The Tortoise and the Hare Racing Deep Geologic Time

And other recently released GSA Bulletin articles

Boulder, Colo., USA - The specific mechanisms by which ubiquitous, ever-present temperature cycling caused by the sun might serve to break down rocks remains debated. Here, M.C. Eppes and colleagues directly measure real-time rock cracking for a boulder sitting on the ground in open sun. They also model how sun-driven temperature cycling can stress the boulder. The team observed that cracking occurs in conjunction with these thermal stresses, but only when the rock surface temperature is disturbed by other factors such as a sudden storm. Thus, they conclude that over deep geologic time, slow cracking driven by daily exposure to simple solar forces represents the "tortoise" of the cracking race by which rocks break down. Except in this story, Eppes and colleagues propose that the "hare" of more rapid crack growth driven by rare events such as freezing, rides the along on the back of the tortoise in order to make further gains at the end of the race.

Deciphering the role of solar-induced thermal stresses in rock weathering
M.C. Eppes et al., Department of Geography and Earth Sciences, University of North Carolina at Charlotte, Charlotte North Carolina 28223, USA. This article is online at

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Other recently published GSA BULLETIN articles are highlighted below:

Late Holocene rupture behavior and earthquake chronology on the Hope fault, New Zealand
N. Khajavi et al., Department of Geological Sciences, University of Canterbury, New Zealand, This paper is online at

The Hope Fault is the second fastest slipping fault in New Zealand and is a major source of seismic hazard. The fault ruptured in a major earthquake in 1888. Understanding the dimensions of the 1888 earthquake was important for assessing whether proposed geometric barriers along the Hope Fault truly limit the extent of earthquake ruptures. We used a variety of techniques including LiDAR, photogrammetry, paleoseismic trenching, OxCal modelling, dendrochronology, OSL and radiocarbon dating alongside the historical accounts of the 1888 earthquake to show the 1888 rupture involved two separate fault segments (the entire Hope River segment and a part of the Hurunui segment) and that some earthquakes on the Hope Fault cross proposed geometric barriers. This result is important for understanding earthquake dynamics on segmented fault systems worldwide, including the San Andreas and North Anatolian faults, which are structurally analogous to the Hope Fault.

Formation of intra-continental basins in the opposite corners of the Tabas Block as coeval structures controlled by transpressional faulting, Iran
A. Konon et al., Faculty of Geology, University of Warsaw, Żwirki i Wigury 93, PL-02-089 Warsaw, Poland. This article is online at

In the manuscript, we present examples of internal deformation within the vertical-axis rotated tectonic blocks bounded by major intra-continental strike-slip faults observed in a region of continental collision. We discuss the coeval development and inversion of two intra-continental basins in the opposite corners of a tectonic block as result of transpressional tectonics. We have considered transpressional faulting and vertical-axis rotation of the blocks as related processes. Deformation according to the ‘bookshelf’ mechanism can be considered as a consequence of accommodation of the shortening of the area north of the Main Zagros Thrust and an externally imposed shearing along the Great Kavir (Doruneh) Fault during collision of the Arabian and Eurasian plates. The processes favored the interaction of the strike-slip faults and thrusts, in their restraining junctions allowing for the formation and then inversion of two intra-continental basins: the Tabas and Abdooghi basins within the Tabas Block.

Development of fracture network in crystalline rocks during weathering: Study of Bishop Creek chronosequence using x-ray computed tomography and 14C-PMMA impregnation method
A. Mazurier et al., IC2MP/HYDRASA, University of Poitiers, France. This article is online at

This contribution quantifies the evolution of the connected pore network of a granitoid rock (Lamarck granodiorite) during a 120,000 year weathering period. The initial steps of regolith production were interpreted using the sequentially altered moraine deposits that form the Bishop Creek chronosequence. Pore network development is characterized by the coevolution of fissure aperture and internal porosity of minerals that are sensitive to alteration. Fissure density is not drastically changed during weathering, long macrocracks being formed by the coalescence of some preexisting microcracks. Use of the 14C-PMMA method was crucial in order to characterize the full spectrum of pore apertures (<0.1 µm to > 1 mm). This article suggests that the influence of pore space structure evolution during the weathering must be considered in order to understand the initial stages of regolith formation.

