Paul Mann

Delivery of dark material to Vesta via carbonaceous chondritic impacts

NASA’s Dawn spacecraft observations of Asteroid (4) Vesta reveal a surface with the highest albedo and color variation of any asteroid we have observed so far. Terrains rich in low albedo dark material (DM) have been identified using Dawn Framing Camera (FC) 0.75 lm filter images in several geologic settings: associated with impact craters (in the ejecta blanket material and/or on the crater walls and rims); as flow-like deposits or rays commonly associated with topographic highs; and as dark spots (likely secondary impacts) nearby impact craters. This DM could be a relic of ancient volcanic activity or exogenic in origin. We report that the majority of the spectra of DM are similar to carbonaceous chondrite meteorites mixed with materials indigenous to Vesta. Using high-resolution seven color images we compared DM color properties (albedo, band depth) with laboratory measurements of possible analog materials. Band depth and albedo of DM are identical to those of carbonaceous chondrite xenolith-rich howardite Mt. Pratt (PRA) 04401. Laboratory mixtures of Murchison CM2 carbonaceous chondrite and basaltic eucrite Millbillillie also show band depth and albedo affinity to DM. Modeling of carbonaceous chondrite abundance in DM (1–6 vol.%) is consistent with howardite meteorites. We find no evidence for large-scale volcanism (exposed dikes/pyroclastic falls) as the source of DM. Our modeling efforts using impact crater scaling laws and numerical models of ejecta reaccretion suggest the delivery and emplacement of this DM on Vesta during the formation of the � 400 km Veneneia basin by a low-velocity (<2 km/s) carbonaceous impactor. This discovery is important because it strengthens the long-held idea that primitive bodies are the source of carbon and probably volatiles in the early Solar System.

Cenozoic tectonic history of the North America‐Caribbean plate boundary zone in western Cuba

Structural studies of well-dated Jurassic to lower Miocene rocks in western Cuba constrain the sequence of structural events affecting this oblique collisional zone between the late Cretaceous island arc and the Jurassic-Cretaceous North America passive margin in the southeastern Gulf of Mexico and Straits of Florida. Results of detailed mapping and collection of fault slip data at 34 sites define a regionally consistent, five phase tectonic model for the period from the late Paleocene to the post-early Miocene. During the late Paleocene to the early Eocene, the Cuban island arc collided with the North American passive margin (Bahamas Platform). Northwest-ward overthrusting during the collision defines tectonic phase I. A NNE-SSW compression concurrent with early Eocene left-lateral strike-slip faulting along the Pinar fault zone defines phase II. This result is consistent with structural mapping showing sinistral shear within the 065° striking Pinar fault zone. An ENE-WSW to E-W compression defining phase III overprinted phase II faults in the lower Eocene and older rocks. Post-early Miocene normal faulting characterizes phase IV. Inversion of fault slip data indicates two contemporaneous directions of tension of 120 and 170. Strike-slip faults that overprint phase IV normal faults yield a 120 compression (phase V). The direction of compression associated with the arc/continent collision rotates clockwise from NW-SE in the late Paleocene/early Eocene (phase I), to NNE-SSW (phase II) and to ENE-WSW by the middle Eocene (phase III). The rotation in the compression direction occurred because the arc turned toward an oceanic area in the present-day area of central and eastern Cuba. Progressive collision led to complete subduction of the remnant oceanic crust by middle to late Eocene time.

