Geodesy in the Year 2000 (1990) / Chapter Skim
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If Only We Had Better Gravity Data...
If Only We Had Better Gravity Data...
Pages 53-84
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From page 53... ...
This revolution in Earth science, which will integrate the 2-dimensional surface kinematic pattern of plate tectonics with a 3-dimensional dynamic model, is largely the outgrowth of advances in global digital networks, supercomputing, and satellite geodesy. There is no question but that progress has been hindered by the lack of a high-resolution, extremely accurate, truly global gravity field.
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From page 54... ...
To realize the full potential of such data, gravity field modeling must be also constrained by better topographic data from the oceans and seismic information on crustal structure. Until such data is forthcoming, we must question the adequacy of the thermal plate model in describing the density structure of fracture zones.
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From page 55... ...
Thermal plate models allow us to calculate the load associated with the cold slab (Toksoz et al., 19711. If we had a gravity or geoid map continuous from the undeformed seafloor, across the outer rise, trench, forearc, and island arc to the overriding plate, we would be able to calculate the stresses acting on the underthrust plate, and thereby learn much about lithosphere/asthenosphere interaction and aspects of mantle rheology, such as the degree to which the lower mantle resists slab penetration (Hager, 1984~.
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From page 56... ...
or clusters (on slow-moving plates) formed by hot spots, we suspect that the vast majority of oceanic volcanoes are smaller features erupted in a less organized, but still non-random, manner (Jordan et al., 1983~; Smith and Jordan, 1987~.
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From page 57... ...
Gravity anomalies over rifts are sensitive to the perturbed crustal structure from lithospheric stretching and any deep thermal anomalies responsible for doming and plate thinning. An outstanding problem in the study of extensional deformation is the disagreement among various measures of extension, such as heat flow, subsidence, and gravity anomalies, as to the total amount of lithospheric thinning in a single vertical column (Royden et al., 1983a,b; Wernicke, 1985~.
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From page 58... ...
Broad constraints on thermal structure would be obtained with gravity data accurate to 1 to 2 mgals with a resolution of 100 km. Specific information on variations in flexural rigidity, given the low elastic plate thicknesses to be expected, would require similar accuracy but a resolution of 20 km or better.
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From page 59... ...
This new model for the structure of mountain belts has thus established the validity of elastic flexure to describe the rheology of continental lithosphere and the existence of subsurface loads to maintain the deflection of foreland basins despite erosion of topographic loads. Despite the importance we now place on buried loads in describing the conditions of mechanical equilibrium at mountain belts, the nature of these buried loads remains obscure.
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From page 60... ...
Thus, an improved constraint on the thickness of the continental lithosphere can be derived based on improved surface gravity data and proper modeling of mantle thermal structure adjacent to the roots of continents. Earth scientists do not have, at present, a precise global gravity field to search for the gravity signal from deep continental thermal structure.
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From page 61... ...
, while other studies, such as the geoid plus seismic tomography study described in Figure 8, indicate that the viscosity of the upper mantle is 10~9 Pa-sec with the viscosity of the lower mantle approximately one order of magnitude higher (Hager and Clayton, 1987~. Studies utilizing nonNewtonian rheology indicate that the viscosity of the lower mantle is probably not constant but changes with a two- to three-orders-ofmagnitude variation across the lower mantle (Karato, 1981~.
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From page 62... ...
Since subducting slabs lie beneath island arcs and typically span the ocean-continent transition, altimetric geoids are not sufficient to study this problem and gravity fields such as those obtained by a gravity-measuring satellite are required. The expected signal strength-will be above 0.1 mgal with a length scale dictated by the angle of a subducting slab and the speed of slab subduction; a typical wavelength will be about 1000 km.
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From page 63... ...
While the volcanoes associated with mantle plumes are lithospheric features, the source and ultimate cause of hot spot activity lies in the mantle below. Candidates for the formation and feeding of hot spots include chimney-like thermal plumes (Morgan, 1972)
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From page 64... ...
Furthermore, subsequent missions of similar design could provide additional time samplings of the field, thereby better resolving ongoing variations. Effects causing changes in gravity include ocean and solid Earth tides, postglacial rebound, secular melting of the ice caps, seasonal variations in ground water (e.g., snowpack)
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From page 65... ...
