Tectonic Underplating Research Articles

Dynamics of sediment subduction‐accretion at convergent margins: Short‐term modes, long‐term deformation, and tectonic implications

A conceptual model is proposed for the short‐term operation of subduction‐accretion systems at convergent margins. The operation is described in terms of the mass fluxes among the four system components, namely, the accretionary (or prowedge) wedge (P), an uplifted plug (U), a retrowedge (R), and the subduction zone. The latter consists of a conduit (C) of slowly moving and deforming material and an underlying subduction channel where material moves and deforms more rapidly. A broad range of convergent margin types can be accommodated within the conceptual model framework, with differing short‐term modes that depend on which of the P‐U‐R‐C components that are active. Finite element geodynamical model experiments are used to investigate whether these modes are dynamically feasible. These experiments determine excitation of modes under controls that include model entry and exit mass fluxes; flexural loading by accreted material and slab pull forces; boundary velocities corresponding to subduction and subduction zone advance/retreat; distribution, thickness, and density of accreted sediment; and internal frictional properties of the Coulomb sediments. That all modes, with the exception of P (in which only a prowedge is created), are seen in the models provides support for the concepts and allows conditions that favor particular modes to be recognized. In addition, the finite deformation predicted by the models is used to infer the long‐term tectonic styles associated with both single‐mode evolutions and those that exhibit mode switching. A steady state mass balance can occur for modes 0 (pure subduction), P‐C (i.e., where components P and C are active), or other modes when the surface mass denudation plus the subduction flux balances accretion plus tectonic underplating. The P‐C mode is favored by subduction zone retreat and/or an increasing subduction load and creates an apparent landward dipping backstop. P‐U‐C is predicted to be the most common mode, and it creates an apparent seaward dipping backstop. P‐U‐R‐C is an inefficient mode but is favored by easily detached retrocrust or sediment and/or a decreasing subduction load.

Geodynamic models of crustal‐scale episodic tectonic accretion and underplating in subduction zones

We investigate the dynamics of crustal‐scale episodic accretion at convergent margins using a finite element model of subduction. The numerical model incorporates effects from flexure, subduction loading, and large‐scale variations in material properties. The model is used to assess the influence of these parameters on the discrete transfer of units from the lower to the upper plate during subduction. The primary controls investigated are spatial variations in density, material strength, and thickness of material layers. These controls correspond to natural examples where episodic accretion results from the thermal structure of subducting lithosphere, giving rise to areas of thermally activated ductile creep; attempted subduction of small crustal blocks (density, frictional and/or ductile strength variations); interaction between well‐attached basement blocks and overlying, detachable sediments; and spatial changes in thickness of sediment and crust. In the experiments, accretion of small continental terranes within a model subduction zone can cause crustal‐scale fold nappes and shear zones to develop, with accompanying tectonic underplating and/or frontal accretion. The model results may provide insight into real cases of episodic accretion, for example, the emplacement of the upper Penninic nappe stack of the Central Alps.

Neoproterozoic terrane assemblages in Eritrea: review and prospects

Four, perhaps five large terranes that are structurally and lithologically distinct constitute the Neoproterozoic of Eritrea. Two are composites of structurally bounded smaller blocks, many of which also show considerable lithological differences. Accretion along regional shear zones was dominated by sinistral strike-slip. The westernmost Barka Terrane is an upper amphibolite to granulite grade complex, characterised by polyphase ductile strains. It contains metasediments and undeformed felsic dykes that have no counterparts in the other terranes. The Hagar Terrane is dominated by immature supra-subduction zone volcanic rocks, but its western parts contain oceanic basalts and pelagic sediments, deformed basic-ultrabasic rocks and a major olistostrome. Parts of this tectonised ophiolite retain high P, low T assemblages, suggesting that the Barka Shear Zone (its western boundary) was initiated as a subduction zone. Other subduction zone metamorphic assemblages in the internal Hagar Terrane may represent tectonic underplating of an evolving volcanic arc. The Adobha Abiy Terrane is a highly deformed post-accretionary sedimentary basin, formed after the close of volcanism. The Nakfa Terrane is a collage of many lithologically different volcano-sedimentary blocks bounded by shear zones, in which metamorphism generally reached greenschist and lower grades. It also includes several mafic-ultramafic belts. Those blocks for which geochemical data are available suggest magmatism in supra-subduction zone environments. In its axial part, the Nakfa Terrane reveals a regional steep belt dominated by high-grade migmatites and gneisses that formed from volcanic rocks subjected to lower crustal conditions. Late syn-kinematic granitoid plutons are regionally concentrated along this steep belt. However, these high-grade rocks show a number of differences from isolated occurrences of gneisses in the Red Sea lowlands that might constitute a fifth entity, the Arag Terrane. Three major episodes of deformation probably correspond to protracted terrane accretion, shortening and thickening of new crustal materials, and crustal reworking dominated by sinistral transpression. The last affects post-accretion sedimentary sequences in the Adobha Abiy Terrane and similar strata in the Nakfa Terrane that show lower metamorphic grade than the earlier volcano-sedimentary associations. This sequence spans a ∼300 Ma period from about 850 to 550 Ma.

