Robert Handler

Along-strike variations in the thermal and tectonic response of the continental Ecuadorian Andes to the collision with heterogeneous oceanic crust

Oblique to strike geological segmentation in the Andean chain has been previously recognised at various scales and is commonly attributed to changes in the convergence vectors of the oceanic and continental plates, as well as the upperplate expressions of differing along-strike subducted slab age, strength and composition. We present new white mica and biotite 40 Ar/ 39 Ar and zircon and apatite fission-track data from several traverses across the Cordillera Real of Ecuador in the northern Andes that reveal distinct along-strike differences in the timing of accelerated crustal cooling during the Cenozoic. The data record elevated cooling rates from temperatures of V380‡C during V65^55 and V43^ 30 Ma from all sampled regions of the Cordillera Real and at V15 Ma and since V9 Ma in the northern Cordillera Real. Each cooling period was probably driven by exhumation in response to the accretion and subduction of heterogeneous oceanic crust. Elevated cooling rates of up to V30^20‡C/Myr were initiated during the Palaeocene and Eocene^early Oligocene along the entire contemporaneous margin of Ecuador and were driven by the accretion of the oceanic Pallatanga Terrane and Pin ‹ on^Macuchi Block, respectively, onto northwestern South America. Both of these geological provinces originated at the southern parts of the leading and trailing boundaries of the Caribbean Plateau and accreted onto the margin during the approximately northeastward migration of the Plateau into its current position. Within Ecuador the development of higher topography and elevated cooling rates of up to 50‡C/Myr at V15 Ma and since V9 Ma are restricted to the region north of 1‡30PS and is situated above the postulated subducted flatslab section of the aseismic Carnegie Ridge. Plate convergence rate calculations suggest the Carnegie Ridge collided with the Ecuador Trench at V15 Ma, which caused the pre-existing coastal provinces to displace to the northeast, subsequently driving extension and marine ingressions in southern Ecuador and compression and uplift in northern Ecuador. fl 2001 Elsevier Science B.V. All rights reserved.

Conjugate shear zones in the Southern Bohemian Massif (Austria): implications for Variscan and Alpine tectonothermal activity

Abstract Mylonitic fabrics developed within conjugate wrench ductile shear and fault systems in the Southern Bohemian Massif display both dextral (NW-SE-trending systems) and sinistral (NE-SW-trending systems) shear senses. Contrasting temperature conditions of deformation can be observed in the different shear zones. Temperatures above 650° C were reached in the Pfahl shear zone. Shearing under greenschist facies conditions took place in the Rodl and Danube shear zones. Brittle deformation dominated in the Vitis and Diendorf shear zones. 40 Ar 39 Ar dating of various size fractions of muscovite formed and/or deformed during mylonitization yield ages of ca. 287 Ma (the NW-SE-trending system) and ca. 288-281 Ma (the NE-SW-trending system). The shear zones are interpreted as a late Variscan conjugate system. 40 Ar 39 Ar age spectra of fine-grained newly grown sericite or rejuvenation of ages give evidence for post-Variscan reactivation of the shear zones. Brittle deformation within the shear zones was probably associated with maintenance of very high fluid pressure during Variscan deformation. Foreland deformation during the Alpine orogeny also played a significant role, leading not only to the development of the Ceske Budejovice Graben but also to local reactivation of the shear zones at higher crustal levels.

40Ar/39Ar muscovite ages from the Penninic‐Austroalpine plate boundary, Eastern Alps

The Radstadt Mountains, Eastern Alps, expose the tectonic boundary between the base of the Austroalpine continental plate (hanging wall) and the Penninic oceanic units (foot wall). Parts of the Austroalpine basement were penetratively deformed because of ongoing rifting of the continental crust during the Permian (290–250 Ma). Austroalpine Permo-Mesozoic cover rocks were deformed during the Cretaceous (∼80 Ma) and the Paleogene (55–50 Ma). These ages are interpreted as representing two distinct events of nappe stacking. The younger ages represent the collision of Austroalpine and Penninic tectonic units. The Penninic nappe complex displays a successive decrease of ages from ∼37 to 25 Ma from high to deep tectonic levels. A second age group of ∼22 Ma was found both in low-temperature release steps and as plateau ages close to the Penninic-Austroalpine boundary. It is attributed to a thermal overprint due to ductile extension of the overthickened orogenic wedge.

