Flat-slab Subduction Research Articles

40Ar/ 39Ar eruption ages and geochemical characteristics of Late Tertiary to Quaternary intraplate and arc-related lavas in interior Alaska

Late Tertiary to Quaternary basaltic rocks from three locations in interior Alaska — Fort Hamlin Hills, Prindle Volcano, and Buzzard Creek — have been characterized geochemically using major and trace element concentrations and radiogenic isotopic ratios. The rocks from Fort Hamlin Hills and Prindle Volcano have well-constrained 40Ar/ 39Ar eruption ages ranging from 32.83 ± 0.16 Ma for the Fort Hamlin Hills basalts, to 0.20 ± 0.06 Ma for the Prindle Volcano basanites. Ages for the Buzzard Creek basalts have large errors because of the low K 2O concentration and young ages of the lavas, which we estimate to be < 0.01 Ma. On the basis of Sr–Nd–Pb–Hf isotope and rare earth element data, we have determined that the primary melts for the Fort Hamlin Hills and Buzzard Creek basalts were produced in the lithospheric mantle at spinel facies depths. The Prindle Volcano basanites were produced in asthenospheric mantle in garnet facies conditions. Although involvement in the source components of small volumes of enriched materials related to Late Jurassic to Early Cretaceous flat-slab subduction is not ruled out for the Fort Hamlin Hills basalts and Prindle Volcano basanites. These lavas do not exhibit the arc signature typified by depletion in high-field-strength elements. We conclude that the volcanism at the Fort Hamlin Hills and Prindle Volcano localities in interior Alaska represents intraplate activity due to lithospheric extension accompanied by decompression melting and additional heating. Buzzard Creek basalts, on the other hand, exhibit an arc signature characterized by troughs at Ta–Nb, and peaks at Pb and Sr on element distribution diagrams. These basalts occur in a 400 km volcano gap between the Aleutian arc system and Wrangell Volcanic Province.

Tectonic evolution of the Gulf of Mexico, Caribbean and northern South America in the mantle reference frame: an update

Abstract We present an updated synthesis of the widely accepted ‘single-arc Pacific-origin’ and ‘Yucatán-rotation’ models for Caribbean and Gulf of Mexico evolution, respectively. Fourteen palaeogeographic maps through time integrate new concepts and alterations to earlier models. Pre-Aptian maps are presented in a North American reference frame. Aptian and younger maps are presented in an Indo-Atlantic hot spot reference frame which demonstrates the surprising simplicity of Caribbean–American interaction. We use the Müller et al. ( Geology 21 : 275–278, 1993) reference frame because the motions of the Americas are smoothest in this reference frame, and because it does not differ significantly, at least since c. 90 Ma, from more recent ‘moving hot spot’ reference frames. The Caribbean oceanic lithosphere has moved little relative to the hot spots in the Cenozoic, but moved north at c. 50 km/Ma during the Cretaceous, while the American plates have drifted west much further and faster and thus are responsible for most Caribbean–American relative motion history. New or revised features of this model, generally driven by new data sets, include: (1) refined reconstruction of western Pangaea; (2) refined rotational motions of the Yucatán Block during the evolution of the Gulf of Mexico; (3) an origin for the Caribbean Arc that invokes Aptian conversion to a SW-dipping subduction zone of a trans-American plate boundary from Chortís to Ecuador that was part sinistral transform (northern Caribbean) and part pre-existing arc (eastern, southern Caribbean); (4) acknowledgement that the Caribbean basalt plateau may pertain to the palaeo-Galapagos hot spot, the occurrence of which was partly controlled by a Proto-Caribbean slab gap beneath the Caribbean Plate; (5) Campanian initiation of subduction at the Panama–Costa Rica Arc, although a sinistral transform boundary probably pre-dated subduction initiation here; (6) inception of a north-vergent crustal inversion zone along northern South America to account for Cenozoic convergence between the Americas ahead of the Caribbean Plate; (7) a fan-like, asymmetric rift opening model for the Grenada Basin, where the Margarita and Tobago footwall crustal slivers were exhumed from beneath the southeast Aves Ridge hanging wall; (8) an origin for the Early Cretaceous HP/LT metamorphism in the El Tambor units along the Motagua Fault Zone that relates to subduction of Farallon crust along western Mexico (and then translated along the trans-American plate boundary prior to onset of SW-dipping subduction beneath the Caribbean Arc) rather than to collision of Chortis with Southern Mexico; (9) Middle Miocene tectonic escape of Panamanian crustal slivers, followed by Late Miocene and Recent eastward movement of the ‘Panama Block’ that is faster than that of the Caribbean Plate, allowed by the inception of east–west trans-Costa Rica shear zones. The updated model integrates new concepts and global plate motion models in an internally consistent way, and can be used to test and guide more local research across the Gulf of Mexico, the Caribbean and northern South America. Using examples from the regional evolution, the processes of slab break off and flat slab subduction are assessed in relation to plate interactions in the hot spot reference frame.

