Showing papers in "Geology in 1995"

Unraveling the effects of potassium metasomatism in sedimentary rocks and paleosols, with implications for paleoweathering conditions and provenance

Abstract: Lutites are commonly metasomatized during diagenesis, but the analysis presented here accounts for most postdepositional change. Potassium metasomatism is particularly common, and typically involves the conversion of kaolin (residual weathering product) to illite by reaction with K + -bearing pore waters. Sandstones also undergo K metasomatism, which involves the replacement of plagioclase by potassium feldspar. These changes can be identified petrographically and are quantitatively accounted for by techniques discussed herein. Bulk chemical analyses and ternary diagrams are used to determine the amount of K addition, premetasomatized sediment composition, and composition of provenance areas. The premetasomatized mineralogy of paleosols can be compared with the mineralogy of recent soil profiles and thus, climate and topographic conditions determined for past weathering events. Some weathering indices lead to erroneous conclusions because, by excluding K 2 O from consideration, correction cannot be made for metasomatic effects.

Fluid flow along potentially active faults in crystalline rock

Abstract: The relationship between in-situ stress and fluid flow in fractured and faulted rock is examined by using data from detailed analyses of stress orientation and magnitude, fracture geometry, and precision temperature logs that indicate localized fluid flow Data obtained from three boreholes that penetrate highly fractured and faulted crystalline rocks indicate that potentially active faults appear to be the most important hydraulic conduits in situ The data indicate that the permeability of critically stressed faults is much higher than that of faults that are not optimally oriented for failure in the current stress field

South China in Rodinia: Part of the missing link between Australia–East Antarctica and Laurentia?

Abstract: Stratigraphic correlations and tectonic analysis suggest that the Yangtze block of South China could have been a continental fragment caught between the Australian craton and Laurentia during the late mesoproterozoic assembly of the supercontinent Rodinia. The Cathaysia block of southeast China may have been part of a 1.9–1.4 Ga continental strip adjoining western Laurentia before it became attached to the Yangtze block around 1 Ga. This configuration provides a western source region for the clastic wedges in the Belt Supergroup of western North America which contain detrital grains of 1.8–1.6 Ga and 1.22–1.07 Ga. The breakup of Rodinia around 0.7 Ga separated South China (Yangtze plus Cathaysia blocks) from the other continents.

Andean tectonics as a cause for changing drainage patterns in Miocene northern South America

Abstract: NewdatafromNeogenestratainnorthernSouthAmericasuggestthatMiocenetectonisminthenortheasternAndeswasresponsible for the genesis of the Amazon River and changes in the drainage patterns of other major rivers such as the Magdalena and the Orinoco. Here we present a new model for the paleogeographic evolution of northern South America during the Miocene. In the early Miocene, a large part of the drainage of northwest Amazonia was directed northward along the paleo‐Orinoco river system to a deltainLakeMaracaibo.UpliftoftheEasternCordillerainthelate middle Miocene caused thefirst development of the Amazon River; however, no connection with the Atlantic was established, and the Amazon fed the paleo‐Orinoco river system, which drained toward the Caribbean. Substantial Andean uplift in the late Miocene resultedinmajorchangesinpaleogeography:theOrinocochangedits course, the Amazon established a connection to the Atlantic, causing the drowning of carbonate platforms, and the Amazon-Caribbeanconnectionwasclosed.Thusthedrainageandpaleogeography ofnorthernSouthAmericaintheMiocenewerestronglycontrolled by tectonic movements in the northeastern Andes.

Plume-lithosphere interaction in generation of the Emeishan flood basalts at the Permian-Triassic boundary

Abstract: The Emeishan flood volcanism that erupted at Permian-Triassic boundary time produced a large igneous province of at least 2.5 X 10 5 km 2 in the western margin of the Yangtze craton, southwestern China. The volcanic successions, suggested to have resulted from a starting mantle plume, comprise thick piles of basaltic flows and subordinate picrites and pyroclastics. The picrites, which have high magnesian contents (MgO ≊ 20–16 wt%), variable degrees of light rare earth element enrichment [(Ce/Yb) N ≊ 4–25] and heterogeneous isotope ratios [ϵ Nd ≊ (T) +4 to −4], are proposed to have been generated by mixing between the dominant plume-derived magmas and small amounts of lamproitic liquids from the continental lithospheric mantle.

