Journal of Geophysical Research: Solid Earth, 1987
In the mid‐late Tertiary (27–6 m.y. ago), volcanism diminished, then ceased in the main Andean Co... more In the mid‐late Tertiary (27–6 m.y. ago), volcanism diminished, then ceased in the main Andean Cordillera in Chile between 29° and 30°30′S. Over the same time span the subduction angle shallowed under this region, which today lies in the modern shallowly dipping, nonvolcanic segment of the south central Andes (28°–33°S). The trace and major element chemistry and mineralogy of the volcanic rocks in this region record the detailed pattern of lithospheric cooling and crustal thickening that accompany this major tectonic transition. Volcanic changes include, in addition to a decreased rate of volcanism, a decrease in the range of compositions from basaltic andesites to rhyolites in the oldest rocks to only rhyolite in the youngest rocks, a change in source and fractionating mineralogy with an increased role for amphibole and garnet in the younger rocks, and an increase in crustal assimilation. These volcanic changes also correlate in time with deformational events that help to account f...
Adakites in the southern and central Andes show a residual garnet signature that can variously be... more Adakites in the southern and central Andes show a residual garnet signature that can variously be related to local slab melting associated with subduction of hot oceanic crust at the Chile Triple Junction, widespread interaction of mafic magmas in regions of thickened crust, and episodic melting of crust removed by forearc subduction erosion, particularly at times of frontal arc migration.
Journal of Geophysical Research: Solid Earth, 2014
The southern Puna Plateau has been proposed to result from a major Pliocene delamination event th... more The southern Puna Plateau has been proposed to result from a major Pliocene delamination event that has previously been inferred from geochemical, geological, and some preliminary geophysical data. Seventy‐five seismic stations were deployed across the southern Puna Plateau in 2007–2009 by scientists from the U.S., Germany, Chile, and Argentina to test the delamination model for the region. The Puna passive seismic stations were located between 25 and 28°S. Using the seismic waveform data collected from the PUNA experiment, we employ attenuation tomography methods to resolve both compressional and shear quality factors (Qp and Qs, respectively) in the crust and uppermost mantle. The images clearly show a high‐Q Nazca slab subducting eastward beneath the Puna plateau and another high‐Q block with a westward dip beneath the Eastern Cordillera. We suggest that the latter is a piece of delaminated South American lithosphere. A significant low‐Q zone lies between the Nazca slab and the S...
Page 1. La Franja de Maricunga: s??ntesis de la evoluci??n del Frente Volc??nico Oligoceno-Miocen... more Page 1. La Franja de Maricunga: s??ntesis de la evoluci??n del Frente Volc??nico Oligoceno-Mioceno de la zona sur Constantino Mpodozis Paula Cornejo Suzanne M. Kay Andrew Tittler de los Andes Centrales Servicio Nacional ...
Fifty years ago (1965) at the dawn of the plate tectonic revolution, the biggest geological news ... more Fifty years ago (1965) at the dawn of the plate tectonic revolution, the biggest geological news was coming from the oceanic crust, which had been long neglected in favor of the more accessible continental crust. the geosynclinals paradigm (e.g. Hans Stille, Marshall Kay) no longer ruled, continental drift was no longer problematic. Corollary to embracing the new plate tectonic rules was the realization that continental crust formed at plate margins, and serves as a repository of plate tectonic history. Within the decade, continental geology surged- John Dewey and Jack Bird's classic mountain building paper on the Northeastern Appalachians came out in 1070. For northeastern geology, two integrative conferences stand out, in particular, because assembled major players of the plate tectonic revolution, but also due to the influential conference volumes that followed. In November 1966, NASA's Goddard Institute hosted the conference entitled "The History of Earth's Crus...