Accretionary orogens through Earth history

Abstract Accretionary orogens form at intraoceanic and continental margin convergent plate boundaries. They include the supra-subduction zone forearc, magmatic arc and back-arc components. Accretionary orogens can be grouped into retreating and advancing types, based on their kinematic framework and resulting geological character. Retreating orogens (e.g. modern western Pacific) are undergoing long-term extension in response to the site of subduction of the lower plate retreating with respect to the overriding plate and are characterized by back-arc basins. Advancing orogens (e.g. Andes) develop in an environment in which the overriding plate is advancing towards the downgoing plate, resulting in the development of foreland fold and thrust belts and crustal thickening. Cratonization of accretionary orogens occurs during continuing plate convergence and requires transient coupling across the plate boundary with strain concentrated in zones of mechanical and thermal weakening such as the magmatic arc and back-arc region. Potential driving mechanisms for coupling include accretion of buoyant lithosphere (terrane accretion), flat-slab subduction, and rapid absolute upper plate motion overriding the downgoing plate. Accretionary orogens have been active throughout Earth history, extending back until at least 3.2 Ga, and potentially earlier, and provide an important constraint on the initiation of horizontal motion of lithospheric plates on Earth. They have been responsible for major growth of the continental lithosphere through the addition of juvenile magmatic products but are also major sites of consumption and reworking of continental crust through time, through sediment subduction and subduction erosion. It is probable that the rates of crustal growth and destruction are roughly equal, implying that net growth since the Archaean is effectively zero.

Dextral shear, terrane accretion and basin formation in the Northern Andes: best explained by interaction with a Pacific-derived Caribbean Plate?

Abstract The structure, stratigraphy and magmatic history of northern Peru, Ecuador and Colombia are only adequately explained by Pacific-origin models for the Caribbean Plate. Inter-American models for the origin of the Caribbean Plate cannot explain the contrasts between the Northern Andes and the Central Andes. Persistent large magnitude subduction, arc magmatism and compressional deformation typify the Central Andes, while the Northern Andes shows back-arc basin and passive margin formation followed by dextral oblique accretion of oceanic plateau basalt and island arc terranes with Caribbean affinity. Cretaceous separation between the Americas resulted in the development of a NNE-trending dextral–transpressive boundary between the Caribbean and northwestern South America, becoming more compressional when spreading in the Proto-Caribbean Seaway slowed towards the end of the Cretaceous. Dextral transpression started at 120–100 Ma, when the Caribbean Arc formed at the leading edge of the Caribbean Plate as a result of subduction zone polarity reversal at the site of the pre-existing Trans-American Arc, which had linked to Central America to South America in the vicinity of the present-day Peru–Ecuador border. Subsequent closure of the Andean Back-Arc Basin resulted in accretion of Caribbean terranes to western Colombia. Initiation of flat-slab subduction of the Caribbean Plate beneath Colombia at about 100 Ma is associated with limited magmatism, with no subsequent development of a magmatic arc. This was followed by northward-younging Maastrichtian to Eocene collision of the trailing edge Panama Arc. The triple junction where the Panama Arc joined the Peru–Chile trench was located west of present-day Ecuador as late as Eocene time, and the Talara, Tumbes and Manabi pull-apart basins directly relate to its northward migration. Features associated with the subduction of the Nazca Plate, such as active calc-alkaline volcanic arcs built on South American crust, only became established in Ecuador, and then Colombia, as the triple junction migrated to the north. Our model provides a comprehensive, regional and testable framework for analysing the as yet poorly understood collage of arc remnants, basement blocks and basins in the Northern Andes.

Andean flat-slab subduction through time

Abstract The analysis of magmatic distribution, basin formation, tectonic evolution and structural styles of different segments of the Andes shows that most of the Andes have experienced a stage of flat subduction. Evidence is presented here for a wide range of regions throughout the Andes, including the three present flat-slab segments (Pampean, Peruvian, Bucaramanga), three incipient flat-slab segments (‘Carnegie’, Guañacos, ‘Tehuantepec’), three older and no longer active Cenozoic flat-slab segments (Altiplano, Puna, Payenia), and an inferred Palaeozoic flat-slab segment (Early Permian ‘San Rafael’). Based on the present characteristics of the Pampean flat slab, combined with the Peruvian and Bucaramanga segments, a pattern of geological processes can be attributed to slab shallowing and steepening. This pattern permits recognition of other older Cenozoic subhorizontal subduction zones throughout the Andes. Based on crustal thickness, two different settings of slab steepening are proposed. Slab steepening under thick crust leads to delamination, basaltic underplating, lower crustal melting, extension and widespread rhyolitic volcanism, as seen in the caldera formation and huge ignimbritic fields of the Altiplano and Puna segments. On the other hand, when steepening affects thin crust, extension and extensive within-plate basaltic flows reach the surface, forming large volcanic provinces, such as Payenia in the southern Andes. This last case has very limited crustal melt along the axial part of the Andean roots, which shows incipient delamination. Based on these cases, a Palaeozoic flat slab is proposed with its subsequent steepening and widespread rhyolitic volcanism. The geological evolution of the Andes indicates that shallowing and steepening of the subduction zone are thus frequent processes which can be recognized throughout the entire system.

