Dallas Abbott - Academia.edu (original) (raw)

Papers by Dallas Abbott

Journal of Geophysical Research: Solid Earth, 2018

The Northern Appalachian Anomaly (NAA), a region of exceptionally low seismic velocities in the a... more The Northern Appalachian Anomaly (NAA), a region of exceptionally low seismic velocities in the asthenosphere beneath southern New England and easternmost New York State, has been interpreted as a site of mantle upwelling. We synthesize a combination of new and previously published data that indicates the following: (1) The upwelling has eroded or delaminated the lithosphere in a localized region centered in southern Vermont that we call the "Green Mountains Anomaly." Forty-second period Rayleigh wave phase velocities, which have peak sensitivity at lithospheric depths, are slow in this region, and S wave receiver functions are dominated by shallow (60-km depth) mantle structures indicating reduced velocities. Thermal springs and sites with anomalously high concentrations of mantle-derived helium-3 are concentrated at the borders of the Green Mountains anomaly, perhaps due to stress concentrations produced by geologically recent, uncompensated delamination of the lower lithosphere. (2) S wave receiver functions indicate intense (>10%), shallow (60-km depth) short wavelength structures along the southern and western edges of the NAA, indicating that the lithosphere there is being intensely altered, possibly by a combination of shearing and introduction of volatiles. And (3) Notwithstanding the localized thinning and alteration, the NAA lithosphere as a whole does not appear to have experienced pervasive heating, for the compressional wave quality factor of Q P = 870 inferred from the decay rate of Po waves is very significantly above the Q P ≈ 60 value previously reported for the NAA asthenosphere. Plain Language Summary At depths between 100 and 300 km (60-180 miles), the Earth beneath southern New England and easternmost New York State (USA) is unusually hot, so much so that the rock is at or near its melting point. This region has been nicknamed the Northern Appalachian Anomaly, or NAA, for short. One previously published explanation is that convection currents in the Earth are bringing up hot material from deeper depths. We follow up on this idea by examining whether the upwelling is heating up and eroding the lithosphere, the roughly100-km-thick (60-mile-thick) layer of cool rock just below the Earth's surface, and causing pieces of it to fall off (delaminate) and sink. We find evidence for erosion and delamination only beneath a region in and around the Green Mountains (Vermont, USA). There, seismic waves are unusually slow and spring temperatures are unusually warm, both of which are signatures of heat being close to the Earth's surface. The lithosphere above the rest of the NAA is less affected by the upwelling, except near its southern and western edges, where some alteration may be beginning.

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Banded iron formation (BIF) is a chemical sedimentary rock containing >15 wt% iron (Fe). Most ... more Banded iron formation (BIF) is a chemical sedimentary rock containing >15 wt% iron (Fe). Most common…

Geological Society of America Abstracts with Programs, 2020

We have found an impact crater that is likely < 6000 years old. Burckle crater is in the centr... more We have found an impact crater that is likely < 6000 years old. Burckle crater is in the central Indian Ocean on the edge of a fracture zone at 30.87° S 61.36°E. The crater is 29±1 km wide and is the inferred source of layers with high magnetic susceptibility in 3 deep sea cores. Each layer goes to the top of the core. Two out of 3 of the cores have basal Pleistocene ages and the basal age of the third is unknown. The high susceptibility layers contain broken plagioclase, spinel periodotite, and chrysotile asbestos. One sample contains pure Ni with drops of oxidized Ni. Because pure Ni melts at 1453°C, it is very likely that the drops formed during an impact. The high susceptibility layers from 2 cores are over 5 times thicker than they should be for a 29 km wide source crater. We also find that a 29 km wide source crater cannot vaporize enough seawater to produce meters of rain, even in a restricted region between 4750 and 7250 km from the crater. Thus, we infer that Burckle cra...

