Maximum Crustal Thickness Research Articles

The crustal structure beneath SE Romania from teleseismic receiver functions

SUMMARY Due to vigorous Neogene geodynamic processes, including oceanic subduction, slab break-off and mountain building in the Carpathian Arc, the architecture of the lithosphere in Romania is quite complicated. To improve the knowledge about the lithosphere‐asthenosphere system in this region the passive seismological CALIXTO99 experiment was conducted in 1999 in the SE part of Romania. Here we present crustal models derived from the analysis of teleseismic recordings with the receiver function (RF) method of the 120 temporarily installed stations and of the permanent GEOFON stations MLR and TIRR. The RF results extend the known crustal models which are based mainly on seismic refraction work and analysis of regional earthquakes. We apply a grid-search inversion at 30 stations and use two different error estimation methods to determine the Moho depth and the average crustal vp/vs ratio. The complex 3-D intracrustal structure, especially the deep sedimentary basins, distorts significantly the RF waveforms within the whole station network. This leads to ambiguous results at some stations. Our model of the Moho depth has a maximum crustal thickness in the SE Carpathian Mountains at station MLR with a depth of about 45 km and an average crustal vp/vs ratio of 1.79. The surrounding crust in SE Romania has a thickness of mainly 35‐40 km. The RFs at MLR are characterized by clear azimuthal effects that can be correlated with the variation of the sediment thickness in the foredeep of the Carpathian Mountains. A RF waveform inversion verifies these results and gains improved 1-D S-wave velocity models at several stations in SE Romania.

Seismic structure of the Carnegie ridge and the nature of the Galápagos hotspot

The Galápagos volcanic province (GVP) includes several aseismic ridges resulting from the interaction between the Galápagos hotspot (GHS) and the Cocos–Nazca spreading centre (CNSC). The most prominent are the Cocos, Carnegie and Malpelo ridges. In this work, we investigate the seismic structure of the Carnegie ridge along two profiles acquired during the South American Lithospheric Transects Across Volcanic Ridges (SALIERI) 2001 experiment. Maximum crustal thickness is ∼19 km in the central Carnegie profile, located at ∼85°W over a 19–20 Myr old oceanic crust, and only ∼13 km in the eastern Carnegie profile, located at ∼82°W over a 11–12 Myr old oceanic crust. The crustal velocity models are subsequently compared with those obtained in a previous work along three other profiles over the Cocos and Malpelo ridges, two of which are located at the conjugate positions of the Carnegie ones. Oceanic layer 2 thickness is quite uniform along the five profiles regardless of the total crustal thickness variations, hence crustal thickening is mainly accommodated by layer 3. Lower crustal velocities are systematically lower where the crust is thicker, thus contrary to what would be expected from melting of a hotter than normal mantle. The velocity-derived crustal density models account for the gravity and depth anomalies considering uniform and normal mantle densities (3300 kg m−3), which confirms that velocity models are consistent with gravity and topography data, and indicates that the ridges are isostatically compensated at the base of the crust. Finally, a two-dimensional (2-D) steady-state mantle melting model is developed and used to illustrate that the crust of the ridges does not seem to be the product of anomalous mantle temperatures, even if hydrous melting coupled with vigorous subsolidus upwelling is considered in the model. In contrast, we show that upwelling of a normal temperature but fertile mantle source that may result from recycling of oceanic crust prior to melting, accounts more easily for the estimated seismic structure as well as for isotopic, trace element and major element patterns of the GVP basalts.

