Sarah M. Roeske

Chelyabinsk airburst, damage assessment, meteorite recovery, and characterization

Deep Impact? On 15 February 2013, the Russian district of Chelyabinsk, with a population of more than 1 million, suffered the impact and atmospheric explosion of a 20-meter-wide asteroid—the largest impact on Earth by an asteroid since 1908. Popova et al. (p. 1069, published online 7 November; see the Perspective by Chapman) provide a comprehensive description of this event and of the body that caused it, including detailed information on the asteroid orbit and atmospheric trajectory, damage assessment, and meteorite recovery and characterization. A detailed study of a recent asteroid impact provides an opportunity to calibrate the damage caused by these rare events. [Also see Perspective by Chapman] The asteroid impact near the Russian city of Chelyabinsk on 15 February 2013 was the largest airburst on Earth since the 1908 Tunguska event, causing a natural disaster in an area with a population exceeding one million. Because it occurred in an era with modern consumer electronics, field sensors, and laboratory techniques, unprecedented measurements were made of the impact event and the meteoroid that caused it. Here, we document the account of what happened, as understood now, using comprehensive data obtained from astronomy, planetary science, geophysics, meteorology, meteoritics, and cosmochemistry and from social science surveys. A good understanding of the Chelyabinsk incident provides an opportunity to calibrate the event, with implications for the study of near-Earth objects and developing hazard mitigation strategies for planetary protection.

Radar-Enabled Recovery of the Sutter’s Mill Meteorite, a Carbonaceous Chondrite Regolith Breccia

The Meteor That Fell to Earth In April 2012, a meteor was witnessed over the Sierra Nevada Mountains in California. Jenniskens et al. (p. 1583) used a combination of photographic and video images of the fireball coupled with Doppler weather radar images to facilitate the rapid recovery of meteorite fragments. A comprehensive analysis of some of these fragments shows that the Sutters Mill meteorite represents a new type of carbonaceous chondrite, a rare and primitive class of meteorites that contain clues to the origin and evolution of primitive materials in the solar system. The unexpected and complex nature of the fragments suggests that the surfaces of C-class asteroids, the presumed parent bodies of carbonaceous chondrites, are more complex than previously assumed. Analysis of this rare meteorite implies that the surfaces of C-class asteroids can be more complex than previously assumed. Doppler weather radar imaging enabled the rapid recovery of the Sutter’s Mill meteorite after a rare 4-kiloton of TNT–equivalent asteroid impact over the foothills of the Sierra Nevada in northern California. The recovered meteorites survived a record high-speed entry of 28.6 kilometers per second from an orbit close to that of Jupiter-family comets (Tisserand’s parameter = 2.8 ± 0.3). Sutter’s Mill is a regolith breccia composed of CM (Mighei)–type carbonaceous chondrite and highly reduced xenolithic materials. It exhibits considerable diversity of mineralogy, petrography, and isotope and organic chemistry, resulting from a complex formation history of the parent body surface. That diversity is quickly masked by alteration once in the terrestrial environment but will need to be considered when samples returned by missions to C-class asteroids are interpreted.

Introduction: An overview of ridge-trench interactions in modern and ancient settings

Virtually all subduction zones eventually interact with a spreading ridge, and this interaction leads to a great diversity of tectonic processes in the vicinity of the triple junction. In the present-day Pacifi c basin, there are seven examples of active or recently extinct spreading ridges and transforms interacting with trenches. In contrast, there are only a few well-documented cases of spreading ridge interactions in the ancient geologic record, which indicates this process is grossly underrepresented in tectonic syntheses of plate margins. Analogies with modern systems can identify some distinctive processes associated with triple junction interactions, yet an incomplete understanding of those processes, and their effects, remains. Additional insights can be gained from well-documented examples of ancient ridge subduction because exhumation has revealed deeper levels of the tectonic system and such systems provide a temporal record of complex structural, metamorphic, igneous, and sedimentary events. This volume focuses on ridge-trench interactions in the Paleogene forearc record of the northern Cordillera (north of the 49th parallel). Insights from this system and modern analogs suggest that there is no single unique signature of ridge subduction events, but a combination of processes (e.g., igneous associations, changes in kinematics, motion of forearc slivers, thermal events, etc.) can be diagnostic, especially when they are time-transgressive along a plate margin. Understanding these processes in both modern and ancient systems is critical to our understanding of the creation and evolution of continental crust and provides a new framework for evaluating the evolution of the onshore and offshore tectonic history of the northern North American Cordillera.