Neotectonics of the Macquarie Ridge Complex, Australia-Pacific plate boundary

New marine geophysical data along the Macquarie Ridge Complex, the Australia-Pacific plate boundary south of New Zealand, illuminate regional neotectonics. We identify tectonic spreading fabric and fracture zones and precisely locate the Australia-Pacific plate boundary along the Macquarie Ridge Complex. We interpret a ∼5–10 km wide Macquarie Fault Zone between the two plates along a bathymetrie high that extends nearly the entire length of the Australia-Pacific plate boundary south of New Zealand. We conclude that this is the active Australia-Pacific strike-slip plate boundary. Arcuate fracture zones become asymptotic as they approach the plate boundary. A broad zone of less intense deformation associated with the plate boundary extends ∼50 km on either side of the Macquarie Fault Zone. Marine geophysical data suggest that distinct segments of the plate boundary have experienced convergence and strike-slip deformation, although teleseismic evidence overwhelmingly indicates strike-slip motion along the entire surveyed boundary today. The McDougall and southernmost Puysegur segments show no evidence for past underthrusting, whereas data from the Macquarie and Hjort segments strongly suggest past convergence. The present-day strike-slip plate boundary along the Macquarie Ridge Complex coincides with the relict spreading center responsible for Australia-Pacific crust in the region. Our conceptual model for the transition from seafloor spreading to strike-slip motion along the Macquarie Ridge Complex addresses the decreasing length of spreading center segments and spacing between fracture zones, as well as the arcuate bend of the fracture zones that become asymptotic to the current transform plate boundary.

Structure and tectonics of the upper Cenozoic Puerto Rico‐Virgin Islands carbonate platform as determined from seismic reflection studies

The Puerto Rico-Virgin Islands carbonate platform was deposited over an area of 18,000 km2 from early Oligocene to Holocene on top of an inactive and subsiding Cretaceous-earliest Oligocene island arc. Regional single-channel and multichannel seismic reflection lines presented in this study provide the first information on the regional stratigraphy and structure of this platform that has previously been known mainly from onshore stratigraphic sections of a relatively small (2250 km2) portion of the platform exposed by late Neogene tectonic uplift along the north coast of Puerto Rico. Seismic reflection lines are used to map the thickness of the carbonate platform strata and to correlate this thickness with onshore outcrop and well data from northern and southern Puerto Rico, St. Croix (U.S. Virgin Islands), and the Saba Bank. Limestone thickness variations from a little over 2 km to almost zero are used to subdivide the Puerto Rico-Virgin Islands platform into five distinct carbonate provinces: (1) north Puerto Rico area including the onshore exposures; (2) Virgin Islands area; (3) St. Croix and Saba Bank area; (4) south Puerto Rico area; and (5) Mona Passage area. Carbonate thickness and structural information from each area are used to test five previously proposed models for the deformation and vertical movements of the platform. The most prominent feature of the platform in the Puerto Rico-Virgin Islands area is a large, east-west trending arch. The northern limb of this arch exhibits a smoother, more uniform dip than the steeper, more abruptly faulted, southern limb. The core of the arch is responsible for the exposure of arc basement rocks on Puerto Rico. The origin of this arch, which occurs over a 300 km wide area, is best explained by north-south shortening and arching, caused by interaction at depth of subducted slabs of the North America and Caribbean plates. Other important evidence for this model can be found in the Benioff zones observed in the earthquake profiles. Loading of the Caribbean plate results in downward flexing of the North America plate and causes the 4 km subsidence of the carbonate platform north of Puerto Rico.

Sequence stratigraphy, structure, and tectonic history of the southwestern Ontong Java Plateau adjacent to the North Solomon Trench and Solomon Islands Arc