The influence of magmatic-hydrothermal activity on brine evolution in closed basins: Searles Lake, California
T.K. Lowenstein et al., Department of Geological Sciences and Environmental Studies, Binghamton University, Binghamton, New York 13902, USA. This article is online at

Hot springs in Long Valley caldera, California, with high alkalinity from magmatic carbon dioxide, discharge into Hot Creek at the headwaters of the Owens River. This hydrothermal source of alkalinity is responsible for the formation of the rare evaporite mineral trona, Na3(CO3)(HCO3)•2H2O, in Searles Lake, a dry salt pan in eastern California, 300 kilometers to the south. Chemical analyses of fluid inclusions in halite from a deep core at Searles Lake indicate that its waters became hyperalkaline around 1 million years ago. Volcanism and hydrothermal activity, associated with a major eruptive episode at Glass Mountain in the Long Valley region around 1 million years ago, provided carbon dioxide and elevated alkalinity to Searles lake inflow waters, which led to the formation of trona. Magmatism in Owens Valley, and the appearance of trona in Searles Lake, occurred 500,000 years before the giant eruptions that formed the Bishop Tuff and Long Valley caldera.

Two episodes of partial melting in ultrahigh-pressure migmatites from deeply subducted continental crust in the Sulu orogen, China
W.-C. Li et al., CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China. This article is online at

Numerical modeling predicts dehydration melting of subducting crustal rocks at the slab top beneath the mantle wedge at subarc depths. Sediment-derived melts are usually considered as one of the slab components in the mantle sources of oceanic and continental arc volcanics. However, a direct record of the anatexis in deeply subducted crustal rocks is still desired in order to prove the presence of slab melting in nature. Since the deeply subducted oceanic slab is rarely exhumed, the contributions of oceanic slab derived melts are only indirectly recognized from island arc volcanics. Nevertheless, the exhumed UHP metamorphosed continental crust provide us an excellent laboratory to directly investigate the melting process at the subarc depths in subduction zones.

Extrinsic and intrinsic controls on mouth bar and mouth bar complex architecture: Examples from the Pennsylvanian (upper Carboniferous) of the central Appalachian Basin, Kentucky, USA
R.M. Jerrett et al., School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Road, Manchester, M139PL, UK. This article is online at

Mouth bars -- bodies of sediment deposited at the mouths of rivers as they enter standing bodies of water (lakes and seas) -- are major sites of sediment deposition, and potential hydrocarbon reservoir sites. Predicting 3D heterogeneity within these sedimentary bodies is important. However, their internal architecture is extremely poorly documented. Here, we present the 3D architecture of two mouth bars, which are preserved and exceptionally well exposed in outcrops of the Pennsylvanian-aged Pikeville and Hyden formations, in eastern Kentucky. The architecture permits us to reconstruct the suite of subaqueous sedimentary transport and deposition processes that lead to the deposition of these bodies of sediment. As the mouth bars deposit and make shallower the receiving standing body of water, a predictable change in these sedimentary processes occurs, from more dominated by the gravitational inertia of the flow, to more dominated by friction roughness of the basin floor.

Holocene volcanism of the upper McKenzie River catchment, central Oregon Cascades, USA
N.I. Deligne et al., Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403, USA. This article is online at

Imagine: Unusual activity suggests a new mafic volcano might soon form in the central Oregon Cascades. What might happen? In Holocene volcanism of the upper McKenzie River catchment, central Oregon Cascades, USA, Deligne et al. examine a ~3000 year old cluster of mafic volcanism in the central Oregon Cascades to further understanding of this type of activity regionally and globally. Detailed mapping and geochemical analysis revealed the cluster comprised of 13 different eruptive units, tapping at least two, likely three magma sources. The cluster produced 23 vents and a lava field covering 76 km2. Using a variety of dating techniques, Deligne et al. found the cluster erupted in a few decades, rather than millennia as previously thought. The multiple magma sources in a small area was a surprise, as was the concentrated period of activity. As a result, we now better understand the possibilities of mafic volcanism in the region.

