Early Accretion Research Articles

Energy requirements

The determination of the appropriate energy and nutritional requirements of a newborn infant requires a clear goal of the energy and other compounds to be administered, valid methods to measure energy balance and body composition, and knowledge of the neonatal metabolic capacities. Providing an appropriate amount of energy to newborn infants remains a challenge considering the great number of newborn infants who suffer in-hospital growth retardation. The energy requirements of a newborn infant are influenced by several factors - basal metabolism, growth, energy expenditure, and energy losses - which change continuously during development. Calculating the energy requirements of preterm infants is subject to error if general recommendations are applied without recognition of the large variation in factors that influence, for example, energy expenditure. Therefore, energy recommendations should be individualized and preferably based on measurements of energy expenditure. In particular, extremely low birth-weight and very low birth-weight infants are prone to develop negative energy and nutrient balances, due to low energy intake, low energy reserves and high energy demands. Early energy accretion is not only essential for growth but also influences neurodevelopmental outcome and physical health in the long term, thereby underlining the importance of adequate neonatal nutrition.

How well can Pb isotopes date core formation?

Timescale and the physics of planetary core formation are essential constraints for models of Earth's accretion and early differentiation. Wood and Halliday use the apparent mismatch in core-formation dates determined from tungsten (W) and lead (Pb) chrono-meters to argue for a two-stage core formation, involving an early phase of metal segregation followed by a protracted episode of sulphide melt addition. However, we show here that crust-;mantle Pb isotope systematics do not require diachronous core formation. Our observations indicate that very early (< or = 35 Myr) core formation and planet accretion remain the most plausible scenario.

Phase‐Space Distributions of Chemical Abundances in Milky Way–Type Galaxy Halos

Motivated by upcoming data from astrometric and spectroscopic surveys of the Galaxy, we explore the chemical abundance properties and phase-space distributions in hierarchically formed stellar halo simulations set in a ΛCDM universe. Our stellar halo metallicities increase with stellar halo mass. The slope of the [Fe/H]-M* trend mimics that of the satellite galaxies that were destroyed to build the halos, implying that the relation propagates hierarchically. All simulated halos contain a significant fraction of old stellar populations accreted more than 10 Gyr ago, and in a few cases some intermediate-age populations exist. In contrast with the Milky Way, many of our simulated stellar halos contain old stellar populations that are metal-rich, originating in the early accretion of massive satellites (M* ~ 109 M☉). We suggest that the (metal-rich) stellar halo of M31 falls into this category, while the more metal-poor halo of the Milky Way is lacking in early massive accretion events. Interestingly, our hierarchically formed stellar halos often have nonnegligible metallicity gradients in both [Fe/H] and [α/Fe]. These gradients extend a few tens of kiloparsecs, and can be as large as 0.5 dex in [Fe/H] and 0.2 dex in [α/Fe], with the most metal-poor halo stars typically buried within the central ~5 kpc of the galaxy. Finally, we find that chemical abundances can act as a rough substitute for time of accretion of satellite galaxies, and, based on this finding, we propose a criterion for identifying tidal streams spatially by selecting stars with [α/Fe] ratios below solar.

The Burst Mode of Protostellar Accretion

We present new numerical simulations in the thin disk approximation that characterize the burst mode of protostellar accretion. The burst mode begins upon the formation of a centrifugally balanced disk around a newly formed protostar. It comprises prolonged quiescent periods of low accretion rate (typically 10-7 M☉ yr-1) that are punctuated by intense bursts of accretion (typically 10-4 M☉ yr-1, with duration 100 yr) during which most of the protostellar mass is accumulated. The accretion bursts are associated with the formation of dense protostellar/protoplanetary embryos, which are later driven onto the protostar by the gravitational torques that develop in the disk. Gravitational instability in the disk, driven by continuing infall from the envelope, is shown to be an effective means of transporting angular momentum outward and mass inward to the protostar. We show that the disk mass always remains significantly less than the central protostar's mass throughout this process. The burst phenomenon is robust enough to occur for a variety of initial values of rotation rate and frozen-in (supercritical) magnetic field and a variety of density-temperature relations. Even in cases where the bursts are nearly entirely suppressed, a moderate increase in cloud size or rotation rate can lead to vigorous burst activity. We conclude that most (if not all) protostars undergo a burst mode of evolution during their early accretion history, as inferred empirically from observations of FU Orionis variables.