The Ontong Java Plateau (OJP) is the largest and thickest oceanic plateau on Earth and one of the few oceanic plateaus actively converging on an island arc. We present velocity determinations and geologic interpretation of 2000 km of two-dimensional, multi-channel seismic data from the southwestern Ontong Java Plateau, North Solomon Trench, and northern Solomon Islands. We recognize three megasequences, ranging in age from early Cretaceous to Quaternary, on the basis of distinct interval velocities and seismic stratigraphic facies. Megasequence OJ1 is early Cretaceous, upper igneous crust of the OJP and correlates with basalt outcrops dated at 122–125 Ma on the island of Malaita. The top of the overlying megasequence OJ2, a late Cretaceous mudstone unit, had been identified by previous workers as the top of igneous basement. Seismic facies and correlation to distant Deep Sea Drilling Project/Ocean Drilling Program sites indicate that OJ2 was deposited in a moderately low-energy, marine environment near a fluctuating carbonate compensation depth that resulted in multiple periods of dissolution. OJ2 thins south of the Stewart Arch onto the Solomon Islands where it is correlated with the Kwaraae Mudstone Formation. Megasequence OJ3 is late Cretaceous through Quaternary pelagic cover which caps the Ontong Java Plateau; it thickens into the North Solomon Trench, and seismic facies suggest that OJ3 was deposited in a low-energy marine environment. We use seismic facies analysis, sediment thickness, structural observations, and quantitative plate reconstructions of the position of the OJP and Solomon Islands to propose a tectonic, magmatic, and sedimentary history of the southwestern Ontong Java Plateau. Prior to 125 Ma late Jurassic and early Cretaceous oceanic crust formed. From 125 to 122 Ma, the first mantle plume formed igneous crust (OJ1). Between 122 and 92 Ma, marine mudstone (OJ2 and Kwaraae mudstone of Malaita, Solomon Islands) was deposited on Ontong Java Plateau. At 92 Ma a second mantle plume caused widespread volcanism on the plateau. From 92 to 15 Ma, pelagic carbonate sediment (OJ3) was deposited. At ∼15 Ma the southern Ontong Java Plateau was deformed by normal faults during its approach toward the North Solomon Trench. Finally, from 4 to 0 Ma, the Malaita Accretionary Prism formed during collision between a substantially thicker portion of the Ontong Java Plateau and the Solomon Islands arc. Flexure of the Ontong Java Plateau near the trench caused coeval normal faulting.

Chelyabinsk meteorite explains unusual spectral properties of Baptistina Asteroid Family

We investigated the spectral and compositional properties of Chelyabinsk meteorite to identify its possible parent body in the main asteroid belt. Our analysis shows that the meteorite contains two spectrally distinct but compositionally indistinguishable components of LL5 chondrite and shock blackened/impact melt material. Our X-ray diffraction analysis confirms that the two lithologies of the Chelyabinsk meteorite are extremely similar in modal mineralogy. The meteorite is compositionally similar to LL chondrite and its most probable parent asteroid in the main belt is a member of the Flora family. Our work confirms previous studies (e.g., Vernazza et al. [2008]. Nature 454, 858–860; de Leon, J., Licandro, J., Serra-Ricart, M., Pinilla-Alonso, N., Campins, H. [2010]. Astron. Astrophys. 517, A23; Dunn, T.L., Burbine, T.H., Bottke, W.F., Clark, J.P. [2013]. Icarus 222, 273–282), linking LL chondrites to the Flora family. Intimate mixture of LL5 chondrite and shock blackened/impact melt material from Chelyabinsk provides a spectral match with (8) Flora, the largest asteroid in the Flora family. The Baptistina family and Flora family overlap each other in dynamical space. Mineralogical analysis of (298) Baptistina and 11 small family members shows that their surface compositions are similar to LL chondrites, although their absorption bands are subdued and albedos lower when compared to typical S-type asteroids. A range of intimate mixtures of LL5 chondrite and shock blackened/impact melt material from Chelyabinsk provides spectral matches for all these BAF members. We suggest that the presence of a significant shock/impact melt component in the surface regolith of BAF members could be the cause of lower albedo and subdued absorption bands. The conceptual problem with part of this scenario is that impact melts are very rare within ordinary chondrites. Of the � 42,000 ordinary chondrites, less than 0.5% (203) of them contain impact melts. A major reason that impact melts are rare in meteorites is that high impact velocities (V > 10 km/s) are needed to generate the necessary shock pressures and temperatures (e.g., Pierazzo, E., Melosh, H.J. [1998]. Hydrocode modeling of oblique impacts: The fate of the projectile. In: Origin of the Earth and Moon, Proceedings of the Conference. LPI Contribution No. 957) unless the target material is highly porous. Nearly all asteroid impacts within the main belt are at � 5 km/s (Bottke, W.F., Nolan, M.C., Greenberg, R., Kolvoord, R.A. [1994]. Collisional lifetimes and impact statistics of near-Earth asteroids. In: Tucson, Gehrels T. (Ed.), Hazards Due to Comets and Asteroids. The University of Arizona Press, Arizona, pp. 337–357), which prevents them from producing much impact melt unless they are highly porous. However, shock darkening is an equally efficient process that takes place at much lower impact velocities (� 2 km/s) and can cause the observed spectral effects. Spectral effects of shock darkening and impact melt are identical. The parent asteroid of BAF was either a member of the Flora family or had the same basic composition as the Floras (LL Chondrite). The shock pressures produced during the impact event generated enough impact melt or shock blackening to alter the spectral properties of BAF, but keep the BAF composition largely unchanged. Collisional mixing of shock blackened/impact melt and LL5 chondritic material could have created the Baptistina Asteroid Family with composition identical to those of the Floras, but with subdued absorption