Eocene continental climates and latitudinal temperature gradients

Abstract: Global climate during the Mesozoic and early Cenozoic is thought to have been warmer than at present, but there is debate about winter temperatures. Paleontological data indicate mild temperatures even at high latitudes and in mid-latitude continental interiors, whereas computer simulations of continental paleoclimates produce winter temperatures closer to modern levels. Foliar physiognomy and floristic composition of 23 Eocene floras from the interior of North America and Australia indicate cold month means generally >2 °C, even where the mean annual temperature (MAT) was

Reef drowning during the last deglaciation: Evidence for catastrophic sea-level rise and ice-sheet collapse

Abstract: Elevations and ages of drowned Acropora palmata reefs from the Caribbean-Atlantic region document three catastrophic, metre-scale sea-level–rise events during the last deglaciation. These catastrophic rises were synchronous with (1) collapse of the Laurentide and Antarctic ice sheets, (2) dramatic reorganization of ocean-atmosphere circulation, and (3) releases of huge volumes of subglacial and proglacial meltwater. This correlation suggests that release of stored meltwater periodically destabilized ice sheets, causing them to collapse and send huge fleets of icebergs into the Atlantic. Massive inputs of ice not only produced catastrophic sea-level rise, drowning reefs and destabilizing other ice sheets, but also rapidly reduced the elevation of the Laurentide ice sheet, flipping atmospheric circulation patterns and forcing warm equatorial waters into the frigid North Atlantic. Such dramatic evidence of catastrophic climate and sea-level change during deglaciation has potentially disastrous implications for the future, especially as the stability of remaining ice sheets—such as in West Antarctica—is in question.

Motion of the Pacific plate relative to Eurasia and its potential relation to Cenozoic extension along the eastern margin of Eurasia

Abstract: Extension along the eastern margin of Eurasia has been regarded commonly as a far-field effect of the India-Eurasia collision. However, some aspects of the timing and location of this extension make a link between the collision and extension difficult. We suggest that some extensional features commonly interpreted as effects of the collision may be related more simply to changes in plate-convergence rates along the eastern plate boundary of Eurasia. An analysis of the motion of the Pacific plate relative to Eurasia suggests that the rate of Pacific-Eurasia convergence varied significantly during the Tertiary Period. From a Late Cretaceous convergence rate of ∼120–140 mm/yr, the rate declined substantially during early Tertiary time and reached a minimum in Eocene time of ∼30–40 mm/yr. In Oligocene to earliest Miocene time, the average convergence rate increased moderately to 70–95 mm/yr, then decreased again to 65–70 mm/yr during early to middle Miocene time. From late Miocene to the present, the rate of convergence increased to an average of 100–110 mm/yr. The Paleocene through middle Miocene episode of relatively slow convergence correlates with a period of widespread extension along the eastern margin of Eurasia. Decreased convergence may have been related to a net reduction in horizontal compressional stress transmitted between the Pacific and Eurasian plates, which resulted in widespread extension adjacent to the margin of Eurasia.

Thin crust, ultramafic exposures, and rugged faulting patterns at the Mid-Atlantic Ridge (22°–24°N)

Abstract: Off-axis rock sampling in the lat 22°–24° N region of the Mid-Atlantic Ridge shows that the emplacement of mantle-derived rocks in the sea floor has been a common process there for the past few million years. We find a good correlation between domains of positive residual gravity anomalies (inferred to have a thin crust) and the distribution of ultramafic samples. We also find that thin-crust domains have a rugged topography, thought to reflect strong tectonic disruption. We propose that these thin-crust domains are made of tectonically uplifted ultramafic rocks, with gabbroic intrusions and a thin basaltic cover. We also suggest that strong tectonic disruption may be a direct consequence of the lithological and rheological heterogeneity of these thin-crust domains.

Eolian dunes: Computer simulations and attractor interpretation

Abstract: A simple computer-simulation algorithm for the transport of sand by wind produces forms resembling barchan, crescentic ridge, linear, and star natural dune classes. Sand is moved as slabs composed of many grains that are picked up at random, transported in a specified direction, and deposited (1) with a probability that depends on the local presence or absence of sand or (2) in shadow zones in the lee of dunes. The simulated dune fields are interpreted as complex systems, with sand-dune classes being dynamical attractors of these systems. The evolution of dunes once formed becomes decoupled from the details of eolian sand transport.