Dextral-slip reactivation of an arc-forearc boundary during Late Cretaceous-Early Eocene oblique convergence in the northern Cordillera

The Border Ranges fault system forms the most fundamental crustal boundary in southern Alaska, separating crystalline basement with Paleozoic and Mesozoic arc sequences, the Wrangellia terrane, from a long-lived accretionary complex, the Chugach terrane. Although the Border Ranges fault system originated as a subduction zone megathrust, few strands of it preserve that original history. This study shows that a dextral fault zone, herein named the Hanagita fault system, has overprinted the subduction zone megathrust. This dextral fault zone is 5-10 km wide and has been traced continuously for ∼250 km. The Hanagita fault system is probably continuous with dextral slip systems described in Glacier Bay and Baranof Island in southeast Alaska, which would extend its length to ∼700 km. Within this fault zone, elements of Wrangellia and the Chugach terrane are complexly intermixed. Geologic mapping and 4 0 Ar/ 3 9 Ar geochronology of white mica crystallized during dextral slip indicate the strike-slip faulting may have begun by ∼85 Ma and was continuous from ∼70 Ma until ∼51 Ma. Accretion of a thick flysch sequence along this margin began in the Maastrichtian (latest Cretaceous) and continued uninterrupted through middle Eocene. Migration of a ridge-trench-trench triple junction along this margin during the late Paleocene-early Eocene introduced hot fluids to the fault zone and plutons on the southern edge of the fault system. Dextral slip occurred before, during, and after the triple junction migration. Thus, the Hanagita fault system records long-lived oblique convergence between two separate oceanic plates and North America and is a relatively well-preserved example of displacement partitioning along the backstop of an accretionary prism. The Hanagita fault system includes several through-going dextral high-angle faults that are continuous on the scale of 10s to 100+ km. These strike WNW, subparallel to the structural grain in the accretionary prism to the south. Between these strands are numerous shear zones and faults that are continuous on the scale of several 100 m to several km. The majority of these also show subhorizontal to oblique slip, but the dip of the fault planes is more variable than the through-going faults. The western part of the fault system has north-dipping thrust and oblique slip faults that formed approximately synchronously with south-dipping oblique slip faults 100 km to the east. The faults formed at progressively lower temperatures through time based on cross-cutting relations and 4 0 Ar/ 3 9 Ar white mica dates of fault fabrics. The oldest shear zones formed at low greenschist facies, and the youngest structures are discrete brittle faults and gouge zones. Rocks within the fault system include crystalline rocks from the inboard side of the fault system and melange of the McHugh Complex, the oldest part of the accretionary prism in this area. The crystalline rocks include abundant Late Jurassic diorite (U/Pb zircon date) with 4 0 Ar/ 3 9 Ar hornblende cooling ages of 142-146 Ma, which correlates with the Chitina batholith of the Wrangell Mountains and the St. Elias Plutonic Suite of the St. Elias range. Other crystalline rocks include an ∼170 Ma diorite (hornblende 4 0 Ar/ 3 9 Ar) associated with abundant metabasite and leucocratic plutonic rock. No known correlative to these units occurs for a minimum of 600 km to the southeast along the Border Ranges fault system. Thus, the total amount of dextral displacement along the Hanagita fault system from the Late Cretaceous to Middle Eocene is conservatively estimated at 600 km but could be >1000 km, based on correlation of the 170 Ma pluton and related rocks to a similar suite on the west side of Vancouver Island. This displacement is also consistent with regional geology elsewhere in the accretionary prism. The timing of displacement on the fault system coincides with dextral slip on several other major faults, parallel but more inboard from the margin, incl

Cambrian initiation of the Las Pirquitas thrust of the western Sierras Pampeanas, Argentina: Implications for the tectonic evolution of the proto-Andean margin of South America