ABSTRACT The central Andes is the type locality for magmas erupted through thickened crust in a c... more ABSTRACT The central Andes is the type locality for magmas erupted through thickened crust in a continental margin arc. A compilation of some 600 chemical analyses shows that 25-0 Ma andesites erupted in the southern Central Volcanic Zone region (25.5- 28.2°S) north of the Chilean flatslab exhibit among the greatest range of trace element and isotopic ratios in continental arcs. This range reflects their evolution in a region with a changing subducting slab geometry, a crust thickening to 65-75 km, an arc and backarc rising to elevations of 5000-6800 m, a frontal arc that migrated ~45 km eastward at 8-3 Ma, and volcanism that terminated to the south. To a first order, all of these lavas evolve towards steeper REE patterns and more enriched chemical signatures reflecting interaction of mantle melts in a thickening crust in line with Pb isotopic ratios, which largely correlate with crustal domains that last equilibrated in Ordovician and late Paleozoic magmatic events. A trend to lower Ba/La ratios might partially reflect a terriginous sediment-free trench by 10 Ma. In more detail, the arc lavas north of the flat-slab at ~28.2-26.8°S show a distinct evolution from those at ~26.8-25.5°S in parallel with tomographic images that show a highly attenuating crust and upper mantle interpreted to reflect partial melting under the backarc in the north and diminished attenuation in the backarc mantle and crust to the south. Andesites at 28.2- 26.8°S show the most variable and extreme heavy REE patterns (Sm/Yb =2-9), Na2O (3-5.5%), HFSE depletion (La/Ta = 15-110) and Ba/La ratios (15-55) with some of the world's highest values occurring in 9-3 Ma lavas erupted as the frontal arc migrated and the slab shallowed to the south. The high Sm/Yb ratios and wt % Na2O are consistent with generation of a garnet-bearing, feldspar-free residue in the thick underlying crust as well as in a mantle wedge into which crust was injected in a peak of forearc subduction erosion as the arc migrated. A correlation of high Sm/Yb and La/Ta ratios temporally links heavy REE retaining residual garnet with residual titanite and amphibole retaining Ti and HFSE. In contrast, andesites at 26.8-25.5°S, west of the Puna where the slab is inferred to have shallowed at 18-7 Ma and then steepened after 7 Ma as lithospheric delamination occurred, mostly lack extreme REE and HFSE ratios and high Na2O. These features along with a more upper crustal-like chemistry reflect eruption in a mixed stress regime and incorporation of radiogenic Puna crust, whose ductile flow to the west was facilitated by crustal and mantle partial melting in the aftermath of lithosphere foundering and slab steepening. Thus although forearc subduction erosion likely accompanied frontal arc migration across the region, the variability of the magma chemistry along strike is largely due to the extensive partial crustal and mantle melting that began in the late Miocene in the north and the late Miocene shallowing of the subducting slab that produced a rigid backstop and backarc cooling in the south.
Evolution of an Andean Margin: A Tectonic and Magmatic View from the Andes to the Neuque´n Basin (35°-39°S lat), 2006
ABSTRACT The evolving chemistry of the Chachahuén volcanic complex provides evidence for transien... more ABSTRACT The evolving chemistry of the Chachahuén volcanic complex provides evidence for transient entry of a subduction zone component into the mantle wedge over a late Miocene shallow subduction zone under the Neuquén Basin. The Chachahuén complex, which is in the backarc of the Andean Southern Volcanic Zone near 37°S and some 500 km east of the Chile Trench, occurs at the intersection of NE and SE fault systems that parallel regional trends. Support for a shallow subduction-zone setting at the time of eruption and during the contractional uplift of the Sierra de Chachahuén comes from K/Ar and new 40Ar/39Ar ages, mineral assemblages, major and trace element chemistry, and Nd-Sr-Pb isotopic compositions. Importantly, the chemistry of the Chachahuén rocks requires an arc-like component in the mantle that is absent in both early Miocene or Pliocene alkaline lavas erupted in the same region. The oldest Chachahuén volcanic rocks are the ca. 7.3–6.8 Ma Vizcachas group orthopyroxenebearing andesites to rhyodacites that erupted from fissures and small centers along the NE-trending fault system. Intraplate chemical tendencies in the most silicic samples are attributed to mantle-derived basalts interacting with a lower crust that has a chemical imprint that reflects older alkaline magmatic events. Younger Chachahuén group volcanic rocks erupted at ca. 6.8–6.4 Ma from vents generally aligned along the NE-trending fault system and ca. 6.3–4.9 Ma magmas that erupted from a trap-door– type caldera and flanking stratovolcanoes along the NW-trending fault system. These high-K basaltic to dacitic rocks contain amphibole phenocrysts and show arc-like high field strength element depletions that are the strongest in basaltic andesite lavas. Parallels between Chachahuén volcanic rocks and uplift of the Sierra de Chachahuén with late Miocene Pocho volcanic rocks and uplift of the Pampean Ranges over the modern Chilean flat-slab support transient Miocene shallow subduction zone under the Neuquén Basin.