Dynamics of deformation and sedimentation in the northern Sierras Pampeanas: An integrated study of the Neogene Fiambala basin, NW Argentina

The thick-skinned Sierras Pampeanas morphotectonic domain of western and northwestern Argentina (27°S–33°S) is characterized by reverse-fault–bounded basement blocks that delimit internally deformed, Neogene sedimentary basins. Foreland-basin evolution in this part of the Andes is still not very well understood. For example, challenging questions exist as to how thick-skinned deformation develops, if there are distinct spatiotemporal trends in deformation and exhumation, how such deformation styles influence sedimentation patterns, and whether or not broken foreland basins are related to regional plate-tectonic processes, such as flat-slab subduction. The Fiambala basin of the northwestern Sierras Pampeanas is the largest of several intermontane basins in the transition to the southern margin of the Puna Plateau. This basin preserves a thick continental Neogene sequence that provides information on the dynamics of thick-skinned deformation and resulting sedimentation. The Fiambala basin contains ~4 km of fluvial-alluvial sedimentary rocks that comprise the Tamberia, Guanchin, and Punaschotter Formations. U-Pb geochronology of ashes intercalated within the Fiambala stratigraphic sequence demonstrates that these sedimentary rocks are late Miocene to Pliocene (8.2 ± 0.3 Ma to 3.05 ± 0.4 Ma) in age. Sedimentology and provenance data indicate that the source of the Tamberia Formation was located to the west of the modern western basin-bounding range. The Guanchin and Punaschotter Formations record input from local sources, including the modern basin-bounding range to the west and the southern Puna Plateau to the north, suggesting reorganization of the catchment area at ca. 5.5 Ma. The coarsening-upward trends recorded by the fluvial Tamberia and Guanchin Formations indicate enhanced tectonics and relief during sedimentation. The Punaschot-ter conglomerates record alluvial-fan sedimentation and local sources. Fault kinematic data document a contractional regime, characterized by E-W and NE-SW shortening, active throughout the middle-late Miocene and Pliocene. Furthermore, a comparison between the Fiambala basin and similar sedimentary basins in the Sierras Pampeanas (e.g., Bermejo foreland basin) and the Eastern Cordillera leads us to propose that the study area originally constituted an integral part of a continuous and more extensive foreland-basin system (thin-skinned) for much of its early history. Our data suggest coeval intrabasin deformation along strike from the Bermejo region northward to the Eastern Cordillera. The coeval change at ca. 6 Ma from a regional to more compartmentalized (thick-skinned) tectono-sedimentary environment in the regions adjacent to the Eastern Cordillera, the southern Puna margin, and other sectors within the Sierras Pampeanas domain may thus reflect a regional tectonic process related to flat subduction. Our data, combined with existing sedimentological and petrological evidence, imply that the passage from steep to flat subduction occurred synchronously from ~30°S to ~26°S.

Early cretaceous subduction-related adakite-like rocks of the Gangdese Belt, southern Tibet: Products of slab melting and subsequent melt–peridotite interaction?

The limited geochronology and geochemistry data available for the Early Cretaceous igneous rocks of the southern Gangdese Belt, southern Tibet, has resulted in the proposal of conflicting geodynamic models for the generation of the widespread Cretaceous igneous rocks in the middle and northern parts of the belt. To explore this issue, we present SHRIMP U–Pb zircon data and geochemical and Sr–Nd–Pb–Hf isotopic data for the Mamen andesites from the southern margin of the Gangdese Belt. The Mamen andesites, emplaced at 136.5 Ma, are sodic (Na 2O/K 2O = 1.2–2.3) and have geochemical characteristics typical of adakites (i.e., high Al 2O 3, high La/Yb ratios and Sr contents, low Y and HREE contents, and positive Eu anomalies), except for high Cr, Ni, and MgO contents. The andesites have initial ( 87Sr/ 86Sr) t ratios of 0.70413–0.70513, positive εNd( t) values of 3.7–5.8, and ( 206Pb/ 204Pb) t ratios of 18.37–18.51, ( 207Pb/ 204Pb) t ratios of 15.59–15.65, and ( 208Pb/ 204Pb) t ratios of 38.43–38.72. In situ Hf isotopic analyses of zircons that had previously been dated by SHRIMP yielded positive initial εHf( t) values ranging from +11.0 to +15.5. A model calculation using trace element and Sr–Nd–Pb isotopic data indicates that several percent of subducted sediment is required to generate the Mamen andesites, which were derived via the partial melting of subducted Neo-Tethyan slab (MORB + sediment + fluid) and subsequently hybridized by peridotite in the mantle wedge. Our data indicate that the Neo-Tethyan oceanic crust was subducted northward beneath the Gangdese Belt during the Early Cretaceous at a high angle. Our results are inconsistent with a tectonic model that advocates the low-angle or flat-slab subduction of Neo-Tethyan oceanic crust in generating the widespread Cretaceous magmatism recorded in the Gangdese Belt.