Paleomagnetism of the Wintering Lake pluton and the Early Proterozoic tectonic motion of the Superior Boundary Zone, Manitoba

This Lithoprobe-funded paleomagnetic study of the Early Proterozoic Wintering Lake granitoid body supports tectonic models that suggest continental accretion of the Trans-Hudson Orogen with the Superior Craton occurred at ~1822 Ma. Thermal demagnetization data for the granitoid specimens suggest that the magnetic remanence carriers are coarse-grained magnetite or titanomagnetite, and saturation isothermal remanence tests suggest that the magnetite is mostly multidomain. Six of seven paleomagnetic contact tests were negative, indicating that the host rocks have been remagnetized and that the granitoid body may have been partially remagnetized near its margins. Acceptable site mean remanence directions for 20 of 21 granitic sites yield a paleopole at 46.8°N, 102.2°W (with semi-axes of the 95% ellipse of confidence about the paleopole of dp = 11° and dm = 11°). The paleopole fits on the extrapolated apparent polar wander path (APWP) for the Superior craton at ~1822 Ma, which is the interpreted emplacement age of the pluton close to the peak of the Trans-Hudson orogeny. This is the first well-constrained paleomagnetic result from the Superior Province that provides direct evidence from concordant paleopoles for the Early Proterozoic accretion of the orogen to the craton. Further, the paleomagnetic results from the pluton's host rocks, along with other recent results from the Superior Boundary Zone, fill in a gap in the APWP for the craton between ~1780 and ~1720 Ma. The Superior path is now shown to form a hairpin as the craton moves from mid to polar paleolatitudes from ~1880 to ~1830 Ma, suffers a stillstand from ~1830 to ~1770 Ma during the peak of the Trans-Hudson orogeny, returns to mid-paleolatitudes from ~1770 to ~1740 Ma, and then moves on to subequatorial paleolatitudes by ~1720 Ma.

The energetics, evolution, and stellar depletion of $\mathsf{^6}$Li in the early Galaxy

Motivated by the recent report of a high Li6 ``plateau'' extending to low metallicities in Galactic halo stars, we study the energetics of an early production of Li6 through the interaction of energetic particles with the interstellar medium. We then explore the potential of various candidate sources of pre-galactic energetic particles and show that, in general, they fail to satisfy the observational and theoretical requirements (especially if the Li6 plateau is at a considerably higher level than observed). Succesful candidates appear to be: supernova explosions with abnormally low metal yield; the massive black hole in the Galactic center (provided that it was formed early on and that it was then radiating much more efficiently than today); and, perhaps, an early accretion phase of supermassive black holes in galaxies. Assuming that Li6 is indeed pre-galactic, we study the galactic evolution of the light isotopes Li7, Li6, and Be in a self-consistent way. We find that the existence of a Li6 plateau is hard to justify, unless a fine-tuned and metallicity-dependent depletion mechanism of Li6 in stellar envelopes is invoked. The depletion of Li6 should be different, both in magnitude and in its metallicity dependence, than the depletion required to explain current observations of Li (mostly Li7) in halo stars. If the recently reported Li6 ``plateau'' is confirmed, our analysis suggests important implications for our understanding of the production, evolution, and stellar depletion of the Li isotopes.

