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JOURNAL OF GEOPHYSICAL RESEARCH VOL. 68, No. 15 AUGUST 1, 1963

The Nature of the Mohorovicic Discontinuity, A Compromise 1

PETER J. WYLLIE

Department of Geochemistry and Minemlogy Pennsylvania State Universit.y, University Park

Abstract available . The experimental data and steady-state calculations make it difficult M to explain the discontinuity beneath both and continents on the basis of the same change. The phase oceanic M discontinuity may be a chemical discontinuity between and peridotite , and a similar chemical discontinuity ma,y thus be expected b�neath the conti­ nents. Since ayailable experimental data place the basalt- phase change at about thE' same depth as the continental M discontinuity, intersections may exist between a zone of chemical discontinuity and a phase , the transition being either basalt-ecloO"ite or feldspathic peridotite-garnet peridotite. Detection of the latter transition by seismic t�'h­ niques ma,y be difficult. The M discontinuity could therefore represent the basalt-eclogite phase change in some localities (e.g. mountain belts) and the chemical discontinuity in others (e.g. oceans and continental shields). Variations in the depth to the chemical discontinuity and in the positions of geoisotherms produce great flexibility in oro genetic models. Interse;­ tions between the two zones at depth could be reflected at the surface by major fault zones separating large structural blocks of different elevations.

Chemical discontinuity. The conyentional termediate between dunite and basalt is view that the Moho discontinuity at the base of qualitatively reasonable for the . the is caused by a chemical change from Ringwood [1962a, c] adopted a specific model basaltic to peridotite has been expounded compounded from the concepts advanced and recently by Hess [1955], Wager [1958], and developed by many petrologists and geochem­