Late Cretaceous Gangdese intrusions of adakitic geochemical characteristics, SE Tibet: Petrogenesis and tectonic implications

The Gangdese batholith emplaced from the Cretaceous to Eocene in southern Tibet has been widely regarded as the major constituent of an Andean-type convergent margin resulting from northward subduction of the Neo-Tethyan oceanic lithosphere under Asia. While the Gangdese batholith consists predominantly of calc-alkaline rocks, we identify from the eastern part of the batholith a suite of epidote-bearing granodiorites that shows adakitic geochemical characteristics, marked with apparently higher La/Yb and Sr/Y, and lower Y and HREE, than other Gangdese rocks and common arc magmas. SHRIMP zircon U–Pb analyses of two of the samples yielded 206Pb/ 238U dates of 80.4 ± 1.1 and 82.7 ± 1.6 (2 σ) Ma, which constrain the emplacement ages of the adakitic rocks. Trace element modeling suggests that these rocks originated from partial melting of a garnet amphibolite source that, on the basis of the Sr and Nd isotope data [I Sr = 0.7044–0.7048; ε Nd( T) = + 3.2 to + 0.9], we interpret to be a newly underplated, mafic lower crust, rather than the subducted Neo-Tethyan oceanic crust. This juvenile crust was produced by Cretaceous basaltic underplating above the mantle wedge and then thickened by the tectonic contraction owing to flattening of the Neo-Tethyan subduction, a process that also led to the adakitic magmatism. Our interpretation involving a Late Cretaceous flat-slab subduction and related orogenesis in southern Tibet is consistent with petrographic data, such as the occurrence of magmatic epidote and muscovite rimmed with resorption texture in the granodiorites, which indicate deep-seated emplacement followed by rapid tectonic exhumation.

Late Miocene high and rapid surface uplift and its erosional response in the Andes of central Chile (33°–35°S)

We address the question of the late Cenozoic geomorphological evolution of the central Chile Andes (33°–35°S), using uplift markers, river incision, previous and new ages of volcanic bodies, and new fission track ages. The uplift markers consist of relicts of high elevated peneplains that evidence &gt;2 km of regional surface uplift lasting ∼2 Ma with variable amount along an E‐W transect. The eastern Coastal Cordillera was uplifted 1.5–2.1 km at 33–34°S and &lt;1 km at 35°S, the western Principal Cordillera was uplifted ∼2 km, and the central eastern Principal Cordillera was uplifted &gt;2.5 at 33°45′S and ∼1.5 km at 34°30′S. Erosional response to uplift was characterized by the retreat of a sharp knickpoint with celerities between 10 and 40 mm a−1. Extrapolation using a stream power law shows that uplift began shortly before 4 Ma or at 10.5–4.6 Ma (7.6 Ma central age) depending on the morphostructural units involved. The first alternative implies simultaneous uplift of the continental margin. The second model (the most reliable one) implies that the uplift affected together the eastern Coastal Cordillera and the Principal Cordillera, while the rest of the western fore arc subsided. This regional uplift can be mostly balanced by crustal thickening resulting from coeval shortening related to the out‐of‐sequence thrusting event in the Principal Cordillera and the uplift of the Frontal Cordillera. Simultaneously, emplacement of the southern edge of the flat slab subduction zone might have partially contributed to this uplift event.

Trans-Alaska Crustal Transect and continental evolution involving subduction underplating and synchronous foreland thrusting