Hf–W evidence for rapid differentiation of iron meteorite parent bodies

Abstract New, high-precision W isotope data on iron meteorites are presented that provide important constraints on the timing of silicate–metal segregation in planetesimals. Magmatic iron meteorites all have e 182 W within error or less radiogenic than initial e 182 W estimated by studies of chondritic meteorites. At face value this implies that iron meteorites are as old and older than refractory calcium–aluminium rich inclusions (CAI), which are widely thought to be the oldest solar system objects. Moreover, different meteorites from the same magmatic groups, believed to be derived from the same planetissimal core, display a range of e 182 W. We suggest that the paradoxical e 182 W values more negative than initial Solar System Initial (SSI) are most readily explained as a result of secondary, spallation reactions with cosmic rays during transit between parent body and the earth. This is supported by the most negative e 182 W being found in meteorites with the oldest exposure ages and the magnitude of the effect is shown to be consistent with known nuclear reactions. On the other hand, it is also striking that none of the magmatic iron group meteorites have e 182 W analyses, outside error, more radiogenic than the estimated solar system initial ratio. This suggests that core formation in parent bodies of magmatic iron meteorites occurred ≤ 1.5 Myr after the formation age of CAI [Y. Amelin, A.N. Krot, I.D. Hutcheon, and A.A. Ulyanov, Lead isotopic ages of chondrules and calcium-aluminum inclusions, Science 297, 1678–1683, 2002]. This extremely early metal–silicate differentiation is coeval with the first chondrules [M. Bizzarro, J.A. Baker, and H. Haack, Mg isotope evidence for contemporaneous formation of chondrules and refractory inclusions, Nature 431, 275–278, 2004, A.N. Krot, Y. Amelin, P. Cassen, and A. Meibom, Young chondrules in CB chondrites from a giant impact in the early Solar System, Nature 436, 989–992, 2005]. Formation of later chondrules, and hence the parent bodies of some chondritic meteorites, must therefore have occurred in the presence of planetesimals large enough to possess iron cores. We conclude that early planetary accretion and differentiation was sufficiently fast for 26 Al-decay to be an important heat source. Non-magmatic iron meteorites, however, display more radiogenic and varied W isotope signatures. This is in keeping with them being generated later, by impact melting during which the metal (partially) re-equilibrated with the then more radiogenic silicate fraction.

Sedimentary evidence from the south Mediterranean region (Sicily, Crete, Peloponnese, Evia) used to test alternative models for the regional tectonic setting of Tethys during Late Palaeozoic-Early Mesozoic time

Abstract The south Mediterranean region, including western Sicily, Crete and mainland Greece (southern Peloponnese and Evia), is critical to an interpretation of the Late Palaeozoic-Early Mesozoic tectonic evolution of Tethys. Several contrasting tectonic models compete to explain the regional evolution. In a divergence-related hypothesis (Model 1) the south Aegean region experienced pulsed rifting along the northern margin of Gondwana that culminated in break-up to form the Pindos ocean in the region of Greece. In an alternative convergence-related hypothesis (Model 2) the south Aegean experienced Late Palaeozoic Early Mesozoic northward subduction, accretion and arc magmatism, culminating in ‘Cimmerian’ suturing of a Palaeotethyan ocean in latest Triassic time. In a third model, southward subduction of a Palaeotethyan ocean took place beneath the North Gondwana margin during Late Palaeozoic-Triassic time, giving rise to back-arc magmatism in an extensional setting. In addition, a more complex setting involving two opposing subduction zones (Andean-type and intra-oceanic) has also been suggested (Model 4), mainly based on lava geochemistry. To test these tectonic alternatives, mainly sedimentary studies were carried out in western Sicily, western and eastern Crete, the Peloponnese and Evia (eastern central Greece). Western Sicily was studied as a proxy for the unexposed deep Mediterranean south of Crete. Most of the available evidence supports the divergence-related (pulsed rift) hypothesis (Model 1). There is no clear evidence of sea-floor spreading (e.g. ophiolites) to the south of what became the Pindos ocean, or of plate convergence (e.g. magmatic arcs, subduction complexes), or collisional deformation in the south Aegean region that could be related to subduction or collision during the Mid-Carboniferous to Triassic, as in Model 2. Model 3 is not supported by evidence from the wider region (northern Greece, Turkey). Model 4 is not supported by evidence independent of igneous geochemistry. In the proposed interpretation, the northern margin of Gondwana initially rifted during Mid-Carboniferous to Early Permian time to form a wide deep-water basin. This was followed by further rifting, associated with volcanism during the Early Triassic; final continental break-up and spreading to form the Pindos ocean to the north during Late Triassic to Early Jurassic time then followed. Mid-Triassic uplift of part of the rift basin is explained as a flexural response to rifting as a precursor to opening of the Pindos ocean. Passive margin subsidence during the Early Mesozoic relates to opening of the Pindos ocean to the north. A subduction geochemical signature within some Triassic volcanic rocks, in this interpretation, is explained by melting of heterogeneous sub-crustal mantle, following an earlier, possibly Hercynian, subduction event.