Seismic structure of Cocos and Malpelo Volcanic Ridges and implications for hot spot‐ridge interaction

The Cocos and Malpelo Volcanic Ridges are blocks of thickened oceanic crust thought to be the result of the interaction between the Galapagos hot spot and the Cocos‐Nazca Spreading Center during the last 20 m.y. In this work we investigate the seismic structure of these two aseismic ridges along three wide‐angle transects acquired during the Panama basin and Galapagos plume—New Investigations of Intraplate magmatism (PAGANINI)‐1999 experiment. A two‐dimensional velocity field with the Moho geometry is obtained using joint refraction/reflection travel time tomography, and the uncertainty and robustness of the results are estimated by performing a Monte Carlo‐type analysis. Our results show that the maximum crustal thickness along these profiles ranges from ∼16.5 km (southern Cocos) to ∼19 km (northern Cocos and Malpelo). Oceanic layer 2 thickness is quite uniform regardless of total crustal thickness variations; crustal thickening is mainly accommodated by layer 3. These observations are shown to be consistent with gravity data. The variation of layer 3 velocities is similar along all profiles, being lower where crust is thicker. This leads to an overall anticorrelation between crustal thickness and bulk lower crustal velocity. Since this anticorrelation is contrary to crustal thickening resulting from passive upwelling of abnormally hot mantle, it is necessary to consider active upwelling components and/or some compositional heterogeneities in the mantle source. The NW limit of the Malpelo Ridge shows a dramatic crustal thinning and displays high lower crustal velocities and a poorly defined crust‐mantle boundary, suggesting that differential motion along the Coiba transform fault probably separated Regina and Malpelo Ridges.

High-resolution body wave tomography beneath the SVEKALAPKO array: I.A priorithree-dimensional crustal model and associated traveltime effects on teleseismic wave fronts

Assessment of contributions from shallow lithosphere to teleseismic wave front distortion is a prerequisite for high-resolution regional teleseismic tomography. Several methods have been proposed in the past for the correction of these effects, e.g. application of station correction terms. We propose an approach that is independent of the subsequent inversion and uses the available a priori knowledge of the crustal structure to calculate crustal traveltime effects of teleseismic wave fronts. Our approach involves the construction of a 3-D crustal model based on controlled source seismology data and calculation of the associated traveltime anomalies for incoming teleseismic wave fronts. The model for central Fennoscandia shows a maximum crustal thickness of 64 km and includes a high-velocity lower crust as derived for parts of the study area by previous authors. Traveltimes calculated using finite differences for teleseismic waves travelling through this crustal model are compared with those from the standard reference model IASP91 and the residuals are used to correct observed teleseismic arrival times at the SVEKALAPKO array. To test the performance of this approach, in a second part of the study a synthetic traveltime data set is obtained by tracing wave fronts through a mantle structure with known velocity anomalies and the 3-D crustal model. This data set is inverted with and without correction for crustal effects. The 3-D crustal effects alone with a homogeneous mantle are also inverted and the results show that the crustal effects propagate down to 450 km. The comparison of the inversion results demonstrates the need to apply appropriate 3-D crustal corrections in high-resolution regional tomography for upper-mantle structure beneath the Baltic Shield.

Crustal evolution over the last 2 m.y. at the Mid‐Atlantic Ridge OH‐1 segment, 35°N

We present the crustal and mantle velocity structure along the strike of the eastern rift mountains at 35°N on the Mid‐Atlantic Ridge. These results were obtained by an inversion of ∼1800 Pg/Pn and ∼450 PmP travel times and by gravity modeling. As commonly observed at slow spreading mid‐ocean ridges, thicker crust (9 km) occurs at the segment midpoint, while thinner crust (7 km) is found toward the segment ends. This along strike variation occurs primarily in the lower crust, which is 7 km thick at the segment center and 4–6 km thick at the segment ends. In contrast, the thickness of the upper crust is relatively constant along strike. At the segment ends, relatively low velocities extend for 10–15 km along strike and from the seafloor to 4 km depth. These low velocities may indicate an attenuated melt supply and/or fracturing and alteration within the shallow to mid‐crust. Directly beneath a cluster of three seamounts at the segment center is a region of relatively high velocity (+0.5 km/s) in the mid‐crust. This feature may correspond to a frozen magma chamber that fed the overlying volcanoes. A synthesis of these results with those from two companion experiments along the rift valley and the conjugate flank provide a detailed record of crustal accretion and evolution at this segment. Specifically, the crustal velocity structures of each flank are nearly identical, and they exhibit a thinner and 16% faster upper crust than is observed on axis. The lower crust is remarkably similar in all three settings, except for a low‐velocity body on axis, which is interpreted as a partially molten zone. The maximum crustal thickness is also similar in all three profiles, but north of the segment center, zero‐age crust is nearly 4 km thinner than beneath the eastern flank and 2 km thinner than beneath the western flank. These differences may indicate that segment‐centered mantle upwelling varies on a timescale of ∼2 m.y.

