Robert J. Stern

Subduction zone infancy: Examples from the Eocene Izu-Bonin-Mariana and Jurassic California arcs

A new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the Izu-Bonin-Mariana (IBM) arc system and then applied to Late Jurassic ophiolites of Cailfornia. The model accounts for several key observations about the earliest stages in the evolution of the IBM system: (1) subduction nucleated along an active transform boundary, which separated younger, less-dense lithosphere in the west from older, more-dense lithosphere to the east; (2) initial arc magmatic activity occupied a much broader zone than existed later; (3) initial magmatism extended up to the modern trench, over a region now characterized by subnormal heat flow; (4) early are magmatism was characterized by depleted (tholeiitic) and ultra-depleted (boninitic) magmas, indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth; (5) early arc magmatism was strongly extensional, with crust forming in a manner similar to slow-spreading ridges; and (6) crust production rates were 120 to 180 km 3 /km-Ma, several times greater than for mature arc systems. These observations require that the earliest stages of subduction involve rapid retreat of the trench; we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault. This resulted in strong extension and thinning of younger, more buoyant lithosphere to the west. This extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere. Addition of water and asthenospheric upwelling led to catastrophic melting, which continued until lithosphere subsidence was replaced by lithospheric subduction . Application of the subduction-zone infancy model to the Late Jurassic ophiolites of California provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger Sierran magmatic arc and the Franciscan subduction complex, provides a mechanism for the formation and subsidence of the Great Valley forearc basin, and explains the limited duration of high-T, high-P metamorphism experienced by Franciscan melanges.

The Saharan metacraton

This article introduces the name “Saharan Metacraton” to refer to the pre-Neoproterozoic––but sometimes highly remobilized during Neoproterozoic time––continental crust which occupies the north-central part of Africa and extends in the Saharan Desert in Egypt, Libya, Sudan, Chad and Niger and the Savannah belt in Sudan, Kenya, Uganda, Congo, Central African Republic and Cameroon. This poorly known tract of continental crust occupies ∼5,000,000 km2 and extends from the Arabian-Nubian Shield in the east to the Tuareg Shield to the west and from the Congo craton in the south to the Phanerozoic cover of the northern margin of the African continent in southern Egypt and Libya. The term “metacraton” refers to a craton that has been remobilized during an orogenic event but is still recognizable dominantly through its rheological, geochronological and isotopic characteristics. Neoproterozoic remobilization of the Saharan Metacraton was in the forms of deformation, metamorphism, emplacement of igneous bodies, and probably local episodes of crust formation related to rifting and oceanic basin development. Relics of unaffected or only weakly remobilized old lithosphere are present as exemplified by the Archean to Paleoproterozoic charnockites and anorthosites of the Uweinat massif at the Sudanese/Egyptian/Libyan boarder. The article explains why the name “Saharan Metacraton” should be used, defines the boundaries of the metacraton, reviews geochronological and isotopic data as evidence for the presence of pre-Neoproterozoic continental crust, and discusses what happened to the Saharan Metacraton during the Neoproterozoic. A model combining collisional processes, lithospheric delamination, regional extension, and post-collisional dismembering by horizontal shearing is proposed.

Crustal evolution in the East African Orogen: a neodymium isotopic perspective

The East African Orogen (EAO) is one of Earth’s great collision zones, where East and West Gondwana collided to form the supercontinent ‘Greater Gondwana’ or ‘Pannotia’ at the end of Neoproterozoic time. There is now sufficient Nd isotopic data for basement rocks of the EAO to yield a useful summary. A total of 449 samples were gleaned from the literature, recalculated to a common value for the La Jolla Nd standard, and entered in Excel spreadsheets. This data set was filtered to exclude samples with 147 Sm/ 144 Nd > 0.165, considered to yield unreliable model ages, leaving 413 suitable data. The crust of the Arabian–Nubian Shield, including Egypt east of the Nile, Sudan east of the Keraf suture, Sinai, Israel, Jordan, most of Arabia, Eritrea, and northern Ethiopia yields overwhelmingly Neoproterozoic model ages. Crust to the east, in the Afif terrane of Arabia, Yemen, Somalia, and Eastern Ethiopia yields much older model ages, averaging 2.1 Ga, demonstrating an abundance of reworked ancient crust. This provides an isotopic link with Madagascar (mean age of 2.4 Ga), which in pre-Jurassic reconstructions lies on the southern extension of this older, remobilized tract. Crust in the far southern extreme of the EAO in Tanzania also yields ancient model ages, averaging 2.3 Ga. The central EAO, in southern Ethiopia and Kenya, yields intermediate ages (mean 1.1–1.2 Ga), interpreted to indicate extensive mixing between Neoproterozoic mantle-derived melts and ancient crust. The Nd isotope data indicate that the northern EAO is composed of juvenile Neoproterozoic crust sandwiched between reworked older crust, whereas the EAO farther south is progressively dominated by ancient crust reworked during Neoproterozoic time. The distribution of juvenile and reworked ancient crust suggests that the most intense collision between East and West Gondwana occurred in the southern part of the EAO. 2002 Elsevier Science Ltd. All rights reserved.