Development of Trans-European Suture Zone in Poland: from Ediacaran rifting to Early Palaeozoic accretion

This contribution summarizes selected results of the Accretion of Poland Project. Emphasis is placed on geochronological, geochemical and palaeomagnetic constraints on the Late Neoproterozoic to Early Palaeozoic development of the Trans-European Suture Zone (TESZ). During the Late Neoproterozoic break-up of Rodinia, a major rift developed in the area of the future TESZ along which Baltica was separated from peri-Gondwana and Laurentia, resulting in opening of the Tornquist Ocean and development of the southwestern Baltica passive margin. This was paralleled by the development of the Cadomian orogenic system along the margin of Gondwana and the eastern and southern margins of Baltica. Some tectonic units involved in the TESZ, such as the Brunovistulian Terrane and the Malopolska Massif characterized by Cadomian basement, were derived from the internal and external parts of the Cadomian Orogen, presumably somewhere at the SE or SW corners of Baltica. Determination of areas where these terrains were originally located depends strongly on the Ediacaran plate model that is adopted for Baltica. The Malopolska Massif was re-accreted to Baltica, presumably due to latest Ediacaran strike-slip tectonics, during the late Middle to Late Cambrian, causing at that time an interruption of its passive margin subsidence pattern and minor erosion. During Late Ordovician to Silurian times, the Caledonian collision of Gondwana-derived East Avalonia Terrane with Baltica gave rise to the development of a foredeep basin along the southwestern margin of Baltica. The proximal part of this foredeep corresponds to the Pomeranian region to the Koszalin-Chojnice Zone, and its distal parts to the Baltic Basin, both of which developed on Baltica basement. During Ordovician and Silurian times clastics were shed into the Koszalin-Chojnice Zone and the Baltic Basin from the evolving Caledonian orogenic wedge, consisting of a subduction-related volcanic arc, obducted ophiolites and accretionary prism, as well as crustal units that were detached from basement of Baltica and Avalonia. The Brunovistulian Terrane was accreted to the Malopolska Massif at the turn from the Silurian to the Devonian. Proximal terranes, such as the Pomerania and Łysogory units remained after Late Neoproterozoic rifting in a position close to the relatively mobile SW margins of Baltica.

Palaeozoic accretion of Gondwana-derived terranes to the East European Craton: recognition of detached terrane fragments dispersed after collision with promontories

Abstract Recent work in Central Europe, combined with emerging information about basement massifs in SE Europe and NW Turkey, permits a new look at the relationships between crustal blocks abutting the East European Craton (EEC) along the Trans-European Suture Zone (TESZ). The simplest model indicates that the end-Cambrian establishment of the Bruno-Silesian, Lysogory and Malopolska terranes close to their present location on the SW margin of the EEC formed a major promontory on this margin of the continent. Moesia may also have formed part of this block. Both late Ordovician accretion of Avalonia and early Carboniferous accretion of the Armorican Terrane Assemblage (ATA) attached new continental material around the Bruno-Silesian Promontory (BSP). Palaeozoic faunal affinities and inherited isotopic signatures similar to those of Avalonia seen in the Istanbul block of NW Turkey, and in minor thrust slices in Moravia and Romania, suggest that easternmost Avalonia was severed, on collision with the BSP, and migrated east along the southern margin of the EEC. Likewise, the similarities to the ATA of the Balkan, Istranca, Sakarya and eastern Pontides blocks suggests that more easterly components of the ATA were detached at the BSP and migrated east. All the newly accreted blocks contain similar Neoproterozoic basement indicating a peri-Gondwanan origin; Palaeozoic plume-influenced metabasite geochemistry in the Bohemian Massif may explain their progressive separation from Gondwana before their accretion to the EEC. Inherited ages from Avalonia contain a 1.5 Ga 'Rondonian' component arguing for proximity to the Amazonian Craton at the end of the Neoproterozoic; Armorican terranes lack such a component, suggesting that they have closer affinities with the West African Craton. Models showing the former locations of these terranes and the larger continents from which they rifted, or later became attached to, must conform to both these constraints and those provided by palaeomagnetic data. In the late Neoproterozoic and Palaeozoic, these smaller terranes, some containing Neoproterozoic ophiolitic marginal basin and magmatic arc remnants, probably fringed the end-Proterozoic supercontinent as part of a 'Pacific-type' margin. When this margin fragmented, most resulting fragments accreted to the EEC.

