Mantle Discontinuities
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REVIEWS OF GEOPHYSICS, SUPPLEMENT, PAGES 783-793, APRIL 1991 U.S. NATIONAL REPORT TO INTERNATIONAL UNION OF GEODESY AND GEOPHYSICS 1987-1990 Mantle Discontinuities CRAIG R. BINA Dept. of Geological Sciences, Northwestern University, Evamton, Illinois INTRODUCT?ON mantle discontinuities which have taken place during the period 1987-1990, as well as briefly discussing some The nature of the discontinuities evident in seismic implications for mantle thermal structure and dynamics. velocity profiles of the Earth's mantle has been a topic of While I have endeavored to compile as complete a refer- active research since the work of Birch in the 1950s. ence list as possible, the lesser number of references 'The chemical, mineralogical, and thermal properties of directly discussed in the text necessarily reflects my own these features have important implications outside the interests and familiarity with the literature. I conclude by fields of seismology and mineral physics, constraining as outlining the picture of mantle structure and composition they do the overall dynamics of the Earth's interior, the which is emerging from these efforts while indicating the deep return flow for plate tectonics, and the geochemical remaining major uncertainties. signatures of source regions of igneous rocks. In the period 1987-1990, great strides have been made in identifying and characterizing mantle discontinuities from a seismological standpoint. Advances in detailed The seismological characterization of mantle discon- observation of reflected and converted phases, analysis of tinuities in both the upper and lower mantle has followed multiple ScS reverberations, stacking of global digital several avenues of inquiry. Investigation has focused data, application of seismometer arrays, and broadband upon establishing the presence or absence of seismic data collection have permitted in~proveddetermination of discontinuities, determining whether they are global or the location, magnitude, and sharpness (depth-extent) of local in extent, measuring the topographic variability of rapid seismic velocity changes in the mantle. Notable their depth of occurrence, and quantifying the magnitude among such advances are the global stacking study of and sharpness of their velocity changes. In addition to Shearer [1990] and the SH-pblarized mantle reverberation numerous local and regional studies, notable advances of studies of Revenaugh and Jordan [1987, 1989, 1990a, the last four years include the use of SH-polarized mantle 1990bl which systematically investigate the fine structure reverberations [Revenaugh and Jordan, 1987, 1989, of seismic velocity variations in the mantle. 1990a, 1990bl and the stacking of five years of global Important progress has also been made in understand- long-period data [Shearer, 19901 to better constrain the ing the physical and chemical properties of mantle fine structure of mantle seismic discontinuities. Work discontinuities by better constraining the effects of pres- continues p. Kennett, pers. comm., 19891 on a new sure, temperature, and compositional variations upon the radial Earth model (IASPR) to fit global body-wave crystal structures and elastic moduli of the mantle's con- travel times more closely than the currently-used radial stituent minerals. Advances in multi-anvil press and model PREM [Dziewonski and Anderson, 19811 while diamond-anvil cell high-pressure technology, calorimetric retaining those features of PREM structure which provide study of mineral thermochemical properties, synchrotron a good fit to free oscillation data. Recently reported radiation analysis of crystal structures, and first-principles depths and magnitudes of mantle seismic discontinuities computation of crystal properties have allowed better are summarized in Table 1. constraints upon both the stability fields and thermoelastic Below the crust-mantle boundary, upper mantle seismic properties of mantle mineral assemblages. Notable velocity structure is dominated by a velocity increase at among these advances are the detailed experimental study about 410 hi depth, followed by an anomalously high of post-spinel phase transformations by Ito and Takahashi velocity gradient throughout the region between this 410 [I9891 and the subsequent thermodynamic analysis by krn discontinuity and the base of the upper mantle near Wood L1989, 19901. 