Generation Of Continental Crust Research Articles

Generation of new continental crust and terrane accretion in Southeastern Alaska and Western British Columbia: constraints from P- and S-wave wide-angle seismic data (ACCRETE)

The ACCRETE study addresses the question of continental assemblage in southeastern Alaska and western British Columbia through accretion of exotic terranes and generation of new crust by magmatic addition in a former continental arc. We present results of wide-angle P- and S-wave seismic interpretation of a 300-km long marine-land seismic line across the contacts between accreted terranes and Coast Mountains. Additional constraints on the model are obtained from correlation with geologic mapping. Our results indicate that the Coast Shear Zone (CSZ) is a nearly vertical fault zone probably related to a transpressive regime. West of the CSZ, the mid-Cretaceous (90 Ma) thrust belt is rooted in the deep crust and is truncated by the CSZ. From the interpretation of the imaged sub-vertical reflecting zones, we infer the positions of the Alexander–Wrangellia terrane boundary (AWB) and of Tertiary extensional grabens within Dixon Entrance near its intersection with the profile. The observed values of Vp and Vp/Vs in the lower crust of the Alexander terrane are similar to those of oceanic crust and distinctly different from the Coast Mountains Batholith (CMB) to the northeast. The crust under the CMB (32 km) is thinner than the average continental crust, and the Moho is sharp (∼200 m) and highly reflective. The low-velocity mantle (7.9 km/s) suggests high temperature consistent with the stability of garnet in mafic rocks in the lower crust. The lower crustal velocity of 6.9 km/s supports a lower crust composed of interlayered garnet pyroxene granulite and quartzofeldspathic-restite related to batholith generation. The crustal section under the CMB is seismically identical to the lower two thirds of normal crust, heated and inflated by intrusions of tonalite, and gabbro interlayered with restites from batholith generation and uplifted during exhumation.

Isotopic evidence for Palaeoproterozoic accretion in the basement of the East European Craton

The Precambrian basement in the western part of the East European Craton (EEC) consists of a series of arcuate tectonic belts metamorphosed in amphibolite and granulite facies. These are separated from each other by crustal discontinuities generally trending NNE–SSW. In conjunction with previously published Sm–Nd and U–Pb isotopic data, the results reported now show that the basement between the Baltic and Ukrainian Shields is Palaeoproterozoic in age. It was formed between 2.1 and 1.8Ga ago.Igneous rocks from the various arcuate belts generally have positive initial ϵNd values between +1 and +3. Thus they are essentially juvenile and contain little material derived from older continental crust. Most metasedimentary rocks, in contrast, contain a distinct component of older crustal materials. This is demonstrated by initial ϵNd values ranging from −1 to −3 and by the results of U–Pb multigrain and single grain (SIMS) analyses of detrital zircons.The oldest magmatic activity in the region occurred 2.1–2.0Ga ago in the southeast, along the margin of the Archaean protocontinent of Sarmatia. Two subsequent episodes of volcanism in the tectonic belts farther west have been dated at ca. 2.0 and 1.87Ga using the U–Pb zircon approach. The Proterozoic volcanic rocks are thus younging towards the northwest, away from Sarmatia. The high-grade metamorphism, that has been dated employing U–Pb in monazite and zircon, is also younging from the southeast to the northwest. Its ages range from 1.95 to less than 1.79Ga.The available data are explained best by accretionary plate tectonics. These involved repeated episodes of generation of juvenile continental crust along and outboard of the margin of the dominantly Archaean Craton in Sarmatia. An Andean-type of geodynamic setting between 2.1 and 2.0Ga ago was succeeded by the formation of several island arcs and accretion of these onto the older continent between 2.0 and 1.8Ga. Subsequent compression of the newly formed crust lasted until ca. 1.7Ga.

Modification of an oceanic plateau, Aruba, Dutch Caribbean: Implications for the generation of continental crust