Harris and Rowell [1960]. Petrological and geo­ ists, and he c oncluded that to a first approxima­ chemical concepts related to the composition of tion the mean chemical composition of the upper the 's mantle and problems of heat dis­ mantle and crust over any e::l.iensive region of tribution have been discussed by Bowen [1928J, the earth is similar to a mixture of 4 parts of Buddington [1943], Ross et al. [1954], Ver­ dunite to approximately 1 part of basalt. Ring­ hoogen [1954J, Birch [1958], Lovering [1958, wood called this material 'pyrolite,' but I prefer 1959], Kuno [1959], MacDonald [1959a, 1959b], to retain the terms 'feldspathic peridotite' and Harris and Rowell [1960], and many others. A 'ga,rnet periciotite' (for the same material in the conclusion that the is composed eclogite facies) which were used long ago by of material having the composition of a feld­ Bowen [1928]. spathic peridotite is in reasonable agreement The main advantage of this model for the with the hypothesis that the oyer-all composi­ upper mantle is that it provides reasonable ex­ tion of the mantle is the same as that of the planations for a wealth of petrological data. silicate fraction of chondritic meteorites. Al­ Petrogenesis demancis that the mantle material though there are divided opinions about the be capable of supplying the basaltic validity of meteorite analogies, the chondritic which has been erupted so frequently at the model is favored in most recent reviews [Mac­ earth's surface. Furthermore, it appears to be Donald, 1959b; Harris and Rowell, 1960; Vino� more likely that this magma is produced as a gradov, 1961; Ringwood, 1962a; Mason, 1962]. result of of crystalline material MacDonald [1959b], after summarizing argu­ than by complete melting, which is what would ments in favor of the chondritic model, con­ be required if the mantle were composed of cluded that material having radioactiyity in- eclogite [Bowen, 1928; Hess, 1960b]. Vinogra­ dov [1961] concluded that original mantle ma­ terial of chondritic composition is affected by 1 Contribution 62-81 from the College of Mineral Industries. zone melting, which leads to a separation into 4611 4612 PETER J. WYLLIE basaltic magma and residual dunite, the basalt earth's crust. Kennedy proceeded to develop eventually forming the earth's crust. plausible explanations of these observations bv Birch [1952] showed that the elastic prop­ assuming that the M discontinuity is repr� erties of feldspathic peridotite do not conform sented by a phase change from basalt to eclo­ with those of mantle material at depths of sev­ gite . MacDonald and Ness [1960] also discussed eral hundred kilometers. This is not an argu­ some of the contradictions with the hypothesis ment against the peridotite hypothesis, however, that the upper mantle is composed of peridotite. because at appropriate depths the basaltic por­ In particular, they questioned the assumption tion of the peridotite will be converted to eclo­ that a sharp chemical discontinuity between gite [Bowen, 1928, p. 318]. The mantle material peridotite and basalt could persist for periods then consists of garnet peridotite (eclogite of longer than 10° years. Birch [1952J, garnet pyrolite of Rl:ngwood Phase change. The suggestion that the upper [1962a]), composed of a mixture of dunite and mantle is composed of eclogite appears to haye eclogite (eclogite here being restricted in chemi­ originated with Fermor [1913, 1914], and the cal composition to the range of basaltic composi­ idea was supported by Goldschmidt [1922] and tions). Birch [1952] concluded that such ma­ Holmes [1927]. This hypothesis has been re­ terial satisfies the elastic requirements even vived by Lovering [1958] and Kennedy [1959J. more fully than a pure dunite would, and Bowen Lovering compared a layered earth with a dif­ [1928] concluded that the mineralogical consti­ ferentiated, parent meteorite body whose COID­ tution of the basaltic fraction of the mantle position is represented by achondrites, and he would make little difference in the generation of concluded that the outer 100 km of the earth is basaltic magma. (Recent experimental work by of basaltic composition. The M discontinuity Yoder and Tilley [1962J indicates that this may can then be explained only as a phase change have an important bearing on the nature of the from basalt to eclogite, its dense chemical basaltic magma produced.) Because of the simi­ equivalent. This hypothesis is attractive because larity of elastic properties of feldspathic perido­ vertical movements of isogeotherms cause M to tite, garnet peridotite, dunite, and eclogite, it is migrate up or down, producing changes in possible that the existence of a discontinuity or crustal thickness and causing contraction or ex­ transition zone at depth between any pair of pansion, with consequent changes in surface these rocks cannot be detected with existing elevation. This process has far-reaching implica­ seismic techniques. tions for orogenetic theories. MacDonald and This hypothesis is satisfactory in many ways, Ness [1960], Wetherill [1961], and Noble [1961] but again there are difficulties. Kennedy [1959] have since calculated the order of magnitude of cited four major observations which were in­ possible effects produced by movement of the M t1dequately e::'\:plained by the traditional view discontinuity. Hess [1955J had earlier discussed that the earth's crust is separated by the M similar processes caused by serpentinization or

discontinuity from denser ultramafic material, dehydration of the upper-mantle material (as­

such as feldspathic peridotite: (1) the rela­ sumed to be peridotite) with specific reference tively uniform values for heat flow in conti­ to suboceanic topography. nental and oceanic basins (an explanation of The phase change from basalt to eclogite in­ this may be forthcoming when more heat-flow volves a change from the mineral assemblage data are available [Birch, 1958]), (2) the per­ plagioclase + pyroxene (+ olivine) to pyrope­ sistence of continents and mountain ranges in rich garnet + jadeitic pyroxene (+ olivine) spite of high erosion rates, which might be ex­ The basalt may be saturated, in which case it

pected to cause of the continental may contain some free quartz but no olivine, or

rocks in the in a relatively short span of it may be undersaturated, containing nepheline geological time, (3) the subsidence of marginal and olivine. Experimentally measured curves troughs in response to loading by low-density for transitions involving these minerals are , and (4) the uplift of plateaus once plotted in Figure 1. Curve 1 for the reaction worn to level. According to Kennedy, items albite + nepheline � j adeite was measured by 3 and 4 appear to require large masses of rock Robertson et al. [1957], and curve 2 for the