Late Cenozoic tectonism, collapse caldera and plateau formation in the central Andes

The evolution of Andean volcanism including the formation of late Miocene to Recent collapse calderas on the Puna plateau is generally interpreted in terms of the kinematic framework of the Nazca and South American Plates. We present evidence that caldera dynamics and associated ignimbrite volcanism are genetically linked to the activity of first-order NW–SE-striking zones of left-lateral transtension on the local and regional scales. Consequently, ages of collapse calderas indicate activity of these fault zones which initiated at about 10 Ma on the Puna plateau. The onset of such faulting points to a change in the deformation regime from dominantly vertical thickening to orogen-parallel stretching upon reaching maximum crustal thickness and critical surface elevation. Horizontal magma sheets that formed at mid-crustal level possibly due to heat advection by volume increase of asthenospheric mantle below thickened crust were tapped by sub-vertical faults. This accounts well for the observed tectono-magmatic phenomena at surface. It follows that formation of collapse calderas and eruption of voluminous ignimbrites appear to be related to the mechanical evolution of the Andean plateau rather than to changes in the geometry of the Wadati–Benioff zone or plate boundary kinematics.

Estimates of Martian crustal thickness from viscous relaxation of topography

Isostatically compensated crustal thickness variations and associated topographic contrasts at the surface of a planet result in lateral pressure gradients, which may cause the lower crust to flow and reduce the relief. Areas of thicker crust are generally associated with more rapid relaxation of topography. On Mars, topographic features such as impact basins and the hemispheric dichotomy have survived for 4 Gyr. We use a finite difference representation of depth‐dependent, non‐Newtonian lower crustal flow to investigate how topography decays with time. For a dry diabase rheology, total radiogenic concentrations ≥80% of terrestrial values, and crustal radiogenic concentrations similar to terrestrial basalts, we find that an upper bound on the mean planetary crustal thickness is ∼100 km. In the probably unrealistic case where all the radiogenic elements are in the crust, this maximum crustal thickness can be increased to ∼115 km. The main uncertainty in these results is the total radiogenic abundances on Mars. Comparing our results with the observed shape of the crustal dichotomy provides no evidence that this slope is primarily the result of lower crustal flow. Both Hellas and the dichotomy are isostatically compensated; if the mechanism is Airy isostasy, then the lower bound on mean crustal thickness is ∼30 km. Crustal thicknesses of 30–100 km on Mars can be produced by mid‐ocean ridge spreading at potential temperatures of 1350°–1600°C. However, for such crustal thicknesses the lithosphere is likely to be positively buoyant, making subduction difficult.

Fluctuation of group velocity of Love waves across a dent in the continental crust

The dispersion characteristics of group velocity of Love waves are measured across a dent in the continental crust that has a maximum thickness of about 50 km; the numerical modeling analyses are performed using the finite difference method. Measurement of the group velocity over the entire width of the mountain root displays several distinct dispersions. The group velocities for the crustal dent structure are lower than those for the stratified medium with a maximum crustal thickness of the dent at short periods of 20–30 s. The period range becomes longer as the sloping angle of the dent increases or as the crustal thickness of the dent increases. The period indicating a group velocity minimum for the crustal dent structure is shorter than that for the stratified medium with a maximum crustal thickness of the dent. An example of the interpretation of observational data is shown. In the example, the group velocities for the propagation path over the Tibetan Plateau have properties concordant with the above dispersion characteristics.