The effect of non-isothermality on the gravitational collapse of spherical clouds and the evolution of protostellar accretion

We investigate the role of non-isothermality in gravitational collapse and protostellar accretion by explicitly including the effects of molecular radiative cooling, gas-dust energy transfer, and cosmic ray heating in models of spherical hydrodynamic collapse. Isothermal models have previously shown an initial decline in the mass accretion rate \dot{M}, due to a gradient of infall speed that develops in the prestellar phase. Our results show that: (1) in the idealized limit of optically thin cooling, a positive temperature gradient is present in the prestellar phase which effectively cancels out the effect of the velocity gradient, producing a near constant \dot{M} in the early accretion phase; (2) in the more realistic case including cooling saturation at higher densities, \dot{M} may initially be either weakly increasing or weakly decreasing with time, for low (T_d ~ 6 K) and high dust temperature (T_d ~ 10 K) cases, respectively. Hence, our results show that the initial decline in \dot{M} seen in isothermal models is definitely not enhanced by non-isothermal effects, and is often suppressed by them. In all our models, \dot{M} does eventually decline rapidly due to the finite mass condition on our cores and a resulting inward propagating rarefaction wave. Thus, any explanation for a rapid decline of $\dot{M}$ in the accretion phase likely needs to appeal to the global molecular cloud structure and possible envelope support, which results in a finite mass reservoir for cores.

Tectonic significance of potential-field anomalies in western Canada: results from the Lithoprobe SNORCLE transect

Potential-field anomalies within the Lithoprobe SNORCLE (Slave – Northern Cordillera Lithospheric Evolution) transect area provide geometrical constraints for regional crustal and lithospheric structures, as well as for local anomalies when coupled with subsurface geometry visible on nearly 2500 km of deep seismic reflection and refraction profiles. Areal distribution of gravity and magnetic anomalies permit structures to be projected away from seismic cross sections, and forward modelling provides tests of different interpretations of deep (crustal and upper mantle) density structures. In a key result from modelling, a Paleoproterozoic subduction zone beneath the Wopmay orogen probably consists of high-density rocks, such as eclogite, within the upper mantle. This result supports the concept of moderate- to low-angle intra-lithospheric sutures. On an even larger scale, applications of bandpass and directional filters assist in detecting anomalies according to wavelength or azimuthal orientation and thus provide means to track patterns across structural grain. For example, gravity and magnetic trends that are associated with Precambrian rocks of the Canadian Shield can, in some cases, be followed across much of the Cordillera. This result is consistent with North American Precambrian rocks composing much of the crust in the Cordillera and thus that the addition of "new" lithosphere during Mesozoic – early Tertiary accretion has been relatively minor.

THE Hf-W ISOTOPIC SYSTEM AND THE ORIGIN OF THE EARTH AND MOON

▪ Abstract The Earth has a radiogenic W-isotopic composition compared to chondrites, demonstrating that it formed while 182Hf (half-life 9 Myr) was extant in Earth and decaying to 182W. This implies that Earth underwent early and rapid accretion and core formation, with most of the accumulation occurring in ∼10 Myr, and concluding approximately 30 Myr after the origin of the Solar System. The Hf-W data for lunar samples can be reconciled with a major Moon-forming impact that terminated the terrestrial accretion process ∼30 Myr after the origin of the Solar System. The suggestion that the proto-Earth to impactor mass ratio was 7:3 and occurred during accretion is inconsistent with the W isotope data. The W isotope data is satisfactorily modeled with a Mars-sized impactor on proto-Earth (proto-Earth to impactor ratio of 9:1) to form the Moon at ∼30 Myr.

Tectonic framework of a Paleoproterozoic arc-continent to continent-continent collisional zone, Trans-Hudson Orogen, from geological and seismic reflection studies