The 40 Ar/ 39 Ar hornblende crystallization ages from the Las Pirquitas thrust of the Sierra de Pie de Palo, Argentina, indicate that imbrication of the proto-Andean margin had initiated by ca. 515 Ma. The ages do not permit the Pie de Palo block to represent basement of the adjacent Argentine Precordillera terrane, but rather require it to be a separate fragment not juxtaposed with the Precordillera prior to the Ordovician. The Pie de Palo block was likely autochthonous to the Famatina arc of the proto-Andean margin in the Early Cambrian. When combined with existing regional ages of magmatism, metamorphism, and deformation, the ages suggest that east-dipping subduction beneath the Famatina arc initiated by the Early to Middle Cambrian. The newly established convergent margin resulted from a westward shift of the plate boundary at the end of the Pampean orogeny in the eastern Sierras Pampeanas. The Famatina arc persisted as an Andean-type active margin until the collision and accretion of the Precordillera terrane in the Middle Ordovician.

Mafic and ultramafic crustal fragments of the southwestern Precordillera terrane and their bearing on tectonic models of the early Paleozoic in western Argentina

New U-Pb zircon ages and initial Nd isotope data from mafic and ultramafic rocks that have been tectonically imbricated on the southwest margin of the Precordillera terrane, western Argentina, indicate that these rocks do not constitute a single ophiolite pseudostratigraphy as previously thought. Rock units include Late Proterozoic (576 ± 17 Ma) microgabbro and Silurian (418 ± 10 Ma) sills, both of which have initial Nd isotope values of +6.0 to +7.3 and identical trace element geochemistry indicating formation in extensional settings. Late Ordovician (450 ± 20 Ma) layered gabbro has a distinct initial Nd isotope value of +0.6. It intrudes quartzofeldspathic gneiss and tectonized ultramafic rocks; this assemblage represents the deep part of a continental margin volcanic arc that formed on the eastern margin of Chilenia, as the ocean basin west of the Precordillera terrane began to close in the Middle Ordovician. The new tectonic interpretations generally support existing hypotheses that the Precordillera terrane rifted and drifted from southeastern Laurentia in the Cambrian to Early Ordovician and that the western margin of the Precordillera terrane represents a suture between two fragments of continental crust.

Multiple migmatite events and cooling from granulite facies metamorphism within the Famatina arc margin of northwest Argentina

The Famatina margin records an orogenic cycle of convergence, metamorphism, magmatism, and extension related to the accretion of the allochthonous Precordillera terrane. New structural, petrologic, and geochronologic data from the Loma de Las Chacras region demonstrate two distinct episodes of lower crustal migmatization. The first event preserves a counterclockwise pressure-temperature path in kyanite-K-feldspar pelitic migmatites that resulted in lower crustal migmatization via muscovite dehydration melting at ∼12 kbar and 868°C at 461 ±1.7 Ma. The shape of the pressure temperature path and timing of metamorphism are similar to those of regional midcrustal granulites and suggest pervasive Ordovician migmatization throughout the Famatina margin. One-dimensional thermal modeling coupled with regional isotopic data suggests Ordovician melts remained at temperatures above their solidus for 20–30 Ma following peak granulite facies metamorphism, throughout a time period marked by regional oblique convergence. The onset of synconvergent extension occurred only after regional migmatites cooled beneath their solidus and was synchronous with the cessation of Precordillera terrane accretion at ∼436 Ma. The second migmatite event was regionally localized and occurred at ∼700°C and 12 kbar between 411 and 407 Ma via vapor saturated melting of muscovite. Migmatization was synchronous with extension, exhumation, and strike-slip deformation that likely resulted from a change in the plate boundary configuration related to the convergence and collision of the Chilenia terrane.