Tectonophysics, 2013

Undisturbed mid Archean crust (stabilized by 3.0-2.9 Ga) has several characteristics that disting... more Undisturbed mid Archean crust (stabilized by 3.0-2.9 Ga) has several characteristics that distinguish it from post Archean crust. Undisturbed mid-Archean crust has a low proportion of internal seismic boundaries (as evidenced by converted phases in seismic receiver functions), lacks high seismic velocities in the lower crust and has a sharp, flat Moho. Most of the seismic data on mid-Archean crust comes from the undisturbed portions of the Kaapvaal and Zimbabwe (Tokwe segment) cratons. Around 67-74% of younger Archean crust (stabilized by 2.8-2.5 Ga) has a sharp, flat Moho. Much of the crust with a sharp, flat Moho also lacks strong internal seismic boundaries, but there is not a one to one correspondence. In cases where its age is known, basaltic lower crust in Archean terranes is often but not always the result of post Archean underplating. Undisturbed mid-Archean cratons are also characterized by lower crustal thicknesses (Archean median range = 32-39 km vs. post-Archean average = 41 km) and lower crustal seismic velocities. These observations are shown to be distinct from those observed in any modern-day tectonic environment. The data presented here are most consistent with a model in which Archean crust undergoes delamination of dense lithologies at the garnet-in isograd resulting in a flat, sharp Moho reflector and a thinner and more felsic-intermediate crust. We discuss the implications of this model for several outstanding paradoxes of Archean geology.

Advances in Space Research, 2016

… Maui Optical and …, 2007

The Quaternary period represents the interval of oscillating climatic extremes (glacial and inter... more The Quaternary period represents the interval of oscillating climatic extremes (glacial and interglacial periods) beginning about 2.6 million years ago to the present. Based on modeling by the Near Earth Object (NEO) community of planetary scientists, the known and validated record of Quaternary impact on Earth by comets and asteroids is seemingly depauperate in terms of larger impactors >10,000 Mt (roughly equal to or larger than about 500 m in diameter). Modeling suggests that an average of between 2-3 and perhaps as many as 5 globally catastrophic (ca. ≥1,000,000 Mt) impacts by asteroids and comets could have occurred on Earth during this period of time, each having catastrophic regional environmental effects and moderate to severe continental and global effects. A slightly larger number of substantive but somewhat less than globally catastrophic impacts in the 10,000-100,000 Mt range would also be predicted to have occurred during the Quaternary. However, databases of validated impact structures on Earth, contain only two examples of Quaternary period impacts in the 10,000-100,000 Mt range (Zhamanshin, Bosumtwi), dating to around a million years ago, while no examples of Quaternary period globally catastrophic impact structures have been yet identified. In addition, all of the 27 validated Quaternary period impact structures are terrestrial-no Quaternary period oceanic impacts have been yet validated. Two likely globally catastrophic probable oceanic impacts events, Eltanin (ca. 1,000,000 Mt at around 2.6 mya), and that associated with the Australasian tektite strewn field (> 1,000,000 Mt at around 0.78 mya), are known due to their debris fields for which craters have not yet been identified and validated. These and the 8-km diameter Bolivian Iturralde candidate impact structure (ca. 10,000 Mt at around 20 kya) round out our list of likely large Quaternary impact structures. This suggests that one or more Quaternary period globally catastrophic impacts and several events in the 10,000-100,000 Mt range occurred in oceanic settings and have not yet been identified. At issue here is the default position of the NEO community that no large impacts have occurred during the past 15,000 years and that there is little evidence for human death by impacts during the past 5000 years of recorded history. This bias, deriving largely from reliance on stochastic models and by selectively ignoring physical, anthropological, and archaeological evidence in support of such impacts, is apparent in the messages being given to the media and general public, and in the general lack of grant support and other assistance to scientists and scholars wishing to conduct fieldwork on impacts that may date to the past 15,000 years. Such a position has a chilling effect on what should otherwise be an important arena of inquiry into the risks and effects of cosmic impact on human society. It potentially limits advancement in our understanding of the recent record and flux of cosmic impact, and diverts attention away from significant research questions such as the possible role of impact in Quaternary period climate change and biological and cultural evolution and process.