Vertical incidence seismic data were collected along a 300 km-long profile across the northwestern flank of the Trans-Hudson Orogen in Saskatchewan (line S2b). The present study integrates the seismic data with previously published geological maps and recent results from the La Ronge – Lynn Lake Bridge Project that provide new constraints on the lithological, structural, and tectonic framework of this collisional zone. An interpretative section, based on the seismic reflectivity and surface geological constraints, is presented. The integrated data suggest the following key elements. (1) Although an early foreland-vergent accretion history is suggested by surficial geological constraints, the predominant fabric is reflected by penetrative hinterlandward vergence. (2) A zone of more reflective lower crust with hinterland-verging fabric may represent "fossil" reflectors from a pre-collisional accretionary phase. (3) The Archean to Paleoproterozoic Sask Craton may extend in the lower middle crust to at least beneath Rottenstone Domain, consistent with what is observed along strike on other Lithoprobe seismic profiles. (4) The Needle Falls and Parker Lake shear zones do not appear to be associated with obvious reflectors, consistent with observations from Reindeer Lake, suggesting that they merely rework the Wathaman Batholith intrusive margin and are not fundamental sutures. (5) The La Ronge and Rottenstone domains show nearly identical seismic properties, consistent with recent mapping results in the Reindeer Lake area that suggest that they are temporally and, at least in part, lithologically related in the area covered by, and to the east of, seismic line S2b.

Self-magnetization of the early Solar System matter

The remnant magnetization of chondrite type meteorite matter indicates the existence of 10 −5–10 −3 T magnetic fields in the early Solar System accretion disk. Taking into account parameter regimes being typical for this evolutionary stage of Sun and planets we consider the protosolar disk matter as partially ionized dusty plasma consisting of massive charged dust grains, neutral gas, electrons and ions. Results of systematic multifluid neutral gas–plasma–dust simulations show that shear flow driven collisional interactions yield a self-magnetization of the early Solar System matter which is able to explain the measured remnant magnetization of meteorite material.

Radial migration and dehydration of phyllosilicates in the solar nebula

While it is currently thought that Earth's water was delivered by hydrous minerals, the origin of these minerals is still debated. Nebular models suggest that the area where the Earth formed was too hot for these minerals to form, leading many to believe that they were delivered by large planetary embryos which formed in the outer asteroid belt region of the solar nebula. Others have argued that the hydrous minerals were present during the early accretion phase of the Earth in order to explain different aspects of its geochemistry and therefore, must have formed locally, implying that the nebula must have been cooler than the models predict. In this paper we explore a new possibility: that these hydrous minerals were formed in the outer asteroid belt region of the solar nebula and were then brought into the hotter regions of the nebula by gas drag where they were incorporated into the planetesimals which formed there. The hydrated minerals were able to survive for long periods of time in this hot region due to the sluggish dehydration kinetics. We find that this process need not have been efficient, requiring only a small amount (∼few percentages) of the material in the outer asteroid belt region of the nebula to be subject to this process. This delivery mechanism provides a way for hydrous minerals to be incorporated early on into the planetesimals which were accreted by the Earth without having to alter the generally accepted solar nebula models that are consistent with meteoritic and asteroidal observations.

Accretion and postaccretion tectonics of the Calamian Islands, North Palawan block, Philippines

Abstract Upper Paleozoic to Mesozoic sedimentary sequences of chert (Liminangcong Formation), clastics (Guinlo Formation) and a number of limestone units (Coron Formation, Minilog Formation and Malajon Limestone) constitute the accretionary complex of the North Palawan block, Philippines. Based on chert‐to‐clastic transitions from different stratigraphic sequences around the Calamian Islands, three accretionary belts are delineated: the Northern Busuanga Belt (NBB), the Middle Busuanga Belt (MBB) and the Southern Busuanga Belt (SBB). The accretion events of these belts along the East Asian accretionary complex, indicated by their sedimentary transitions, began with the Middle Jurassic NBB accretion, followed by the Late Jurassic MBB accretion and the Early Cretaceous SBB accretion. Several limestone blocks that formed over the seamounts became juxtaposed with chert–clastic sequences during accretion. During the Late Cretaceous, accretion‐subduction along the East Asian margin subsided bringing tectonic stability to the region. The seafloor spreading during the mid‐Oligocene disconnected the entire North Palawan block from the Asian mainland and then migrated southward. The collision between the North Palawan block and the Philippine Island Arc system in the middle Miocene generated a megafold structure in the Calamian Islands as a result of the clockwise turn of the accretionary belts in the eastern Calamian from originally northeast–southwest to northwest–southeast.

The Laramide Orogeny: What Were the Driving Forces?

