechanical and stress characteristics of young lithosphere. In addition, it is necessary to better understand processes of magma generation, ascent, storage, eruption and intrusion and how these interact with tectonic processes within a segment. Are the deeper processes that control magma generation and spreading coherent over distances larger than a single segment- or do segments behave independently? If so, is this primarily due to the interaction of the upper mantle with the young and evolving lithosphere, or the termporal and spatial variability of upper mantle processes? 3. Thermal and Mechanical Processes Affecting the Evolution of Oceanic Lithosphere. The oceanic lithosphere is defined as the cool, mechanically strong boundary layer that overlies weaker ductile mantle. A thorough understanding of the spreading process requires knowledge of the mechanisms affecting the creation and subsequent evolution of the lithosphere. It is first necessary to understand the cooling processes, including hydrothermal convection and thermal conduction, that cause the lithosphere to form and thicken. Simultaneously, distributed horizontal extension or stretching will thin the lithosphere. These processes will be interrelated: for example, the stretching process will create faults and fissures that will allow seawater to penetrate the crust and mantle. The importance of these processes emphasizes the need to understand the temporal and spatial distribution, both along and across strike, of faulting and extensional deformation. Deformation of the lithosphere will be controlled by the mechanical properties of the rock comprising it and the forces or stresses acting upon it. Stresses within the lithosphere will result from differential contraction due to cooling, normal and shear stresses exerted at the base of the lithosphere by mantle flow and melt segregation processes, forces due to ridge axis topography, and forces transmitted to the ridge axis through the lithospheric stress guide. One important objective of mid-ocean ridge studies is to understand the relative importance of these various forces in controlling deformation at the ridge axis. 32
Page 2 and 3: The Mid-Oceanic Ridge A Dynamic G
Page 4 and 5: OCEAN STUDIES BOARD WALTER H. MUNK,
Page 6 and 7: PREFACE Recent discoveries of the w
Page 8 and 9: o Regional Scale: selected subsets
Page 10 and 11: 1 INTRODUCTION The global mid-ocean
Page 12 and 13: papers were compiled, printed, and
Page 14 and 15: 2 STATEMENT OF GOALS AND OBJECTIVES
Page 16 and 17: experience from land volcanic and m
Page 18 and 19: solid understanding of both the bio
Page 20 and 21: NEXT STEPS TOWARD A RIDGE INITIATIV
Page 22 and 23: 4 REPORTS OF THE WORKING GROUPS INT
Page 24 and 25: Critical Investigations The specifi
Page 26 and 27: Seismic refraction methods provide
Page 28 and 29: New Instrument and Laboratory Capab
Page 30 and 31: GROUP 2: GEOMETRY AND DYNAMICS OF M
Page 32 and 33: FORMATION OF OCEAN CRUST AT SPREADI
Page 34 and 35: 3. Regional Surveys. The third comp
Page 36 and 37: provide efficient and economically
Page 38 and 39: 3. Adaptation of tomographic invers
Page 42 and 43: Thermal stresses have been suggeste
Page 44 and 45: electromagnetic fields must be meas
Page 46 and 47: capability should include at least
Page 48 and 49: In order to understand subseafloor
Page 50 and 51: fossil hydrothermal systems in ophi
Page 52 and 53: instrumented to record the environm
Page 54 and 55: GROUP 5: BIOLOGY OF HYDROTHERMAL SY
Page 56 and 57: There are several key questions abo
Page 58 and 59: Dispersal of Organisms What are the
Page 60 and 61: Strategy Biological studies will em
Page 62 and 63: GROUP 6: WATER COLUMN DYNAMICS AND
Page 64 and 65: On the ridge segment scale, detaile
Page 66 and 67: standard transect sampling and Lagr
Page 68 and 69: APPENDIX I The Mid-Oceanic Ridge: A
Page 70 and 71: Forsyth, Donald Department of Geolo
Page 72 and 73: Lonsdale. Peter Scripps Institution
Page 74 and 75: Reeve, Michael National Science Fou
Page 76: Tunnicliffe, Verena University of V

Workshop Report - Ridge 2000 Program

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