The central Andean retroarc thrust belt is characterized by a southward transition at ∼22°S in structural style (thin‐skinned in Bolivia, thick‐skinned in Argentina) and apparent magnitude of ...Cenozoic shortening (>100 km more in the north). With the aim of evaluating the abruptness and cause of this transition, we conducted a geological and geo‐thermochronological study of the Cachi Range (∼24–25°S), which is a prominent topographic feature at this latitude. Our U‐Pb detrital zircon results from the oldest exposed rocks (Puncoviscana Formation) constrain deposition to mainly Cambrian time, followed by major, Cambro‐Ordovician shortening and ∼484 Ma magmatism. Later, Cretaceous rift faults were locally inverted during Cenozoic shortening. Coupled with previous work, our new (U‐Th)/He zircon results require 8–10 km of Miocene exhumation that was likely associated with fault‐propagation folding within the Cachi Range. After Miocene shortening, displacement on sinistral strike‐slip faults demonstrates a change in stress state to a non‐vertically orientedσ3. This change in stress state may result from an increase in gravitational potential energy in response to significant crustal thickening and/or lithospheric root removal. Our finding of localized Cenozoic shortening in the Cachi Range increases the estimate of the local magnitude of shortening, but still suggests that significantly less shortening was accommodated south of the thin‐skinned Bolivian fold‐thrust belt. Our results also underscore the importance of the pre‐existing stratigraphic and structural architecture in orogens in influencing the style of subsequent deformation.
Key Points
Fault‐propagation folding resulted in major exhumation in the region
Miocene shortening was followed by a change in stress state
Shortening magnitude varies greatly along‐strike in the central Andes
The Danubian domain basement of the South Carpathians, Romania, comprises two Neoproterozoic continental crustal fragments, the Drăgşan and Lainici-Păiuş terranes, which were sutured by the closure ...of an intervening oceanic domain, the Tişoviţa terrane. Magmatic and detrital zircons extracted from an orthogneiss, four granitoid plutons, two metasedimentary units, and a Liassic sandstone were dated by zircon U/Pb LA-ICP-MS. The Făgeţel augen gneiss from the Drăgşan terrane basement yielded an age of 803.2
±
4.4
Ma, the oldest well-constrained crystallization age reported from the Romanian Carpathians basement. The Tismana, Şuşiţa, Novaci and Olteţ granitoid plutons, which intrude the Lainici-Păiuş terrane basement, yielded ages of 600.5
±
4.4, 591.0
±
3.5, 592.7
±
4.9, and 588
±
2.9
Ma, respectively. The Tismana granitoid age of 600
Ma and the youngest detrital zircon ages of 637–622
Ma from a metaquartzite within the Lainici-Paiuş terrane, constrain the deposition of the metaquartzite protolith to ca. 620–600
Ma. The 803
Ma age represents an old Pan-African age, whereas the younger Neoproterozoic ages suggest Pan-African/Cadomian thermotectonic events. Detrital and inherited zircon ages within the Drăgşan and Lainici-Paiuş terranes attest to a peri-Amazonian, Avalonian-type provenance for the Drăgşan terrane and possibly a Ganderian-type provenance for the Lainici-Păiuş terrane. The Lainici-Păiuş terrane rifted off Gondwana before the Drăgşan terrane. Both terranes were attached to Moesia during the Early Paleozoic.
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► The Danubian terranes are Avalonian fragments sutured to West Moesia during the Late Ordovician. ► Lainici-Păiuş terrane was subjected to extensive granitoid plutonism at 600–590
Ma. ► It originated in the active margin of a post-Rodinian continental fragment.. ► Făgeţel gneiss is the oldest igneous protolith in the Romanian Carpathians proved by zircon dating. ► During the Variscan Orogeny, the Danubian domain basement was subjected to granitic plutonism.