to be transferred laterally near the base of the same reaction at lower temperatures was meas- THE MOHOROVICIC DISCONTINUITY 4613 125 40 GEOTHERMAL! gite transition. However, the effect of other GRADIENT ../ components (especially CaO and FeO occurring 100&! in solid solution in natural plagioclase and py­ lJ.I I­ roxene) on these transitions is not known, and �YJ31 lJ.I the position of the transition is obviously sensi­ � 7 � �I:.' ../'�., 75 tive to the degree of silica saturation of the 52 � original basalt (curves 1 and 3). The experi­ �� 50F mentally determined curves do indicate that .. c.. �J . . ;�:�1 25 � crystalline basalt would be converted to eclo­ �)('- )Jr.}' gite at no great depth within the earth, but ...... , � o .. o there are large margins of uncertainty as to the o 400 800 12 0 1600 0 depth at which the change occurs. TEMPERATURE °C To locate the position within the earth of a Fig. 1. El;:perimentaliy measured reactions in­ phase change from basalt to eclogite, or from volving dense silicate minerals. Ab, albite; Ne, nepheline; J d, ja.deite; Q, quartz; En, enstatite; feldspathic peridotite to garnet peridotite, it is Sa, sanidine; Si, sillimanite; Alm., almandine; H, necessary to know not only the pressure-tem­ hercynite; C, iron cordierite; F, fayalite. The perature range for the phase transition but also dotted line is an average after the geothernlal gradient within the earth. N ei­ Howell [19591. See text for references. ther of these is adequately known, as is indi­ cated by the two e:\.'treme estimates shown in ured by Hoffer and Dachille, using apparatus Figure 2 [Kennedy, 1959 j Ringwood, 1962a]. in which a shear component had been added to These estimated positions of the phase change the hydrostatic pressure in order to improve the were based partly on the transition curves of kinetics of the reaction [Dachille and Roy, Figure 1 and partly on preliminary experi­ 1962]. The reaction albite � jadeite + quartz mental work with natural basaltic glasses.

(curve 3) was measured by Birch and LeComte Kennedy [1959J reported the conversion of

[1960]. Kennedy published preliminary curves basaltic glass to a product with eclogitic affin­ 4, 5, and 6 for a more complex sequence of re­ ities at 500°C and pressures just above 10 kb. actions involving solid solution between albite and nepheline, and conversion of these minerals (() jadeite. Boyd and England [1959] determined � :A I, 40 "' 12 the stability field of pyrope, bounded on its low­ , , 5 1962)/ .. / J pressure side by less dense crystalline phases (RINGWOOD, /_//� U'l en -"- I" '/� 0:: (curve 7). Yoder's [1955] preliminary curve for 0:: IOO � <{3D GARNET " _ -/- I ;:;:f en ' the stability of almandine (curve 8) suggests PERIDOTITE ' --./ , ::; w g , :;; " .. '. ,' " o that the garnet in , which contains some 52 , ' , 75� almandine in solid solution, would be stable at w 20 ,/ ,/ /FELDSPATHIC 0:: / __/ ,/ PERIDOTITE somewhat lower pressures than pure pyrope. => U'l C B' 1\ F Although these simple mineralogical reactions U'l " ' 50 c.. � 10 ECL w OGITE /, (KEN Cl give an indication of the position and slope of '" NEDY I 1959) c.. A 25 the basalt-eclogite transition, they are not suffi­ .. ,, �...... �.-:.. ... , cient to permit us to locate it precisely. Robert­ ��--�4�0�0--��80�0���1�20�0�168�0 Bon et al. [1957] and Birch and LeComte [1960] TEMPERATURE °C nated cautiously that their experimental data Fig. B!lsalt-eclogite phase were too few to permit them to decide whether 2. tnmsition and geo­ thermal gradients after Kennedy [1959] and Ring­ the M discontinuity was phase change, and a u)ood [1962a]. Estimated geothermal gradients: A, they pointed out the difficulty of accounting for in oceanic regions; B, in orogenic regions; C, in the oceanic discontinuity in terms of the basalt­ continental shield regions. According to Kennedy. eclogite transformation. Boyd and England the M discontinuity could be caused by the phase transition in all three environments. According to [1959] suggested that the pyrope and jadeite Ringwood, the transition lies within peridotite of stability curves (7 and 1) could together be the mantle. Ringwood [1962c] has since modified taken as an approximation of the basalt-eclo- this diagram. 4614 PETER J. WYLLIE

