image: Figure 5 from Hansen is a cross-sectional illustrations of Venus and Earth showing endogenic and exogenic geodynamic processes through time. view more
Credit: V.L. Hansen and Lithosphere
Boulder, Colo., USA - Earth was a completely different planet more than 2.5 billion years ago. Little is known about this critical time when cratonic continental seeds formed; life emerged; and precious mineral resources concentrated. Our knowledge is limited because plate tectonic processes destroyed most of this early record. In contrast, Earth's sister, Venus -- similar in size, density, bulk composition, and distance from the Sun -- never developed plate tectonics.
Venus also lacks a water cycle. Like siblings, Venus and Earth were most similar in their youth; however, Venus preserves a more complete geological record of its infancy, including both exogenic and endogenic features. Applying clues from Venus, Vicky L. Hansen proposes a new hypothesis for the formation of Earth's cratons. Large bolides pierced early thin lithosphere causing massive partial melting in the ductile mantle; melt escaped upward, forming cratonic crust; meanwhile strong, dry, buoyant melt residue formed cratonic roots, serving as unique buoyant life preservers during future plate-tectonic recycling.
FEATURED ARTICLE
Impact origin of Archean cratons
V.L. Hansen, Department of Earth and Environmental Sciences, University of Minnesota Duluth, 1114 Kirby Drive, Duluth, Minnesota 55218, USA. Published online ahead of print on 24 Aug. 2015; http://dx.doi.org/10.1130/L371.1.
Other articles published online ahead of print in Lithosphere during July and August 2015 are highlighted below:
Orocline formation at the core of Pangea: A structural study of the Cantabrian Orocline, NW Iberian Massif
J. Shaw et al., School of Earth and Ocean Sciences, University of Victoria, P.O. Box 1700 STN CSC, Victoria, BC, Canada, V8W 2Y2, Canada. Published online ahead of print on 24 Aug. 2015; http://dx.doi.org/10.1130/L461.1.
What processes were involved in the construction of Earth's most recent and renowned supercontinent? While most earth scientists agree that the 350-million-year-old Variscan mountain range of Western Europe was constructed through continental collisions that formed Pangea, new evidence to the contrary is presented in this Lithosphere paper by Ph.D. candidate Jessica Shaw and her colleagues at the universities of Victoria, British Columbia, and Salamanca, Spain. Across Spain and Portugal, the ancient Variscan mountain range bends back on itself in a tight S-shape. By analyzing hundreds of small folds within rocks of NW Spain, Shaw and her colleagues provide evidence suggesting that the mountain range was once linear, shortening lengthwise into its modern S-shape after its initial construction. These findings imply that the Variscan must predate Pangea, and that the true record of supercontinent formation is not construction of the mountain range, but its subsequent map-view deformation.
Birth of the northern Cordilleran orogen, as recorded by detrital zircons in Jurassic synorogenic strata and regional exhumation in Yukon
Maurice Colpron, Yukon Geological Survey, P.O. Box 2703 (K-14), Whitehorse, Yukon, Y1A 2C6, Canada. Published online ahead of print on 22 July 2015; http://dx.doi.org/10.1130/L451.1.
Sedimentary strata in the Jurassic Whitehorse trough were deposited during the early stages of terrane accretion and mountain building that led to development of the modern Canadian Cordillera. New detrital zircon dates and a review of regional data suggest that subsidence in Whitehorse trough was accompanied by significant uplift of metamorphic rocks surrounding the trough, and was followed by first indications of compression in the Canadian Cordillera, and onset of deposition in the foreland basin to the east. It is suggested that "collision" of island-arc terranes with western North America first occurred in the northern Canadian Cordillera and propagated southward between Early and Middle Jurassic.
Spatial and temporal variation in penetrative strain during compression: Insights from analog models
C.M. Burberry, Dept. of Earth and Atmospheric Sciences, University of Nebraska, Lincoln, Nebraska, USA. Published online ahead of print on 24 Aug. 2015; http://dx.doi.org/10.1130/L454.1.
Short-term observations of tectonic plate movement have often failed to match up with geologically based estimates of how much and how quickly plates contract, or shorten, when they collide. This study by Caroline Burberry shows that tracking relatively small deformations in Earth's crust -- deformations collectively known as penetrative strain -- can help account for this "missing" shortening. The author simulated different amounts of shortening across eight sandbox models, which reasonably estimated the magnitude, timing, and distribution of a penetrative strain when compared with patterns observed along a well-known deformation belt in Wyoming. By characterizing this strain behavior during various stages of deformation, the study could help reconcile longstanding discrepancies in geological measurement to more accurately predict total plate contraction across mountainous regions and other landforms produced by tectonic collisions.