A Neoproterozoic supra-subduction terrane in northern Eritrea, NE Africa

The western part of a tectonic block in the complex Pan-African architecture of northern Eritrea contains partly re-equilibrated low- T , high- P assemblages (550°C, 14.5 kbar), basalts with oceanic geochemistry and a major olistostrome containing lenses of serpentinite. These features demonstrate subduction zone products. East of this accretionary wedge the block is dominated by island-arc volcanic and volcaniclastic rocks, and various calc-alkaline plutons. A bimodal volcanic suite of primitive andesitic basalts and evolved dacites and ignimbrites represents early submarine arc development and emergent volcanism, respectively. Intense, polyphase deformation culminated in sinistral transpression with crustal-scale imbrication between several east-verging thrusts. A tectonically bounded unit in the internal part of this arc association also contains relict high- P assemblages (650°C, 10–13 kbar) in oceanic basalts, which are possibly products of tectonic underplating as the arc developed. Oceanwards collapse of the tectonically inflating arc may explain formation of evolved, subaerial volcanic rocks within small extensional basins. Down-to-west extensional structures may have been inverted during terrane accretion to act as east-verging shear zones during late thickening. The area remains to be correlated with those areas in Arabia that were juxtaposed with it before Red Sea opening.

Tectonic shortening and crustal thickness in the Central Andes: How good is the correlation?

Exceptional thickening of continental crust beneath the Central Andes is believed to be mainly a result of tectonic shortening of the South American plate in Neogene time. This shortening has been estimated to have contributed as much as 70%–80% of the present crustal volume. A compilation of published shortening values and our own estimates based on balanced cross sections from the Central Andes between lat 3°S and 40°S suggest that 70%–80% is a maximum rather than an average value for this part of the Andes. Tectonic shortening and the crustal cross-section area are only loosely correlated. Variations in tectonic shortening are more abrupt than those of crustal areas, particularly near the northern and southern ends of the Altiplano-Puna high plateau, where thick crust is associated with relatively small amounts of shortening. Shortening there may account for no more than about 30% of the present crustal cross-section area. The processes that created the remaining crustal area are not clear, but are likely to involve poorly constrained pre-Neogene tectonic shortening, moderate magmatic additions to the crust, tectonic underplating of material derived from the forearc, and possible flow of ductile lower crust along strike.

THE EVOLUTION OF THE ALTIPLANO-PUNA PLATEAU OF THE CENTRAL ANDES

▪ Abstract The enigma of continental plateaus formed in the absence of continental collision is embodied by the Altiplano-Puna, which stretches for 1800 km along the Central Andes and attains a width of 350–400 km. The plateau correlates spatially and temporally with Andean arc magmatism, but it was uplifted primarily because of crustal thickening produced by horizontal shortening of a thermally softened lithosphere. Nonetheless, known shortening at the surface accounts for only 70–80% of the observed crustal thickening, suggesting that magmatic addition and other processes such as lithospheric thinning, upper mantle hydration, or tectonic underplating may contribute significantly to thickening. Uplift in the region of the Altiplano began around 25 Ma, coincident with increased convergence rate and inferred shallowing of subduction; uplift in the Puna commenced 5–10 million years later.