The generation of the continental crust may be connected to mantle plume activity. However, the nature of this link, and the processes involved, are not well constrained. An obstacle to understanding relationships between plume-related mafic material and associated silicic rocks is that later tectonic movements are liable to obscure the original relationships, particularly in ancient greenstone belts. Studies of younger analogous regions may help to clarify these relationships. On the island of Aruba in the southern Caribbean, a sequence of partly deformed mafic volcanic rocks intruded by a predominantly tonalitic batholith is exposed. The mafic lavas show geochemical and isotopic affinities with other basaltic, picritic and komatiitic rocks that crop out elsewhere in the Caribbean—these are well documented as belonging to an 88–91 Ma plume-related oceanic plateau, which is allochthonous with respect to the Americas, and is thought to have been formed in the Pacific region. The ∼85 to ∼82 Ma tonalitic rocks share some geochemical characteristics (high Sr and Ba, low Nb and Y) with Archaean tonalite–trondhjemite–granodiorite (TTG) suites. Field relationships suggest that deformation of the plateau sequence, possibly related to collision with a subduction zone, was synchronous with intrusion of the Aruba batholith. New incremental heating 40Ar/ 39Ar dates, combined with existing palaeontological evidence, show that cooling of the batholith occurred shortly after eruption of the plateau basalt sequence. Sr–Nd isotopic data for both rock suites are uniform ( 87Sr/ 86Sr i≈0.7035 , εNd i ≈+7), whereas Pb isotopes are more variable (Plateau sequence: 206Pb/ 204Pb =18.6–19.1 , 207Pb/ 204Pb =15.54–15.60 , 208Pb/ 204Pb =38.3–38.75 ; Aruba batholith: 206Pb/ 204Pb =18.4–18.9 , 207Pb/ 204Pb =15.51–15.56 , 208Pb/ 204Pb =38.0–38.5 ). This suggests that there has been a minor sedimentary input into the source region of the batholith. However, the limited time interval between basaltic and tonalitic magmatism makes a normal subduction-related origin for the tonalites improbable. Instead, models involving derivation of tonalite from partial melting of the plateau sequence, or alternatively, genesis in an unusual subduction zone environment, are investigated.

Simulations of crustal anatexis: Implications for the growth and differentiation of continental crust

There is overwhelming and incontrovertible petrological and geophysical evidence for the significant role played by mantle‐derived mafic magma in the generation and growth of continental crust. Likewise, intrusion of mafic magmas beneath or into continental crust is very likely the major source of enthalpy that drives intracrustal differentiation. A simple dynamical model has been constructed to examine the critical factors that govern the evolution and time‐scale of crustal anatexic events when driven by basaltic magma underplating or injection into crust. Critical factors include the intensity of the enthalpy input (i.e., the power dissipated) and the rheological properties, bulk composition and compositional structure of the source rocks undergoing partial fusion. We have performed numerical simulations to evaluate these factors using phase equilibria and thermochemical and transport property data applicable to the binary eutectic system CaAl2Si2O8‐CaMgSi2O6 as a rough analog to study anatexis of mafic lower crust. The role of enthalpy power is tested by varying the enthalpy (or temperature) along the base of the crustal block while maintaining a fixed temperature at the top. As ratio Tbot/Ttop increases modestly from 1.05 to 1.15, the average fraction of melt at steady state in the anatexic region increases from 50% to 75%. Time to attain steady state scales inversely with Tbot/Ttop with an increase by a factor of 2 for a 10% decrease in Tbot/Ttop. Typical anatexic timescales are in the range 103 to 105 years for length scales in range 102 to 103 m. The consequences of different rheological models, especially the importance of Darcy percolative flow relative to en masse flow within the partial melt region was also investigated. A value of the solid fraction called the critical value (ƒscrit) is set such that for 0<ƒs<ƒscrit no relative motion is allowed between solid and melt. The relevant viscosity is the viscosity of the magmatic suspension which is a function of the local value of ƒs. In contrast, for values of ƒs such that ƒscrit<ƒs<1, momentum transport is accomplished by Darcy flow so that solid is considered immobile and melt is free to percolate through the solid matrix of varying composition. Numerical experiments have been performed for ƒscrit of 0.7, 0.5, 0.3 and 0.0 to assess the importance of the rheological model. At high values of ƒscrit (e.g., >0.5) relatively large volumes of nearly homogeneous melt are rapidly generated. In distinction, for ƒscrit small, more enthalpy is stored in the solid and less melt is produced. However, the compositional spectrum of liquids generated within the anatexic region is significantly larger for small ƒscrit. Because of the sensitive dependence of solidi and liquidi on bulk composition, melt productivity is related to bulk composition given a particular enthalpy power input and rheological model. There is a factor of 2 difference in volume of melt generated when the bulk composition of the source is 10 modal percent less refractory. Compositionally zoned melts form by melting of either homogeneous or layered sources, although composition‐frequency relations are sensitive to the initial (subsolidus) compositional structure. The effects of anatexic events are to fundamentally reorganize the pattern of compositional structure within the crust. Intracrustal differentiation is a long‐term inevitable process associated with the underplating or injection of mafic magma into preexisting crust.

