Lowermost Continental Crust Research Articles

Composite Melt–Rock Interactions in the Lowermost Continental Crust: Insights from a Dunite–Pyroxenite–Gabbronorite Association of the Mafic Complex from the Ivrea–Verbano Zone (Italian Alps)

Abstract The processes leading to the building of the continental crust through magmatic underplating are fundamentally unknown, mainly because of the rare accessibility to deep level sections of the continental crust. The Italian Alps expose the Permian Mafic Complex, an 8-km-thick gabbronorite–diorite batholith that intruded the lower continental crust during the post-Variscan transtensional tectonics. We present here a petrological and geochemical study of a concentric dunite–pyroxenite–gabbronorite association, called Monte Mazzucco sequence, enclosed at deep levels of the Mafic Complex, thereby allowing us to provide new insights into the magmatic processes driven by emplacement of mantle melts in deep crustal continental areas. The studied sequence includes a ~ 60-m-thick dunite lens, in which olivine (82 mol % forsterite) is associated with accessory Cr-spinel including blebs and lamellae made up of magnetite. The dunite lens is permeated by mm- to cm-scale thick magmatic veins, which range in composition from hornblende lherzolite to olivine hornblendite and hornblende websterite. The lens is mantled by a m-scale ring consisting of amphibole-bearing (≤1 vol %) websterite, and the websterite ring is in turn enclosed by amphibole-free gabbronorites. Both magmatic veins within the dunites and mantling websterites typically include an oxide association of Al-spinel and magnetite. Remarkably, the hornblende websterite veins and the mantling websterites are typically plagioclase-free and include clinopyroxene and amphibole with chondrite-normalized rare earth element patterns characterized by negative Eu anomaly. The mantling websterites display a subtle, gradual outward decrease of Mg# for orthopyroxene, clinopyroxene and accessory olivine, coupled with an increase of the negative Eu anomaly in clinopyroxene and amphibole. The enclosing gabbronorites are amphibole-free and have a chemically evolved signature. We propose a petrogenetic scenario including two major events of melt–dunite interaction. The first resulted from focused reactive melt infiltration and formed the magmatic veins within dunites. The hornblende lherzolite and the olivine hornblendite veins were produced by focused reactive melt migration through dunite grain boundaries, involving dissolution of olivine and recrystallization of Cr-spinel into Al-spinel and magnetite, whereas the hornblende websterite veins crystallized from melts penetrating through narrow fractures and recording earlier plagioclase fractionation. Most likely, the infiltrating melts were overall derived from an evolving H2O-rich magma emplaced below the dunite body. The second event of melt–dunite reactive interaction developed the websterite ring around dunites. We envision that the outermost domain of the dunite body was replaced by websterites in response to reaction with an invading H2O-poor melt, which had previously undergone plagioclase fractionation. The dunite replacement occurred under dynamic conditions, which promoted the reaction progress, thereby leading to total or almost total dissolution of precursor olivine, and started to form the lens shape of the Monte Mazzucco ultramafic association. The gabbronorites closely adjacent to the websterite ring represent the crystallization products of the invading melt.

The peridotite-pyroxenite sequence of Rocca d'Argimonia (Ivrea-Verbano Zone, Italy): Evidence for reactive melt flow and slow cooling in the lowermost continental crust

