Computational Models of Classical Conditioning guest editors’ introduction

In the present special issue, the performance of current computational models of classical conditioning was evaluated under three requirements: (1) Models were to be tested against a list of previously agreed-upon phenomena; (2) the parameters were fixed across simulations; and (3) the simulations used to test the models had to be made available. These requirements resulted in three major products: (a) a list of fundamental classical-conditioning results for which there is a consensus about their reliability; (b) the necessary information to evaluate each of the models on the basis of its ordinal successes in accounting for the experimental data; and (c) a repository of computational models ready to generate simulations. We believe that the contents of this issue represent the 2012 state of the art in computational modeling of classical conditioning and provide a way to find promising avenues for future model development

Revised Lithostratigraphy of the Sonsela Member (Chinle Formation, Upper Triassic) in the Southern Part of Petrified Forest National Park, Arizona

BACKGROUND: Recent revisions to the Sonsela Member of the Chinle Formation in Petrified Forest National Park have presented a three-part lithostratigraphic model based on unconventional correlations of sandstone beds. As a vertebrate faunal transition is recorded within this stratigraphic interval, these correlations, and the purported existence of a depositional hiatus (the Tr-4 unconformity) at about the same level, must be carefully re-examined. METHODOLOGY/PRINCIPAL FINDINGS: Our investigations demonstrate the neglected necessity of walking out contacts and mapping when constructing lithostratigraphic models, and providing UTM coordinates and labeled photographs for all measured sections. We correct correlation errors within the Sonsela Member, demonstrate that there are multiple Flattops One sandstones, all of which are higher than the traditional Sonsela sandstone bed, that the Sonsela sandstone bed and Rainbow Forest Bed are equivalent, that the Rainbow Forest Bed is higher than the sandstones at the base of Blue Mesa and Agate Mesa, that strata formerly assigned to the Jim Camp Wash beds occur at two stratigraphic levels, and that there are multiple persistent silcrete horizons within the Sonsela Member. CONCLUSIONS/SIGNIFICANCE: We present a revised five-part model for the Sonsela Member. The units from lowest to highest are: the Camp Butte beds, Lot's Wife beds, Jasper Forest bed (the Sonsela sandstone)/Rainbow Forest Bed, Jim Camp Wash beds, and Martha's Butte beds (including the Flattops One sandstones). Although there are numerous degradational/aggradational cycles within the Chinle Formation, a single unconformable horizon within or at the base of the Sonsela Member that can be traced across the entire western United States (the "Tr-4 unconformity") probably does not exist. The shift from relatively humid and poorly-drained to arid and well-drained climatic conditions began during deposition of the Sonsela Member (low in the Jim Camp Wash beds), well after the Carnian-Norian transition

Structural and temporal requirements for geomagnetic field reversal deduced from lava flows

Reversals of the Earth's magnetic field reflect changes in the geodynamo-flow within the outer core-that generates the field. Constraining core processes or mantle properties that induce or modulate reversals requires knowing the timing and morphology of field changes that precede and accompany these reversals(1-4). But the short duration of transitional field states and fragmentary nature of even the best palaeomagnetic records make it difficult to provide a timeline for the reversal process(1,5). Ar-40/Ar-39 dating of lavas on Tahiti, long thought to record the primary part of the most recent 'Matuyama-Brunhes' reversal, gives an age of 795 +/- 7 kyr, indistinguishable from that of lavas in Chile and La Palma that record a transition in the Earth's magnetic field, but older than the accepted age for the reversal. Only the 'transitional' lavas on Maui and one from La Palma (dated at 776 +/- 2 kyr), agree with the astronomical age for the reversal. Here we propose that the older lavas record the onset of a geodynamo process, which only on occasion would result in polarity change. This initial instability, associated with the first of two decreases in field intensity, began similar to 18 kyr before the actual polarity switch. These data support the claim(6) that complete reversals require a significant period for magnetic flux to escape from the solid inner core and sufficiently weaken its stabilizing effect(7)

Geomagnetic Field, Polarity Reversals

International audienceBernard Brunhes (1906) was the first to measure magnetization directions in rocks that were approximately antiparallel to the present Earth’s field. Brunhes (1906) recorded magnetizations in baked sedimentary rocks that were aligned with reverse magnetization directions in overlying Miocene lavas from central France (Puy de Dome). In so doing, Brunhes (1906) made first use of a field test for primary thermal remanent magnetization (TRM) that is now referred to as the “baked contact” test. Matuyama (1929) was the first to attribute reverse magnetizations in (volcanic) rocks from Japan and China to reversal of geomagnetic polarity, and to differentiate mainly Pleistocene lavas from mainly Pliocene lavas based on the polarity of the magnetization. In this respect, Matuyama (1929) was the first person to use the sequence of geomagnetic reversals as a means of ordering rock sequences. The reality of geomagnetic reversals was then progressively established with the studies of Hospers (1951, 1953) in Iceland, and Roche (1950, 1951, 1956) in the Massif Central of France. The work of Hospers on Icelandic lavas was augmented by Rutten and Wensink (1960) and Wensink (1966) who subdivided Pliocene-Pleistocene lavas in Iceland into three polarity zones from young to old: N-R-N. Magnetic remanence measurements on basaltic lavas combined with K/Ar dating, pioneered by Cox et al. (1963) and McDougall and Tarling (1963a, b, 1964), resulted in the beginning of development of the modern geomagnetic polarity timescale (GPTS). These studies, and those that followed in the mid-1960s, established that rocks of the same age carry the same magnetization polarity, at least for the last few million years. The basalt sampling sites were scattered over the globe. Polarity zones were linked by their K/Ar ages, and were usually not in stratigraphic superposition. Doell and Dalrymple (1966) designated the long intervals of geomagnetic polarity of the last 5 Myrs as magnetic epochs, and named them after pioneers of geomagnetism (Brunhes, Matuyama, Gauss, and Gilbert). Then, the discovery of marine magnetic anomalies confirmed seafloor spreading (Vine and Matthews 1963), and the GPTS was extended to older times (Vine 1966; Heirtzler et al. 1968; Lowrie and Alvarez 1981). Since then, the succession of polarity intervals has been extensively studied and used to construct magnetostratigraphic timescales linking biostratigraphies, isotope stratigraphies, and absolute ages (see Opdyke and Channell 1996, “Magnetic stratigraphy”, for a review)

Boudinage of a stretching slablet implicated in earthquakes beneath the Hindu Kush

As the fragments of Gondwana (Africa, Arabia, India and Australia) moved northward, arc-shaped belts with intervening basins formed in the Alpine–Himalayan mountain chain during and after collision. This was accompanied by subduction (or sinking) of the ancient Tethyan oceanic plate (or slab) into the underlying mantle. The arc-like shapes could in part be the end result of processes related to drips forming in the less-viscous mantle layer at the base of the Earth's rigid outer shell and then falling into the deeper mantle. Alternatively, the arcs could have formed because slabs constituted of intervening small ocean basins were independently subducted during convergence, and have now disappeared. The subducting slabs tend to stretch, tear and eventually break off, leaving behind thin, vertical strips of colder material that can easily be mistaken for mantle drips. Previous work indicates the presence of such remnant material beneath the Hindu Kush region, close to the collision zone between the Indian and Eurasian continental plates. Here, we analyse a cluster of intermediate-depth earthquakes beneath this region and suggest the existence of an elongate boudin, a lens-shaped feature bounded by ductile faults or shear zones. Our data do not support mantle drip and instead offer a snapshot into the process of break-off, as a thin strip of vertically stretching slab tears free before descending deeper into the underlying mantle