Convergent plate boundaries are key sites for continental crustal formation and recycling. Quantifying the evolution of crustal thickness and paleoelevation along ancient convergent margins ...represents a major goal in orogenic system analyses. Chemical and in some cases isotopic compositions of igneous rocks formed in modern supra‐subduction arcs and collisional belts are sensitive to Moho depths at the location of magmatism, implying that igneous suites from fossil orogens carry information about crustal thickness from the time they formed. Several whole‐rock chemical parameters correlate with crustal thickness, some of which were calibrated to serve as “mohometers,” that is, quantitative proxies of paleo‐Moho depths. Based on mineral‐melt partition coefficients, this concept has been extended to detrital zircons, such that combined chemical and geochronological information extracted from these minerals allows us to reconstruct the crustal thickness evolution using the detrital archive. We discuss here the mohometric potential of a variety of chemical and isotopic parameters and show that their combined usage improves paleocrustal thickness estimates. Using a MATLAB® app developed for the underlying computations, we present examples from the modern and the deeper time geologic record to illustrate the promises and pitfalls of the technique. Since arcs are in isostatic equilibrium, mohometers are useful in reconstructing orogenic paleoelevation as well. Our analysis suggests that many global‐scale correlations between magma composition and crustal thickness used in mohometry originate in the sub‐arc mantle; additional effects resulting from intracrustal igneous differentiation depend on the compatible or incompatible behavior of the involved parameters.
Plain Language Summary
Understanding how continental crust formed and evolved is one of the major goals of geology. Chemical composition of igneous rocks formed at modern convergent plate margins correlates with crustal thickness and elevation at the time of magmatism. Therefore, when averaged over local and regional scales, a series of chemical parameters act as excellent tracers of crustal thickness. These parameters in turn are used on ancient igneous rocks in order to determine crustal thickness and elevation of mountain ranges during magmatism; they are referred to as chemical mohometers and paleo‐altimeters. Here, we review several proposed parameters and recommend new ones. We provide a MATLAB® app that ingests a variety of geochemical parameters and calculate paleocrustal thickness and elevation of diverse possible geologic applications.
Key Points
Chemical and isotopic compositions of igneous rocks are sensitive to crustal thickness and paleoelevation of magmatic arcs
Composition‐Moho depth correlation models in modern arcs and collisional belts help estimate crustal thickness of ancient orogens
Mantle melting and crustal differentiation contribute to composition‐Moho depth correlations in arc magmas
Magmatic processes that occur during the transition from oceanic to continental subduction and collision in orogens are critical and still poorly resolved. Oceanic slab detachment in particular is ...hypothesized to mark a fundamental change in magmatism and deformation within an orogen. Here, we report on two Quaternary volcanic centers of Myanmar that may help us better understand the process of slab detachment. The Monywa volcanic rocks are composed of low‐K tholeiitic, medium‐K calk‐alkaline, and high‐K to shoshonitic basalts with arc signatures, while the Singu volcanic rocks show geochemical characteristics similar to asthenosphere‐derived magmas. These volcanic rocks have low Os concentrations but extremely high 187Os/186Osi ratios (0.1498 to 0.3824) due to minor (<4%) crustal contamination. The Monywa arc‐like rocks were generated by small degrees of partial melting of subduction‐modified asthenospheric mantle at variable depths from the spinel to garnet stability fields. Distinct from the Monywa arc‐like rocks (87Sr/86Sri = 0.7043 to 0.7047; εNdi = +2.3 to +4.7), the Singu OIB‐like rocks exhibit higher 87Sr/86Sri (0.7056 to 0.7064) and lower εNdi (+0.8 to +1.6) values. These isotopic characteristics indicate a large contribution of an isotopically enriched asthenosphere layer beneath the Burmese microplate, which possibly flowed from SE Tibet. We interpret that this short‐lived, small‐scale, and low‐degree melting Quaternary volcanism in Myanmar was triggered by its position above a slab window resulting from the tearing of the oceanic lithosphere from buoyant continental lithosphere of the Indian plate.