The present study investigates the origin of a ∼ 400 m thick peridotite-pyroxenite sequence, locally known as Rocca d'Argimonia, which is encased within the lowest levels of the ∼8 km thick Mafic Complex, a gabbro-dioritic batholith from the lower continental crust section of the Ivrea-Verbano Zone (Southern Alps). The broad purpose of this work is to elucidate the evolution of mantle-derived magmas emplaced in the lowermost continental crust. We also wish to provide new geochronological constraints on the intrusion and the cooling evolution of the Lower Mafic Complex, which are still mostly unknown.Zircon grains separated from two gabbronorites enclosing the peridotite-pyroxenite sequence document a whole reset of the original UPb isotopic system at 286 ± 2 Ma. A Rocca d'Argimonia peridotite provided a consistent age of 296 ± 19 Ma, based on a LuHf clinopyroxene-amphibole alignment (initial εHf = +7.9). These results are interpreted to date the early cooling evolution of the Lower Mafic Complex. The clinopyroxene-amphibole peridotite pair also gave a RbSr alignment corresponding to an age of 250 ± 16 Ma (initial 87Sr/86Sr = 0.7045), which is reconciled with the subsequent, slow cooling evolution of the Lower Mafic Complex.The Rocca d'Argimonia peridotites (dunites to harzburgites and lherzolites) have whole-rock εNd(286 Ma) and εHf(286 Ma) values ranging from +0.4 to −1.1 and from +8.8 to +3.4, respectively, and 87Sr/86Sr(286 Ma) ratios varying from 0.7048 to 0.7060. The peridotites are crosscut by gabbronorite dykes and associated with olivine-free orthopyroxene-dominated pyroxenites. The gabbronorite dykes and the pyroxenites share the Nd-Hf-Sr signature of the peridotites. The gabbronorites enclosing the peridotite-pyroxenite sequence differ in the relatively enriched Nd-Hf-Sr isotopic fingerprint (εNd(286 Ma) = −4.5 to −5.6, εHf(286 Ma) = +0.2 to −2.5, 87Sr/86Sr(286 Ma) = 0.7073 to 0.7086). Remarkably, pyroxenes from the pyroxenites typically contain higher Cr2O3 than the peridotite counterparts (e.g., 0.23–0.37 wt% Cr2O3 in peridotite orthopyroxenes and 0.43–0.50 wt% Cr2O3 in pyroxenite orthopyroxenes).We propose that the peridotite-pyroxenite sequence formed by reactive melt percolation through an olivine-rich spinel-bearing matrix. In particular, the relatively high Cr2O3 content of the pyroxenitic pyroxenes is attributed to redistribution of Cr released by the breakdown of accessory spinel that was originally present in the olivine-dominated matrix, in response to a high melt/solid ratio. The melt residual after the crystallization of the pyroxenites continued to percolate the olivine-dominated matrix, thereby forming spinel-bearing lherzolites, harzburgites and dunites with the progression of the melt migration process. The chemically most primitive dunite records Nd-Hf-Sr isotopic disequilibrium conditions, thereby providing further information about the process of reactive melt flow. It is inferred that the original olivine-dominated matrix crystallized from a mantle magma having a slightly enriched Nd-Hf-Sr isotopic signature, whereas the percolating melts overall had a relatively depleted Nd-Hf-Sr isotopic fingerprint, despite being variably contaminated by the continental crust.

Spatial distribution of ultrahigh-temperature granulites of the Highland Complex of Sri Lanka: Lowermost continental crust above an ultrahot palaeo-Moho

The spatial distribution of isolated ultra-high-temperature (UHT) granulites in Sri Lanka corresponds to the exposure level of the deepest granulites of the Highland Complex (HC). Their spatial distribution, and that of ultramafic bodies in the HC, defines a relatively thin (several km) lowermost crustal layer that has been folded into a N-plunging asymmetric fold (F3 in our scheme), Z-shaped looking N. We postulate that the UHT granulites represent suitable lithologies within this lowermost crustal layer above an ultrahot palaeo-Moho. Prior to D3, this layer had already been imbricated and intensely folded with the HT granulites and with ultramafic units after peak metamorphism, leading to a thick package of HT granulites with isolated UHT assemblages. This concept has some important implications for granulite terrains in general: (i) UHT granulites elsewhere may also represent a thin zone above an ultrahot palaeo-Moho and overlain by HT granulite terrains, and their spatial relationships may have been complicated by ductile deformation during early cooling; (ii) many other HT granulite terrains may be underlain by a similar zone of UHT granulites, and, if so, new discoveries of UHT terrains may be expected.