Sequence of Thrusting Within A Thick‐Skinned Tectonic Wedge: Evidence From 40Ar/39Ar and Rb‐Sr Ages From the Austroalpine Nappe Complex of the Eastern Alps

The sequence of thrusting within a regional‐scale Alpine thick‐skinned tectonic wedge has been constrained by Rb‐Sr and 40Ar/39Ar dating of whole‐rock, constituent white mica and biotite from structural units of the Austroalpine Nappe Complex, Eastern Alps (Austria). Preservation of Variscan and pre‐Variscan isotopic signatures in pre‐Alpine basement units indicates a non‐penetrative metamorphic Alpine overprint. Alpine nappe assembly and accompanying ductile deformation occurred during maintenance of lower greenschist‐facies metamorphic conditions (c. 300°–450°C). Analyses from greenschist‐facies metamorphic mylonites and penetratively ductile deformed cover sequences display decreasing 40Ar/39Ar plateau ages structurally downward. Thrusting and accompanied ductile deformation commenced at c. 100–90 Ma in the uppermost structural levels. Footwall fault propagation effected internal deformation and out‐of‐sequence thrusting within intermediate structural levels at c. 95–80 Ma. This was contemporaneous with regional exhumation and cooling of deeper crustal basement units at c. 88–82 Ma and was associated with extension and strike‐slip faulting, leading to the formation of synorogenic sedimentary basins. Additional foot‐wall thrust propagation was associated with ductile deformation within lowermost structural units at c. 80–70 Ma. The structural and new geochronologic results presented herein suggest that (1) the Austroalpine Nappe Complex represents a forward propagating thrust complex that formed as a result of footwall propagation of a master fault and associated piggyback transport of hangingwall structural units; (2) most parts of the Austroalpine block occupied a lower‐plate tectonic setting during Cretaceous tectonics subsequent to subduction of the Meliata‐Hallstatt ocean; and (3) the Austroalpine Nappe Complex had already been formed when subduction of Penninic oceanic domains was active.

Geology, age and tectonic evolution of the Sierra Maestra Mountains, southeastern Cuba

We summarize the available geological information on the Sierra Maestra Mountains in southeastern Cuba and report new zircon fission track and biotite Ar-Ar ages for this region. Two different and genetically unrelated volcanic arc sequences occur in the Sierra Maestra, one Cretaceous in age (pre-Maastrichtian) and restricted to a few outcrops on the southern coast, and the other Palaeogene in age, forming the main expression of the mountain range. These two sequences are overlain by middle to late Eocene siliciclastic, carbonatic and terrigenous rocks as well as by late Miocene to Quaternary deposits exposed on the southern flank of the mountain range. These rocks are britle deformed and contain extension gashes filled with calcite and karst material. The Palaeogene volcanic arc successions were intruded by calc-alkaline, low- to medium-K tonalites and trondhjemites during the final stages of subduction and subsequent collision of the Caribbean oceanic plate with the North American continental plate. U-Pb SHRIMP single zircon dating of five granitoid plutons yielded 206Pb/238U emplacement ages between 60.5 ± 2.2 and 48.3 ± 0.5 Ma. These granitoids were emplaced at pressures ranging from 1.8 to 3.0 kbar, corresponding to depths of ca. 4.5-8 km. 40Ar/39Ar dating of two biotite concentrates yielded ages of 50 ± 2 and 54 ± 4 Ma, indicating cooling through ca. 300 oC. Zircon and apatite fission track ages range from 32 ± 3 to 46 ± 4 Ma and 31 ± 10 to 44 ± 13 Ma, respectively, and date cooling through 250 ± 50 oC and 110 ± 20 oC. The granitoids are the result of subduction-related magmatism and have geochemical characteristics similar to those of magmas from intra-oceanic island-arcs such as the Izu Bonin-Mariana arc and the New Britain island arc. Major and trace element patterns suggest evolution of these rocks from a single magmatic source. Geochemical features characterize these rocks as typical subduction-related granitoids as found worldwide in intra-oceanic arcs, and they probably formed through fractional crystallization of mantlederived low- to medium-K basalts. Several distinct phases of deformation were recognized in the Sierra Maestra, labelled D1 to D6, which define the transition from collision of the Palaeogene island arc to the formation of the Oriente Transform Wrench Corridor south of Cuba and later movement of the Caribbean plate against the North American plate. The first phase (D1) is related to the intrusion of a set of extensive subparallel, N-trending subvertical basalt-andesite dykes, probably during the early to middle Eocene. Between the late-middle Eocene and early Oligocene (D2), rocks of the Sierra Maestra were deformed by approximately east-west trending folds and north-vergent thrust faults. This deformation (D2) was linked to a shift in the stress regime of the Caribbean plate from mainly NNE-SSW to E-W. This shift in plate motion caused the abandonment of the Nipe-Guacanayabo fault system in the early Oligocene and initiation of a deformation front to the south where the Oriente Transform Wrench corridor is now located. Compressive structures were overprinted by widespread extensional structures (D3), mainly faults with southward-directed normal displacement in the Oligocene to early Miocene. During this period the plate boundary jumped to the Oriente fault. This event was followed by transpressive and transtensive structures (D4–D6) due to further development of the sinistral E-trending Oriente Transform wrench corridor. These structures are consistent with oblique convergence in a wide zone of left-lateral shear along an E-W-oriented transform fault.