Post-Laramide removal of the Farallon slab, western United States

Abstract: I propose that post-Laramide removal of the subhorizontally subducting Farallon slab occurred by buckling downward along an approximately east-northeast–trending axis. This process was accomplished by a tearing or necking separation of the subducted slab near the northern and southern boundaries of the United States and propagation of the separated edges toward the central axis of downwelling, accompanied by aesthenosphic upwelling behind the trailing edges. Initial buckling probably began near 50 Ma, and slab removal was complete by 20 Ma. This model is based primarily on the space-time evolution of the “ignimbrite flare-up” (a major mid-Tertiary igneous event of mantle origin), which involved two propagating fronts of initiation of volcanism that followed the proposed motions of the separated slab edges as they converged on central Nevada from the north and southeast. Post-Laramide uplift, extension, establishment of the Cascadia subduction zone, and active magmatism may be consequences of lithosphere-scale modifications caused by the Laramide removal of the slab and the resulting asthenospheric upwelling.

Oblique plate motion and continental tectonics

Abstract: Three-dimensional deformation is necessarily associated with oblique plate convergence and commonly results in partitioning of deformation between contractional and transcurrent components along plate margins. Kinematic models of strike-slip partitioning for transpression and transtension allow the exact relation among three critical parameters—plate motion, instantaneous strain axes, and degree of strike-slip partitioning—to be calculated. Application to two end-member tectonic environments characterized by a low (South Island, New Zealand) and high (central California) degree of strike-slip partitioning demonstrates a remarkable consistency among the three parameters, suggesting that strike-slip partitioned transpression is a valid model for deformation in these regions. The extreme degree of strike-slip partitioning in wrench-dominated systems, such as central California, is tentatively associated with a fundamental misorientation of finite and instantaneous strain axes.

Experimental study of avulsion frequency and rate of deposition

Abstract: In existing models of alluvial architecture it is typically assumed that mean avulsion frequency is independent of sedimentation rate However, if avulsion is driven by superelevation of a river bed above its surrounding flood plain, one might expect avulsion rate to increase with sedimentation rate We have carried out a series of experiments with laboratory-scale fluvial fans in which we measured the frequency of apical avulsions as a function of mean sedimentation rate on the fan Avulsion frequency increased strongly with increasing sedimentation rate and then stabilized as mass flows began to influence deposition In the regime of increasing avulsion frequency, the added volume of sediment needed to trigger an avulsion decreased with increasing sedimentation rate Although our experimental results cannot simply be scaled up to natural rivers, they suggest the possibility of coupling between avulsion frequency and sedimentation rate that would be strong enough to change qualitatively the results of existing models of alluvial architecture These models should be applied with caution until avulsion mechanics are better understood

Chronology and dynamics of a large silicic magmatic system: Central Taupo Volcanic Zone, New Zealand

Abstract: The central Taupo Volcanic Zone in New Zealand is a region of intense Quaternary silicic volcanism accompanying rapid extension of continental crust. At least 34 caldera-forming ignimbrite eruptions have produced a complex sequence of relatively short-lived, nested, and/or overlapping volcanic centers over 1.6 m.y. Silicic volcanism at Taupo is similar to the Yellowstone system in size, longevity, thermal flux, and magma output rate. However, Taupo contrasts with Yellowstone in the exceptionally high frequency, but small size, of caldera-forming eruptions. This contrast reflects the thin, rifted nature of the crust, which precludes the development of long-term magmatic cycles at Taupo. 11 refs., 4 figs., 1 tab.

Microdiamond in high-grade metamorphic rocks of the Western Gneiss region, Norway

Abstract: Three grains of microdiamond were recovered from high-grade gneiss exposed in the Western Gneiss region, Norway. Identification and characterization of the diamond grains by Raman and infrared spectroscopy indicate the presence of substitutional impurities of H and N. Primary fluid inclusions in garnet and quartz in the diamond-bearing rock demonstrate the evolution of metamorphic volatile fluids from reduced N 2 -CO 2 compositions during the peak phase of metamorphism, to N 2 -CH 4 ± H 2 O–bearing compositions during retrograde metamorphism. Compatible geologic, petrologic, and fluid composition data imply a metamorphic origin for the microdiamonds; if so, the metamorphic and fluid conditions recorded by the microdiamonds and gneissic host may be applicable to microdiamond investigations in other high-pressure, regionally metamorphosed orogens.