Crustal thickness and structure along three contrasting spreading segments of the Mid‐Atlantic Ridge, 33.5°–35°N

The crustal thickness and crustal and upper mantle structure along the rift valleys of three segments of the northern Mid‐Atlantic Ridge with contrasting morphologies and gravity signatures are determined from a seismic refraction study. These segments lie between the Oceanographer and Hayes transforms and from north to south have progressively deeper axial valleys with less along‐axis relief and smaller mantle Bouguer gravity lows. Major variations in seismic crustal thickness and crustal velocity and density structure are observed along these segments. The thickest crust is found near the segment centers, with maximum crustal thicknesses of 8.1, 6.9, and 6.6±0.5 km, decreasing from north to south. However, the mean crustal thickness is similar for each segment (5.6±0.4, 5.7±0.4 and 5.1±0.3 km). Near the segment ends, crustal thickness is 2.5 to 5±0.5 km with no systematic variation from north to south. At segment ends, both crustal velocities and vertical velocity gradients are anomalous and may indicate fracturing and alteration of thin igneous crust and underlying mantle. Away from segment ends, the thickness of the upper crust is relatively uniform along axis (∼3 km), although its internal structure is laterally heterogeneous (velocity anomalies of ±0.6 km s−1 over distances of 5 km), possibly related to the presence of discrete volcanic centers. The along‐axis crustal thickness variations are primarily accommodated in the lower crust. The center of the northern segment (OH‐1) has an unusually thick crustal root (excess thickness of 2–4 km and along‐axis extent of 12 km). Our results are consistent with an enhanced supply of melt from the mantle to the segment centers and redistribution of magma along axis at shallow crustal levels by lateral dike injection. Along this portion of the Mid‐Atlantic Ridge, our results suggest that differences in axial morphology, seismic crustal thickness, and gravity anomalies are correlated and the result of variations in melt flux from the mantle. A surprising result is that the melt flux per segment length is similar for all three segments despite their different morphologies and gravity signatures. This argues against excess melting of the mantle beneath segment OH‐1. Instead, we suggest that the thickened crust at the segment center is a result of focusing of melt, possibly due to the influence of the thermal structure of the Oceanographer fracture zone on melt migration in the mantle.

The crustal structure beneath the Central Andean forearc and magmatic arc as derived from seismic studies — the PISCO 94 experiment in northern Chile (21°–23°S)

In this study, we present an interpretation of seismic refraction profiles from the PISCO 94 experiment in northern Chile. As the PISCO experiment was a combined active and passive seismological study, we also discuss results of the passive part in the context of the seismic refraction model. Previous seismic refraction and gravimetric studies indicate a maximum crustal thickness of about 70 km beneath the Pre- and Western Cordillera. The new seismic refraction data lead to a differentiated image of the Andean crust which shows strong varying characteristics. The crustal discontinuities (up to five are detected) dip from W to E. The upper crust has a thickness of 18 km (Precordillera) to 23 km (magmatic arc) underlain by the recent middle crust down to 35–45 km where the velocity increases to about 7 km/s at its base. This crustal level is interpreted as old continental lower crust and its base as blurred continental (paleo) Moho. Beneath the Precordillera, a strong discontinuity at 70 km depth with a velocity increase to about 8 km/s was detected, interpreted as the recent geophysical Moho. For the magmatic arc, this deep discontinuity could not be found by active seismic measurements. The tomographic models of the seismological studies, in general, confirm the seismic refraction results. Anomalously high v p/ v s ratios in the deeper part of the forearc indicate a hydrated mantle wedge consisting of serpentine and amphibole-bearing peridotite and the 70 km discontinuity is interpreted as the boundary between these two different stages of the hydrated mantle wedge. A zone of high attenuation (Qp) and high v p/ v s ratios beneath the magmatic arc coincides with the low velocity zones and indicates partially molten rocks from a depth of 20 km down to the asthenospheric wedge.