SEG Technical Program Expanded Abstracts 2011, 2011
ABSTRACT Using exploration seismic data processing software like Vista 10.0 to process earthquake... more ABSTRACT Using exploration seismic data processing software like Vista 10.0 to process earthquake seismic data gives earthquake seismologists the advantage of having well-developed functions such as CMP stacking and migration available together with multiple display options (hundreds of traces per screen or more). After being converted into SEG-Y format, seismic data from large scale, high density arrays like the EarthScope USArray network can be processed in Vista 10.0 and looked at in a new way to detail lithospheric structure using depth phase precursors. To do this, we have extracted recordings of intermediate to deep (150 km to 700 km) earthquakes from subduction zones around the Pacific Rim from the EarthScope open dataset. Using proper phase alignment, filtering and coherency enhancement, the precursors to pP from slab earthquakes beneath South America can be used to image the underside reflections from Moho (pmP), from LAB (plP) and from possible 410 (p410P) discontinuities. The redundancy provided by the EarthScope USArray network provides us with the ability to enhance signal-to-noise ratios by stacking within common reflection point bins (CMP bins in Vista 10.0) and applying multichannel coherency filters. In the Andean case, we gathered the underside reflection points in 2 km by 30 km bins for stacking, resulting in substantial improvement in the resulting image. Synthetic seismograms confirm that the observed arrivals are appropriate for reflections from the Moho and LAB. The observation of well-defined reflections from the underside of the LAB is particularly significant, as this boundary has proven difficult to image by traditional seismic techniques. Comparable results have been obtained from deep earthquakes beneath western Pacific island arcs for the adjacent oceanic lithosphère and interpretation of these reflections needs to be further considered. We suggest that the power of applying exploration seismic software like Vista 10.0 to earthquake data sheds new light on processing dense passive seismic network like EarthScope's USArray.
The recognition of accreted terranes and their importance in orogenesis has spurred the search fo... more The recognition of accreted terranes and their importance in orogenesis has spurred the search for allochthonous fragments along the western and southern margins of South America. Here we present stratigraphic and petrologic data from Chile and Argentina between 29° and 33°S latitude that demonstrate the “suspect” nature of several major terranes, which we infer to have been accreted during the Paleozoic. Three lower‐middle Paleozoic terranes are described (from east to west): (1) the Pampeanas terrane, a Cambrian‐Devonian magmatic and metamorphic province built on late Precambrian basement at the margin of South America, (2) the Precordillera terrane, a Cambrian‐Devonian shelf‐slope‐oceanic basin assemblage bounded by mélanges on both sides and bearing many stratigraphic similarities to the lower‐middle Paleozoic of the Northern Appalachians, and (3) the “Chilenia” terrane, which has largely been obliterated by late Paleozoic magmatism and metamorphism. The distribution of Carbonif...
Late Oligocene-Early Miocene volcanic activity along the magmatic front of the south Central Ande... more Late Oligocene-Early Miocene volcanic activity along the magmatic front of the south Central Andes in Northern Chile was associated with the formation of epithermal gold-silver deposits in the Maricunga mineral belt (28° S to 26° S). Magmatic activity began at approximately ...