Middle-late Miocene (>10 Ma) formation of the Main Boundary thrust in the western Himalaya

Abstract: Three independent data sets from northwestern India and Pakistan suggest initial displacement along >1000 km of the Main Boundary thrust prior to 10 Ma, at least 5 m.y. earlier than previously reported. Regionally extensive changes in the depositional characteristics and rates of the foreland-basinfill between 11 and 9.5 Ma are interpreted to reflect new hinterland loading due to the formation of the Main Boundary thrust. Sediment-accumulation rates, sandstone-siltstone ratios, and thickness and amalgamation of individual sandstone bodies all substantially increase after 11 Ma in well-dated stratigraphic sections from Pakistan to Nepal across the Indo-Gangetic foreland basin. In the Himachal Pradesh reentrant of northwestern India, a newly discovered 8.7 Ma conglomerate derived from the hanging wall of the Main Boundary thrust indicates that source-area uplift and denudation must have occurred prior to 9 Ma and probably prior to 10 Ma, assuming a gravelprogradationrateof3cm/yr.Threeapatitefission-trackages fromstructuresattheleadingedgeoftheMainBoundarythrustin the Kohat region of northwest Pakistan indicate that rapid cooling below ;105 C1‐2 m.y. earlier. These data indicate that the Main Boundary thrust in the western Himalaya formed synchronously along strike in the middle-late Miocene, has a displacement rate of ;10 mm/yr, and has a displacement history that is coeval with late displacement on the Main Central thrust.

Pseudotachylyte controversy: Fact or friction?

Abstract: High-speed slip experiments performed on Westerly granite using friction welding apparatus reveal that comminution is an essential precursor to melting by friction. Observations of slip surfaces via analytical scanning electron microscopy (SEM) document the following sequence of events occurring in 2 s with increasing velocity (up to 2 m/s): fracture; progressive comminution; surface melting of mineral fragments; fragment-to-fragment adhesion; and, finally, production of a fragment-laden, melt-supported suspension. Explosive dehydration and melting of the epidote-group mineral allanite indicates that temperatures of at least 1000 °C were realized at the interface. This is corroborated by calculation of the temperature rise for the known operating conditions. Contrary to earlier proposals, these results show that comminution and frictional melting are complementary and not mutually exclusive processes. Depending on the velocity–shear stress–displacement relations prevailing during frictional slip, rocks produced in seismogenic zones can be predominantly comminuted wall rock or fragment-melt mixes (pseudotachylytes).

Contrasting plate-tectonic styles of the Qinling-Dabie-Sulu and Franciscan metamorphic belts

Abstract: The Dabie Mountains are part of the >2000-km-long Qinling-Dabie-Sulu suture zone juxtaposing the Sino-Korean and Yangtze cratons. An eastern extension apparently crosses Korea and lies along the Japan Sea side of Honshu as the Imjingang and Sangun terranes, respectively; a northeastern segment may be present in Sikhote-Alin, Russian Far East. This orogenic belt records late Paleozoic ocean-floor consumption and the Triassic collision of two Precambrian continental massifs in east-central China. Coesite and microdiamond inclusions in strong, refractory minerals of eclogite facies ultrahigh-pressure (UHP) metamorphic rocks in the Dabie-Sulu area attest to profound subduction of a leading salient of the old, cold Yangtze craton, now recovered through tectonic exhumation and erosion. Northward increase in intensity of subsolidus recrystallization of the suture complex is analogous to the internal progression in grade of high-pressure (HP) and UHP metamorphism documented in the Western Alps. In both regions, subduction of narrow prongs of continental material, UHP metamorphism, and return toward midcrustal levels of relatively lower density, buoyant microcontinental blocks resulted from delamination of these rocks from the descending, higher density, oceanic-crust-capped lithospheric plate. Such salients of continental crust represent an integral structural part of the downgoing slab, remain intact, and may be dragged to great depths before disengaging and rising differentially as coherent blocks. UHP parageneses include trace mineralogic relics requiring peak metamorphic conditions of 700–900 ° C and 28–35 kbar or more. In contrast, Pacific-type HP metamorphic belts, as represented by the Franciscan Complex of western California, recrystallized under physical conditions up to 200–500 ° C, 10 ± 3 kbar. In this setting, voluminous quartzo-feldspathic and graywacke debris was carried downward on oceanic-crust-capped lithosphere, choking the subduction zone with incompetent material. Sited between both plates, and strongly adhering to neither, this buoyant, largely sedimentary complex decoupled at 25–30 km depth, and ascended toward the surface. In both Alpine-type intracontinental collision and Pacific-type underflow, light sialic material displaced dense mantle; thus, the return to midcrustal levels was propelled dominantly by body forces.