Accurate satellite "radiometer missions could monitor the small changes in the global gravity field caused by the redistribution of mass from the glaciers to the oceans. The current estimate of yearly sea level rise, caused by melting of small glaciers, is 5 x 10-4 m.
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From page 66... ...
which in turn would reveal the viscoelastic structure of the lower mantle and core. Winter in the Northern Hemisphere is accompanied by a dramatic increase in continental water storage associated with the development of ice deposits and snowpacks (Hinnov and Wilson, 1987~.
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From page 67... ...
Clearly, no one gravity mission could possibly satisfy all of these requirements, especially if we consider the exciting possibility of monitoring changes in gravity over year to decade time periods. Many of the problems in the marine environment, such as ridge crest processes, fracture zone structure, origin of midplate swells and plateaus, seamount distribution, and some aspects of small-scale convection, require accuracy of only a few mgals but resolution of 50 km or better.
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From page 68... ...
A global potential map is outside the range of even airborne surveys, but for some of the problems described in the oceanic and continental lithosphere sections requiring high resolution data in a limited area, this strategy might be feasible. ACKNOWLEDGEMENT This report has been prepared from contributions to a NASA-sponsored workshop held during February 1987, in Colorado Springs.
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From page 69... ...
H Jordan, Slab Penetration Into the Lower Mantle Beneath the Mariana and Other Island Arcs of the Northwest Pacific J
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From page 70... ...
and B Parsons, Cooling of the Oceanic Lithosphere: Evidence From Geoid Anomalies Across the Udintsev and Eltanin Fracture Zone System, Earth Planet.
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From page 71... ...
B Watts, Gravity Anomalies and Flexure of the Lithosphere at Mountain Ranges, J
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From page 72... ...
P., Some Remarks on Heat Flow and Gravity Anomalies, J Geophys.
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From page 73... ...
F Haxby, Thermal Stresses in the Oceanic Lithosphere: Evidence From Geoid Anomalies at Fracture Zones J
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From page 74... ...
C., Local Gravity Anomalies Produced by Dislocation Sources, J Geophys.
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From page 75... ...
C Fletcher, Extension of Continental Lithosphere: A Model for Two Scales of Basin and Range Deformation, J
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From page 76... ...
Major episodes of melt segregation occur near regions A-E in upper mantle, leading to replenishment and swelling of axial magma chamber. Magma migrates downhill along-strike, away from sources of melt.
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From page 77... ...
The continuous curves give the expected relationship between geoid slope and average age for thermal plate models with lithospheric thicknesses of 90 km and 60 km.
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From page 78... ...
Schematic cross section of a midplate swell. The volcano with height ht and density PO loads an elastic plate of thickness Te, which corresponds to the depth to the thermal-controlled elastic/ductile transition.
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From page 79... ...
79 o o e~ ~ :L, To to a _ ~ o 0 a 0 it: o SO O O ~ 0 ,,l o o 3 , O O ,= ~ U)
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From page 80... ...
80 'mime" Normal Fruiting ~. ~ Moho ~=~~ ~Ductilc' Stre~d~ng He-'~;~~ i Dilation by l~ru Astheno~e ,~l ~- ', ,~~8rime-Ductik Tronsiffon 1 ~ ~ ~ Con pectins (0J Asthenosphere Incipient Detoc~men1 ~\'':~;~ _It~e _ Ash Surfoce mbr~cote Norma Faulting ~.
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From page 81... ...
The flexure of the elastic plate on the left side is maintained by the weight of either an abducted block from the overriding plate or some intracrustal load. The gravity anomaly is characterized by a positivenegative couple in which the low in the foreland is due to flexural depression of the basement and the high in the hinterland is caused by the excess mass of the buried load.
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From page 82... ...
Strength in the lower crust and at the base of the elastic plate is limited by ductile flow laws for quartz and olivine, respectively. For the case of low plate curvature (a)
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From page 83... ...
drive convection in the mantle with an order-of-magnitude viscosity increase between the upper and lower mantle. The total geoid is now the sum of the effects of the density anomalies themselves plus the deformations they induce on the Earth's free surface and the core/mantle boundary.
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From page 84... ...
From Wagner and McAdoo (19869. 10 in -= 6 8 3 4 Cal c, a: 2 r ; Oceanic Lithosphere Continental Lithosphere Oceanography\ Mantle \.
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