Evidence from ophiolites, blueschists, and ultrahigh-pressure metamorphic terranes that the modern episode of subduction tectonics began in Neoproterozoic time

Earth is the only known planet with subduction zones and plate tectonics, and this fact demonstrates that special conditions are required for this mode of planetary heat loss. Sinking of cold, dense lithosphere in subduction zones is the principal plate-driving force, so plate tectonics could not have begun until Earth cooled sufficiently to allow lithosphere to collapse into the underlying asthenosphere. Direct geologic evidence for when the modern episode of subduction tectonics began focuses on the first appearance of ophiolitic graveyards, blueschist facies metamorphic rocks, and ultrahigh-pressure metamorphic terranes. Ophiolites manifest two modes of lithospheric motion expected from subduction tectonics: seafloor spreading and obduction. High-pressure, low-temperature metamorphic blueschists and ultrahigh-pressure terranes indicate subduction and exhumation of oceanic and continental crust, respectively. These lines of evidence indicate that the modern style of subduction tectonics began in Neoproterozoic time. This revolution in the functioning of the solid Earth may have driven wild fluctuations in Earth’s climate, described under the ‘‘snowball Earth’’ hypothesis. These conclusions may be controversial, but suggest fruitful avenues for research in geodynamics and paleoclimate.

Sutures and shear zones in the Arabian-Nubian shield

Abstract Deformational belts in the Arabian-Nubian Shield (ANS) are divided into: (1) those associated with sutures, both arc-arc and arc-continental; and (2) post-accretionary structures which include north trending shortening zones and northwest trending strike-slip faults. The arc-arc sutures manifest collision between arc terranes at -800-700 Ma. They are orientated east to northeast in the northern part of the ANS and north to north-northeast in the south. North or south verging ophiolitic nappes are associated with the east to northeast trending sutures. These nappes were steepened by upright folds associated with the final stages of collision between terranes. East or west verging ophiolitic nappes are associated with the north to north-northeast trending sutures. These were deformed by upright folds and strike-slip faults related to oblique collision between terranes and/or post-accretionary deformations. The arc-continental sutures define the eastern and western boundaries of the ANS and are marked by north trending deformational belts which accompanied collision of the ANS with east and west Gondwana at -750-650 Ma. The post-accretionary structures were developed between -650-550 Ma due to continued shortening of the ANS. This produced north trending shortening zones which offset the east to northeast trending sutures in the northern part of the ANS but were superimposed as co-axial deformation on the north to north-northeast trending sutures in the south. The shortening deformation culminated with the development of northwest trending strike-slip faults and shear zones.

Origin of back-arc basin magmas: trace element and isotope perspectives

The compositions of back-arc basin basalts (BABB) can usefully be viewed as products of four factors: (1) the composition of inflowing mantle and its preconditioning during flow to the site of melting; (2) the influx of a subduction component into the arc-basin system; (3) the nature of the interaction between the mantle and subduction components; (4) the melting of water-rich mantle and the assimilation/ crystallization history of the resulting hydrous magma. Geochemical mapping using Nb/Yb as a mantle flow tracer indicates that mantle flow in arc-basin systems varies according to contributions from subduction-driven corner flow, flow around subduction edges, and deflection by barriers to flow. Geochemical mapping using Ba/Nb as a subduction tracer indicates that the magnitude of the subduction input is a function primarily of basin evolution, mantle flow patterns, and arc proximity. The subduction component may reach the back-arc by mixing with ambient mantle, by direct addition of a subduction component, by addition of a hydrous mantle melt, or by incorporation of a component stored in mantle lithosphere. Trace element and water contents of back-arc glasses indicate that decompression melting beneath back-arc basins is augmented by flux melting but suppressed by mixing with depleted mantle. Cl-K systematics indicate that water in back-arc basin magmas may be augmented by assimilation of hydrated ocean crust. The increased water content of primary BABB magmas leads to enhanced olivine and oxide crystallization, and to fluid exsolution at depth, both of which will influence the composition and architecture of the resulting back-arc oceanic crust.