Transformation of a magmatic arc and an orogenic root during oblique collision and it’s consequences for the evolution of the European Variscides (Mid-German Crystalline Rise)

The architecture of the European Variscides has been subdivided by Kossmat (1927) into paleogeographically coherent units which are presently interpreted as former plate fragments. The Mid-German Crystalline Rise (MGCR) separates two fragments (Rhenohercynian and Saxothuringian belts) at the site of an inferred plate boundary and reequilibrated orogenic root. The commonly favoured model interprets the MGCR as the magmatic arc on Saxothuringian crust above a south-dipping subduction zone in Upper Devonian and Carboniferous times. Data from the MGCR, the kinematic evolution of the Mid-European Variscides, and first order volume balancing suggest a reinterpretation of this unit which challenges classical views on the MGCR as well as on the subdivision of Variscan architecture. The MGCR is composed of two rock groups with different tectonic identity. A Lower Carboniferous low pressure-high temperature magmatic arc association on Lower Paleozoic basement rests tectonically on a stack of medium pressuremedium temperature rocks of inferred Rhenohercynian origin. The latter were tectonically accreted to the base of the overriding plate by tectonic underplating. The entire process was controlled by oblique convergence. This led to regional partitioning of the plate kinematic vector into contractional domains (lower Rhenohercynian plate and back-arc area of the upper Saxothuringian plate), bulk heterogeneous plate margin parallel extensional domains (MGCR), and plate margin parallel wrench domains (MGCR boundaries). During this process material was continually transferred from the lower plate to the upper plate, uplifted and exhumed by net crustal extension. The concomitant removal of parts of the former arc and the entire orogenic root necessitates a reappraisal of Variscan architecture and evolution.

A Comparison of Structures in the Andean Orogen of Northern Chile and Exhumed Midcrustal Structures in Southern California, USA: An Analogy in Tectonic Style?

The modern Andean Cordillera has proven to be a good modern analog for the Mesozoic and early Tertiary tectonic evolution of the US Cordillera, particularly for the transition between the Sevier and Laramide orogenies. A detailed version of this analogy, based on the tectonic evolution of the northern Chilean Andes, may explain the tectonic style of intra-arc exhumation and the southward migration of tectonism associated with arc extinction in southern California. Two regionally extensive episodes of deformation and exhumation are identified in southern California; the first occurred in an intra-arc setting in mid-Late Cretaceous time, and the second followed extinction of the magmatic arc and tectonic underplating by a blueschist/greenschist-grade metagraywacke terrane. We develop a model of Laramide oblique subduction of an aseismic oceanic ridge to explain these observations, based on modern subduction of the Juan Fernandez Ridge beneath the northern Chilean Andes. Laramide oblique ridge collision and ...

Cenozoic evolution of the Savu Basin, Indonesia: forearc basin response to arc-continent collision

A seismo-stratigraphic analysis of the Savu Basin relates the evolution of a complexly evolved forearc basin to the late Miocene-Recent collision with Australia. Initial early Miocene subsidence of the outer forearc basin and the subsequent development of a middle-late Miocene volcanic ‘proto’ arc, located in the south of the basin, both resulted from the late Oligocene-early Miocene eastward propagation of the Java/Timor Trench. Block-faulting of the relatively thick continental forearc basement probably determined the rather irregular and scattered pattern of middle-late Miocene volcanic centres. Just north of the accretionary prism, the forearc basement is flexed down and reshaped into a trenchward dipping backstop that facilitates backthrusting of the accretionary prism. The southern forearc basement probably acted as a barrier against compression. The thickness of this continental basement is suggested to be a critical factor in the response of the forearc to continent collision. Owing to its strength, the Savu Basin did not react to collision by the development of northward verging ‘inter-arc’ thrusts. In contrast, it responded to the underthrusting of continental crust by reactivation of the basement ridges. This resulted in the differentiation of the forearc basin into a presently extinct and uplifted Miocene South Savu Basin and a Pliocene-Recent active North Savu Basin. The simultaneous late Miocene-Recent uplift of large segments of the outer forearc and subsidence of the North Savu Basin is interpreted to result from tectonic underplating of continental material and uplift caused by the buoyancy of the subducted continental crust. The relatively recent decoupling of this crust from the oceanic slab, and the subsequent isostatic rebound, may superimpose the initial effects of post-collision extension on arc-continent collision.