Journal of Geophysical Research: Solid Earth, 1987
In the mid‐late Tertiary (27–6 m.y. ago), volcanism diminished, then ceased in the main Andean Co... more In the mid‐late Tertiary (27–6 m.y. ago), volcanism diminished, then ceased in the main Andean Cordillera in Chile between 29° and 30°30′S. Over the same time span the subduction angle shallowed under this region, which today lies in the modern shallowly dipping, nonvolcanic segment of the south central Andes (28°–33°S). The trace and major element chemistry and mineralogy of the volcanic rocks in this region record the detailed pattern of lithospheric cooling and crustal thickening that accompany this major tectonic transition. Volcanic changes include, in addition to a decreased rate of volcanism, a decrease in the range of compositions from basaltic andesites to rhyolites in the oldest rocks to only rhyolite in the youngest rocks, a change in source and fractionating mineralogy with an increased role for amphibole and garnet in the younger rocks, and an increase in crustal assimilation. These volcanic changes also correlate in time with deformational events that help to account f...
Adakites in the southern and central Andes show a residual garnet signature that can variously be... more Adakites in the southern and central Andes show a residual garnet signature that can variously be related to local slab melting associated with subduction of hot oceanic crust at the Chile Triple Junction, widespread interaction of mafic magmas in regions of thickened crust, and episodic melting of crust removed by forearc subduction erosion, particularly at times of frontal arc migration.
Journal of Geophysical Research: Solid Earth, 2014
The southern Puna Plateau has been proposed to result from a major Pliocene delamination event th... more The southern Puna Plateau has been proposed to result from a major Pliocene delamination event that has previously been inferred from geochemical, geological, and some preliminary geophysical data. Seventy‐five seismic stations were deployed across the southern Puna Plateau in 2007–2009 by scientists from the U.S., Germany, Chile, and Argentina to test the delamination model for the region. The Puna passive seismic stations were located between 25 and 28°S. Using the seismic waveform data collected from the PUNA experiment, we employ attenuation tomography methods to resolve both compressional and shear quality factors (Qp and Qs, respectively) in the crust and uppermost mantle. The images clearly show a high‐Q Nazca slab subducting eastward beneath the Puna plateau and another high‐Q block with a westward dip beneath the Eastern Cordillera. We suggest that the latter is a piece of delaminated South American lithosphere. A significant low‐Q zone lies between the Nazca slab and the S...
Page 1. La Franja de Maricunga: s??ntesis de la evoluci??n del Frente Volc??nico Oligoceno-Miocen... more Page 1. La Franja de Maricunga: s??ntesis de la evoluci??n del Frente Volc??nico Oligoceno-Mioceno de la zona sur Constantino Mpodozis Paula Cornejo Suzanne M. Kay Andrew Tittler de los Andes Centrales Servicio Nacional ...
Fifty years ago (1965) at the dawn of the plate tectonic revolution, the biggest geological news ... more Fifty years ago (1965) at the dawn of the plate tectonic revolution, the biggest geological news was coming from the oceanic crust, which had been long neglected in favor of the more accessible continental crust. the geosynclinals paradigm (e.g. Hans Stille, Marshall Kay) no longer ruled, continental drift was no longer problematic. Corollary to embracing the new plate tectonic rules was the realization that continental crust formed at plate margins, and serves as a repository of plate tectonic history. Within the decade, continental geology surged- John Dewey and Jack Bird's classic mountain building paper on the Northeastern Appalachians came out in 1070. For northeastern geology, two integrative conferences stand out, in particular, because assembled major players of the plate tectonic revolution, but also due to the influential conference volumes that followed. In November 1966, NASA's Goddard Institute hosted the conference entitled "The History of Earth's Crus...