The Laramide orogeny is the Late Cretaceous to Paleocene (80 to 55 Ma) orogenic event that gave rise to the Laramide block uplifts in the United States, the Rocky Mountain fold-and-thrust belt in Canada and the United States, and the Sierra Madre Oriental fold-and-thrust belt in east-central Mexico. The Laramide orogeny is believed to post-date the Jurassic and late Early Cretaceous accretion of the terranes that make up much of the North American Cordillera, precluding a collisional origin for Laramide orogenesis. Instead, the deformation belt along much of its length likely developed 700-1500 km inboard of the nearest convergent margin. The purpose of this paper is to show, through a review of proposed mechanisms for producing this inboard deformation (retroarc thrusting, "orogenic float" tectonics, flat-slab subduction and Cordilleran transpressional collision), that the processes responsible for orogeny remain enigmatic.

Landscapes, soils, and mound histories of the Upper Indus Valley, Pakistan: new insights on the Holocene environments near ancient Harappa

The site of Harappa in central Pakistan has been the primary source of information on Indus Valley cultural and natural landscapes in the Upper Indus Basin. While the site has been excavated for over 100 years, little was known of the pre-occupation history and environments responsible for the culture's emergence in the third millennium BC. Recent geoarcheological investigations of sites sharing the same landform as Harappa—the Bari Doab—along the ancient Beas River have synthesized sediment, soil, and cultural stratigraphies that place Harappa in the context of a regional landscape history. Two key sites—Lahoma Lal Tibba and Chak Purbane Syal—illustrate that larger floodplains of the Indus system stabilized sometime in the early Holocene, when soil development exceeded rates of alluviation. The site of Harappa was revisited to procure radiocarbon dates beneath initial occupation horizons. The stratigraphy and dates established the Pleistocene/Holocene interface and confirmed coeval trends of diminished Early Holocene floodplain accretion and sustained soil formation. Pedogenesis continued at least to the beginning of the fifth millennium BP when occupation began to intensify along the Beas and elsewhere in the Upper Indus. Site formation sequences at Lahoma Lal Tibba and Chak Purbane Syal mirror those of Harappa proper, albeit on a smaller scale.

EL Nino Influence on Holocene Reef Accretion in Hawai'i

New observations of reef accretion from several locations show that in Hawai'i accretion during early to middle Holocene time occurred in areas where today it is precluded by the wave regime, suggesting an increase in wave energy. Accretion of coral and coralline algae reefs in the Hawaiian Islands to- day is largely controlled by wave energy. Many coastal areas in the main Ha- waiian Islands are periodically exposed to large waves, in particular from North Pacific swell and hurricanes. These are of sufficient intensity to prevent modern net accretion as evidenced by the antecedent nature of the seafloor. Only in areas sheltered from intense wave energy is active accretion observed. Analysis of reef cores reveals patterns of rapid early Holocene accretion in several loca- tions that terminated by middle Holocene time, ca. 5000 yr ago. Previous anal- yses have suggested that changes in Holocene accretion were a result of reef growth ''catching up'' to sea level. New data and interpretations indicate that the end of reef accretion in the middle Holocene may be influenced by factors in addition to sea level. Reef accretion histories from the islands of Kaua'i, O'ahu, and Moloka'i may be interpreted to suggest that a change in wave energy con- tributed to the reduction or termination of Holocene accretion by 5000 yr ago in some areas. In these cases, the decrease in reef accretion occurred before the best estimates of the decrease in relative sea-level rise during the mid-Holocene high stand of sea level in the main Hawaiian Islands. However, reef accretion should decrease following the termination of relative sea-level rise (ca. 3000 yr ago) if reef growth were ''catching up'' to sea level. Evidence indicates that rapid accretion occurred at these sites in early Holocene time and that no permanent accretion is occurring at these sites today. This pattern persists despite the availability of hard substrate suitable for colonization at a wide range of depths between � 30 m and the intertidal zone. We infer that forcing other than rela- tive sea-level rise has altered the natural ability to support reef accretion on Hawaiian insular shelves. The limiting factor in these areas today is wave energy. Numbers of both large North Pacific swell events and hurricanes in Hawai'i are greater during El Nino years. We infer that if these major reef- limiting forces were suppressed, net accretion would occur in some areas in Hawai'i that are now wave-limited. Studies have shown that El Nino/Southern Oscillation (ENSO) was significantly weakened during early-mid Holocene time, only attaining an intensity similar to the current one ca. 5000 yr ago. We speculate that this shift in ENSO may assist in explaining patterns of Holocene Hawaiian reef accretion that are different from those of the present and appar- ently not related to relative sea-level rise.