We investigate the crustal structure and tectonic evolution of the North American continent in Alaska, where the continent has grown through magmatism, accretion, and tectonic under-plating. In the 1980s and early 1990s, we conducted a geological and geophysical investigation, known as the Trans-Alaska Crustal Transect (TACT), along a 1350-km-long corridor from the Aleutian Trench to the Arctic coast. The most distinctive crustal structures and the deepest Moho along the transect are located near the Pacific and Arctic margins. Near the Pacific margin, we infer a stack of tectonically underplated oceanic layers interpreted as remnants of the extinct Kula (or Resurrection) plate. Continental Moho just north of this underplated stack is more than 55 km deep. Near the Arctic margin, the Brooks Range is underlain by large-scale duplex structures that overlie a tectonic wedge of North Slope crust and mantle. There, the Moho has been depressed to nearly 50 km depth. In contrast, the Moho of central Alaska is on average 32 km deep. In the Paleogene, tectonic underplating of Kula (or Resurrection) plate fragments overlapped in time with duplexing in the Brooks Range. Possible tectonic models linking these two regions include flat-slab subduction and an orogenic-float model. In the Neogene, the tectonics of the accreting Yakutat terrane have differed across a newly interpreted tear in the subducting Pacific oceanic lithosphere. East of the tear, Pacific oceanic lithosphere subducts steeply and alone beneath the Wrangell volcanoes, because the overlying Yakutat terrane has been left behind as underplated rocks beneath the rising St. Elias Range, in the coastal region. West of the tear, the Yakutat terrane and Pacific oceanic lithosphere subduct together at a gentle angle, and this thickened package inhibits volcanism.

A Rheic cause for the Acadian deformation in Europe

The Acadian (mid-Devonian) deformation in NW Europe has typically been interpreted as the culminating event of the Silurian closure of the Iapetus Ocean. This view has been challenged by the recognition of an intervening early Devonian transtensional event across part of the assembled Laurussian continent. Instead, the Acadian shortening must be driven by a renewed ‘push from the south’, involving subduction of the Rheic Ocean, and either flat-slab subduction or impingement of another Gondwana-derived continental fragment. A problem with either hypothesis is the lack of Acadian deformation or even correlative unconformity in the segment of the Rhenohercynian Zone between the Acadian belt and the Rheic suture. The possibility is explored that this Rhenohercynian segment was juxtaposed with the Acadian belt and the Midland Microcraton only during latest Acadian and/or Variscan tectonics. If so, a major lithospheric suture lies buried just south of the Variscan Front, along the Bristol Channel Fault Zone, and the missing Acadian terranes must now lie elsewhere along the orogen. A case is made that they are related to the allochthonous terranes of NW Iberia. In any case, the Acadian event in Europe should properly be regarded as proto-Variscan rather than late Caledonian.

Geometry and brittle deformation of the subducting Nazca Plate, Central Chile and Argentina

SUMMARY We use data from the Chile Argentina Geophysical Experiment (CHARGE) broad-band seismic deployment to refine past observations of the geometry and deformation within the subducting slab in the South American subduction zone between 30 ◦ S and 36 ◦ S. This region contains a zone of flat slab subduction where the subducting Nazca Plate flattens at a depth of ∼100 km and extends ∼300 km eastward before continuing its descent into the mantle. We use a grid-search multiple-event earthquake relocation technique to relocate 1098 events within the subducting slab and generate contours of the Wadati-Benioff zone. These contours reflect slab geometries from previous studies of intermediate-depth seismicity in this region with some small but important deviations. Our hypocentres indicate that the shallowest portion of the flat slab is a

Crustal structure of the south-central Andes Cordillera and backarc region from regional waveform modelling

SUMMARY We investigate the crustal structure in the Andes Cordillera and its backarc region using regional broadband waveforms from crustal earthquakes. We consider seismic waveforms recorded at regional distances by the CHile-ARgentina Geophysical Experiment (CHARGE) during 2000‐2002 and utilize previous seismic moment tensor inversion results. For each single station-earthquake pair, we fixed the source parameters and performed forward waveform modelling using ray paths that sample the crust of the highest elevation Cordillera and the accreted terranes in the backarc region. Our investigation indicates that synthetic seismograms for our earthquake-station geometry are most sensitive to crustal parameters and less sensitive to mantle parameters. We performed a grid search around crustal thickness, P-wave seismic velocity (Vp) and P -t oS-wave seismic velocity ratio (Vp/Vs), fixing mantle parameters. We evaluated this waveform analysis by estimating an average correlation coefficient between observed and synthetic data over the three broadband components. We identified all acceptable crustal models that correspond to high correlation coefficients that provide best overall seismogram fits for the data and synthetic waveforms filtered mainly between 10 and 80 s. Our results indicate along strike variations in the crustal structure for the north‐south high Cordillera with higher P-wave velocity and thickness in the northern segment (north of 33 ◦ S), and persistently high Vp/Vs ratio (>1.85) in both segments. This is consistent with a colder mafic composition for the northern segment and a region of crustal thickening above the flat slab region. In contrast, the results for the current volcanic arc (south of 33 ◦ S) agree with a warmer crust consistent with partial melt related to Quaternary volcanism presumably of an intermediate to mafic composition. A distinctive feature in the backarc region is the marked contrast between the seismic properties of the Cuyania and Pampia terranes that correlates with their heterogeneous crustal composition. The Cuyania terrane, composed of mafic-ultramafic rocks, exhibits high Vp, high Vp/Vs and a thicker layered crust versus the thinner more quartzrich crust of the eastern Sierras Pampeanas associated with low Vp and low Vp/Vs. These differences may have some effect on the mechanism that unevenly generates crustal seismicity in the upper ∼30 km in this active compressional region. In particular, the seismic properties of the Cuyania terrane, which shows evidence for a high Vp and high Vp/Vs crust, may be reflecting the complex tectonic evolution history of this terrane including accretion-rifting and re-accretion processes since the Palaeozoic that promote a high level of crustal seismicity in the upper ∼30 km, enhanced by the flat slab subduction in the segment between 31 ◦ S and 32 ◦ S. Another possible mechanism could be directly related to the presence of a strong lower crust above the flat slab that efficiently transfer stresses from the slab to the upper crust generating higher seismicity in the Cuyania terrane between 30 ◦ S and 34 ◦ S.