Making continental crust

The continental crust has an andesitic bulk composition, which cannot have been produced by the basaltic magmatism that dominates sites of present-day crustal growth—at both convergent margins and within plates. These observations suggest that there may have been a different mode of continental crust generation in Archaean times, and may point to delamination of the lower crust as an important recycling process.

The Generation of Continental Crust: An Integrated Study of Crust-Forming Processes in the Archaean of Zimbabwe

The Generation of Continental Crust: An Integrated Study of Crust-Forming Processes in the Archaean of Zimbabwe

Isotopic abundances relevant to the identification of magma sources

The behaviour of natural radiogenic isotope tracers in the Earth that have lithophile and atmophile geochemical affinity is reviewed. The isotope tracer signature of oceanic and continental crust may in favourable circumstances be sufficiently distinct from that of the mantle to render a contribution from these sources resolvable within the isotopic composition of the magma. Components derived from the sedimentary and altered basaltic portion of oceanic crust are recognized in some island arc magmas from their Sr, Nd and Pb isotopic signatures. The rare-gas isotope tracers (He, Ar, Xe in particular) are not readily recycled into the mantle and thus provide the basis of an approach that is complementary to that based on the lithophile tracers. In particular, a small mantle-derived helium component may be readily recognized in the presence of a predominant radiogenic component generated in the continents. The importance of assessing the mass balance of these interactions rather than merely a qualitative recognition is emphasized. The question of the relative contribution of continental-oceanic crust and mantle to magma sources is an essential part of the problem of generation and evolution of continental crust. An approach to this problem through consideration of the isotopic composition of sediments is briefly discussed.

Archean to Early Proterozoic tectonics and crustal evolution: a review

Hypothesis on the formation and evolution of the Earth's continental crust in the Early Precambrian have to account for the independent and directionally consistent motion of crustal segments of considerable dimensions since at least 3,5 Ga ago, and at minimum velocities comparable to those of today. It seems established, therefore, that plate tectonics has governed lithospheric evolution since the generation of rigid crustal segmenta, and it is speculated that a change in style of plate interaction, rather than a change in fundamental mechanisms, has determined non-uniformitarian crustal evolution towards the present Wilson cycle. Uncertainties in the reliable reconstruction of Archaean tectonic settings are caused by low-temperature rock alteration that may obliterate primary magma tic trends, by the variable possible sources and melting processes for the generation of bimodal and calc-alkaline associations, and by disagreement on the interpretation of rocks with primitive isotopic characteristics. It is suggested that magmatic underplating played a major role in lithospheric growth and that reworking of continental crust during large-scale crustal differentiation was more important in the Archaean than recognized by most currently popular models. The generation and survival of first continental crust is seen in analogy with the evolution of Iceland, and later greenstone belt development was probably largely intracontinental, following a rift-and-sag mede in response to crustal rearrangements. Most high-grade terrains are considered to be older than neighbouring or overlying greenstone associations and were brought to the surface through low-angle thrusting. Archaean lithospheric growth built stable cratons that experienced little internal deformation in early Proterozoic times. Less stabilized crust reacts to large-scale distortions predominantly by stretching, elongate basin-formation and ensialic orogeny through mechanisms ranging from crust restacking to transform shearing, finally generating mobile belts. Locally, modern-type convergent plate margins become recognizable at about 2.2 Ga ago while the full Wilson cycle is finally established in the late Proterozoic.

Geodynamics—Where Are We and What Lies Ahead?

The introduction and evolution of the plate tectonics hypothesis during the past two decades has sparked the current renaissance of research in the earth sciences. An outgrowth of active geophysical and geological exploration of the oceans, the plate tectonics model has come under intense scrutiny by geologists, geochemists, and geophysicists who have attempted to apply the model to the origin and growth of continents, the generation of oceanic and continental crust, and the nature of the lithosphere, asthenosphere, and underlying mantle with respect to their evolution through time and to the driving mechanism or mechanisms for plate tectonics. The study of other terrestrial planets and moons has been helpful in understanding the earth model. The unequal distribution of geological features, both in the continents and oceans, emphasizes the need for ongoing studies of international scope such as the recently completed International Geodynamics Project and its successor, the International Lithosphere Program, both stressing studies related to the dynamics of the lithosphere.