Evolution of mantle melts intruding the lowermost continental crust: constraints from the Monte Capio–Alpe Cevia mafic–ultramafic sequences (Ivrea–Verbano Zone, northern Italy)

This study presents a new petrological–geochemical data set for the Monte Capio and Alpe Cevia mafic–ultramafic sequences, which are exposed in the deepest levels of the Ivrea–Verbano Zone. These sequences are composed of a peridotite core, with dunite in the center, mantled by minor orthopyroxene-dominated pyroxenites and subordinate hornblende gabbronorites. Amphibole is ubiquitous in the peridotites and the pyroxenites (≤ 15 vol % and 10–40 vol %, respectively), and the peridotite–pyroxenite associations are frequently crosscut by amphibole-rich (45–90 vol %) veins/dykes showing sinuous-to-sharp planar boundaries towards host rocks. The whole-rock Mg# [100 × Mg/(Mg + Fetot2+)] decreases from the peridotites to the pyroxenites and the crosscutting amphibole-rich dykes (84–81, 80–77, and 73–66, respectively), consistently with the Mg# variations shown by included orthopyroxene, clinopyroxene, and amphibole. Olivine has relatively low forsterite and NiO amounts (84–78 mol % and ≤ 0.14 wt%), and spinel is characterized by low Cr# [100 × Cr/(Cr + Al)] of 7–24. The anorthite content of plagioclase varies from 91 to 88 mol% in plagioclase-bearing pyroxenites to 91–75 mol% in amphibole-rich dykes. The chondrite-normalized REE patterns of amphibole from peridotites and pyroxenites show nearly flat MREE–HREE, no evident Eu anomaly, and LREE that are slightly depleted to slightly enriched with respect to MREE. Amphibole from the amphibole-rich veins/dykes exhibits slight LREE depletion. Whole-rock and amphibole separates show substantial variations in initial Nd–Sr isotopic compositions (e.g., whole-rock eNd calculated at 290 Ma ranges from − 0.3 to − 4.7), irrespective of the rock-type and of incompatible element amphibole compositions. We propose that the Monte Capio–Alpe Cevia dunites formed by cooling of magma lenses that intruded the lowermost continental crust of the Ivrea–Verbano Zone. The chemically evolved signature of the dunites documents earlier crystallization of chemically primitive dunites at lower levels, or olivine fractionation within the dunites during melt ascent. Associated pyroxene-bearing peridotites show a magmatic evolution ruled by reaction of a melt-poor crystal mush with migrating melts relatively rich in SiO2 and H2O, which developed orthopyroxene and amphibole at the expenses of olivine ± clinopyroxene. These migrating melts may be reconciled with those feeding the crosscutting amphibole-rich veins/dykes, whose compositions suggest formation by chemically evolved H2O-rich basalts with an arc-type incompatible trace-element fingerprint. Unraveling the origin of the Monte Capio–Alpe Cevia pyroxenites is hampered by the complex open-system magmatic evolution, which also included assimilation of material released by basement metasediments and/or involvement of primary melt batches with different compositions.

Ductile extensional shear zones in the lower crust of a passive margin

We describe and interpret an unpublished set of ION Geophysical seismic reflection profile showing strong organized seismic reflectors at the base of the continental crust of the Uruguayan volcanic rifted margin. We distinguish two main groups of reflectors in the lowermost continental crust. A first group, at depths ranging from 32 km below the continent to 16 km in the continent–ocean transition, comprises reflectors continuous over tens of kilometers, peculiarly visible near the mantle–crust boundary. A second group of reflectors dipping toward the ESE (oceanward) is widely distributed in the lower crust. These reflectors are slightly curved and tend to merge and become sub-parallel with the first group of reflectors. Together they draw the pattern of thick shallow-dipping top-to-the continent shear zones affecting the lower continental crust. Such sense of shear is also consistent with the continentward dip of the normal faults that control the deposition of the thick syn-tectonic volcanic formations (SDR). A major portion of the continental crust behaved in a ductile manner and recorded a component of top-to-the continent penetrative simple shear during rifting indicative of a lateral movement between the upper crust and the mantle.