Oaxaquia, a Proterozoic microcontinent accreted to North America during the late Paleozoic

Abstract: Grenville-age granulite facies rocks in southern, central, and northeastern Mexico have distinctive geologic features that suggest a common tectonic evolution. The similarities include northwest-trending structural grain from Oaxaca to Tamaulipas, massif-type anorthosite-charnockite complexes, protoliths rich in sedimentary rocks of shallow-marine platform or continental rift-related facies devoid of calc-alkaline volcanic rocks, common metamorphism under granulite facies conditions, U-Pb zircon ages of about 1.0 Ga., and an apparently common history of uplift and cooling. Altogether, this evidence strongly suggests a coherent geologic history for this block. Paleontologic data from the overlying sedimentary sequences indicate that Oaxaquia was not part of Laurentia during most of the Paleozoic. This precludes emplacement of Oaxaquia in its present position by simple lateral displacement from the southern United States as well as a Taconic time of emplacement. Oaxaquia was probably emplaced to its present position during late Paleozoic time. The concept of a Mesoproterozoic “Oaxaquia” microcontinent extending for about 1000 km in Mexico needs to be considered in the reconstruction of the Grenville orogen as a whole and for the Paleozoic tectonic interactions between eastern Laurentia and western Gondwana.

Early Jurassic mass extinction: A global long-term event

Abstract: The end-Pliensbachian extinction event (187 Ma) has been interpreted either as one of 10 global periodically recurring mass extinctions of the past 250 m.y. or as a minor localized European event. Elevated levels of family extinction spanned five ammonite zones during the late Pliensbachian and the early Toarcian, an interval of ∼7.5 m.y., and were distributed unequally in the Boreal, Tethyan, and Austral realms. Detailed sampling of invertebrate macrofaunas through complete expanded sequences in northwest Europe shows that most species extinctions occurred in the early Toarcian, following a regional anoxic event. The Early Jurassic mass-extinction event took place over a long time scale, and it was global in extent.

Influence of syntectonic sedimentation on thrust wedges in analogue models

Abstract: Scaled analogue sandbox models have been used to simulate the growth of Coulomb thrust wedges that include syntectonic sedimentation during thrusting. Increases in the syncontractional sedimentation rate dramatically change the shape of the Coulomb thrust wedge, decreasing the number of thrust slices, the internal shortening, and the critical taper of the wedge. Thrust trajectories change from ramp-flat to sigmoidal with increased syncontractional sedimentation. The analogue models show many similarities to thrust-fault geometries in accretionary prisms and foreland fold-and-thrust belts in which thrust slices prograded into basins with active sedimentation.

Constraints on Pb closure temperature in titanite based on rocks from the Ungava orogen, Canada: Implications for U-Pb geochronology and P-T-t path determinations

Abstract: On the basis of multiequilibrium metamorphic pressure-temperature ( P-T ) estimates, we have determined that 660–700 °;C is a reasonable minimum estimate of the closure T of Pb diffusion in titanite, a temperature significantly higher than previously suggested. Activity-corrected reactions that produce titanite define narrow T stability fields (≈40 °;C) that correspond closely to metamorphic P-T determinations derived for coronitic garnet-clinopyroxene-hornblende samples. Growth of titanite, occurring between 1789 and 1814 Ma in the underthrust Archean basement of the Paleoproterozoic Ungava orogen (northern Quebec), is associated with retrograde metamorphism during underthrusting of the Archean basement (Approx.)1 b.y. after formation of the original granulitic assemblages.