Three‐dimensional crustal structure of the Mendocino Triple Junction region from local earthquake travel times

The large‐scale, three‐dimensional geometry of the Mendocino Triple Junction at Cape Mendocino, California, was investigated by inverting nearly 19,000 P wave arrival times from over 1400 local earthquakes to estimate the three‐dimensional velocity structure and hypocentral parameters. A velocity grid 175 km (N‐S) by 125 km (E‐W) centered near Garberville, California, was constructed with 25 km horizontal and 5 fan vertical node spacing. The model was well resolved near Cape Mendocino, where the earthquakes and stations are concentrated. At about 40.6° N latitude a high‐velocity gradient between 6.5 and 7.5 km/s dips gently to the south and east from about 15 km depth near the coast. Relocated hypocenters concentrate below this high gradient which we interpret as the oceanic crust of the subducted Gorda Plate. Therefore the depth to the top of the Gorda Plate near Cape Mendocino is interpreted to be ∼15 km. The Gorda Plate appears intact and dipping ∼8° eastward due to subduction and flexing downward 6°–12° to the south. Both hypocenters and velocity structure suggest that the southern edge of the plate intersects the coastline at 40.3° N latitude and maintains a linear trend 15° south of east to at least 123° W longitude. The top of a large low‐velocity region at 20–30 km depth extends about 50 km N‐S and 75 km E‐W (roughly between Garberville and Covelo) and is located above and south of the southern edge of the Gorda Plate. We interpret this low velocity area to be locally thickened crust (8–10 km) due to either local compressional forces associated with north‐south compression caused by the northward impingement of the rigid Pacific Plate or by underthrusting of the base of the accretionary subduction complex at the southern terminous of the Cascadia Subduction Zone. South of Cape Mendocino and southwest of the Garberville fault, high velocities indicative of oceanic crust are detected at 15 km depth. We interpret this evidence as tectonic underplating of the Pacific oceanic crust beneath the coastal Franciscan Complex, similar to the situation seismically imaged in central coastal California (Page and Brocher, 1994). This geometry indicates that the westernmost portion of the slab window has been filled by tectonic underplating and crustal thickening, while deep‐seated mantle upwelling is confined to an area centered about 75 km north of Clear Lake (Benz et al, 1992; Zandt et al., 1992).

Comparison of deep structure along three transects of the western North American Continental Margin

Similarities in geology and potential field data that have in the past been noted among the regions of southern Alaska, southern Vancouver Island, and central California are now seen to be accompanied by similarities in deep crustal structure. A number of tectonic elements have been identified in the deep structure along transects in these three regions, although not all elements are present along each transect. These elements are (A) an actively subducting oceanic plate and (B) an overriding continental plate that consists of (1) a Cenozoic accretionary prism, (2) a Mesozoic accretionary prism, (3) a backstop to the Mesozoic prism, (4) a tectonically underplated body of oceanic rocks, and (5) a crustal root. The Mesozoic prism is in some cases an underthrust body (type 2a) but in other cases forms the principal component of a landward verging tectonic wedge (type 2b). The technically underplated body of oceanic rocks extends landward from the fault contact between the Cenozoic and Mesozoic prisms to a point beneath the backstop. The crustal root lies beneath the backstop and landward of the underplated body. All of these elements are interpreted to be present along the Alaskan and Vancouver Island transects. In Alaska the underplated body is interpreted to be fragments of the Kula plate; the same may be true at Vancouver Island. These two transects appear to differ in that, in Alaska, the Mesozoic prism, in one interpretation, is the principal component of a tectonic wedge (type 2b), whereas at Vancouver Island, it is an underthrust body (type 2a). Along the central California transect, active subduction is no longer taking place, and the San Andreas fault has removed the Cenozoic prism from this region of the North American plate. On the North American plate (i.e., east of the San Andreas fault), the Mesozoic prism, interpreted as the main component of a tectonic wedge (type 2b), and the backstop to the Mesozoic prism are present. There is, however, no clear evidence of tectonically underplated oceanic rocks, and the crust is thin (no root). In both Alaska and Vancouver Island, the Mesozoic prisms above the underplated bodies experienced low‐pressure/high‐temperature metamorphism at about the time of tectonic underplating; no such metamorphism is currently exposed in California. The metamorphism may have been caused by the underplating of young, hot oceanic crust, or, alternatively, by subduction of an oceanic ridge. The presence of a tectonic wedge (type 2b) in Alaska and California and the absence of such a wedge at Vancouver Island could arise either from the fact that in the former two locations the Mesozoic prisms were more voluminous, owing to either more rapid trench sedimentation or more rapid convergence, or to the possibility that at the latter location the Mesozoic prism was juxtaposed with the backstop primarily by strike‐slip faulting.

