June 2010 Lithosphere highlights

Boulder, CO, USA – Lithosphere presents international science focusing on tectonic processes at all scales. Highlights include work in New Zealand that may facilitate comparison of short-term plate movement using GPS data with medium-term geologic rates; new field data from the Puna Plateau, Argentina; modeling studies of the southern Mexico lithosphere; how measuring channel steepness, erosion, and uplift rates can lead to better understanding of earthquake risk in southern Italy; and a study out of Turkey on pull-apart basins.

View abstracts for the complete issue of Lithosphere at http://lithosphere.gsapubs.org/current.dtl.

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Revised slip rates for the Alpine fault at Inchbonnie: Implications for plate boundary kinematics of South Island, New Zealand

R.M. Langridge et al., GNS Science, Earthquake Geology, P.O. Box 30-368, Lower Hutt, Wellington 5300, New Zealand. Pages 139-152.

The Alpine fault, which accommodates both right-lateral and reverse slip along the Australia-Pacific plate boundary, transitions into the Marlborough Fault System in the central South Island, New Zealand. In this paper, a team of scientists from New Zealand and Spain present new slip rate estimates for the Alpine fault near this transition at Inchbonnie. Progressive avulsion and abandonment of the Taramakau River floodplain aided by fault movements during the late Holocene have preserved a multi-event fault scarp that grows in height to the northeast of the river. Surveyed dextral and vertical displacements at a left stepover in the fault are combined with precise radiocarbon soil ages, yielding dextral, vertical, and reverse-slip rates of ~13.6, ~3, and ~3.4 mm/yr, respectively. These values are larger (dextral) and smaller (dip-slip) than previous estimates for this site, but reflect advances in the chronology of surfaces of up to ~1.7 k.y. These new rates have important implications for understanding the partitioning of strain across transpressional plate margins and for the comparison of short term strain from Global Positioning System data with medium term geologic rates. A local kinematic circuit of fault slip rates around the transition in central South Island shows that ~50% of the motion from the Central segment of the Alpine fault is transferred to the Hope and Kakapo faults at the southwestern edge of the Marlborough Fault System.

Pliocene intraplate-type volcanism in the Andean foreland at 26°10’S, 64°40’W (NW Argentina): Implications for magmatic and structural evolution of the Central Andes

A. Gioncada et al., University of Pisa, Dipartimento di Scienze della Terra, via S.Maria, 53, Pisa 56126, Italy. Pages 153-171.

The Central Andes represent a continental margin active since Jurassic time. The long-lived subduction of the Pacific plate under the South America continent led to a complex magmatic and tectonic evolution and to substantial modifications of the lithospheric mantle and crust, which in this sector reaches the maximum thickness in the globe (50-70 km). The Miocene to present magmatic arc forms a continuous N-S-striking belt of calcalkaline volcanoes and calderas representing the Western Cordillera. Behind the arc, between 24° and 27°S, magmatic belts developed eastward, for about 300 km from the active arc, across the 4000-m-high Puna plateau. The ascent of magmas along these belts is linked to extensive NW-SE-trending strike-slip fault systems. Recently, the studies of the backarc lithospheric magmas have suggested the existence of different chemical domains in the subcontinental lithospheric mantle. Most reconstructions, based on geophysical and petrological data, refer to a lithosphere piecemeal delamination process leading to thinning of lithospheric mantle with the consequent upraise and melting of asthenosphere, which induced partial melting of lower and upper crust and lithosphere itself. This study by an interdisciplinary team from Italy, Argentina, and France focuses on the Pliocene Antilla magmatic complex (26°10’S and 64°40’W; Salta Province, NW Argentina), placed at about 650 km from the Pacific trench, in a key location at the boundary among the morphotectonic units of the Eastern Cordillera, Sierras Pampeanas, and Santa Barbara System and at the intersection of the NW-trending Archibarca fault zone with the NE-trending Tucumán lineament. This work presents a new geologic field survey, K/Ar chronology, and geochemical, petrological, and isotopic data on the Antilla rocks, and discusses the petrogenesis and tectonic setting of this volcanism in the regional geologic context, by comparing these rocks with those of Miocene-Pliocene age emplaced in the southern Puna plateau. The new data, focused on a small volcanic center far from the active arc, are of great importance for characterizing the lithospheric mantle in the backarc at 26°S and contributing to define the limits, in space and time, of lithospheric delamination, one of the most important geological processes that controlled the evolution of the Central Andes in the past 15 Ma.

Analogue model of inversion tectonics explaining the structural diversity of Late Cretaceous shortening in southwestern Mexico

M. Cerca et al., Ivestigador Asociado C, Universidad Nacional Autonoma de Mexico, Centro de Geociencias, Blvd Juriquilla 3001, Juriquilla, Queretaro, Queretaro, 6230, Mexico. Pages 172-187.