Formation of the 1300-km-wide intracontinental orogen and postorogenic magmatic province in Mesozoic South China: A flat-slab subduction model

Research Article| February 01, 2007 Formation of the 1300-km-wide intracontinental orogen and postorogenic magmatic province in Mesozoic South China: A flat-slab subduction model Zheng-Xiang Li; Zheng-Xiang Li 1Tectonics Special Research Centre, School of Earth and Geographical Sciences, University of Western Australia, Crawley, WA 6009, Australia Search for other works by this author on: GSW Google Scholar Xian-Hua Li Xian-Hua Li 2Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, P.O. Box 1131, Guangzhou, China Search for other works by this author on: GSW Google Scholar Geology (2007) 35 (2): 179–182. https://doi.org/10.1130/G23193A.1 Article history received: 13 Jul 2006 rev-recd: 25 Sep 2006 accepted: 29 Sep 2006 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Zheng-Xiang Li, Xian-Hua Li; Formation of the 1300-km-wide intracontinental orogen and postorogenic magmatic province in Mesozoic South China: A flat-slab subduction model. Geology 2007;; 35 (2): 179–182. doi: https://doi.org/10.1130/G23193A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We propose a flat-slab subduction model for Mesozoic South China based on both new sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon data and a synthesis of existing structural, geochronological, and sedimentary facies results. This model not only explains the development of a broad (∼1300-km-wide) intracontinental orogen that migrated from the coastal region into the continental interior between ca. 250 Ma and 190 Ma, but can also account for the puzzling chain of events that followed: the formation of a shallow-marine basin in the wake of the migrating foreland fold-and-thrust belt, and the development of one of the world's largest Basin and Range–style magmatic provinces after the orogeny. The South China record may serve as an example of the multiple effects of flat-slab subduction, including migrating orogenesis and foreland flexure, synorogenic sagging behind the active orogen, postdelamination lithospheric rebound, and the development of a Basin and Range–style broad magmatic province. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

How does the Nazca Ridge subduction influence the modern Amazonian foreland basin?: COMMENT and REPLY: REPLY

In their Comment of our Geology paper ([Espurt et al., 2007][1]), [Clift and Ruiz (2008)][2] argue that: 1) the flat-slab subduction of the Nazca Ridge is unlikely to have produced uplift of the Fitzcarrald Arch in the Amazonian retroforeland basin (using geologic data from the forearc area); 2)

Giant Porphyry Deposits: Characteristics, Distribution, and Tectonic Controls

More than half of the 25 largest known porphyry copper deposits, defined in terms of contained copper metal, formed during three time periods: the Paleocene to Eocene, Eocene to Oligocene, and middle Miocene to Pliocene. These giant deposits are clustered within three provinces, central Chile, northern Chile, and southwest Arizona-northern Mexico. Other giant deposits occur in Montana, Utah, Panama, Peru, Argentina, Irian Jaya, Mongolia, and Iran. Compressive tectonic environments, thickened continental crust, and active uplift and erosion were associated with the formation of many of these deposits. Calc-alkalic magmas are most favorable for the formation of giant porphyry copper deposits, although several of the largest systems are associated with high K calc-alkalic intrusions. The 25 largest gold-rich porphyry deposits are concentrated in the southwest Pacific and South America, with other occurrences in Eurasia, British Columbia, Alaska, and New South Wales. Many of the deposits formed in the last 13 m.y. The largest of the deposits are associated with high K calc-alkalic intrusions. Many calc-alkalic porphyritic intrusions have also produced giant gold-rich porphyries. In the last 20 m.y., the formation of giant porphyry copper-molybdenum and copper-gold deposits in the circum-Pacific region has been closely associated with subduction of aseismic ridges, seamount chains, and oceanic plateaus beneath oceanic island and continental arcs. In several examples, these tectonic perturbations have promoted flat-slab subduction, crustal thickening, uplift and erosion, and adakitic magmatism coeval with the formation of well-endowed porphyry and/or epithermal mineral provinces. Similar tectonic features are inferred to be associated with the giant porphyry copper-molybdenum provinces of northern Chile (Eocene-Oligocene) and southwest United States (Cretaceous-Paleocene). Topographic and thermal anomalies on the downgoing slab appear to act as tectonic triggers for porphyry ore formation. Other factors, such as sutures in the overriding plate, permeability architecture of the upper crust, efficient processes of ore transport and deposition, and, in some cases, formation and preservation of supergene enrichment blankets are also vital for the development of high-grade giant ore deposits. A low-grade geochemical anomaly may be the final product of mineralization, if ore-forming processes do not operate efficiently, even in the most favorable geodynamic settings.