The points of intersection of his estimated curve for the basalt-eclogite transition with estimated geothermal gradients give the depths 40 125 to M in three environments. The three dashed (!) curves represent geothermal gradients increas­ 0:: IOO � ing from ocean basins (Al continental shields « 30 to m W I­ (Cl, and reaching maximum values beneath g W SC mountain belts (B). Because the geothermal 75 :2: w20 3 curves have approximately the same slope as gj (YODER AND :2 the basalt-eclogite transition, the depths to M (!) TILLEY,I96 �(!) 50 I: increase in the same sequence. According to this 10 b: 0... interpretation, the 1\1 discontinuity can be ex­ 25 � plained everywhere in terms of a basalt-eclogite L O VERING,I9581 ��L-'4do� �� phase transition. 0��-C8�0�O��-"12�OYO'- 160 � TEMPERATURE °C Ringvlood [1962aJ adopted a standard chon­ dritic model for the upper-mantle composition Fig. 3. Published estimates of the position of the and concluded that the basalt-eclogite transition basalt-eclogite phase transition. is located within the mantle, where it is ex­ pressed as a change from feldspathic peridotite oceans is less than it is beneath the continents ('pyrolite') to garnet peridotite ('pyrolite') . and orogenic regions, whereas in Ringwood's When Boyd and England [1959J crystallized a model the geothermal gradient beneath the basalt glass at 1200°C between 33- and 40-kb oceans is greater than the estimated gradients pressure, they reported the presence of minor in the other two environments. If Ringwood's calcic plagioclase as well as garnet, clinopyrox­ estimated geothermal gradient for the area ene, and some free silica. Ringwood interpreted beneath the oceans were used in Kennedy's these results as indicating that the runs lay model, the M discontinuity beneath the oceans within the basalt-eclogite transition interval. would be deeper than M beneath the continents. His original estimated position for this interval Four published estimates of the basalt-eclo­ is illustrated by the stippled band in Figure 2. gite phase transition are compared in Figure 3;

This has a much steeper slope than Kennedy's two of these have already been discussed in estimate, and the transition therefore extends connection with Figure 2. Lovering [1958] con­ to much higher pressures before being inter­ cluded from his achondrite analogy that the M sected by melting curves. On the basis of more discontinuity must be a phase transition from recent work by Boyd and England [1961J on basalt to eclogite, and he drew the boundary the stability of anorthite, Ringwood [1962cJ through two points with temperatures and pres­ suggested that the transit.ion zone between sures believed to exist at the base of the crust plagioclase pyrolite and garnet pyrolite would beneath (a) ocean basins and (b) continents. be wider, 'extending to lower pressures at the Yoder and Tilley [1962] presented a prelimi­ expense of plagioclase pyrolite.' He further sug� nary diagram for the transition between natural gested that the transition in pyrolite involved basalt and eclogite and for the melting relations reactions some\vhat different from those in­ of both rocks, based on their own e}.,"periments volved in the basalt-eclogite transition, with with natural samples as well as those of Boyd pyroxene predominating. Ringwood's later esti­ and England [1959] and Kennedy [1959]. The mated transition is therefore closer to the esti� experimental data are meager, but the few runs mate of Yoder and Tilley [1962J, which is completed provide limits for the gradient of the plotted in Figure 3. phase transition and show reasonable agreement Figure 2 illustrates not only the effect.s of with extrapolations from studies of the mineral uncertainties in estimating the position of the components. The width of the transition interval basalt-eclogite transition on the basis of avail­ remains uncertain. able data but the uncertainties in estimates of The position of this transitional interval illus­ the geothermal gradient at depth. In Kennedy's trates more clearly than the scattered curves in model, the geothermal gradient beneath the Figure 1 that basalt, or the basaltic portion of THE MOHOROVICIC DISCONTINUITY 4615