Key Points
Southward tearing of oceanic lithosphere from continental lithosphere of the Indian plate occurred in central Myanmar
Active slab detachment triggers both arc‐like and OIB‐like Quaternary basaltic volcanism during highly oblique continental subduction
An exotic asthenosphere layer beneath the Burmese microplate flowed from central and SE Tibet
Garnet pyroxenites are the most common deep lithospheric xenolith assemblages found in Miocene volcanic rocks that erupted through the central part of the Sierra Nevada batholith. Elemental ...concentrations and isotope ratios are used to argue that the Sierra Nevada granitoids and the pyroxenite xenoliths are the melts and the residues/cumulates, respectively, resulting from partial melting/fractional crystallization at depths exceeding 35–40 km. The estimated major element chemistry of the protolith resembles a basaltic andesite. Effectively, at more than about 40 km depth, batholith residua are eclogite facies rocks. Radiogenic and oxygen isotope ratios measured on pyroxenites document unambiguously the involvement of Precambrian lithosphere and at least 20–30% (mass) of crustal components. The mass of the residual assemblage was significant, one to two times the mass of the granitic batholith. Dense garnet pyroxenites are prone to foundering in the underlying mantle. An average removal rate of 25–40 km3/km Myr is estimated for this Cordilleran‐type arc, although root loss could have taken place at least in part after the cessation of arc magmatism. This rate is matched by the average subcrustal magmatic addition of the arc (∼23–30 km3/km Myr), suggesting that the net crustal growth in this continental arc was close to zero. It is also suggested that in order to develop a convectively removable root, an arc must have a granitoid melt thickness of at least 20–25 km. Residues of thinner arcs should be mostly in the granulite facies; they are not gravitationally unstable with respect to the underlying mantle.
The Santa Catalina‐Tortolita‐Rincon Mountains of Southeast Arizona are a classic metamorphic core complex (MCC) and represent footwall exposures of crustal rocks exhumed by a detachment system. This ...study presents new evidence for the formation of the majority of ductile deformation during the Eocene (~46 Ma), synchronous with the emplacement of the regionally significant Wilderness Sills Suite (57–45 Ma). The evidence is provided by Eocene U‐Pb ages of syn‐ to late kinematic dikes emplaced in the principal ductile mylonitic fabric of the Catalina forerange, earlier than the brittle normal fault system and the formation of Tucson basin beginning with the latest Oligocene. Well‐documented shear sense indicators may not reflect extension at that time (Eocene), but more likely the direction of crustal flow now rotated during later extension. Muscovite‐plagioclase Rb‐Sr isochron ages of three mylonitic rocks are all clustered around 34 Ma, which is inferred to be the last age when these rocks were being deformed under ductile conditions following the emplacement of the Wilderness Sills Suite and various related dikes. Biotite‐plagioclase Rb‐Sr ages on the same rocks demonstrate that the section cooled below ~300°C at 25–26 Ma during the development of normal faulting. Normal faulting was synchronous with the emplacement of the Catalina Intrusive Suite. New U‐Pb age results for Catalina Intrusive Suite indicate a combined mean age of 24.9 Ma. Chemical compositions of hornblende‐plagioclase pairs were obtained on six Catalina Intrusive Suite samples; depth estimates for the emplacement of the Catalina Intrusive Suite average of about 6 km. These results suggest that the exposed Catalina ductile detachment system was at about 5 km beneath the surface at 25 Ma. These new data bring new light into the development of this core complex and suggest that the similarity in orientations of principal ductile and brittle fabrics at the Catalina MCC locality are coincidental. Neither was the principal ductile fabric developed during the low‐angle normal faulting of the latest Oligocene nor was this exposed section a midcrustal one at that time. Transient, pluton emplacement‐enhanced, and extension‐related ductile deformation at shallow crustal levels operated locally at ~25 Ma but that does not account for the development of the majority of the Catalina MCC mylonites.