New field and thermobarometric work in the Californian Salinian block clarifies current and pre-Tertiary relationships between the schist of Sierra de Salinas and Cretaceous arc-related granitic ...rocks. The contact is variably preserved as a brittle fault and high-temperature mylonite zone, the Salinas shear zone, which represents the contact between North America and sediments accreted above the Farallon slab between ∼
76 Ma and ∼
70 Ma. Near granulite facies, prograde replacement of hornblende with clinopyroxene is associated with deformation of plutonic rocks at the base of the upper plate. In the lower plate, the schist of Sierra de Salinas, garnet–biotite thermometry indicates decreasing temperatures down-section from at least 714 °C to ∼
575 °C over an exposed thickness of ∼
2.5 km, consistent with petrologic evidence of an inverted metamorphic gradient. The measured temperatures are significantly higher than observed at shallow levels above subducting slabs or predicted by 2D computational models assuming low shear stresses. Previous workers have called upon shear heating to explain similar observations in the correlative Pelona schist, an unlikely scenario given the results of recent rock deformation experiments which predict that feldspar–quartz–mica aggregates are far too weak to withstand stresses of ∼
70 MPa required by the shear heating hypothesis. As an alternative, we propose that high temperatures resulted from conductive heating while the leading edge of the schist traveled ∼
150 km beneath the recently active Salinian continental arc during the initiation of shallow subduction. Weakening of the schist due to high temperatures helped facilitate the collapse of the Salinian arc as the schist was emplaced. Schist emplacement coincided with loss of lower, mafic portions of the arc, and therefore evolution of the Southern California crust towards a more felsic composition.
Current end-member models for the geodynamic evolution of orogenic plateaus predict (
a
) slow and steady rise during crustal shortening and ablative subduction (i.e., continuous removal) of the ...lower lithosphere or (
b
) rapid surface uplift following shortening, which is associated with punctuated removal of dense lower lithosphere and or lower crustal flow. This review integrates results from recent studies of the modern lithospheric structure, geologic evolution, and surface uplift history of the Central Andean Plateau to evaluate the geodynamic processes involved in forming it. Comparison of the timing, magnitude, and distribution of shortening and surface uplift, in combination with other geologic evidence, highlights the pulsed nature of plateau growth. We discuss specific regions and time periods that show evidence for end-member geodynamic processes, including middle-late Miocene surface uplift of the southern Eastern Cordillera and Altiplano associated with shortening and ablative subduction, latest Oligocene-early Miocene and late Miocene-early Pliocene punctuated removal of dense lower lithosphere in the Eastern Cordillera and Altiplano, and late Miocene-early Pliocene crustal flow in the central and northern Altiplano.
The role of magmatic processes as a significant mechanism for the generation of voluminous silicic crust and the development of Cordilleran plateaus remains a lingering question in part because of ...the inherent difficulty in quantifying plutonic volumes. Despite this difficulty, a growing body of independently measured plutonic-to-volcanic ratios suggests the volume of plutonic material in the crust related to Cordilleran magmatic systems is much larger than is previously expected. To better examine the role of crustal magmatic processes and its relationship to erupted material in Cordilleran systems, we present a continuous high-resolution crustal seismic velocity model for an ~800 km section of the active South American Cordillera (Puna Plateau). Although the plutonic-to-volcanic ratios we estimate vary along the length of the Puna Plateau, all ratios are larger than those previously reported (~30:1 compared to 5:1) implying that a significant volume of intermediate to silicic plutonic material is generated in the crust of the central South American Cordillera. Furthermore, as Cordilleran-type margins have been common since the onset of modern plate tectonics, our findings suggest that similar processes may have played a significant role in generating and/or modifying large volumes of continental crust, as observed in the continents today.
The Coast Ridge Belt (CRB, Santa Lucia Mts., central California) comprises mid‐crustal rocks (750–800°C and 0.8 GPa) of the California magmatic arc. We estimated the bulk composition of the CRB and ...converted our results to seismic velocities expected at the observed pressures and temperatures. This transformation allows us to compare calculated velocities for the CRB with in situ measurements in similar arcs. The bulk composition of this arc section changes abruptly at 25 km depth from a granodiorite to a quartz‐diorite or diorite. These data are in agreement with geophysical results from other Cordilleran batholiths, and suggest 1.5 to 2 times thicker felsic columns than usually interpreted for modern continental arcs, and a relatively sharp transition between a felsic upper crustal batholith, and a mafic deep crust. This implied rheological boundary may have significant implications for intracrustal faulting or convective removal of the roots of batholiths.