ABSTRACT The central Andes is the type locality for magmas erupted through thickened crust in a c... more ABSTRACT The central Andes is the type locality for magmas erupted through thickened crust in a continental margin arc. A compilation of some 600 chemical analyses shows that 25-0 Ma andesites erupted in the southern Central Volcanic Zone region (25.5- 28.2°S) north of the Chilean flatslab exhibit among the greatest range of trace element and isotopic ratios in continental arcs. This range reflects their evolution in a region with a changing subducting slab geometry, a crust thickening to 65-75 km, an arc and backarc rising to elevations of 5000-6800 m, a frontal arc that migrated ~45 km eastward at 8-3 Ma, and volcanism that terminated to the south. To a first order, all of these lavas evolve towards steeper REE patterns and more enriched chemical signatures reflecting interaction of mantle melts in a thickening crust in line with Pb isotopic ratios, which largely correlate with crustal domains that last equilibrated in Ordovician and late Paleozoic magmatic events. A trend to lower Ba/La ratios might partially reflect a terriginous sediment-free trench by 10 Ma. In more detail, the arc lavas north of the flat-slab at ~28.2-26.8°S show a distinct evolution from those at ~26.8-25.5°S in parallel with tomographic images that show a highly attenuating crust and upper mantle interpreted to reflect partial melting under the backarc in the north and diminished attenuation in the backarc mantle and crust to the south. Andesites at 28.2- 26.8°S show the most variable and extreme heavy REE patterns (Sm/Yb =2-9), Na2O (3-5.5%), HFSE depletion (La/Ta = 15-110) and Ba/La ratios (15-55) with some of the world's highest values occurring in 9-3 Ma lavas erupted as the frontal arc migrated and the slab shallowed to the south. The high Sm/Yb ratios and wt % Na2O are consistent with generation of a garnet-bearing, feldspar-free residue in the thick underlying crust as well as in a mantle wedge into which crust was injected in a peak of forearc subduction erosion as the arc migrated. A correlation of high Sm/Yb and La/Ta ratios temporally links heavy REE retaining residual garnet with residual titanite and amphibole retaining Ti and HFSE. In contrast, andesites at 26.8-25.5°S, west of the Puna where the slab is inferred to have shallowed at 18-7 Ma and then steepened after 7 Ma as lithospheric delamination occurred, mostly lack extreme REE and HFSE ratios and high Na2O. These features along with a more upper crustal-like chemistry reflect eruption in a mixed stress regime and incorporation of radiogenic Puna crust, whose ductile flow to the west was facilitated by crustal and mantle partial melting in the aftermath of lithosphere foundering and slab steepening. Thus although forearc subduction erosion likely accompanied frontal arc migration across the region, the variability of the magma chemistry along strike is largely due to the extensive partial crustal and mantle melting that began in the late Miocene in the north and the late Miocene shallowing of the subducting slab that produced a rigid backstop and backarc cooling in the south.
Evolution of an Andean Margin: A Tectonic and Magmatic View from the Andes to the Neuque´n Basin (35°-39°S lat), 2006
ABSTRACT The evolving chemistry of the Chachahuén volcanic complex provides evidence for transien... more ABSTRACT The evolving chemistry of the Chachahuén volcanic complex provides evidence for transient entry of a subduction zone component into the mantle wedge over a late Miocene shallow subduction zone under the Neuquén Basin. The Chachahuén complex, which is in the backarc of the Andean Southern Volcanic Zone near 37°S and some 500 km east of the Chile Trench, occurs at the intersection of NE and SE fault systems that parallel regional trends. Support for a shallow subduction-zone setting at the time of eruption and during the contractional uplift of the Sierra de Chachahuén comes from K/Ar and new 40Ar/39Ar ages, mineral assemblages, major and trace element chemistry, and Nd-Sr-Pb isotopic compositions. Importantly, the chemistry of the Chachahuén rocks requires an arc-like component in the mantle that is absent in both early Miocene or Pliocene alkaline lavas erupted in the same region. The oldest Chachahuén volcanic rocks are the ca. 7.3–6.8 Ma Vizcachas group orthopyroxenebearing andesites to rhyodacites that erupted from fissures and small centers along the NE-trending fault system. Intraplate chemical tendencies in the most silicic samples are attributed to mantle-derived basalts interacting with a lower crust that has a chemical imprint that reflects older alkaline magmatic events. Younger Chachahuén group volcanic rocks erupted at ca. 6.8–6.4 Ma from vents generally aligned along the NE-trending fault system and ca. 6.3–4.9 Ma magmas that erupted from a trap-door– type caldera and flanking stratovolcanoes along the NW-trending fault system. These high-K basaltic to dacitic rocks contain amphibole phenocrysts and show arc-like high field strength element depletions that are the strongest in basaltic andesite lavas. Parallels between Chachahuén volcanic rocks and uplift of the Sierra de Chachahuén with late Miocene Pocho volcanic rocks and uplift of the Pampean Ranges over the modern Chilean flat-slab support transient Miocene shallow subduction zone under the Neuquén Basin.