Continent-ocean-transition across a trans-tensional margin segment: off Bear Island, Barents Sea

A 410 km long Ocean Bottom Seismometer profile spanning from the Bear Island, Barents Sea to oceanic crust formed along the Mohns Ridge has been modelled by use of ray-tracing with regard to observed P-waves. The northeastern part of the model represents typical continental crust, thinned from ca. 30 km thickness beneath the Bear Island to ca. 13 km within the Continent–Ocean-Transition. Between the Hornsund FZ and the Knolegga Fault, a 3–4 km thick sedimentary basin, dominantly of Permian/Carboniferous age, is modelled beneath the ca. 1.5 km thick layer of volcanics (Vestbakken Volcanic Province). The P-wave velocity in the 3–4 km thick lowermost continental crust is significantly higher than normal (ca. 7.5 km s–1). We interpret this layer as a mixture of mafic intrusions and continental crystalline blocks, dominantly related to the Paleocene-Early Eocene rifting event. The crystalline portion of the crust within the south-western part of the COT consists of a ca. 30 km wide and ca. 6 km thick high-velocity (7.3 km s–1) body. We interpret the body as a ridge of serpentinized peridotites. The magmatic portion of the ocean crust accreted along the Knipovich Ridge from continental break-up at ca. 35 Ma until ca. 20 Ma is 3–5 km thicker than normal. We interpret the increased magmatism as a passive response to the bending of this southernmost part of the Knipovich Ridge. The thickness of the magmatic portion of the crust formed along the Mohns Ridge at ca. 20 Ma decreases to ca. 3 km, which is normal for ultra slow spreading ridges.

Geochemical signatures of Variscan eclogites from the Saxonian Erzgebirge, central Europe

From the abundant metre to km-sized eclogite bodies in the Variscan crystalline complex of the Saxonian Erzgebirge we have investigated 19 samples from the ultrahigh pressure area at the Saidenbach reservoir. Twenty-two samples were from the south-western Erzgebirge, and from occurrences located only some km away from the reservoir. These samples were analysed for major and trace elements using X-ray fluorescence (XRF) spectrometry and inductively coupled plasma mass spectrometry (ICP-MS). The non-Saidenbach eclogites (SiO 2=49–53 wt%) can be derived from N-mid-ocean ridge basalts (MORBs) partially transitional to P-MORBs (e.g., (Nb) N: 3–36; (Sr) N: 4–17; (La/Sm) N<1.5 (in most instances <0.7) and (Sm/Yb) N around 1.2). Eclogites from the Saidenbach reservoir (SiO 2=49–61 wt%) are characterised by (Nb) N: 20–170; (Sr) N: 9–43; (La/Sm) N: 1.2–3.0; (Sm/Yb) N: 1.4–8.8, and a clear negative Eu anomaly for the Si-rich samples, thus, being significantly different from the other investigated eclogites. These signatures point to protoliths related to within plate igneous rocks. However, we also discuss the possibilities of (1) protoliths related to a magmatic arc along an active continental margin and (2) the formation by melting of crustal material in the deep mantle and final crystallisation in the lowermost continental crust similar to the adjacent diamondiferous quartzofeldspathic rocks. Due to the specific geochemical signatures of eclogites in the Saidenbach area including other facts, this ultrahigh pressure region is believed to represent a section of lowermost crust not outcropping in other portions of the Saxonian Erzgebirge.