Quantification of submarine lava-flow morphology through analog experiments

Abstract: We developed a technique for determining paleoeffusion rates and emplacement times for submarine lava flows, using observations of flow morphology and estimates of eruption volume, eruption temperature, lava viscosity, and preflow topography. Using laboratory simulations, and correlating these results with sea-floor observations, four submarine lava-flow morphologies are considered to be diagnostic of specific effusion rates: jumbled, folded, and lineated sheets, and striated pillows. We applied this approach to the CoAxial flow, emplaced in less than 14 days on the Juan de Fuca Ridge in early summer, 1993, and we calculated an effusion rate of ∼100 m3/s, giving an emplacement time of ∼10 days.

Eocene arc-continent collision in New Caledonia and implications for regional southwest Pacific tectonic evolution

Abstract: New Caledonia preserves evidence that constrains models for the tectonic evolution of the southwest Pacific region. Onland geology reflects four main tectonic phases: (1) early Mesozoic development of subduction-related terranes and their accretion to the Gondwana margin; (2) Cretaceous passive margin development and sea-floor spreading during the Gondwana breakup; (3) foundering of an oceanic basin and the Eocene arrival of thinned Gondwana margin crust at a southwest-facing subduction zone, resulting in collisional orogenesis and obduction of an ophiolitic nappe from the northeast; and (4) detachment faulting during extensional collapse, resulting in unroofing of metamorphic core complexes. The last phase explains supposedly anomalous metamorphic gradients in the northeast of the island.

Continuous mixing of crystal mush and replenished magma in the ongoing Unzen eruption

Abstract: Zoning profiles of magnetite phenocrysts in the successively effused dacite of the ongoing Unzen eruption from 1991 wereanalyzed to estimate the time scale of magma mixing. The dacite was formed by mixing of relatively high- and low-temperature ( T ) end-member magmas. The magnetite phenocrysts derived from the low- T magma are reversely zoned by the mixing with high- T magma. A diffusion calculation for reequilibration of the reverse zonings gives the time interval from magma mixing to quenching. For the mixed dacite erupted from May 1991 to May 1993, the typical diffusion time was estimated to be a few months regardless of the effused sequence for 2 yr. This indicates that the mixing was continuous during the effusion. The invariability of the other mixing signatures, such as the thickness of reaction rims around biotite phenocrysts, also supports the continuous mixing model. Low-T end-member magma is estimated by mass-balance calculation to be a crystal-rich mush of dacitic composition. These observations lead to a model wherein the highly crystallized remnant magma of the preceding activity has been mixed with the newly injected hot magma of similar bulk composition just prior to the effusion. The proposed mechanism implies that this type of magma mixing is an inevitable process in periodically erupting polygenetic volcanoes.

Iceberg production, debris rafting, and the extent and thickness of Heinrich layers (H-1, H-2) in North Atlantic sediments

Abstract: The pattern of Heinrich-layer distribution for the last two events (H-1, ∼14.5 and H-2, ∼21.1 ka), mapped from magnetic susceptibility analysis of more than 50 North Atlantic Ocean cores, provides the most detailed information to date on their extent and thickness. An integrated spatial average thickness for the layers is 10–15 cm, and there is a strong distance decay eastward. The pattern of deposition over the North Atlantic is similar for events H-1 and H-2, indicating that icebergs followed similar drift tracks. Rates of iceberg production and sediment flux from the Hudson Strait drainage basin of the North American Laurentide ice sheet, the major iceberg source for the events, were calculated by using a mass-balance approach. This provides an envelope of sedimentation rates and the prediction that it would take between 50 and ∼1250 yr of iceberg sediment delivery to accumulate a Heinrich layer averaging 10 cm thick over the North Atlantic, depending on the model assumptions used. The most likely duration of Heinrich events is 250–1250 yr.