High precision U-Pb ages for granulite metamorphism and deformation in the Archean Kapuskasing structural zone, Ontario: implications for structure and development of the lower crust

Precise U-Pb isotopic ages for two subhorizontal granulite gneisses in the Kapuskasing structural zone indicate that formation and emplacement occurred more than 50 m.y. after cessation of volcanism in the steeply folded nearby greenstone belts by successive stages of ductile underplating across a zone of crust-mantle delamination. Volcanism and batholith formation had ceased by 2700 and 2680 Ma, respectively, while granulite formation occurred between 2660 and 2640 Ma and ductile deformation continued until 2585 Ma. Emplacement of the oldest granulite caused the development of shear planes and melting in the older overlying tonalite, while emplacement of the younger granulite caused rupturing and rehydration of the overlying older granulite. Widespread tectonic underplating is implied since all Superior Province granulites have similar young ages. Anomalous Ar ages that post-date accretion by about 150 m.y., and scrambled Pb-Pb and Rb-Sr whole-rock systems, also suggest that late low-temperature alteration accompanied this event. Temporal relationships and the presence of post-volcanic (2670 Ma) conglomerate cobbles at granulite grade support a model of ductile underplating of a post-accretion, thermally softened back arc-fore arc complex, with components welded onto an overlying volcanic-plutonic complex at successively deeper levels. Multistage deformation in active fault zones and hydrothermal emplacement of major gold deposits in greenstone belts are coeval with ductile underplating with ages between 2670 and 2585 Ma. Anomalously abundant gold production from Archean rocks may indicate that shallow underplating, granulitization and devolatization was a widespread phenomenon at this time. Post-accretion ductile underplating must be considered as a primary cause of seismic reflectors in Archean terrains.

Nd and Sr isotope chronostratigraphy of Colorado Plateau lithosphere: implications for magmatic and tectonic underplating of the continental crust

Nd-Sr isotopes and major and trace element abundances were measured in lower and upper crustal xenoliths carried in Tertiary minettes and kimberlitic diatremes of the Colorado Plateau. The objective was to establish the vertical distribution of mantle extraction ages for an undisturbed Precambrian crustal province characterized by normal thickness and low cratonic heat flow, and to thereby evaluate the importance of magmatic or tectonic underplating in crustal growth. Mid-Proterozoic-age (1.7–2.0 Ga) material is found at all levels of the crust. Although mafic granulites and amphibolites have major and trace element compositions compatible with an origin as magmatically underplated material, Nd model ages do not indicate that there has been significant mantle contribution to the lower crust by igneous processes (such as magmatic underplating) since the main crust-forming event ca. 1.85 Ga ago. The meta-igneous lower crustal xenoliths have relatively radiogenic Nd isotopic compositions and unradiogenic Sr compositions. There is little variability in Nd model ages in spite of large variability in Sm/Nd. High and variable Sm/Nd ratios were therefore established early in the history of the protolith, during igneous or metamorphic processes coincident with or shortly after extraction from the mantle. Metapelitic xenoliths, tectonically emplaced into the lower crust, have the same Proterozoic model ages as meta-igneous parts of the lower crust. These paragneisses are probably arc-associated sediments that were underthrust during terrane accretion in the Proterozoic. The isotopic data indicate that eclogite xenoliths are likely to represent Proterozoic lithosphere rather than subducted Mesozoic MORB, as previously proposed. A number of the eclogite xenoliths have experienced a metasomatic event which introduced LREE, SiO 2 and Na 2O and shifted the Nd and Sr isotopic compositions. The least-altered eclogite xenoliths have major and trace element and radiogenic Nd isotopic compositions similar to those of other Proterozoic mafic, lower crustal xenoliths. A more plausible protolith for the eclogites is Proterozoic lithosphere that was metamorphosed to eclogite facies conditions either during Precambrian continental assembly or in more recent times.