Segmentation and melt supply at the Southwest Indian Ridge

The segmentation of mid-ocean ridge axes and its variation with spreading rate provide constraints on models of mantle upwelling and the supply of basaltic melt to the crust. We present seismic refraction results from three segments of the very slow spreading Southwest Indian Ridge. From the absence of oceanic layer 3 beneath segment boundaries, and from significant variations in maximum crustal thickness between segments, we infer that melt generation and/or delivery is focused at segment midpoints and varies between segments. Anomalously thin crust—2.0–2.5 km beneath the nontransform segment boundaries and 3.5–6.0 km beneath the segment mid-points—is caused by restricted melt generation resulting from conductive heat loss from the upwelling mantle under the spreading center.

Evolution métamorphique des métapélites du Massif hercynien des Rehamna (Maroc): implications tectonothermales

In central Rehamna (Moroccan Hercynian Belt), the maximum crustal thickness was attained after the development of the Ouled Zedness Thrust, which emplaced the Jorf El Ahmar Formation (Eastern Rehamna) over the Skhour Nappe (central Rehamna) during a metamorphic event of intermediate P/T reaching amphibolite-facies conditions. Later on, the El Broumi detachment caused the emplacement of a Devonian-Visean unit (El Broumi-Ouled Hassine and Lalla Tittaf Formations), previously metamorphosed in the low T part of the amphibolite-facies, in the top of the structural ensemble during retrogressive metamorphic conditions. The structural and metamorphic study of the syn-peak mineral assemblages apparently suggests a similar tectonothermal evolution for the Skhour and Devonian-Visean formations. Nevertheless, mineral compositions in the developed metamorphic assemblages are indicative of higher metamorphic conditions in the first formation. Thermobarometric P/T conditions of the regional thermal peak were calculated at 560°C and 9 kbar in the Skhour Nappe, and 532°C and 7 kbar in eastern Rehamna. These P/T conditions are also mainly coincident with those of the pressure peak. This suggests a very limited influence of decompressive thermal relaxation in the final tectonothermal evolution of the Rehamna Massif, interpreted as due to rapid exhumation of the belt as a result of syn-orogenic crustal extension. The syn-extensional part of the P-T path shows slightly retrogressive decompression, which finally reached low pressure conditions in the andalusite stability field.

Seismic images of crustal duplexing and continental subduction in the Brooks Range

We have interpreted an integrated vertical incidence to wide‐angle seismic data set to develop a consistent migrated seismic reflection image and seismic velocity model of the Brooks Range fold and thrust belt in north central Alaska. The common midpoint (CMP) reflection data image the principal structures comprising the Brooks Range: the Endicott Mountains allochthon (EMA), the crustal scale Doonerak duplex, the master detachment, a 1.0–1.5 s thick zone of lower crustal reflectivity just above the crust‐mantle boundary, and a complex crustal root. The master detachment separates the crust into units which have been uplifted and deformed in the fold and thrust belt from those which have not. Least squares inversion of both reflection and refraction travel time data produced a velocity model consistent with the CMP image of the Brooks Range as well as with the Bouguer gravity data. The different layers comprising the seismic velocity model correlate well with the principal structural elements identified in the seismic reflection data, and seismic velocities in the model compare favorably to petrophysical data from Brooks Range rock samples. Maximum crustal thickness in the Brooks Range is 49 km, at an asymmetric root located under the EMA. At the root we observe an offset in the lower crustal reflectivity and two deep zones of reflections north of the root. We interpret these as a Moho offset of some 5 km near the range front, which in our favored interpretation resulted from subduction of the Brooks Range lower crust northward beneath the North Slope. A mantle reflective zone which we interpret as the subducted lower crust can be traced to depths as great as 65 km. Above this zone at considerably shallower depth is the original North Slope Moho. Proximity of the continental subduction zone to the crustal scale Doonerak duplex suggests that the development of the fold and thrust belt has been at least partially controlled by the lower crust/mantle subduction.