The distribution of radiogenic heat production as a function of depth in the Sierra Nevada Batholith, California

Geochemical analyses and geobarometric determinations have been combined to create a depth vs. radiogenic heat production database for the Sierra Nevada batholith, California. This database shows that mean heat production values first increase, then decrease, with increasing depth. Heat production is ∼2 μW/m 3 within the ∼3-km-thick volcanic pile at the top of the batholith, below which it increases to an average value of ∼3.5 μW/m 3 at ∼5.5 km depth, then decreases to ∼0.5–1 μW/m 3 at ∼15 km depth and remains at these values through the entire crust below 15 km. Below the crust, from depths of ∼40–125 km, the batholith's root and mantle wedge that coevolved beneath the batholith appears to have an average radiogenic heat production rate of ∼0.14 μW/m 3. This is higher than the rates from most published xenolith studies, but reasonable given the presence of crustal components in the arc root assemblages. The pattern of radiogenic heat production interpreted from the depth vs. heat production database is not consistent with the downward-decreasing exponential distribution predicted from modeling of surface heat flow data. The interpreted distribution predicts a reasonable range of geothermal gradients and shows that essentially all of the present day surface heat flow from the Sierra Nevada could be generated within the ∼35 km thick crust. This requires a very low heat flux from the mantle, which is consistent with a model of cessation of Sierran magmatism during Laramide flat-slab subduction, followed by conductive cooling of the upper mantle for ∼70 m.y. The heat production variation with depth is principally due to large variations in uranium and thorium concentration; potassium is less variable in concentration within the Sierran crust, and produces relatively little of the heat in high heat production rocks. Because silica content is relatively constant through the upper ∼30 km of the Sierran batholith, while U, Th, and K concentrations are highly variable, radiogenic heat production does not vary directly with silica content.

Seismic ridge subduction and topography: Foreland deformation in the Patagonian Andes

The Patagonian Andes recorded several episodes of active ridge subduction in the last 80 million years. An analysis of the spatial and temporal relation between the present segment of collision of the Chile ridge and the digital topography of the foreland shows a correlation with the beginning of deformation and uplift in the inner sector of the Patagonia fold and thrust belt. Several magmatic episodes related to the collision such as near trench magmatism, adakite emplacement, OIB plateau basalts in the retroarc, and the arc volcanic gap, are associated with the uplift and deformation of the Patagonian Cordillera. Based on these correlations, a collision of the Aluk (or Phoenix)-Farallon ridge during Paleogene times south of 43°30′ is identified. Changes in magmatic patterns, molasses deposits, deformation and uplift of the Patagonian Cordillera constrain the region affected by the collision. Similar evidence implies a third period of collision in the Late Cretaceous, based on the occurrence of adakitic rocks, arc magmatic gap, and deformation along the southern Patagonian Andes. This earliest hypothesized collision would require the existence of a new oceanic microplate between the Pacific and the Aluk plates during Late Cretaceous times. Present rapid isostatic rebound related to the continental ice cap retreat in the Patagonian Andes is restricted to the region south of Chile triple junction (46°30′S). The uplift rate here is more than two times more rapid than normal isostatic rebounds recorded in the Northern Hemisphere, and requires an abnormally hot mantle with low viscosity. This abnormal mantle may be a consequence of several episodes of ridge collision and development of asthenospheric windows that are inconsistent with periods of cold flat-slab subduction proposed by some authors to explain the arc volcanic gaps.