The Laramide fold-and-thrust belt in southern Mexico is characterized by N-S-trending structures in its central and eastern part and by NW-SE-trending structures in its western part. This team of scientists from Mexico and Italy, headed by Mariano Cerca of the Universidad Nacional Autonoma de Mexico (UNAM), investigate, experimentally, the possibility that the Laramide structures of southern Mexico may be the result of inversion of previously thinned lithosphere zones under oblique compression. A revision of the geology of this region shows that the presence of two extensional basins, representing relatively weak blocks within more rigid lithosphere, strongly controlled the subsequent deformation pattern. For modeling purposes, they divided the southern Mexico lithosphere into blocks with different strength profiles: (1) a stable craton; (2) a weak block composed of the Guerrero Morelos Platform; (3) a relatively strong block exposing the pre-Cretaceous Tejupilco schist and the Early Cretaceous Teloloapan volcanic arc (Tejupilco anticlinorium); and (4) a weak block represented by the Arcelia-Palmar Chico basin. A series of physical experiments simulating the mechanical response of an analogue lithosphere composed of five simplified strength profiles was constructed. The model lithosphere was thinned orthogonally and shortened obliquely. Shortening was accommodated mainly by reactivation of preexisting extensional structures. The resulting orogenic deformation in the models is not entirely sequential and foreland-progressive. Inversion tectonics of extensional basins is thus proposed as an explanation for the structural diversity observed in Late Cretaceous shortening of southwestern Mexico. The predictions of their lithospheric model may be tested when more geophysical information about the structure of the southern Mexico lithosphere becomes available.

Quantifying rock uplift rates using channel steepness and cosmogenic nuclide-determined erosion rates; examples from northern and southern Italy

A.J. Cyr et al., Dept. of Earth and Atmospheric Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, Indiana 47907, USA. Pages 188-198.

Knowing the spatial variation of rock uplift (the change in elevation above sea level) across a landscape can help in the identification of active faults, which improves scientists’ ability to assess earthquake hazard, as well as improve their understanding of the dynamics of active mountain belts. However, determining uplift rates over the recurrence time scales of large earthquakes (hundreds to thousands of years) can be a difficult task because there are few tools that can be used to precisely and accurately measure them over similar time scales. One geologic system that changes over a similar time scale as the largest earthquakes (hundreds to thousands of years) is the hillslope and river channel system. Generally, when uplift rates are high, river channels and hillslopes get steeper and the rates at which rivers cut through the landscape and hillslopes erode increases. In this study, scientists from the U.S. and Italy test this by measuring river channel steepness, determined from digital topographic data, and hillslope erosion rates, averaged over a few hundreds to a few thousands of years, and comparing those to well-known uplift rates in northern and southern Italy. In all cases, measurements of river channel steepness and hillslope erosion rates change proportionally with uplift rates, demonstrating the possibility of using channel steepness and erosion rates to measure uplift rates in landscapes where it might not be possible to quantify uplift rates by other means. This project was supported by National Science Foundation Continental Dynamics Program grant EAR-0208169.

Geometric characteristics of pull-apart basins

A. Gurbuz, Ankara Universitesi, Muhendislik Fakultesi, Jeoloji Muhendisligi Bolumu, Ankara Universitesi, Muhendislik Fakultesi, Tandogan-Ankara, Ankara 06100, Turkey. Pages 199-206.

Sedimentary basins are areas with a long history of subsidence and within which a thick sequence of sediments record the evolution of Earth’s crust. Although they are the dustbins of continental weathering, erosion, and transport, they are the most important reservoirs for economic resources. Depending on the type of potential resource (oil, gas, ore deposits, or groundwater), sedimentary thickness is the most important parameter in understanding the attainability of the resource. Pull-apart basins are a type of sedimentary basin that is controlled by strike-slip faults. It is known that pull-apart basins are associated with geometrical irregularities of these faults. They have been studied for many decades, and have become increasingly well understood because of these natural, experimental, and numerical studies, but many uncertainties complicate the interpretations of their geometrical aspects, which makes it difficult to predict the sedimentary thickness of a basin in the feasibility stage of a project. In this comparative study, researcher Alper Gurbuz from the University of Ankara in Turkey has defined a quantitative relationship between the three dimensions of pull-apart basins. This study suggests that the sedimentary thickness (depth) of a pull-apart basin is related to the length and width parameters of the basin. The length and width of a pull-apart basin can be easily measured from any physiographic map or image, and these can be used to calculate the depth data.


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