Earthquakes in the deep continental crust - insights from studies on exhumed high-pressure rocks

SUMMARY Recent studies on two exhumed anorthositic and gabbroic high-pressure complexes in the Bergen Arcs and Western Gneiss Region of Norway demonstrate that deeply subducted dry rocks in the lowermost continental crust may resist metamorphic re-equilibration for geologically significant periods of time. This persistent metastability exercised a profound control on the mechanical properties of these rocks, as evidenced by abundant pseudo-tachylyte veins, cataclasites and fractures, documenting eclogite facies seismic failure at depths exceeding 60 km in the thickened Caledonian crust. Our observations indicate that the brittle seismogenic state of some lower crustal rocks, and their propensity to remain metamorphically metastable despite prevailing conditions of T = 600‐800 ◦ C and P = 1.5 to > 2.0 GPa, is largely controlled by

Rapid exhumation of lower crust during continent-continent collision and late extension: Evidence from 40Ar/39Ar incremental heating of hornblendes and muscovites, Caledonian orogen, western Norway

Resolving enigmatic aspects of the age relations of deep crustal rocks in the Bergen Arcs area provides key insights into the controversial tectonic evolution of the Caledonian orogen in western Norway. In the Bergen Arcs area, eclogites of the lower crustal Lindas nappe (upper plate) structurally overlie eclogite-bearing basement gneiss of the Western Gneiss region (lower plate) across the Bergen Arcs shear zone. Laser 40 Ar/ 39 Ar incremental heating data for hornblendes and muscovites on mineral separates, in tandem with detailed thermobarometry and structural observations, allow evaluation of the tectonic cooling and exhumation histories of the deep crustal rocks as well as offset and movement history across tectonic boundaries. In the Lindas nappe the eclogites formed at ≈700 °C and pressures >17 kbar. Hornblende and three separate muscovite grains from a single eclogite sample reveal well-defined 40 Ar/ 39 Ar isotope plateaus at 447.7 ± 4.2, 432.6 ± 0.5, 429.2 ± 0.9, and 429.5 ± 0.6 Ma (2σ errors given here and below), respectively, indicating relatively rapid cooling of the high-pressure rocks (10–15 °C/m.y.). Apparently older plateau ages from muscovites from other eclogites at 451.3 ± 1.1 and 462.6 ± 0.8 Ma may be related to indiscernible excess argon. High-grade amphibolites (≈690 °C, 8–12 kbar) of the Lindas nappe yield ages of 439.3 ± 3.5 and 455.4 ± 1.5 Ma. In contrast, hornblende from gneiss in the adjacent Western Gneiss region (≈670 °C, 8 kbar) to the east across the Bergen Arcs shear zone reveal plateau ages of 409.3 ± 2.7 and 394.8 ± 2.2 Ma, similar to other hornblende and muscovite 40 Ar/ 39 Ar dates in the Western Gneiss region immediately to the north. To the west of the Lindas nappe allochthonous basement gneiss of the Oygarden Gneiss complex (≈670 °C, 8 kbar) yields a plateau age of 408.1 ± 3.4 Ma. The temperature estimates from all the samples analyzed are above the closure temperatures for hornblende and muscovite and thus indicate that the argon ages reflect cooling after peak metamorphic conditions. The argon ages for the high-pressure eclogites of the Lindas nappe imply extreme crustal thickening (>50 km) prior to ca. 450 Ma. Together the data indicate that subduction of lowermost continental crust, possibly related to continent-continent collision, occurred early (ca. 450 Ma) in the tectonic evolution of the orogen, leading to the development and exhumation of the lower crustal eclogites and high-grade amphibolites within the Lindas nappe (between 450 and 430 Ma). In contrast, the high-pressure eclogites of the Western Gneiss region, which may have formed significantly later (ca. 425 Ma), were exhumed and cooled along with the basement gneiss late in the history of the orogen (ca. 400 Ma). The 40 m.y. offset across the Bergen Arcs shear zone, which separates the Lindas nappe (upper plate) and the Western Gneiss region (lower plate), is consistent with extensional movement along the fault resulting in the rapid exhumation and cooling of the region late in the tectonic evolution of the orogen (ca. 400 Ma). Thus, the deep crustal rocks in the overlying Caledonian nappes in western Norway appear to have been cooled and exhumed rapidly related to continent-continent collision during a predominantly contractional phase early in the tectonic evolution of the orogen, whereas lower crustal rocks of the underlying basement appear to have been exhumed extremely rapidly, coincident with late orogenic to postorogenic extension. Temporally and possibly tectonically distinct mechanisms are illustrated for the exhumation of deep crust within the tectonic evolution of a continent-continent collisional orogen.