Key Points
The classic Catalina core complex main mylonitic fabric did not form synchronous with the brittle detachment fault but much earlier
Ductile fabric is developed during the Eocene and is constrained by age of late kinematic dikes
The entire section was located shallow beneath the surface (some 5 km) at the time of brittle Basin and Range detachment faulting (25 Ma)
The transition zone between the Alps and Dinarides is a key area to investigate kinematic interactions of neighboring orogens with different subduction polarities. A study combining field kinematic ...and sedimentary data, microstructural observations, thermochronological data (Rb‐Sr and fission track), and regional structures in the area of Medvednica Mountains has revealed a complex polyphase tectonic evolution. We document two novel stages of extensional exhumation. The first stage of extension took place along a Late Cretaceous detachment following the late Early Cretaceous nappe stacking, burial, and greenschist facies metamorphism. Two other shortening events that occurred during the latest Cretaceous‐Oligocene were followed by a second event of extensional exhumation, characterized by asymmetric top‐NE extension during the Miocene. Top‐NW thrusting took place subsequently during the Pliocene inversion of the Pannonian Basin. The Cretaceous nappe burial, Late Cretaceous extension, and the Oligocene(‐Earliest Miocene) contraction are events driven by the Alps evolution. In contrast, the latest Cretaceous‐Eocene deformation reflects phases of Dinaridic contraction. Furthermore, the Miocene extension and subsequent inversion display kinematics similar with observations elsewhere in the Dinarides and Eastern Alps. All these processes demonstrate that the Medvednica Mountains were affected by Alpine phases of deformations to a much higher degree than previously thought. Similarly with what has been observed in other areas of contractional polarity changes, such as the Mediterranean, Black Sea, or New Guinea systems, the respective tectonic events are triggered by rheological weak zones which are critical for localizing the deformation created by both orogens.
Key Points
Kinematic and thermochronological study of interacting neighboring orogens
Two stages of extensional exhumation influenced the Alpine‐Dinaridic junction
Structures of the Alps extend far into the Dinarides
An intrinsic feature of Cordillera-style orogenic systems is a spatial trend in the radiogenic isotopic composition of subduction-related magmatism. Magmatism is most isotopically juvenile near the ...trench and becomes increasingly evolved landward. A compilation of radiogenic isotopic data from the central Andes, U.S. Cordillera, and Tibet (the most well-studied examples of modern and ancient Cordilleran systems) demonstrate such spatial trends are long-lived and persist throughout the life of these continental subduction margins. The consistency of the isotopic trend through time in magmatic products is surprising considering the plethora of orogenic processes that might be expected to alter them. In addition to longevity, spatial isotopic trends encompass a broad spectrum of geochemical compositions that represent diverse petrogenetic and geodynamic processes. The two end-members of the spatial isotopic trends are represented by melts sourced within isotopically juvenile asthenospheric mantle and melts sourced from isotopically evolved continental lithospheric mantle and/or lower crust. Mantle lithosphere generally thins toward the magmatic arc and trench in Cordilleran orogens because sub-lithospheric processes such as delamination, subduction erosion, and subduction ablation, operate to thin or remove the continental mantle lithosphere. With time, magmatic additions may impart the isotopic composition of the mantle source on the lower crust, giving rise to an isotopically homogenous deep lithosphere. The results of this analysis have significant implications for interpreting temporal and spatial shifts in isotopic composition within Cordilleran orogens and suggest that the continental mantle lithosphere may be a significant source of magmatism in orogenic interiors.
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•There is an inherent spatial isotopic trend in Cordilleran magmatism.•Arc migration produces apparent temporal isotopic trends.•Mantle lithosphere is an important source of magmatism in orogenic interiors.
Jurassic volcanismin Southern-Central Chile (35°–39°S) is represented mainly by two units. The first characterized by the volcanic and subvolcanic mostly andesitic deposits of the Altos de Hualmapu ...Formation, located in actual Coastal Cordillera (35°–35°30′S) and the second corresponding to the lower basaltic and upper andesitic to dacitic upper member of Nacientes del Biobio Formation, in actual Main Cordillera (39°S). Both units mark the transition between northern and patagonian segments of Early Andean Magmatic Province (EAMP) that reach its maximum magmatic activity in this area during late Middle Jurassic in Coastal Cordillera, after two minor pulses of activity between Upper Triassic and Lower Jurassic. No evidence of arc activity is recorded in this area after 155 Ma, when volcanic axis seems to shift to actual Main Cordillera until Lower Cretaceous when it is resumed again to the west. The discontinuity of the arc front suggest the presence of a major cutoff in axis at ≈36–37°. Whole rock geochemical and isotopical Sr-Nd-Pb data shows that this areal discontinuity coincides with an enrichment of the magmas that suggest ≈20–30% of participation of an enriched source in the genesis of the magmas. Given the mostly extensional to transtensional tectonic regime of Western Gondwana during Jurassic and Lower Cretaceous it is unlike to assume high degrees of assimilation at shallow levels, so the observed enrichment should reflect the addition of fertile asthenospheric mantle dragged by the slab as result of the massive roll back and tearing of the oceanic plate under the Arc in Patagonia during Upper Triassic and Middle Jurassic.