An Overview of the Izu‐Bonin‐Mariana Subduction Factory

The Izu-Bonin-Mariana (IBM) arc system extends 2800km from near Tokyo, Japan to Guam and is an outstanding example of an intraoceanic convergent margin (IOCM). Inputs from sub-arc crust are minimized at IOCMs and output fluxes from the Subduction Factory can be more confidently assessed than for arcs built on continental crust. The history of the IBM IOCM since subduction began about 43 Ma may be better understood than for any other convergent margin. IBM subducts the oldest seafloor on the planet and is under strong extension. The stratigraphy of the western Pacific plate being subducted beneath IBM varies simply parallel to the arc, with abundant off-ridge volcanics and volcaniclastics in the south which diminish northward, and this seafloor is completely subducted. The Wadati-Benioff Zone varies simply along strike, from dipping gently and failing to penetrate the 660 km discontinuity in the north to plunging vertically into the deep mantle in the south. The northern IBM arc is about 22km thick, with a felsic middle crust; this middle crust is exposed in the collision zone at the northern end of the IBM IOCM. There are four Subduction Factory outputs across the IBM IOCM: (1) serpentinite mud volcanoes in the forearc, and as lavas erupted from along (2) the volcanic front of the arc and (3) back-arc basin and (4) from arc cross-chains. This contribution summarizes our present understanding of matter fed into and produced by the IBM Subduction Factory, with the intention of motivating scientific efforts to understand this outstanding example of one of earths most dynamic, mysterious, and important geosystems.

MORB mantle and subduction components interact to generate basalts in the southern Mariana Trough back-arc basin

We report the results of the first geochemical and isotopic survey of basaltic glasses dredged along the spreading ridge of the southern Mariana Trough (SMT; 15–17°N). This ridge is divided into two segments that have different axial depths, major and trace element compositions, water contents, and isotopic compositions of Sr, Nd, and Pb. Glasses from the shallower, northern segment (N-SMT; 16–17°N) are OL- and QZ-tholeiites that have compositions consistent with a higher degree of mantle melting relative to that of the OL tholeiites from the southern ridge segment (S-SMT; 15–16°N). The N-SMT glasses are similar to basalts erupted near 18°N in the Mariana Trough that have been the focus of previous studies. The more extensive melting inferred for the N-SMT correlates well with higher abundances of water and relative abundances of large ion lithophile and light Rare Earth elements that indicate involvement of a subduction component. The southern ridge segment is deeper and erupts compositions characteristic of lower degrees of melting; this correlates well with a lower proportion of the subduction component, including a suite that is indistinguishable from MORB. The strong correlation between degree of melting, water contents, and LIL elements indicates that hydrous fluxing as well as adiabatic decompression control melting of MORB-like mantle beneath back-arc basins. Details regarding the nature of this hydrous fluxing agent are not known, but it could be water-rich melts related to behind-the-arc volcanoes. These melts may be diverted by the back-arc convective regime, to become entrained in the zone of adiabatic upwelling, where they further stimulate melting.

The Late Precambrian Timna igneous complex, Southern Israel: Evidence for comagmatic-type sanukitoid monzodiorite and alkali granite magma