feldspathic peridotite, must undergo a transi­ siderable insight into some aspects of these tion to eclogite at depths within the earth which problems. Results obtained for the thicknesses �re roughly equivalent to, or somewhat deeper of sediments which can accumulate in a sinking than, the position of the ]\.;I discontinuity be­ geosyncline, the variation in the thickness of the net1th the continents. It does not support the crust in various environments, the amount of Lovering contention of [1958] and Kennedy elevation in plateau and mountain regions, and [1959J that the ]\.;I discontinuity at shallow the time span for these processes are of reason­ depths beneath the oceanic crust could be caused able orders of magnitude. by the basalt-eclogite phase change (compare The available evidence does not favor the Figures 2 and 3). The calculations of MacDon­ hypothesis that the J\1 discontinuity beneath the ald and N e88 [1960J and Wetherill [1961], using oceans is represented by the transition from steady-state or equilibrium approximations, also basalt to eclogite. Wide variations in heat-flow indicate that the ]\.;I discontinuity beneath the measurements from the Pacific Ocean reported oceans is unlikely to be caused by the same by Yon IIerzen [1959, 1963J indicate that the phase transition. There remains a possibility M discontinuity beneath the oceans is not caused that 1\1 beneath the oceans could be caused by a by a phase change. If lVI beneath the oceans is different phase transition, such as one from par­ a phase change, the wide range of heat-flow tially serpentinized peridotite to unserpentinized measurements should be accompanied by varia­ peridotite [Hess, 1960a]. This would imply that tions in thickness of the oceanic crust. However, the ]\.;I discontinuity beneath the oceans is a initial seismic studies indicate no variation in 'fossil isotherm.' the depth to 11 which can be correlated with The available evidence thus indicates that the the heat-flow variations. For petrogenic argu­ M discontimiity beneath the continents, but not ments in continental regions this hypothesis beneath the oceans, could be caused by a phase is less satisfactory than the chemical eliseo n­ transition from basalt to eclogite. The width of tinuity hypothesis. It is difficult to reconcile the transition interval appears to be quite ap­ Lovering's [1958J suggestion that the upper 100 preciable (Figure 3), but Yoder and Tilley km of the mantle are composed of eclogite with

[1962J suggested that the part of the reaction the widely accepted model of a chondritic man­ producing garnet might make the largest con­ tle. Eclogite, which is the chemical equivalent of tribution to the seismic velocity change, so that basalt, would have to be completely melted to if a phase change from basalt to eclogite does provide basaltic magma. As was pointed out occur at the M discontinuity the effective change earlier, it is more likely that magma derived from in seismic velocity might be realized over a much the earth's mantle is generated by partial fusion smaller depth interval than is indicated by the [Bowen, 1928; Hess, 1960bJ. It has been sug­ transition band in Figure 3. There appe;HS to be gested that, if the M discontinuity is a phase some question as to just how sharp the 11 dis­ change dependent upon pressure and tempera­

eontinuity may be [Yoder and Tilley, 1962; ture, there should be a correlation between Tuve et al., 1954J . crustal thicknei3s and mean surface temperature, The advantages of a model in which the lVI and between crustal thickness and surface ele­ discontinuity is caused by a phase transition vation, although these could be masked by other

from basalt to eclogite were outlined by Lover­ effects such as compositional variations. So far, ing [1958], developed in detail by Kennedy no significant correlations have been established [1959J, and placed on a semiquantitative hasis [Steinhart, 1960 ; Hou'ell and Woodtli, 1961]. by MacDonald and Ness [1960J, Wetherill The problem of the width of the phase transi­

[1961], and Noble [1961]. (See also M eM ath tion compared with the width or sharpness of