660 krn depth. Recent work also indicates the presence Taken together, these advances allow more meaningful of a smaller velocity increase near 520 km depth, as well correlation of seismological features with mineralogical as suggesting that reported velocity discontinuities near changes. While this permits the placing of stronger con- 220 km depth cannot be globally coherent features. straints upon the composition, structure, and dynamics of Lower mantle velocity structure is dominated by a sharp the interior, there remain significant uncertainties and velocity increase at about 660 km depth, marking the ambiguities. Here I review fundamental advances in both boundary between the upper and lower mantle, and by the seismological and mineralogical understanding of the core-mantle boundary (CMB) at 2890 km depth. The km Copyright 1991 by the American Geophysical Union. lowermost 200-300 of the mantle (the D" layer), just above the CMB, is characterized by anomalous velocity Paper number 91RG00805. structures, consistent with the presence of a velocity 8755-1209/91/91RG00805 515.00 discontinuity with strongly laterally varying properties. 784 BMA MANTLE DISCO;UT?NUTIES TABLE 1. Surnrnary of Reported Mantle Seisnlic Discontinuities Depth (km) Study Reference Characteristics 67 S Revenaugh [I9891 5-8% impedance contrast 86 S Revenaugh and Jordan [I9891 obserued, W Pacific 75-225 S Graves and Helmberger [I9881 LVZ, old Pacific upper 100 S Revenaugh [I9891 LVZ onset, oceanic and tectonic, base not detected 165-215 P LeFevre and Helmhrger [I9891 LVZ, Canadian shield 200 P Bowman and Kennett [1990] 3.3% Vp increase, NW Australia -220 P,S Shearer [1990] not detected -220 S Revenaugh and Jordan 119891 not detected, W Pacific 232 S Revenaugh [I9891 4% impedance contrast, NWC Australia 300 P'IY Wajeman [I9881 Eurasia 400 P Bowman and Kennett [1990] 5.6% Vp increase, NW Australia 405 P LeFevre and Helmberger 119891 5% Vp increase, Canadian shield 405 S Graves and Helmberger [I8891 3.6% Vs increase, old Pacific 410 P,S Shearer [l990] topography 40km 414 S Revenaugh and Jordan [1987, 1989, 1990a,b] refl. coef. 0.04610.010, topog. -12 km 415 P'F' Nakanishi [1988,1989] intermittent 520 P,S Shearer [l990] 3% impedance contrast 520 S Revenaugh and Jordan [1990b] reflection coef. 0.014 630 P Bowman and Kennett [I9901 3.9% Vp increase, NW Australia 655 P'IY Nakanishi [1988,1989] sharp 659 S Graves and Helmberger [I8891 6.8% Vs increase, old Pacific 660 P LeFevre and Helrnberger [I9891 4% Vp increase, Canadian shield 660 P,S Shearer [I9901 topography 40km 660 S Revenaugh and Jordan [1987, 1989, 1990a,b] refl. coef. 0.07210.010, topog. -12 km, slab def. <40 km 670 P'P' Davis et al. [I9891 not detected + 10 km topog. 670 P'P' Wajeman [I9881 7.5% Vp contrast, Eurasia 670 P+S Paulssen [I9881 sharp 660-680 S+P Richards and Wicks [1990] slab def. <50 km, 'Tonga 710 S Revenaugh and Jordan [1989, 1990bI refl. coef. -0.020, tentative 840 P,S Shearer [l990] tentative 900 S Revenaugh and Jordan 11989, 1990b] tentative 2546 P Baumgardt [I9891 2.75% Vp increase 2600 P Davis and Weber [1990] 3% Vp increase, N Siberia 2610 S Young and Lay [1987a] 2.75% Vs increase, Iridian Ocean ---2610 S Revenaugh and Jordan [I9891 not detected, W Pacific Studies are of P-waves (P), S-waves (S), both (P,S), underside reflections (P'P'), and converted phases (P+S, S-+P). LVZ denotes low velocity zone. Magnitudes are given as velocity increases, impedance contrasts, or normal-incidence reflection coefficients. Also noted are long-wavelength topography and apparent deformation by subducting slabs. Wile there is a general lack of evidence for additional evidence for such a feature. This suggests that the significant global discontinuities, minor velocity discon- discontinuity, which appears to be required in certain tinuities tentatively have been suggested at intermediate regional studies, cannot be a globally coherent feature. depths in the lower mantle, and seismic reflectors also Revenaugh and Jordan [1989], using SH-polarized mantle have been observed in subducting slabs. An additional reverberations, find no evidence for this discontinuity conclusion of recent studies is that the 410 and 660 km beneath the western Pacific. In modeling short-period discontinuities exhibit only small variations in depth, waveforms from a hybrid array, however, Bowman and comprising perhaps 20-40 km of relief. Kennett [I9901 require the presence of a 3.3% velocity increase near 200 km depth in their upper mantle P-wave Shallow Mantle Discontinuities velocity profile (NWB-1) for northwest Australia. Furth- ermore, Revenaugh L19891 finds evidence for a 4% Using SH-polarized mantle reverberations, Revenaugh impedance contrast at 232 km depth beneath west-central [I9891 reports a reflector with a 54% impedance con- and northern Australia using SH-polarized mantle rever- trast at 67 km depth in the mantle, which he atmbutes to berations. LeFevre and Helmberger's 119891 upper man- the phase transition from spinel lherzolite to garnet Iher- tle P-wave velocity profile (S25) for the Canadian shield, zolite. In addition, Revenaugh and Jordan [I9891 find determined from waveform modeling