The depletion of tungsten in the bulk silicate earth: Constraints on core formation

The depletion of the siderophile element tungsten (W) in the bulk silicate Earth (BSE), due to core formation, provides clues to the formation and early evolution of the Earth. This study significantly improves our knowledge of the abundance of W in the continental crust, an important reservoir for W in the BSE. Tungsten is a highly incompatible element, whose absolute concentrations are variable due to igneous processes. Therefore, the abundance of W is normalized to the highly incompatible lithophile element Th, to correct for igneous fractionation since the end of core formation. Similar W/Th ratios are observed in several terrestrial reservoirs, including the depleted mantle (nodules and Mid-Ocean Ridge Basalts), the old continental crust (upper continental sediments and Archean granulites), and the young continental crust (continental and oceanic arcs). The use of Th as a normalizing element is inappropriate, however, in the case of granulite xenoliths from the lowermost continental crust. These samples have higher W/Th ratios due to loss of Th (and U) compared to other incompatible elements including Ba and W. The few komatiite samples investigated have high ratios of W to the normalizing elements, possibly due to loss of the normalizing elements during alteration. The depletion of W in the BSE is determined, by using a mass balance calculation based on the W/ Th ratios of mantle and crustal reservoirs, and including the uncertainty in the initial abundance of W in the Earth, based on W abundances in chondritic meteorites. The resulting depletion of W relative to the refractory lithophile element Th is 0.06, with a range from 0.03–0.1. The depletion range for W overlaps the depletions of the compatible siderophile elements Co and Ni. This observation is consistent with the heterogeneous accretion theory for the Earth ( Newsom, 1990; O'Neill, 1991). In contrast, the observed depletions for W, Co, and Ni are not consistent with their experimental high temperature equilibrium metal-silicate partition coefficients (( Hillgren and Drake, 1994). The depletion of W also provides important constraints on the timing of core formation, based on the decay of the now extinct isotope 182Hf to the isotope 182W in the early solar system ( Harper and Jacobsen, 1994).

Experimental high-pressure granulites: Some applications to natural mafic xenolith suites and Archean granulite terranes

High-pressure granulite assemblages have been produced as residues in partial melting experiments on a natural (alkalic basalt) amphibolite between pressures of 12 and 18 kbar. In particular, at 18 kbar, partial melting of the hornblende + plagioclase ± quartz assemblage under fluid-absent conditions produces garnet + clinopyroxene + new albitic plagioclase + melt. Seismic velocities ( V P ) are estimated from the modal data for the experimental assemblages and range from 6.90 km/s for hornblende-bearing residues to 7.62 for the dominantly garnet- clinopyroxene residues. These values are typical for rock types in the lowermost crust, transitional to mantle. The experimental results help place additional pressure-temperature- a (H 2 O) constraints on the source region for the natural high-pressure granulites. The experimental residue assemblage formed at 18 kbar has been described in natural xenolith suites from the Delegate Pipes in Australia, where the pipes intrude Upper Ordovician and Lower Devonian continental crust, and in the Bearpaw Mountains, Montana, where the xenolith- bearing magmas intrude older crust of Archean age. The combination of these data shows that the xenoliths may indeed represent lowermost continental crust and furthermore helps interpret the nature of the crust-mantle boundary in these areas. The Delegate Pipes xenoliths suggest that the crust-mantle boundary may be the site for partial melting and assimilation, whereas the Bearpaw Mountains samples indicate that magmatic underplating may have been a major process in generating thick continental Archean crust. The experimental data and Bearpaw Mountains xenoliths suggest that the large range of rock types found in Archean granulite terranes may not be representative of the lowermost continental crust.