Depth extent of cratons as inferred from tomographic studies

Abstract: We address the question of the depth extent of mantle high-velocity zones under ancient cratons by using seismic velocity maps from recent mantle tomographic studies. We divided old continents into two age provinces: 800–1700 Ma (Middle Proterozoic) and older than 1700 Ma (Archean and Early Proterozoic). The areas included in these age provinces are cross-correlated with the tomography at different depths to quantify the global occurrence of high-velocity anomaly (HVA) extensions beneath cratons. Statistically significant HVAs underlie the oldest cratons to a depth of 250 km. There is no significant consistent correlation between tomography and younger cratons. The older cratons have correlated HVAs extending to depths varying from <100 to 450 km. There is no significant difference in the average velocity anomaly beneath the Early Proterozoic part and the Archean part of the Canadian Shield, whereas the Middle Proterozoic part does not contain any HVA. The mantle under Early Precambrian crust is distinctly different from mantle elsewhere. We propose two different hypotheses to explain these results: (1) cratons of Early Proterozoic age or older contain ancient and deep, physically attached roots, or (2) these cratons contain a small “permanent” physical root that induces cold downwellings beneath it. A root thickness of ∼200 km can be explained by an FeO-poor, olivine-rich mineralogy, whereas a requirement of high strength probably demands, in addition, a volatile-poor composition. Different root depths for cratons of similar age may be related to their location in the various supercontinents in which they were involved, and to their subsequent deformational history, or they may represent different snapshots in time of transient “roots,” such as thermal boundary layers.

Evidence for a 2 Ga subduction zone: Eclogites in the Usagaran belt of Tanzania

Abstract: U-Pb geochronology on metamorphic minerals from a 35-km-long belt of eclogite-facies rocks in central Tanzania yields a Paleoproterozoic age of 2 Ga for the time of metamorphism. Peak metamorphic conditions found in eclogites (± kyanite) and metapelites reached about 750 °C and 18 kbar. A clockwise pressure-temperature path is deduced from mineral zonations, inclusion relations, and retrograde reaction textures. Near-isothermal decompression can be explained by erosion or tectonically controlled exhumation that followed tectonic thickening of the crust during subduction. Trace and rare earth element geochemistry indicates a mid-ocean ridge basaltlike mantle source for the precursors of the mafic members of the eclogite-facies rock suite. All the observations combined indicate that these high-pressure rocks are the oldest-known large-scale outcrops of eclogites formed during subduction of oceanic lithosphere. Linking eclogite formation to a Paleoproterozoic subduction event adds credibility to models of crust dynamics that advocate the operation of plate-tectonic processes early in Earth9s history. The paucity of Precambrian eclogites may then be addressed as a problem of preservation rather than lack of formation.

Detrital zircon reference for Cambrian to Triassic miogeoclinal strata of western North America

Abstract: U-Pb analyses of 656 single zircon grains from Cambrian to Triassic miogeoclinal strata provide a latitudinal and temporal reference for the ages of grains that accumulated along the western margin of North America. Comparisons between this detrital zircon reference and the ages of grains in potentially displaced terranes outboard (west) of the miogeocline should help establish when the terranes first arrived in sedimentary proximity to western North America. North-south variations in the ages of grains in Cambrian and Devonian to Triassic strata, which reflect the north-south changes in the age of cratonal rocks near the margin, should also help place constraints on a terrane's paleolatitude during these time periods. The technique cannot be used to determine paleolatitude during Ordovician time, however, because miogeoclinal strata from northern Canada to northern Mexico are dominated by grains shed from the Peace River arch (northwestern Canada).

Bivergent extension in orogenic belts: The Menderes massif (southwestern Turkey)

Abstract: The central Menderes massif is characterized by an overall dome-shaped foliation pattern and a north-northeast‐trending stretching lineation. The asymmetry of shear bands and quartz c-axis fabrics on either side of the structural dome demonstrate a top to thenorth-northeastshearsenseinthenorthernpartandatoptothesouth-southwestshear sense in the southern part of the submassif, i.e., a bivergent downdip movement. This suggests a symmetric collapse of the Alpine Menderes orogenic belt along two extensional shearzones.Conjugateshearbandsandsymmetricquartzc-axisfabricsintheeast-trendingtransitionzonedemonstrateacoaxialdeformationbetweenthetwoextensiondomains. Bivergent extension in the Menderes massif is in contrast to asymmetric extension in the AegeanSea.Here,thestill-activeHellenicsubductionzoneevolvedfromanadvancingplate boundary associated with crustal thickening into a retreating plate boundary in Oligocene-Miocene time. Southward rollback of the subducting plate during continuous northward subduction allowed asymmetric top to the north-northeast extension in the back-arc region during the exhumation of the Cycladic core complexes. In western Turkey, the arrival of the thick continental crust of the Menderes massif halted subduction and probably caused the symmetric collapse of the massif because the high potential energy of the thickened crust was no longer supported by subduction.