Late Ordovician adakitic rocks in the Central Tianshan block, NW China: Partial melting of lower continental arc crust during back-arc basin opening
X. Zhang et al., Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China. This article is online at

Late Ordovician (ca. 451 Ma) calc-alkaline adakitic granodiorite and granite, associated with coeval S-type granites, were discovered in the Central Tianshan block, NW China. Geochemical features and 87Sr/86Sr ratios suggest that these adakitic rocks were derived from the partial melting of heterogeneous basaltic materials at different crustal levels, which resulted from the incipient opening of the South Tianshan back-arc basin due to the roll-back of the Central Tianshan continental arc along northern Tarim during Ordovician rime.

Long-term Late Cretaceous oxygen- and carbon-isotope trends and planktonic foraminiferal turnover: A new record from the southern midlatitudes
F. Falzoni et al., Francesca Falzoni, Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano, via Mangiagalli 34, 20133 Milano, Italy. This article is online at

Foraminifera are marine protozoan secreting a shell that is usually calcitic (calcium carbonate). These organisms represent a powerful tool for paleontologists, because their shells register precious environmental information (e.g., the temperature of calcification) that can be now inferred through geochemical analyses. In this study, we used foraminifera from a southern mid-latitude site (Exmouth Plateau, Australia) to trace the first complete record of Late Cretaceous greenhouse climate evolution, which is of particular interest due to its potential similarities with predicted conditions for the Earth’s future. Our study demonstrates that (1) the mid-Cretaceous climate optimum was at least five-million-years longer than previously estimated, (2) the supergreenhouse phase was followed by a prolonged cooling and by a sequence of shorter term cooling and warming episodes until the Cretaceous/Paleogene Boundary, and that (3) a global climate change combined with species competition controlled the extinction and speciation of planktonic foraminiferal lineages in the Late Cretaceous.

Highstand shelf fans: The role of buoyancy reversal in the deposition of a new type of shelf sandbody
E. Steel et al., Department of Earth Science, University of California, Santa Barbara, California 93106, USA. This article is online at

Conventionally, high sea-level is thought to confine sandy sediment to the coastline and hinder transport of sediment across the continental shelf to deeper water. This study challenges conventional models by providing evidence of highstand shelf fans on the continental shelf of the Santa Barbara Channel in Southern California. Over the past 10,000 years, extreme flooding in small rivers draining the Transverse Ranges initiated dense underflows known as turbidity currents that transported sediment across the sea-floor. These turbidity currents underwent a process referred to as buoyancy reversal, which produces deposits with unique characteristics that can help to explain a number of enigmatic deposits that do not fit typical models for turbidity current deposition. Turbidity currents that undergo buoyancy reversal provide a mechanism through which high quality reservoir sands can form in continental shelf environments, which are typically thought to be sediment-starved during high sea level.

The catastrophic landslide of medieval Portonovo (Ancona, Italy)
A. Montanari et al., Osservatorio Geologico di Coldigioco, Cda. Coldigioco 4, 62021 Apiro, Italy. This article is online at

This paper is a case study about a huge landslide involving 5 million cubic meters of rock, which collapsed from the northeastern flank of the Monte Conero anticline crashing over the littoral of the Portonovo bay, about 9 km south from the Adriatic port city of Ancona (Italy). Detailed geologic mapping, geomorphologic and stratigraphic analysis, and radiocarbon dating, along with reinterpretation of historical documentations, lead to the conclusion that the landslide occurred probably in the early 14th century, contrary to common believe that it occurred in prehistory. The same structural-rheologic and lithologic settings, which predisposed the collapse, are presently found on the slopes adjacent to the western side of the medieval detachment scar, thus representing a serious threat for the stability of the area and a high risk for imminent large landsliding, which may have grave consequences on present-day infrastructures and inhabitants of the popular Portonovo beach resort.

Late Jurassic to Early Cretaceous plutonism in the Colombian Andes: A record of long-term arc maturity
C. Bustamante et al., Instituto de Geociências, Universidade de São Paulo, Rua do Lago 562, CEP 05508-080, São Paulo, SP, Brazil. This article is online at

The entire Andean range is mainly composed by granitic rocks formed during the Jurassic period. This kind of rocks also characterizes the western margin of North American. We proposed an alternative model for the origin of the Jurassic granitic rocks of the Northern Andes (Colombia and Ecuador). This geologic event occurred during almost 70 million years, and built most of the actual continental crust of the western margin of the Americas.