Preliminary study of the gravity field of the southern Urals along the URSEIS '95 seismic profile

The gravity field in the southern Urals is investigated using two gridded data sets of the Bouguer gravity that were analyzed and modelled by a two-dimensional method. The first data set covers the entire southern Urals with an underlying grid spacing of 18 km by 18 km. The second data set covers the area of the URSEIS '95 seismic profile and its nearest surroundings with a grid of 9 km by 9 km. For modelling and gravity field data processing, topographical data sets with the same resolution are available for the area. Initial applications of wavelength filtering, depth estimation, and 2D modelling show that most of the Bouguer anomaly variation is caused mainly by two lithospheric density domains. Moho depression causes a large-scale negative gravity effect with wavelengths of about 600 km and a dense, shallow oceanic terrane in the Magnitogorsk Zone causes a positive gravity effect characterized by a wavelength shorter than 180 km. A moderate crustal root is obtained in the area of the southern Urals where the depth of the crust-mantle interface varies from some 40 km up to 50 km. The maximum crustal thickness is located below the Magnitogorsk and East Uralian zones. Preliminary analysis of seismic studies indicates a thickened crust for the lower parts of the model which is not reflective. From the viewpoint of density modelling it is necessary to introduce a transitional zone with a density of 3240 kg/m 3 between the earth's crust (density 2910 kg/m 3) and mantle (3350 kg/m 3).

Constraints on melt production rate beneath the mid-ocean ridges based on passive flow models

We present a model for computing the total melt production rate from the decompression partial melting region beneath a mid-ocean ridge, and the maximum oceanic crustal thickness created at the ridge axis assuming an ideal melt migration mechanism. The calculations are based on a self-consistent numerical model for the thermal structure and steady-state mantle flow field at a mid-ocean ridge. The model includes the effect of decreasing the melt production rate within the partial melting region by melt extraction as the residual mantle matrix becomes increasingly difficult to melt. Thus the melt fraction depends not only on temperature and pressure determined by the location beneath the ridge axis (the Eulerian description) but also on the accumulated melt extraction since the upwelling mantle matrix enters the partial melting region determined by the location along the flow-line path (the Langrangian description). This effect has been neglected by previous models. The model can predict the size of the melting region and the locations of the boundaries between mantle, residual mantle, and the partial melting region for a given spreading rate, also the distribution of the melt depletion and the mean melting depth. Given the observed average thickness of oceanic crust (∼6 km), which is relatively independent of spreading rate, the model results also provide a constraint on the overall efficiency of melt migration to the ridge axis; the efficiency must decrease from 100% at 10 mm/yr to about 60% at fast spreading rates (>50 mm/yr). Although this reduction may be partially due to the increasing size of the melting region with increasing spreading rate, it still requires less efficient melt migration near the ridge axis at fast spreading rate. We found that the calculated crustal thickness is very sensitive to the mantle temperature. For a normal mantle temperature of 1350°C, the model can generate the observed 6 km oceanic crust over the global range of spreading rates, while the anomalous thicker crusts of the Iceland hotspot and the Reykjanes Ridge are related to higher mantle temperatures associated with the hotspot. Finally, by comparing our model results with previous ones we found that neglecting variations of the melting relations of the residual mantle matrix with melt removal will overestimate the crustal thickness by at least a factor of 1.7.