[1962J and Noble [1962].) The model provides the M discontinuity as measured by seismic reasonable solutions for major geological prob­ techniques has already been discussed. lems such as the difference between continents Compromise. There is no convincing evi­

and ocean basins, the permanence of the conti­ dence to support the view that the 1\1 discon­

nents, the elevation of large plateaus, the forma­ tinuity beneath the oceans is caused by a phase tion of geosynclines, and the origin of mountain change from basalt to eclogite, and there are belts. The steady-state calculations provide con- few objections to the hypothesis that it is a 4616 PETER J. WYLLIE composition change from to ultramafic material. If the upper-mantle material beneath the oceanic crust is composed of feldspathic peridotite, it is reasonable to conclude that the upper mantle beneath the continents has a similar composition. This implies that there is

a chemical discontinuity of global extent be­ tween material of basaltic composition and material with the composition of feldspathic peridotite. The depth to this discontinuity cer­ tainly differs between oceanic and continental areas, and it probably differs between continen­ tal shield areas and orogenic regions. Fig. 4. Possible arrangements within the earth In addition to the chemical discontinuity, it is of a chemical discontinuity (dashed line), a phase now well established that there must be another transition zone (simplified to a boundary, the discontinuity, at the appropriate depth, marking dotted line), and the M discontinuity (the lower the position of the basalt-eclogite phase transi­ solid line M M). B, basalt ; E, eclogite; F P., feld­ spathic peridotite, G P., garnet peridotite. The tion. This may be expressed as a transition from crust is stippled. basalt to eclogite or as a transition from feld­ spathic peridotite to garnet peridotite, depending upon the depth to the chemical discontinuity. The model for the upper mantle and crust

The depth to the phase transition zone varies presented here is a compromise between the according to the geothermal gradient. Where the rival hypotheses discussed earlier. Four possible chemical discontinuity occurs at a higher level arrangements for the two discontinuities are illus­ than the phase transition, as in the oceanic trated schematically in Figure 4. The phase regions, the chemical discontinuity is recorded transition zone is shown as a dotted line, and the

as the M discontinuity by seismic measure­ IvI discontinuity, the solid line MM, may be a ments. It would probably be difficult, using chemical discontinuity (dashed line) in some present seismic techniques, to detect the deeper localities (e.g. beneath ocean basins and parts transition from feldspathic peridotite to garnet of the continents, such as stable continental

peridotite, because the change in the velocity of shields), and a phase transition in other locali­

seismic waves between these rocks would be ties (e.g. in orogenic belts and other active re­ small. If the phase transition occurs at a higher gions of the continents). level than the chemical discontinuity, the transi­ The occurrence of intersections between the tion from basalt to eclogite would be recorded chemical discontinuity and the phase transition by seismic measurements as the M discontinuity, zone introduces an additional element to be con­ and the deeper chemical discontinuity between sidered in the discussion of orogenetic processes. eclogite and garnet peridotite might be unde­ Regional variations in the depth to the chemical tected. Only the major discontinuities involving discontinuity, and local variations in geothermal elastic properties and density would be detected gradients which cause variations in depth to the by seismic methods, i.e. the discontinuities be­ phase transition zone, produce great flexibility tween basalt and feldspathic peridotite, or be­ in orogenetic models. tween basalt and eclogite. Refinement of seismic Applications. As previously stated, the phase techniques or of other geophysical methods may change hypothesis for M is attractive because

ultimately provide a test for the existence of a vertical movements of isogeotherms cause M to second discontinuity below the M discontinuity. migrate upward or downward with result,ant Recent crustal magnetotelluric measurements in changes in surface elevation. A regional depres­ Massachusetts are of interest in this connection sion of isogeotherms converts basalt to eclogite [Cantwell and Madden, 1960]. The results indi­ (Figure 3), which is accompanied by upward

cate that a rapid change of resistivity must movement of M, contraction, and downward occur around 70 km, which is deeper than the sinking of the surface. Conversely, regional up­ M discontinuity in this region. rise of isogeotherms converts eclogite to basalt, THE MOHOROVICIC DISCONTINUITY 4617

which is accompanied by expansion, downward the intersection of the chemical discontinuity