Foreland-directed propagation of high-grade tectonism in the deep roots of a Paleoproterozoic collisional orogen, SW Montana, USA
C.B. Condit et al., Dept. of Geological Sciences, University of Colorado Boulder, 2200 Colorado Avenue, Boulder, Colorado 80309, USA. Published online ahead of print on 24 Aug. 2015; http://dx.doi.org/10.1130/L460.1.
The study of eroded ancient mountain belts allows geoscientists to understand the processes that may be occurring in the deep crust in currently deforming areas. In this study, C.B. Condit and colleagues observe a space-time pattern of high-grade metamorphism that represents propagation and growth of the core of this ancient mountain belt in SW Montana. This pattern can be applied to and observed in other active and ancient mountain belts.
The case for a temporally and spatially expanded Mazatzal orogeny
E.M. Duebendorfer, School of Earth Sciences and Environmental Sustainability, Box 4099, Northern Arizona University, Flagstaff, Arizona 86011, USA. Published online ahead of print on 24 Aug. 2015; http://dx.doi.org/10.1130/L412.1.
The Mazatzal orogeny, a tectonic event associated with a major period of crustal growth in southwestern Laurentia between 1780 and 1630 Ma (million years ago), has generally been considered to have terminated by 1630 Ma. The effects of this orogeny have traditionally been confined to Arizona, New Mexico, and southern Colorado. In this paper, E.M. Duebendorfer and colleagues document evidence for a continuum of deformation, magmatism, metamorphism, and new mineral growth from 1.65 to 1.58 billion years ago from southern Wyoming to Sonora, Mexico. Their research suggests that the Mazatzal orogeny may have extended in time to ca. 1580 Ma and in space to the Cheyenne belt, southern Wyoming.
Quaternary deformation in SE Sicily: Insights into the life and cycles of forebulge fault systems
D. Di Martire et al., Dept. of Earth Sciences, Environment and Geo-resources, University of Naples Federico II, 80138 Naples, Italy. Published online ahead of print on 6 July 2015; http://dx.doi.org/10.1130/L453.1.
Integrated geological, geomorphological, and DInSAR data have been used to constrain the timing and modes of activity of Quaternary fault systems in the Hyblean Plateau. This area grew since Middle Miocene times as a doubly plunging forebulge associated with slab rollback during NW directed subduction. Syn-rift sediments and post-rift marine terraces allowed geoscientists to define the timing of activity of an Early Pleistocene flexure-related fault system, thus constraining the duration of a typical foreland extensional tectonic event to about 1.5 million years ago. Subsequent deformation was dominated by strike-slip faulting associated with NW oriented horizontal compression. During this latest stage, regional uplift was accompanied by differential uplift accommodated by dip-slip components of motion along active NNW trending faults. Active tectonics and regional uplift can be explained within the framework of intra-plate shortening and foreland rebound following complete slab detachment, a major event occurred at around 0.7 million years ago in southern Italy.
A Jurassic Oceanic Core Complex in the High-P Monviso Ophiolite (Western Alps, NW Italy)
A. Festa et al., Dipartimento di Scienze della Terra, Università di Torino, Via Valperga Caluso, 35, 10125 Torino, Italy. Published online ahead of print on 24 Aug. 2015; http://dx.doi.org/10.1130/L458.1.
This paper by A. Festa and colleagues documents, for the first time, the occurrence in the eclogitized units of the Western Alps in Italy of an ancient Oceanic Core Complex (OCC), comparable with those described from the modern Mid-Atlantic Ridge. The eclogite-facies Monviso (MO) ophiolite displays, in fact, a well preserved record of extensional detachment faulting that exhumed the lithospheric mantle and produced an OCC during the initial stages of the development of the Jurassic Ligurian-Piedmont Ocean. This OCC subsequently experienced a strong overprint of subduction zone and continental collision tectonics. The multiply deformed MO indicates that incomplete and highly deformed meta-ophiolitic successions that are juxtaposed across major shear zones in high-P belts do not always represent the product of a subduction channel, even though that is where and how they might have come up after going down during ocean closure.
Influence of high strain rate deformation on 40Ar/39Ar mica ages from marble mylonites (Syros, Greece)
A. Rogowitz et al., Dept. of Geodynamics and Sedimentology, University of Vienna, Althanstrasse 14, Vienna, A-1090, Austria. Published online ahead of print on 6 July 2015; http://dx.doi.org/10.1130/L455.1.
From the abstract: Interpreting isotopic ages as deformation ages when they are acquired from moderate-temperature metamorphic environments can be a challenging task. Syros Island (Cyclades, Greece) is famous for Eocene high-pressure metamorphic rocks reworked by localized Miocene greenschist-facies deformation. In this work, A. Rogowitz and colleagues investigate phengites from coarse-grained marbles, which experienced the high-pressure event, and phengites from fine-grained localized marble shear zones attributed to the low-grade Miocene deformation. Based on structural criteria, both events can be easily discriminated because of their opposing kinematics.
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Lithosphere