Crustal-thickness variations in the central Andes

We estimated the crustal thickness along an east-west transect across the Andes at lat 20°S and along a north-south transect along the eastern edge of the Altiplano from data recorded on two arrays of portable broadband seismic stations (BANJO and SEDA). Waveforms of deep regional events in the downgoing Nazca slab and teleseismic earthquakes were processed to isolate the P-to-S converted phases from the Moho in order to compute the crustal thickness. We found crustal-thickness variations of nearly 40 km across the Andes. Maximum crustal thicknesses of 70–74 km under the Western Cordillera and the Eastern Cordillera thin to 32–38 km 200 km east of the Andes in the Chaco Plain. The central Altiplano at 20°S has crustal thicknesses of 60 to 65 km. The crust also appears to thicken from north (16°S, 55–60 km) to south (20°S, 70–74 km) along the Eastern Cordillera. The Subandean zone crust has intermediate thicknesses of 43 to 47 km. Crustal-thickness predictions for the Andes based on Airy-type isostatic behavior show remarkable overall correlation with observed crustal thickness in the regions of high elevation. In contrast, at the boundary between the Eastern Cordillera and the Subandean zone and in the Chaco Plain, the crust is thinner than predicted, suggesting that the crust in these regions is supported in part by the flexural rigidity of a strong lithosphere. With additional constraints, we conclude that the observation of Airy-type isostasy is consistent with thickening associated with compressional shortening of a weak lithosphere squeezed between the stronger lithosphere of the subducting Nazca plate and the cratonic lithosphere of the Brazilian craton.

The Moho depth distribution in Fennoscandia and the regional tectonic evolution from Archean to Permian times

From a variety of seismic refraction and reflection profiling results, a new detailed Moho depth map for the Fennoscandian part of the Baltic Shield has been compiled. A complementary crystalline crustal thickness map was compiled for Denmark, the Skagerrak Sea, offshore western Norway and adjacent areas where sedimentary strata reach maximum thicknesses of 10 km and even more. These maps are discussed in relation to tectonic evolution and crustal ages of Fennoscandia, seismic velocities, Bouguer gravity and seismicity. Areas deviating from the proposed age-thickness relationships are the Kola Peninsula (Archean) and the Oslo Rift (Permian). The average crustal thickness in the former area is around 44 km, while the latter area is associated with a Moho elevation of 3–5 km. Maximum crustal thicknesses in excess of 50 km have been found in southwestern and Central Finland and correlate with a crust of Proterozoic age. The Caledonides of western Norway do not exhibit, as elsewhere, a crustal root. Sub-Moho P n and S n seismic velocities are relatively small in that area and Bouguer gravity values are strongly negative. Intracrustal P-velocity variations do not seem to reflect geological age or specific tectonic features. Although the seismicity is moderate in Fennoscandia, most of the earthquake activity and, in particular, the largest earthquakes are confined to areas where the crustal thinning is most pronounced.

Isotopic evidence for preservation of Cordilleran lithospheric mantle during the Sevier-Laramide orogeny, western United States

It has been inferred that Sevier-Laramide flat subduction imparted a large basal shear traction force to the overriding North American plate, resulting in Laramide Rocky Mountain foreland deformation and significant thinning of Cordilleran lithosphere. Additional inferred consequences include regional refrigeration of Cordilleran crust and suppression of synorogenic extensional collapse. Nd isotopic data from Cenozoic mafic volcanic rocks indicate that normal-thickness lithospheric mantle was preserved beneath the Cordilleran craton from Precambrian to late Cenozoic time. Therefore, Cordilleran lithosphere was not thinned significantly, and extensional collapse of orogenically thickened crust was not suppressed by refrigeration. Analogy with the South American Cordillera suggests that flat subduction does not apply significant shear stress to the base of overriding plate lithosphere beyond the fore-arc region and therefore does not mechanically thin overriding plate lithosphere (except in the fore-arc region). The principal result of flat subduction was enhanced mechanical coupling between the underriding and overriding plates along the fore-arc region. This increased the horizontal end load, due to relative plate convergence, placed on the North American plate. Horizontal end-load stress was transmitted laterally across both (1) a hinterland that had attained maximum crustal thickness and (2) the rigid Colorado Plateau into the Laramide Rocky Mountain foreland. Early Tertiary waning of plate-convergence rates then allowed radial extensional collapse of the southern Cordilleran region to drive the Colorado Plateau block northward, resulting in late Laramide Rocky Mountain foreland deformation.