Abstract New data from a geochemical, geochronological and isotopic study of the Late Precambrian Timna igneous complex suggest the formation of alkali granites from a LIL-enriched, mantle derived, sanukitoid-type monzodiorite (a silica oversaturated rock with Mg# >60). These data also provide new insights into the petrology, timing and regional tectonic control of the transition from the calc-alkaline to the alkaline magmatic activity in the northern Arabian-Nubian Shield (ANS) during the Late Precambrian. The Timna alkali granite was formed by fractional crystallization from the monzodioritic magma in a quasi-stratified magmatic cell which formed 610 Ma ago in the 625 Ma old calc-alkaline, porphyritic granite crust. These monzodiorites are mantle-derived, as demonstrated by their high Mg# (63), Cr (230 ppm), and Ni (120 ppm). They are characterized by initial 87 Sr 86 Sr of 0.7034, ϵ-Nd (610 Ma) = +3.4, and are enriched in K2O (2.9%), Sr (840 ppm), Ba (1290 ppm) and LREE [ ( La Lu ) n = 10–25 ]. The chemical characteristics and REE patterns of the monzodiorites and andesitic dykes of Timna are very similar to Dokhan andesites from northeastern Egypt and the Archean sanukitoids from Canada. The isotopic, geochemical and geochronologic data all indicate that Timna monzodiorites are comagmatic with the alkali granite. The alkali granite is a typical post-orogenic, borderline A-type granite. It is enriched in potassium (K2O=4.68–6.64%), has a negative europium anomaly ( Eu Eu ∗ =0.058–0.38 ) and ϵ-Nd (610 Ma) of +3.9. The calc-alkaline granite is a typical I-type granite with a small positive europium anomaly ( Eu Eu ∗ =1.02–1.16 ). Its age and the Sr, Nd and Pb isotopic characteristics with ϵ-Nd (625 Ma) of +5.6 to +5.9 are significantly different from these of the alkali granite and monzodiorites, and indicate little interaction with the monzodiorite during the formation of the alkali granite. The alkali granites are correlative with the post-collisional extensional granites in Jordan and Egypt while the porphyritic granites can be correlated with the late orogenic types. Crustal thickening associated with orogenic compression resulted in crustal anatexis to form the I-type granitic rocks, whereas crustal thinning associated with extension allowed LIL-enriched mantle melts to rise very near to the surface, where space was available for these to pond and fractionate to alkali granite.

Petrogenesis and tectonic setting of late Precambrian ensimatic volcanic rocks, central eastern desert of Egypt

Abstract Early stages in the geologic evolution of the central eastern desert of Egypt (CED) reflect an intense episode of ensimatic volcanic activity similar to modern magmatism of the ocean floors and island arcs. This paper reports results from studies of the petrology and petrogenesis, and interprets the significance of these Late Precambrian volcanic rocks. A three-fold stratigraphy is preserved in the basement of the CED. A basal section of oceanic crust includes ultramafics, gabbros and pillowed basalts. These older metavolcanics (OMV) are conformably succeeded by dominantly volcanogenic metasediments, which are in turn succeeded by a dominantly andesitic, calc-alkaline sequence of younger metavolcanics (YMV). The OMV and YMV are largely restricted to the CED in Egypt, but analogous terranes are found in northern Arabia. (40–400 ppm) and Ni (30–260 ppm). They are poor in K2O (0.05–0.92%), Rb (0.3–5.0 ppm) and Ba (11–89 ppm). On Ti-Zr-Cr-V-Ni-P discriminant diagrams, the OMV plot in the field of modern abyssal tholeiites. High K/Rb (450–1800) and light REE depletions support this inference, although K/Ba (25–45) is lower than modern mid-ocean ridge basalts (MORB). The sum of OMV geochemical characteristics requires that these magmas were derived by the fractional fusion of the mantle. It is suggested that the OMV were generated by 20–25% fractional melting of previously depleted mantle at depths of less than 60 km. Relatively little fractionation accompanied ascent to the surface, where the OMV were erupted in a primitive crustal environment, either a small oceanic rift or a back-arc basin. Metamorphism of the YMV resulted in little elemental redistribution. These andesites have sub-alkaline clinopyroxenes and major-element geochemical characteristics indistinguishable from modern calc-alkaline andesites. YMV andesites in the central and western CED have K/Rb = 400–600, K/Ba = 20–40 and are light REE-enriched and heavy REE depleted. High concentrations of Cr (50–150 ppm) and Ni (20–100 ppm) and low initial 87Sr/86Sr ratios (0.7028–0.7030) indicate that these magmas were generated by melting in the mantle. Modelling studies and consideration of experimental data indicate that these andesites were formed by 2–10% fractional fusion of hydrous, undepleted, garnet therzolite at depths of 65 km or more in the mantle. The data show that an intense episode of instability, convection, and widespread melting occurred in the mantle beneath Afro-Arabia at the end of the Precambrian.