•Southern Central Chile Jurassic magmatism is geochemical and isotopically enriched.•Slab tear influence in Jurassic magmatism of Southern Central Chile.•Main peak of Jurassic arc activity take place during Late Middle Jurassic.•A discontinuity in the arc front position is observed at 36–37°S.•Transition between Central and Patagonian Early Andean Magmatic Province.
The Pleistocene (1.65 Ma) Crystal Knob volcanic neck in the California Coast Ranges is an olivine‐plagioclase phyric basalt containing dunite and spinel peridotite xenoliths. Crystal Knob erupted ...through the Nacimiento belt of the Franciscan complex and adjacent to Salinian crystalline nappes. Its xenoliths sample the mantle lithosphere beneath the outboard exhumed remnants of the southern California Cretaceous subducting margin. This sample set augments previously studied xenolith suites in the Mojave Desert and Sierra Nevada, which linked the mantle lithosphere architecture and crustal structure of the western Cordillera. We examine six peridotite samples ranging from fertile lherzolites to harzburgite residues. Time‐corrected (εNd) of 10.3–11.0 and 87Sr/86Sr of 0.702 are characteristic of underplated suboceanic mantle. Pyroxene exchange geothermometry shows equilibration at 950–1060 °C. Phase stability, Ca‐in‐olivine barometry, and 65‐ to 90‐mW/m2 regional geotherms suggest entrainment at 45‐ to 75‐km depth. The samples were variably depleted by partial melting, and re‐enrichment of the hottest samples suggests deep melt‐rock interaction. We test the Crystal Knob temperature depth array against model geotherms matching potential sources for the mantle lithosphere beneath the Coast Ranges: (A) a shallow Mendocino slab window, (B) a young Monterey plate stalled slab, and (C) Farallon plate mantle nappes, underplated during the Cretaceous and reheated at depth by the Miocene slab window. Models B and C fit xenolith thermobarometry, but only model C fits the tectonic and geodynamic evolution of southern California. We conclude that the mantle lithosphere beneath the central California coast was emplaced after Cretaceous flat slab subduction and records a thermal signature of Neogene subduction of the Pacific‐Farallon ridge.
Plain Language Summary
The Crystal Knob volcanic neck is a small, deeply sourced lava that erupted near in the California Coast Ranges 1.65 million years ago. It carried xenoliths, hand‐sized fragments of the mantle, which can help us understand how the western edge of the North American continent was formed. We focused on six samples from Crystal Knob with different mineralogical characteristics, all of which originated in the mantle. Their chemistry suggests that they were not originally part of North America. Some xenoliths had been partially melted, and many had additional material added, changing their composition from pristine mantle. Mineral phases record their temperature (950–1060°C) and depth (45–75 km) prior to eruption. This rare direct record of the temperature of the upper mantle allows us to test several options for the formation of theunder pinnings of the central California coast. The most viable option is that the mantle beneath the edge of North America was tectonically pushed under the continent ∼75 million years ago. It heated from below ∼24 million years ago at the end of Farallon plate subduction. This history fits with extensive evidence that much of the deep architecture of the California coast is inherited from the Cretaceous period.
Key Points
The 1.65‐Ma Crystal Knob volcanic pipe hosts xenoliths that sample the deep mantle lithosphere near the California coast
Spinel peridotites equilibrated near 1000 °C at depths between 45 and 75 km and show a signature of deep melt interaction
The mantle lithosphere beneath the Coast Ranges was tectonically underplated during the Cretaceous and heated by the Neogene slab window
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