Continental-scale links between the mantle and groundwater systems of the western United States: Evidence from travertine springs and regional He isotope data

to understand regional mantle degassing, we compiled new and existing helium isotope data measured in hot springs, gas fields, and travertine-depositing cool springs and compared these geochemical data with mantle velocity structure determined from tomographic studies. these data suggest heterogeneous mantle degassing, with regions of highest He/He in groundwaters (hence, highest mantle helium contribution) corresponding to regions of lowest mantle velocity, a reflection of tectonically active and partially molten mantle. new He isotope and water chemistry data from travertinedepositing cool springs of the western United States show marked variability consistent with mixing between surface water recharge and inputs from deep crustal and mantle sources. the deeply sourced end-member fluids of these mixing trends have high He/He, high dissolved cO 2 , and high salinity compared to shallow recharge waters, and commonly have elevated trace element concentrations. consequently, these fluids cause degradation of water quality in western U.S. aquifers. Our conclusions highlight a connection between neotectonics (e.g., mantle degassing) and water quality in the western United States. INTRODUCTION Distributed deformation associated with the western north American plate margin extends >1000 km inboard from the San Andreas fault zone to the rocky Mountain and western Great Plains regions. this region forms an orogenic plateau with high average heat flow and is characterized by relatively low upper mantle P-wave velocities with marked heterogeneity (Godey et al., 2003; Humphreys et al., 2003). Progressive geochemical depletion of the upper mantle during generation of basaltic GSA Today: v. 15, no. 12, doi: 10.1130/1052-5173(2005)015 2.0.cO;2 melt likely occurred in several episodes since the Proterozoic (Karlstrom et al., 2005). the mantle was hydrated by flat-slab subduction during the laramide orogeny (Humphreys et al., 2003) and now is partially molten, leading to small-scale convective exchange between an upwelling asthenosphere (Gao et al., 2004) and compositionally variable lithosphere (Dueker et al., 2001; Karlstrom et al., 2005). the mantle underlying western north America is marked by one of the largest known shear wave velocity contrasts on earth (van der lee and nolet, 1997). At the continental scale, this transition reflects the heterogeneous thinning and warming of north America’s lithospheric keel as the plate moved southwest in absolute plate motion in the cenozoic into a wide zone of warm asthenosphere (cDrOM Working Group, 2002). We hypothesize that cO 2 -rich mineral springs and related travertine deposits in the western United States are a manifestation of this mantle tectonism, and hence the geochemistry of spring waters and gases can be used in conjunction with geophysical data sets to understand mantle heterogeneity and the processes of lithosphere-asthenosphere interaction. We report new water and gas chemistry with associated carbon and helium isotope data in the context of a synthesis of the existing noble gas isotope chemistry database for western north America. Our literature synthesis (table Dr1) builds on previous work in the area, with the regional helium isotope data presented in the context of a tomographic image of today’s mantle. We also show that travertine-depositing cool springs contain mantle-derived volatiles in a variety of locations and tectonic settings throughout the western United States, such that many aquifer systems are influenced by mixing of deeply sourced and circulated waters. HE ISOTOPES—BACKGROUND the isotope geochemistry of noble gases is a sensitive tracer of mantle-derived volatiles even with a large input of volatiles derived from earth’s crust. this is because the mantle has retained a significant fraction of the terrestrial inventory of the primordial isotope He acquired during earth formation (clarke et al., 1969), and it is still leaking to earth’s surface. in contrast, the crust has been extensively reworked over geological time and has retained very little He: its helium inventory is dominated by radiogenic He produced from the decay of Uand th-series nuclides. consequently, helium presently emanating from regions of mantle melting, such as mid-oceanic ridges or helium trapped in glass and phenocrysts in mid-oceanic-ridge basalts (MOrB), is characterized by a relatively high He/He ratio (r) of 8 ± 1 times that of air (r A ), which has a He/He ratio of 1.4 × 10 (Graham, 2002). indeed, values as high as 37 × r A have been observed in some ocean island basalts (Hilton et al., 1999) and are thought to be related to deep plumes tapping less degassed mantle reservoirs. When mantle-derived fluids are injected into the crust, mantle helium becomes progressively diluted by crustal helium characterized by low He/He ratios of ~0.02 r A . therefore, any value higher than 0.1 GSA Data repository item 2005199, a description of sampling and analytical methods and geochemical data tables Dr1–Dr3, is available online at www. geosociety.org/pubs/ft2005.htm or on request from Documents Secretary, GSA, P.O. Box 9140, Boulder, cO 80301-9140, USA, or editing@geosociety.org.

The Laramide Orogeny: What Were the Driving Forces?

The Laramide orogeny is the Late Cretaceous to Paleocene (80 to 55 Ma) orogenic event that gave rise to the Laramide block uplifts in the United States, the Rocky Mountain fold-and-thrust belt in Canada and the United States, and the Sierra Madre Oriental fold-and-thrust belt in east-central Mexico. The Laramide orogeny is believed to post-date the Jurassic and late Early Cretaceous accretion of the terranes that make up much of the North American Cordillera, precluding a collisional origin for Laramide orogenesis. Instead, the deformation belt along much of its length likely developed 700-1500 km inboard of the nearest convergent margin. The purpose of this paper is to show, through a review of proposed mechanisms for producing this inboard deformation (retroarc thrusting, "orogenic float" tectonics, flat-slab subduction and Cordilleran transpressional collision), that the processes responsible for orogeny remain enigmatic.