Role of crustal thickening and extensional collapse in the tectonic evolution of the Sevier-Laramide orogeny, western United States

The effects of crustal thickening and extensional collapse of continental crust, combined with variations in plate-convergence forces, provide a coherent explanation for evolution of the Cretaceous-early Tertiary Sevier-Laramide orogeny. Crustal thickening of the Sevier-Laramide hinterland region was triggered by the combination of compressive plate-convergence forces and subduction-induced conductive heating of the crust. Eastward progradation of the locus of Sevier-Laramide deformation through time reflects the progressive widening of the region of crustal thickening. The region of maximum crustal thickening migrated outward until about 80 to 75 Ma, when it encountered excessively strong lithosphere of the Colorado Plateau. The locus of deformation was then transmitted laterally across both a hinterland that had attained maximum crustal thickness and the rigid Colorado Plateau, into the Laramide Rocky Mountain foreland. An episode of vigorous extensional collapse of orogenically thickened crust affected the southern Cordillera between about 75 and 35 Ma. Early Tertiary waning of plate-convergence rates and continued subduction-induced conductive heating of thickened crust allowed extensional collapse along the southern Cordillera region to drive the Colorado Plateau block northward. The result was the late Laramide phase of Rocky Mountain foreland deformation. Because of this, compressive deformation in the central and southern Laramide Rocky Mountain region continued through Eocene time, rather than ceasing at 56 Ma, as it did in the northern Cordillera.

Anatexis and metamorphism in tectonically thickened continental crust exemplified by the Sevier hinterland, western North America

The generation of granitoid magmas by partial melting of crustal rocks during continental thickening events is well documented in many geological provinces throughout the world, including the late Mesozoic Sevier belt of western North America. We present a thermal and petrologic model of anatexis and metamorphism in regions of crustal thickening where the only mantle contribution is the normal conductive supply of heat through the base of the lithosphere (i.e. advection of mass and energy are excluded). We distinguish between formation of migmatites and generation of mobile granitoid magmas and examine the temporal and spatial relationships between these two distinct anatectic processes, between anatexis and regional deformation and between anatexis and metamorphism. A fundamental conclusion is that, if protoliths rich in hydrous minerals are present, regional anatexis is the end-product of classical Barrovian metamorphism in thickened continental crust, even in the absence of a free water-rich fluid phase. Barrovian metamorphic facies series are predicted with thickening ratios (maximum crustal thickness attained/initial crustal thickness) as low as 1.3, but mobile granitoid magmas are not formed if this ratio is less than approximately 1.5. Above these lower bounds, Barrovian metamorphism and anatectic granitoid magmatism occur independently of the magnitude of thickening and of the way in which thickening is accomplished. Both processes are sensitive to a diminished heat supply; lowering either Moho heat flow or crustal radioactive heat production results in blueschist-eclogite metamorphism and inhibits the formation of mobile granitoid magmas. We model anatexis under fluid-absent conditions and show that, with such a constraint, migmatization is always a syn-kinematic process (relative to the crustal thickening event), whereas generation of mobile granitoid magmas is in most cases post-kinematic (relative to crustal thickening) but can be syn-kinematic if thickening takes more than approximately 50 Myr. The typical time intervals for melting are consistent with geological observations; mobile granitoid magmas are predicted by most of our models within approximately 10 Myr of the end of the crustal thickening event. This “incubation period” results primarily from the temperature increase required for the dehydration-melting reactions capable of producing large melt fractions to occur. The energetic requirements of anatexis are relatively minor compared to conductive crustal thermal budgets, as shown by the fact that once the necessary P-T conditions are attained, melting reactions are completed within time intervals on the order of 1 Myr, i.e. 1–2 orders of magnitude smaller than the characteristic time scales of the tectonic processes involved in crustal thickening.