SEG Technical Program Expanded Abstracts 2011, 2011
ABSTRACT Using exploration seismic data processing software like Vista 10.0 to process earthquake... more ABSTRACT Using exploration seismic data processing software like Vista 10.0 to process earthquake seismic data gives earthquake seismologists the advantage of having well-developed functions such as CMP stacking and migration available together with multiple display options (hundreds of traces per screen or more). After being converted into SEG-Y format, seismic data from large scale, high density arrays like the EarthScope USArray network can be processed in Vista 10.0 and looked at in a new way to detail lithospheric structure using depth phase precursors. To do this, we have extracted recordings of intermediate to deep (150 km to 700 km) earthquakes from subduction zones around the Pacific Rim from the EarthScope open dataset. Using proper phase alignment, filtering and coherency enhancement, the precursors to pP from slab earthquakes beneath South America can be used to image the underside reflections from Moho (pmP), from LAB (plP) and from possible 410 (p410P) discontinuities. The redundancy provided by the EarthScope USArray network provides us with the ability to enhance signal-to-noise ratios by stacking within common reflection point bins (CMP bins in Vista 10.0) and applying multichannel coherency filters. In the Andean case, we gathered the underside reflection points in 2 km by 30 km bins for stacking, resulting in substantial improvement in the resulting image. Synthetic seismograms confirm that the observed arrivals are appropriate for reflections from the Moho and LAB. The observation of well-defined reflections from the underside of the LAB is particularly significant, as this boundary has proven difficult to image by traditional seismic techniques. Comparable results have been obtained from deep earthquakes beneath western Pacific island arcs for the adjacent oceanic lithosphère and interpretation of these reflections needs to be further considered. We suggest that the power of applying exploration seismic software like Vista 10.0 to earthquake data sheds new light on processing dense passive seismic network like EarthScope's USArray.
The recognition of accreted terranes and their importance in orogenesis has spurred the search fo... more The recognition of accreted terranes and their importance in orogenesis has spurred the search for allochthonous fragments along the western and southern margins of South America. Here we present stratigraphic and petrologic data from Chile and Argentina between 29° and 33°S latitude that demonstrate the “suspect” nature of several major terranes, which we infer to have been accreted during the Paleozoic. Three lower‐middle Paleozoic terranes are described (from east to west): (1) the Pampeanas terrane, a Cambrian‐Devonian magmatic and metamorphic province built on late Precambrian basement at the margin of South America, (2) the Precordillera terrane, a Cambrian‐Devonian shelf‐slope‐oceanic basin assemblage bounded by mélanges on both sides and bearing many stratigraphic similarities to the lower‐middle Paleozoic of the Northern Appalachians, and (3) the “Chilenia” terrane, which has largely been obliterated by late Paleozoic magmatism and metamorphism. The distribution of Carbonif...
Late Oligocene-Early Miocene volcanic activity along the magmatic front of the south Central Ande... more Late Oligocene-Early Miocene volcanic activity along the magmatic front of the south Central Andes in Northern Chile was associated with the formation of epithermal gold-silver deposits in the Maricunga mineral belt (28° S to 26° S). Magmatic activity began at approximately ...
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