Evolution of deep structure along the Trans‐Alaska Crustal Transect, Chugach Mountains and Copper River Basin, southern Alaska

One of the most important results of the Trans‐Alaska Crustal Transect investigations is the discovery that more than one third of the North American plate in southern Alaska (Chugach Mountains and Copper River basin) consists of tectonically underplated oceanic lithosphere. In southern Alaska, exposed accreted tectonostratigraphic terranes, include, from south to north along the transect, the Prince William, Chugach, Peninsular, and Wrangellia terranes. Chief results from seismic refraction data include the following: (1) A sequence of layers of low and high velocity (5.7–7.8 km/s), more than 10 km thick, dips gently northward, with its top at the surface in the southernmost Chugach terrane and at more than 20‐km depth beneath the southern Peninsular terrane. A high‐velocity layer at the top of this sequence corresponds to metabasalt in the southernmost Chugach terrane. This sequence is interpreted to be tectonically underplated fragments of the Kula plate and its sedimentary overburden. (2) An intermediate‐velocity layer (6.35–6.5 km/s) at 9‐km depth beneath both the northern Chugach and southern Peninsular terranes appears to extend without offset across the deep projection of the suture between these two terranes, the Border Ranges fault system. (3) A crustal “root” between depths of 19 and 57 km beneath the northern Peninsular and Wrangellia terranes appears to extend without offset across the deep projection of the suture between these two terranes, the West Fork fault system. This crustal root also appears to abut the tectonically underplated sequence to the south. Unfortunately, in results 2 and 3 the apparent crosscutting and abutting relationships can not be unambiguously resolved. Three scenarios, A, B, and C, have been offered to explain the evolution of this structure. Scenarios A and B differ in interpretation of the depth extent of the northern Chugach and southern Peninsular terranes. In scenario A these terranes extend to only 9‐km depth, where they rest on an unknown middle crust, interpreted as a deeper part of the Peninsular terrane. In this scenario, the northern Chugach and southern Peninsular terrenes moved landward as a tectonic wedge, during tectonic underplating in the latest Cretaceous or early Tertiary, and uplifted the upper crust of the Peninsular terrane to the north. In scenario B, the northern Chugach and southern Peninsular terranes extend to 20‐km depth. In this scenario, they were compressed but not detached and displaced during accretion. Scenarios A, B, and C differ in interpretation of the deep crustal root that appears to abut the tectonically underplated sequence. Since the Peninsular and Wrangellia terranes were at or below sea level prior to the mid‐Cretaceous, the root was probably emplaced in mid‐Cretaceous or younger times. In scenario A the root is lower crust of North America that moved southward during tectonic underplating of the Kula plate. In scenario B the root is, in part, tectonically underplated rocks similar to the Kula plate sequence to the south. In scenario C the root is, in part, magmatically underplated rocks.

Intracrustal detachments and implications for crustal evolution within the Lachlan fold belt, southeastern Australia

Seismic reflection profiling in the southern Lachlan fold belt, Australia, indicates geometrically complex mid- to lower crustal detachments possibly associated with crustal thickening by tectonic underplating of collided microplates and/or oceanic plateaus. Low-angle detachments either intersect or sole into the crust-mantle boundary, forming a crustal-stacking wedge typical of lithospheric- or A-type subduction. Upper crustal reflections show that major, moderately to steeply dipping reverse faults exposed at the surface flatten with depth and have listric form. The chevron-folded quartz-rich turbidite sequence of the Lachlan fold belt is therefore allochthonous with respect to the lower crust. Like the northern Appalachian maritimes, the Lachlan fold belt has a composite crust with diverse lower crustal blocks overlain by allochthonous, imbricated terranes of the surface rocks.

Deep seismic reflection profiling and the growth of the continental crust

Deep seismic reflection profiling provides evidence for the style and magnitude of Phanerozoic crustal growth by both lateral and vertical accretion. Profiles across convergent zones have been interpreted as showing tectonic underplating of the continental crust by fragmented oceanic lithosphere. Profiling across extensional zones provides evidence for magmatic underplating of the crust. In addition to studies over active areas, deep reflection profiling over large areas of western Europe, North America and Australia shows that the lower crust is strongly reflective and seismically distinct from the upper crust and upper mantle. The layering is detected beneath many different terranes of varying crustal type and basement age, from Archaean to Cenozoic, and is often apparently younger than the tectonic or metamorphic age of the exposed upper crust. Thus continental growth curves estimated from upper crustal measurements alone may underestimate Phanerozoic crustal additions, and mean crustal ages similarly estimated may be too high.