Middle to late Eocene exhumation of the Greater Himalayan Sequence in the Central Himalayas: Progressive accretion from the Indian plate
R. Carosi et al., Dipartimento di Scienze della Terra, Università di Torino, 10125 Torino, Italy; This article is online at

Occurrence of tectonic and metamorphic discontinuities within the metamorphic core of the Himalayan belt at different structural level/position; oldest exhumation detected in the uppermost part of the metamorphic core by a top-to-the SW ductile shearing; hermobarometry and pseudosection modelling allowed to reconstruct the P-T path of the rocks involved in the Kalopani shear zone; U-Th-Pb in situ monazite geochronology constrains the activity of the ductile shear zone at 28-41 Ma. The inferred P-T-d-t path support a tectonic framework pointing to an in-sequence shearing by accretion of crustal slices from the Indian plate.

Testing the limits of chronostratigraphic resolution in the Appalachian Basin, Late Devonian (Middle Frasnian), eastern North America: New U-Pb zircon dates for the Belpre Tephra Suite
A. Lanik et al., Department of Geoscience, University of Calgary, Calgary, Alberta T2N 1N4, Canada; This paper is online at

Ancient volcanic ash beds (tephra) in Devonian strata contain zircon crystals that give unprecedented precise radioisotope dates -- within plus or minus 120,000 years -- for this time interval. These dates tied in with global fossil relative dating that utilizes conodonts from the same strata, accurate to approximately ± 250,000 years, test the limits of time resolution. The time framework allows measurement, refinement, and comparison of depositional rates, evolution, and occurrence of global events. The Belpre Tephra suite in Tennessee consists of eight distinct volcanic beds; two corresponding beds have been found in New York State. The discrepancy in the radioisotope dates -- they are not in chronometric order relative to the fossil biostratigraphy -- is accounted for by being within the analytical range of the measurements and/or the diachronous nature of fossil preservation and first occurrences.

Linking Silicate Weathering to Riverine Geochemistry—a Case Study from a Mountainous Tropical Setting in West Central Panama
R.S. Harmon et al., Department of Marine, Earth Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina 27695, USA. This article is online at

From the abstract: Chemical analyses from 71 watersheds across an ∼450 km transect in west-central Panama provide insight into controls on weathering and rates of chemical denudation and CO2 consumption across an igneous arc terrain in the tropics.

Large igneous province or long-lived magmatic arc along the eastern margin of Australia during the Cretaceous? Insights from the sedimentary record
R.T. Tucker et al., Department of Earth and Oceans, James Cook University, Townsville, Qld 4811, Australia. This article is online at

From the abstract: An extensive Cretaceous volcanic terrain, now limited to the Whitsunday Igneous Association, was once located along the northeastern margin of Australia. Results from this study support the hypothesis that the Whitsunday igneous association was the main source of Cretaceous sediment to the Eromanga Basin, and likely for sediment transported across the continent southward and into the Ceduna Delta system offshore South Australia.

Active faulting on the Wallula fault zone within the Olympic-Wallowa Lineament, Washington State, USA
B.L. Sherrod et al., U.S. Geological Survey at ESS, University of Washington, Box 351310, Seattle, WA 98195, USA. This article is online at

From the abstract: The Wallula fault zone is an integral feature of the Olympic-Wallowa lineament, an ∼500-km-long topographic lineament oblique to the Cascadia plate boundary, extending from Vancouver Island, British Columbia, to Walla Walla, Washington. The structure and past earthquake activity of the Wallula fault zone are important because of nearby infrastructure, and also because the fault zone defines part of the Olympic-Wallowa lineament in south-central Washington and suggests that the Olympic-Wallowa lineament may have a structural origin.

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16 July 2016
GSA Release No. 16-41

Kea Giles Managing Editor,
GSA Communications

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Eppes et al Fig 1
Figure 1 from Eppes et al. Modified from Warren et al. (2013). Click on the image for a larger version