On the formation of granulites

The tectonic settings for the formation and evolution of regional granulite terranes and the lowermost continental crust can be deduced from pressure–temperature–time (P–T–time) paths and constrained by petrological and geophysical considerations. P–T conditions deduced for regional granulites require transient, average geothermal gradients of greater than 35°C km−1, implying minimum heat flow in excess of 100 mWm−2. Such high heat flow is probably caused by magmatic heating. Tectonic settings wherein such conditions are found include convergent plate margins, continental rifts, hot spots and at the margins of large, deep‐seated batholiths. However, particular P–T–time paths do not allow specific tectonic settings to be distinguished at this time. Under different conditions, both clockwise, CW (Pmax attained before Tmax), and anticlockwise, ACW (Pmax attained slightly after Tmax), paths are possible in the same tectonic setting. Both CW and ACW end‐member paths can yield nearly isobaric cooling, IBC, paths. Such cooling paths are clearly not an artefact of thermobarometry, but can be constrained by solid–solid and devolatilization equilibria and geophysical modelling.In terms of understanding the evolution of the deep crust, a potentially significant group of regional granulite terranes are those that show evidence for ACW‐IBC paths. Such paths are the likely result of: (i) episodic igneous activity resulting in intrusions within all levels of the crust, (ii) thickening of the crust by magmatic underplating, (iii) slow uplift as a result of the formation of a deep, garnet‐rich crustal root and (iv) excavation resulting from a later tectonic event unrelated to that resulting in the formation of the granulites. The later event might be triggered by the delamination of the garnet‐rich, lowermost crust.

A theory for buckling of the mantle lithosphere and Moho during compressive detachments in continents

Current knowledge of rock mechanics suggests a strength minimum in the lowermost continental crust, along which simple shear could occur if a strong force drove the mantle lithosphere. We examine the change of shape of the Moho during this process, assuming plane strain, a depth-dependent, effectively-Newtonian crustal viscosity, and small-amplitude sinusoidal waves. Under the assumption that effective crustal viscosity is η min exp[ k( c− z)/ c], waves will propagate at speeds relative to the surface of between V m / k and V m , where c is crustal thickness, z is depth, and V m is mantle lithosphere velocity. If the mantle lithosphere is in relative horizontal tension (“pulled”), then all wavelengths are damped. If it is in relative horizontal compression (“pushed”), waves with wavelengths less than a critical value will grow exponentially. The wavelength of the fastest-growing wave varies in proportion to: the crustal thickness, c; the amount of horizontal compression I to a power ⩽ 1 2 ; the maximum viscous compliance (1/ η min ) to a power ⩽ 1 6 ; and the viscous flexural rigidity of the mantle lithosphere D to a power ⩽ 1 6 . For reasonable parameter values the wavelength that eventually dominates will be in the range 80–300 km. This wave propagates relative to the surface at 70–100% of V m , and for some parameter combinations is almost fixed in the mantle lithosphere reference frame. Thus this is best understood as a buckling process. After deformation ends, the rise in effective viscosities would tend to retard the decay of Moho topography. Slow isostatic equilibration could allow these waves to be expressed as parallel surface structures.

Origin of Granulite Terranes and the Formation of the Lowermost Continental Crust

Differences in composition and pressures of equilibration between exposed, regional granulite terranes and suites of granulite xenoliths of crustal origin indicate that granulite terranes do not represent exhumed lowermost crust, as had been thought, but rather middle and lower-middle crustal levels. Application of well-calibrated barometers indicate that exposed granulites record equilibration pressures of 0.6 to 0.8 gigapascal (20 to 30 kilometers depth of burial), whereas granulite xenoliths, which also tend to be more mafic, record pressures of at least 1.0 to 1.5 gigapascals (35 to 50 kilometers depth of burial). Thickening of the crust by the crystallization of mafic magmas at the crust-mantle boundary may account for both the formation of regional granulite terranes at shallower depths and the formation of deep-seated mafic crust represented by many xenolith suites.