Late Paleozoic orogeny in Alaska's Farewell terrane

Evidence is presented for a previously unrecognized late Paleozoic orogeny in two parts of Alaska’s Farewell terrane, an event that has not entered into published scenarios for the assembly of Alaska. The Farewell terrane was long regarded as a piece of the early Paleozoic passive margin of western Canada, but is now thought, instead, to have lain between the Siberian and Laurentian (North American) cratons during the early Paleozoic. Evidence for a late Paleozoic orogeny comes from two belts located 100–200 km apart. In the northern belt, metamorphic rocks dated at 284–285 Ma (three 40 Ar/ 39 Ar white-mica plateau ages) provide the main evidence for orogeny. The metamorphic rocks are interpreted as part of the hinterland of a late Paleozoic mountain belt, which we name the Browns Fork orogen. In the southern belt, thick accumulations of PennsylvanianPermian conglomerate and sandstone provide the main evidence for orogeny. These strata are interpreted as the eroded and deformed remnants of a late Paleozoic foreland basin, which we name the Dall Basin. We suggest that the Browns Fork orogen and Dall Basin comprise a matched pair formed during collision between the Farewell terrane and rocks to the west. The colliding object is largely buried beneath Late Cretaceous flysch to the west of the Farewell terrane, but may have included parts of the so-called Innoko terrane. The late Paleozoic convergent plate boundary represented by the Browns Fork orogen likely connected with other zones of plate convergence now located in Russia, elsewhere in Alaska, and in western Canada. Published by Elsevier B.V.

Alternating asymmetric topography of the Alaska range along the strike‐slip Denali fault: Strain partitioning and lithospheric control across a terrane suture zone

Contrasting lithospheric strength between terranes often results in the concentration of strain and deformation within the weaker material. Dramatic alternating asymmetric topography of the central and eastern Alaska Range along the active Denali fault is due to contrasting lithospheric strength between terranes and a suture zone, controlled by fault location with respect to the irregular boundary of a relatively stronger terrane backstop. Highest topography and greatest Neogene exhumation in the central Alaska Range occur on the concave side of the arcuate Denali fault, yet to the north and on the convex side of the fault in the eastern Alaska Range. The Denali fault largely lies along a Mesozoic suture zone between two large composite terranes (Yukon and Wrangellia composite terranes: YCT and WCT), but the McKinley strand of the fault cuts across an embayment of weaker suture-zone rocks (Alaska Range suture-zone, ARSZ) within the irregular southern boundary of the YCT (Hines Creek fault). Deformation (and uplift of the Alaska Range) is driven by slip and partitioning of strain along the Denali fault, occurring preferentially in weaker rocks of the ARSZ against the stronger YCT. Where the YCT lies well north of the McKinley strand, deformation is primarily to the north of the fault (eastern Alaska Range). Where the YCT is close to the fault, deformation is primarily to the south (central Alaska Range). While the trace of the McKinley strand approximates a small circle, two restraining bends (McKinley and Hayes) pinned equidistant from the ends of this strand localize uplift and exhumation.

Strike-slip faulting and block rotation along the contact fault system, eastern Prince William Sound, Alaska

The Chugach and Prince William terranes compose a Mesozoic through Paleogene accretionary complex rimming the Gulf of Alaska. In eastern Prince William Sound, we studied the Contact fault system, which juxtaposes flysch of the latest Cretaceous Valdez Group (Chugach terrane) with flysch of the late Paleocene through early Eocene Orca Group (Prince William terrane). Near Valdez, we found three lithotectonic belts separated by right-lateral strike-slip faults. Anomalous structural trends between the two faults appear to be the result of clockwise block rotation in response to dextral shear. In a second area, to the east near Cordova, early formed folds were rotated clockwise before burial to depths sufficient to form axial-planar cleavage. Apparently, synchronous thrusting and dextral-slip faulting occurred during accretion. The relations above, combined with earlier studies, suggest that the fundamental nature of motion on faults composing the Contact fault system is dextral. Near Cordova, deposition, accretion, and block rotation occurred between early Eocene and early middle Eocene time. Near Valdez, block rotation appears to have occurred prior to accretion of the rocks near Cordova. Current plate tectonic models show that convergence became progressively more oblique during Eocene time, correlating well with the initiation of strike-slip faulting. The presence of early Tertiary dextral-slip faults, such as these, helps account for the northward displacement of the Chugach-Prince William terrane with respect to the Peninsular and Wrangellia terranes suggested by paleomagnetic and geologic studies.