Global seismicity characteristics of subduction-to-strike-slip transitions

There are at least 30 major plate boundary segments worldwide where the plate boundary changes from subduction to strike-slip; these include six triple junctions and 24 two-plate boundaries. This study investigates earthquake seismicity in the 24 two-plate subduction-to-strike-slip transition (SSST) regions by utilizing recently published earthquake relocations, ternary diagrams of focal mechanisms, and moment rate calculations. To facilitate cross-regional comparisons, we categorize the geometry of SSST plate boundaries in terms of (1) their radius of curvature, (2) their sense of curvature, that is, whether they are convex or concave as viewed from the downgoing plate, and (3) their tectonic complexity, that is, the variability of crustal thickness and the segmentation of the plate boundary trace. We observe three main trends in SSST regions: (1) there is a conspicuous scarcity of strike-slip earthquakes along plate boundary segments that plate motion models indicate are strike-slip boundaries; (2) in these apparent strike-slip segments, both the rate of occurrence of earthquakes of any kind and the moment release rate are low compared to adjacent subduction segments; and (3) there were few observable differences in seismicity between convex and concave boundaries. The observation that transform zones exhibit moment rate deficiencies, that is, have few large-magnitude earthquakes in the historical record, may have important implications for seismic hazard assessment in SSST regions. In particular, is motion along these boundaries aseismic with little seismic hazard, or is motion expressed in very large magnitude, infrequent, but potentially devastating earthquakes? In at least three such regions, New Zealand, the Philippines, and the Dominican Republic, paleoseismic evidence and the historical record of seismicity suggest that very large, infrequent earthquakes do occur.

A mineralogical characterization of biogenic calcium carbonates precipitated by heterotrophic bacteria isolated from cryophilic polar regions

Precipitation of calcium carbonate (CaCO3(s) ) can be driven by microbial activity. Here, a systematic approach is used to identify the morphological and mineralogical characteristics of CaCO3(s) precipitated during the heterotrophic growth of micro-organisms isolated from polar environments. Focus was placed on establishing mineralogical features that are common in bioliths formed during heterotrophic activity, while in parallel identifying features that are specific to bioliths precipitated by certain microbial phylotypes. Twenty microbial isolates that precipitated macroscopic CaCO3(s) when grown on B4 media supplemented with calcium acetate or calcium citrate were identified. A multimethod approach, including scanning electron microscopy, high-resolution transmission electron microscopy, and micro-X-ray diffraction (μ-XRD), was used to characterize CaCO3(s) precipitates. Scanning and transmission electron microscopy showed that complete CaCO3(s) crystal encrustation of Arthrobacter sp. cells was common, while encrustation of Rhodococcus sp. cells did not occur. Several euhedral and anhedral mineral formations including disphenoid-like epitaxial plates, rhomboid-like aggregates with epitaxial rhombs, and spherulite aggregates were observed. While phylotype could not be linked to specific mineral formations, isolates tended to precipitate either euhedral or anhedral minerals, but not both. Three anhydrous CaCO3(s) polymorphs (calcite, aragonite, and vaterite) were identified by μ-XRD, and calcite and aragonite were also identified based on TEM lattice-fringe d value measurements. The presence of certain polymorphs was not indicative of biogenic origin, although several mineralogical features such as crystal-encrusted bacterial cells, or casts of bacterial cells embedded in mesocrystals are an indication of biogenic origin. In addition, some features such as the formation of vaterite and bacterial entombment appear to be linked to certain phylotypes. Identifying phylotypes consistent with certain mineralogical features is the first step toward discovering a link between these crystal features and the precise underlying molecular biology of the organism precipitating them.