Evolution of Phanerozoic Eastern Australian Lithosphere: Isotopic Evidence for Magmatic and Tectonic Underplating

Evolution of Phanerozoic Eastern Australian Lithosphere: Isotopic Evidence for Magmatic and Tectonic Underplating

The composition and petrogenesis of the lower crust: A xenolith study

Granulite facies lower crustal xenoliths from a single basaltic vent (Hill 32) in the McBride volcanic province, north Queensland, Australia, illustrate the extreme lithologic diversity of the deep crust. These xenoliths are dominantly mafic, but intermediate and felsic granulites represent a significant proportion (∼20%) of the xenolith population. All xenoliths have high‐grade mineralogies and generally well‐equilibrated textures, as well as decompression features which are indicative of derivation from deep crustal levels (0.7–1.0 GPa). Major and trace element chemistry for 12 xenoliths, chosen to span the observed lithologic range, are used to constrain the petrogenesis of these rocks. In the smaller, layered samples, metamorphic differentiation may lead to nonrepresentative sampling. However, in most instances such secondary processes can be identified and the original chemical characteristics delineated. The mafic xenoliths formed through a variety of processes, including crystallization of mafic magma, crystal accumulation from mafic and felsic magmas, and partial melting of intermediate compositions to yield a mafic residuum. The two analyzed intermediate xenoliths are metasediments, based upon high alumina to alkali ratios and rare earth element patterns, whereas the two felsic xenoliths have compositions similar to igneous rocks. Comparisons of noncumulate and/or restite xenoliths with unmetamorphosed rock types show that K and Rb are variably depleted whereas Th and U are strongly depleted, in all rock types. The large ion lithophile element depletion patterns for xenoliths are similar to those of rocks from granulite facies terrains, suggesting similar processes were operative. Suggestions of an anomalously Ba‐ and Sr‐rich lower crust are not supported by the data. By using the observed lithologic proportions of xenoliths at Hill 32, and the chemical analyses presented here, a weighted mean composition of the lower crust can be obtained and compared with recently proposed lower crustal compositions. The weighted mean composition is mafic, and formed through a combination of basaltic underplating, crystal accumulation, tectonic underplating, and partial melting. No simple model of lower crust formation through basaltic or andesitic underplating nor intracrustal melting is sufficient to explain lower crust formation in Phanerozoic continental margin settings.

The crustal structure of the axis of the Great Valley, California, from seismic refraction measurements

In 1982 the U.S. Geological Survey collected six seismic refraction profiles in the Great Valley of California: three axial profiles with a maximum shot-to-receiver offset of 160 km, and three shorter profiles perpendicular to the valley axis. This paper presents the results of two-dimensional raytracing and synthetic seismogram modeling of the central axial profile. The crust of the central Great Valley is laterally heterogeneous along its axis, but generally consists of a sedimentary section overlying distinct upper, middle, and lower crustal units. The sedimentary rocks are 3–5 km thick along the profile, with velocities increasing with depth from 1.6 to 4.0 km/s. The basement (upper crust) consists of four units: 1. (1) a 1.0–1.5 km thick layer of velocity 5.4–5.8 km/s, 2. (2) a 3–4 km thick layer of velocity 6.0–6.3 km/s, 3. (3) a 1.5–3.0 km thick layer of velocity 6.5–6.6 km/s, and 4. (4) a laterally discontinuous, 1.5 km thick layer of velocity 6.8–7.0 km/s. The mid-crust lies at 11–14 km depth, is 5–8 km thick, and has a velocity of 6.6–6.7 km/s. On the northwest side of our profile the mid-crust is a low-velocity zone beneath the 6.8–7.0 km/s lid. The lower crust lies at 16–19 km depth, is 7–13 km thick, and has a velocity of 6.9–7.2 km/s. Crustal thickness increases from 26 to 29 km from NW to SE in the model. Although an unequivocal determination of crustal composition is not possible from P-wave velocities alone, our model has several geological and tectonic implications. We interpret the upper 7 km of basement on the northwest side of the profile as an ophiolitic fragment, since its thickness and velocity structure are consistent with that of oceanic crust. This fragment, which is not present 10–15 km to the west of the refraction profile, is probably at least partially responsible for the Great Valley gravity and magnetic anomalies, whose peaks lie about 10 km east of our profile. The middle and lower crust are probably gabbroic and the product of magmatic or tectonic underplating, or both. The crustal structure of the Great Valley is dissimilar to that of the adjacent Diablo Range, suggesting the existence of a fault or suture zone throughout the crust between these provinces.