Sediment Provenance, 2017

Madagascar chevrons are V-shaped, coastal dunes of disputed origin. Three distinct chevron comple... more Madagascar chevrons are V-shaped, coastal dunes of disputed origin. Three distinct chevron complexes were sampled in this chapter: Fenambosy, Ampalaza, and Faux Cap. Chevrons contain abundant carbonate marine microfossils, some partially dolomitized, deposited over along-strike distances of 12 to >40 km. Marine microfossils in the dunes resemble filled-in marine foraminifers, differing from the hollow tests that dominate local beach deposits, and therefore arguing against modern beach deposits being the source of the chevrons' carbonate sands. Chevron dunes sampled in this study are typically not well sorted, with sediment surfaces that slope ≤10 degree. Both observations are inconsistent with an aeolian origin for the deposits. AMS 14 C dating of three carbonate-rich sands yields ages between 13,835 ± 40 and 11,415 ± 35 years before present (year BP), significantly older than modern marine carbonate sediments aged 800 to 418 year BP (i.e., the marine reservoir correction). Assuming that the age range stipulated for carbonate sand represents the maximum plausible age of deposition of the chevrons also argues against a modern aeolian origin. Taken together, characteristics of the chevrons instead support the inference that they are megatsunami deposits possibly related to a Holocene landslide or bolide impact.

Journal of Siberian Federal University, 2010

We have found seven discrete layers in a bog core from Cornwall, NY about 80 km away from the Atl... more We have found seven discrete layers in a bog core from Cornwall, NY about 80 km away from the Atlantic Ocean. All but two layers contain material that is unlikely to be locally derived. In most cases, the material in the layers has been transported thousands of kilometers from its source area. Six out of the seven layers are difficult to explain except through impact processes. If all of these layers are derived from impacts that produced craters, the data imply a very high impact rate during late Holocene time. In addition, we have been able to associate two of the impact ejecta layers with dated tsunami events that span the Atlantic Ocean. If this discovery is validated by further research, it implies a much larger tsunami hazard in the Atlantic Ocean than previously reported.

Journal of Geophysical Research, 1996

This paper tests two hypotheses for the influence of thick lithosphere (the tectosphere) upon pla... more This paper tests two hypotheses for the influence of thick lithosphere (the tectosphere) upon plate motion. The first is that the tectosphere reaches down into high-viscosity regions of the asthenosphere, effectively slowing the motion of continental plates. The second is that the tectosphere reaches down below the slow-moving convective boundary layer into regions of rapid mantle convection and effectively speeds (or slows) the motion of continental plates. Our data imply that the tectosphere speeds plate motion rather than impedes plate motion and are thus most consistent with the second hypothesis. This coupling has been termed the "mantle drag" force by Fo•yth and Uyeda [1975], who found that this force was small compared to other forces and that mantle drag tends to resist, rather than assist, plate motion. Their analysis, however, did not include the effect of variations in lithospheric thickness, which substantially complicate the simple picture of the asthenosphere being dragged along by an overlying, rapidly moving plate. If the base of a plate has littie or no relief and the plate is driven by boundary forces such as slab pull, then the asthenosphere will move most quickly near the base of the plate (the plate moves the mantle). If, however, a plate is driven by internal buoyancy forces, the asthenosphere will move most rapidly at some depth within the mantle (the mantle moves the plate). In either case, lithospheric thickness variations can affect the motion of a plate by varying its interaction with the asthenosphere. Of course, both types of forces move the plates: boundary forces as exemplified in slab pull and buoyancy forces as exemplified in ridge push; however, in most plates with attached slabs, the effects of boundary forces greatly overwhelm the effects of buoyancy forces, and it is difficult to resolve the component of buoyancy-driven flow. Continental plates with no attached slab therefore provide the best opportunity to examine the role of buoyancy forces in driving plate motions. In this paper, we examine the effect• of thick cratonic keels upon plate motion and use our results to make inferences about the nature of buoyancy-driven flow in the mantle. Depending upon the depth of the keel, it may either intersect a region of horizontal convective counterflow, or it may remain within the region where 'all horizontal mantle flow is in the upper half of the convection ceil. In the former case, plate motion is impeded by the keel, while in the latter, the keel will assist plate motion.

bott, Dee Breger, and Lloyd Burckle Lamont-Doherty Earth Observatory, Columbia University, 61 Rou... more bott, Dee Breger, and Lloyd Burckle Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9W Palisades, NY 10964. pag2107@columbia.edu Lamont-Doherty Earth Observatory. dallas@ldeo.columbia.edu Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104 deebre@coe.drexel.edu Lamont-Doherty Earth Observatory, Columbia University 61 Route 9W Palisades, NY 10964 burckle@ldeo.columbia.edu