Chapter 11 Cenozoic el mamey group of northern hispaniola: a sedimentary record of subduction, collisional and strike-slip events within the north America-Caribbean plate boundary zone

Abstract Cretaceous and Cenozoic paleogeographic and plate tectonic reconstructions of the Greater Antilles (Hispaniola, Cuba, Jamaica, and Puerto Rico) are complicated by large-offset, Eocene? to Recent strike-slip movements between the North America and Caribbean plates. Moreover, Eocene?-Recent oblique subduction of the Bahama carbonate platform presently affects the Hispaniola region and further complicates the reconstruction of this wide and complex plate boundary zone. This paper describes a detailed sedimentological study of deformed and uplifted Eocene to Lower Pliocene sedimentary rocks (El Mamey Group) between the North America and Caribbean plates in northern Hispaniola (Dominican Republic). Paleocene to Lower Pliocene siliciclastic and carbonate rocks of the El Mamey Group, which crop out within a 500 km 2 area of the central Cordillera Septentrional and are well exposed in road and stream cuts, formed the object of this regional tectonic study. On the basis of its compositional, age and facies character, we divide the sedimentary succession of the El Mamey Group of the central Cordillera Septentrional into three lithologically distinct, stratigraphic sequences which we relate to three tectonic phases that affected this segment of the North America-Caribbean plate boundary from the Eocene to Recent. Phase 1 (Paleocene to Middle Eocene). Sedimentary and facies characteristics of an approximately 250-m-thick section of Upper Paleocene to Lower Eocene siliciclastic and carbonate rocks (Los Hidalgos Formation), suggest that these rocks were deposited in a deep-marine, hemipelagic environment adjacent to an active volcanic arc. Calc-alkaline volcanic flows and sills are interbedded with these deep-marine sedimentary rocks. Termination of deposition and volcanism in Early to Middle Eocene time coincides with a major folding and uplift event, which we believe was caused by the early attempted subduction of the Bahama carbonate platform beneath the arc-related basin. This event terminated arc activity in the Hispaniola volcanic arc and forearc. Phase 2 (Late Eocene to Early Miocene). Sedimentary and facies characteristics of a 4000-m-thick, Upper Eocene to Lower Miocene siliciclastic succession (Altamira and Las Lavas formations of the El Mamey Group) suggest that these rocks were deposited as submarine turbidites and other types of mass-flow deposits within a west-northwest-trending, elongate basin. Petrographic analysis of framework grains of sandstones within the section shows two distinct sandstone populations separated by a linear, 100–400-m-wide left-lateral strike-slip fault zone. Petrographic differences across the fault zone are especially prominent in coeval Oligocene sedimentary rocks and suggest that the two basins were juxtaposed by lateral fault movement sometime after Oligocene time. The end of deep-marine siliciclastic deposition in both basins coincides with a gentle Middle Miocene folding event believed to be related to transpressional strike-slip faulting. Phase 3 (Late Miocene to Recent). Sedimentary and facies characteristics of an approximately 250-m-thick section of Upper Miocene to Lower Pliocene carbonate rocks (Villa Trina Formation) suggest that these rocks were deposited as a shallow carbonate bank above slightly folded, Early Miocene siliciclastic rocks. Carbonate deposition was terminated in Early Pliocene time by a folding and uplift event believed to be related to transpression along a restraining bend in the Septentrional fault zone.

Link between the potentially hazardous Asteroid (86039) 1999 NC43 and the Chelyabinsk meteoroid tenuous

We explored the statistical and compositional link between Chelyabinsk meteoroid and potentially haz- ardous Asteroid (86039) 1999 NC43 to investigate their proposed relation proposed by Borovicka et al. (Borovicka, J., et al. (2013). Nature 503, 235-237). First, using a slightly more detailed computation we confirm that the orbit of the Chelyabinsk impactor is anomalously close to the Asteroid 1999 NC43. We find � (1-3) � 10 � 4 likelihood of that to happen by chance. Taking the standpoint that the Chelya- binsk impactor indeed separated from 1999 NC43 by a cratering or rotational fission event, we run a for- ward probability calculation, which is an independent statistical test. However, we find this scenario is unlikely at the � (10 � 3 -10