movement of M, and uplift of the surface. In and the phase transition zone are thus elevated

the compromise model outlined in the preceding by greatly different amounts and a fault, or section, an additional element is introduced by fault zone, would tend to develop between the the e xistence of intersections between a chemical two blocks. Lateral migration of the intersection discontinuity and a phase transition zone. at depth (whose amount depends upon the Kennedy [1959J, discussing applications of steepness of the chemical discontinuity) tends the phase change hypothesis, concluded that to favor the formation of a fault zone rather

the uplift of extensive regions, such as the Colo­ than a single fault plane. The crust thickens rado plateau, could be effected by a regional up­ beneath the elevated block as the phase transi­

rise of isogeotherms. The lateral limits of such tion zone migrates downwards. Figure 5 illus­ uplifts would coincide with the limits of the trates what can happen when the phase transi­ temperature increase at depth, and the elevated tion zone migrates without appreciable change

regions would grade into the surrounding areas. in the position of the chemical discontinuity.

In fact, many elevated areas are terminated Much more flexibility is introduced if changes laterally by faults or fault zones. The model in the position of the chemical discontinuity are

illustrated in Figure 5 shows how lateral bound­ considered as well.

aries of this kind could be provided by the In the conventional picture of the orogenic

intersection at depth of a chemical discontinuity cycle, the chemical discontinuity is moved down­

with a phase transition zone, without a necessity ward, possibly as a result of the a ction of con­

for rather abrupt changes in the distribution of vection cells, with the formation of a tectogene. isogeotherms. Sediments accumulate in the subsiding trough A regional rise of isogeotherms causes the which is formed at the surface. If the initial

phase transition zone to move downward from situation is as illu2trated in Figure 4A, and if

its original position in Figure 5A to the lower the formation of the tectogene is accompanied position illustrated in Figure 5B. The position of by cooling at depth, then, as the chemical dis­

the intersection between the chemical discon­ continuity moves downward, the phase transi­

tinuity and the phase transition zone migrates tion moves upward to meet it. When the two

, laterally down the slope of the chemical dis­ intersect, a pocket of eclogite is developed at the continuity. Conversion of garnet peridotite to root of the tectogene corresponding to the con­

feldspathic peridotite causes a moderate amount dition illustrated in Figure ill. Once this stage

of expansion and surface elevation, but conver­ is reached, continued conversion of basalt to

sion of eclogite to basalt causes very much more eclogite accentuates the subsidence of the trough expansion and correspondingly greater surface at the surface. Subsequent heating at depth elevation. The crustal blocks on either side of would cause the phase transition zone to move downward relative to the chemical discontinuity, and the sediments accumulated in the trough A. would be elevated. When the phase transition zone passes below the chemical discontinuity at the base of the tectogene, the continued depres­

sion of the phase transition in peridotite ceases to have much effect on the surface level. The intersection of the phase transition zone with Fig. 5. The effect of an intersection between the chemical discontinuity (dashed line) and the the chemical discontinuity could thus provide a phase transition zone (simplified to a line, dotted) beginning and an end to the more vigorous The when the regional geoisotherms are raised. changes in surface level occurring during oro­ phase transition migrates downward, causing coo­ genic cycles. This model will be discussed in version of eclogite to basalt with resultant surface mo e detail elsewhere. elevation. Differential elevation occurs between r the crustal blocks overlying eclogite and perido­ tite, with the formation of a fault (or fault zone) Aclmowledgrnents. I wish to thank B. F. How­ ell, Jr., R. H. Jahns, and O. F. Tuttle for their between two blocks of different elevation. The crust (stippled) thickens beneath the elevated critical comments, A. M. Kelly for bringing crustal region. magnetotelluric measurements to my attention, 4618 PETER J. WYLLIE and the National Science Foundation for covering Lovering, J. F., The nature of the M ohorovicic the costs of preparation and publication of the discontinuity, Trans. Am. Geophys. Union, 89, manuscript. 947-955, 1958.

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