The Sebeş–Lotru terrane in the South Carpathians mountain range comprises a lower, Neoproterozoic metamorphic unit (Lotru) and an upper, Ordovician metamorphic unit (Cumpăna) that were juxtaposed ...during the Variscan orogeny. Two orthogneisses from the Lotru metamorphic unit yield U/Pb LA-ICP-MS zircon crystallization ages of 549.3
±
3.8
Ma and 587.5
±
3
Ma, respectively. Two orthogneisses from the Cumpăna metamorphic unit yield zircon crystallization ages of 458.9
±
3.5
Ma and 466.0
±
4.2
Ma, respectively. High U zircons from two other orthogneisses from the Cumpăna metamorphic unit have ages ranging from 400
Ma to 320
Ma, which are interpreted to reflect protracted zircon recrystallization during the regionally significant Variscan collisional event. Detrital zircons from a metasedimentary gneiss in the Cumpăna metamorphic unit have ages ranging from ~
0.5
Ga to 2.8
Ga. The 0.5
Ga age constrains the maximum sediment deposition age to be late Cambrian. The source most compatible with the range of Precambrian detrital ages in the Sebeş–Lotru terrane is northeastern Gondwana.
The Sebeş–Lotru terrane was part of a continental subduction/collision system as a lower plate after about 400
Ma and reached peak metamorphic conditions between 350 and 320
Ma (e.g. Medaris et al., 2003). A cross-cutting granite vein has a zircon U–Pb crystallization age of 321.5
±
3.1
Ma, which constrains minimum age of ductile deformation during the Variscan collision in this region. The trace of the Rheic suture within the South Carpathians is located between the Ordovician upper part of the Sebeş–Lotru terrane and the Drăgşan pre-Alpine terrane of the Danubian domain.
The basement of the Romanian Carpathians is comprised of pre-Variscan metasedimentary and metaigneous units. Age patterns from corresponding detrital zircons show similarities to those from the ...eastern Mediterranean region. Consistently, these patterns suggest a northern Gondwanan origin for the Carpathian terranes with the most common detrital sources representing fragments of the Pan-African Orogen, Arabian-Nubian Shield, Kibaran Orogen, and West African Craton (Eburnean Orogen and Liberian-Leonian Orogens), as well as the Saharan metacraton. Some contributions from the Indian Craton are also possible. The youngest detrital zircon ages constrain the maximum age of the sedimentary rock protoliths to Middle Cambrian. On the other hand, the U/Pb zircon ages of the metaigneous protoliths indicate predominantly Early Ordovician ages and bracket most magmatism in the Carpathian pre-Variscan basement units between Middle Cambrian and Late Ordovician. An exception is the Neoproterozoic lower part of the Sebes–Lotru terrane, whose igneous ages are equivalent to the Danubian Domain in the South Carpathians. The Carpathian pre-Variscan terranes were originally located along the eastern extension of the Galatian superterrane, and thus within a Late Cambrian to Ordovician extensional tectonic setting.
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.
Constraints on bulk conductivity from magnetotelluric measurements and petrological analyses of late Quaternary peridotite xenoliths from the southern Sierra Nevada allow evaluation of models ...commonly used to relate electrical conductivity to the physical and chemical state of the upper mantle. In these models, two conductive melts (basalt and sulfide) are embedded in a resistive matrix. Bounds on the amount of sulfide (0.06–0.4%) and the bulk conductivity (0.03–0.1 S/m) place constraints on the degree of interconnection between the melts. Because the sulfide melt is very conductive, even a small fraction of well‐connected melt results in a bulk conductivity larger than 0.1 S/m. Similarly, completely disconnected melts result in bulk conductivities much less than 0.03 S/m. The only models which matched both the bulk conductivity and sulfide bounds consisted of a small fraction (<1%) interconnected basalt melt with a discontinuous sulfide phase. Such a texture is observed in laboratory experiments with much larger sulfide melt fractions, but has not been reported for small melt fractions. A variant of the Hashin‐Shtrikman model and a hybrid model consisting of cascaded Hashin‐Shtrikman calculations were successful in matching the magnetotelluric and petrologic constraints. With a model that appropriately simulates the melt interconnectivity, we suggest that electrical conductivity may be used to infer in situ melt properties in the mantle.
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