40Ar/39Ar age evidence for Altyn fault tectonic activities in western China

Four40Ar/39Ar age groups of mica, hornblende and K-feldspar were obtained from Proterozoic and early Paleozoic metamorphic rocks in the Aksay-Dangjin Pass area, western China. The samples away from the middle shear zone of the Altyn fault belt yield two plateau age groups in the range of 461-445.2 Ma and 414.9-342.8 Ma, respectively. They represent the tectono-thermal events that had been recorded in the rocks that were displaced by the Altyn strike-slip fault in late Ordovician-early Silurian and Devonian, respectively. These two age groups should be related to the closures of Northern and Southern Qilian Oceans. The deformed granitic gneiss from the northern belt gives a plateau age group of 178.4-137.5 Ma, which is interpreted as the active age of the Altyn fault in the middle-late Jurassicearly Cretaceous and should be related to the accretion of Lhasa block to the north. The sample from the middle shear zone of the Altyn fault belt yields two plateau ages of 36.4 and 26.3 Ma, respectively, suggesting the strike-slip movement with strong metamorphism at greenschist facies along the Altyn fault in the late Eocene. This event occurred in the most areas of the northern Tibet Plateau and should be in response in the north to the collision between Indian and Eurasian continents. The present study demonstrates that the Altyn fault is characterized by multiple pulse-style activities under the tectonic setting of convergence between the Indian and Eurasian continents.

40Ar/39Ar dating of a Langhian biotite-rich clay layer in the pelagic sequence of the Cònero Riviera, Ancona, Italy

A nearly complete and undisturbed Miocene carbonate sequence is present in the easternmost part of the Umbria-Marche basin, Italy, which is ideal for detailed and integrated stratigraphic investigations of the Miocene Epoch. In this study, we were trying to obtain evidence for the presence or absence of distal ejecta from the 15 Ma Ries impact structure in southern Germany, located about 600 km to the north–northwest of the Umbria-Marche basin. The first step is to find coeval strata in the Umbria-Marche sequence. At the La Vedova section, Conero Riviera, we dated a volcaniclastic biotite-rich clay layer, the Aldo Level, which is situated within planktonic foraminiferal Zone N8, at 14.9±0.2 Ma, using the 40Ar/39Ar method. Together with detailed geologic and stratigraphic information about the Aldo Level, the resulting age can be used confidentially to calibrate the Langhian stage. Besides providing new constraints on Miocene geochronology, this age can now be used for impact stratigraphic studies. To directly correlate the biotite ages of the La Vedova section with rocks from the Ries impact event, Ries impact glass was also analyzed and found to be coeval. Although unrelated to this impact event, the biotite-rich clay layer should help in the search for evidence of distal ejecta related to the Ries crater.

Late Paleozoic–Mesozoic tectonics of the Dinarides revisited: Implications from 40Ar/39Ar dating of detrital white micas

Detrital white mica has been dated by the single-grain 40 Ar/ 39 Ar technique from three distinct stratigraphic levels and tectonic settings within the Dinarides in order to monitor the geodynamic evolution of this segment of Alpine orogen, which is located near the transition of Variscan to Pan-African continental crust. Detrital micas of a Pennsylvanian foreland basin suggest erosion of medium to deep crustal levels of the Variscan orogen. The 40 Ar/ 39 Ar ages indicate almost no preservation of older, pre-Variscan crust, except for a low proportion of late Pan-African ages. These data are in accordance with previous findings from the Variscan belt that suggest that the whole upper level of Variscan crust had been nearly entirely eroded prior to molasse deposition. Variscan micas also dominate in graywackes of the Middle to Late Jurassic trench-filling deposits exposed within the Dinaric ophiolite belt, indicating a nearby Variscan continent. No mica from a Mesozoic metamorphic accretionary complex has been detected. By contrast, the Upper Cretaceous Vardar Flysch postdates the emplacement of the Dinaric ophiolite belt as part of the Dinaric-Carpathian metamorphic orogenic wedge, and likely represents a collapse basin postdating Early to early Late Cretaceous nappe stacking. The studied micas comprise a dominant Variscan group and a subordinate Early Cretaceous group from the eroding metamorphic orogenic wedge.

Paleomagnetism and 40Ar/39Ar Age Determinations of Impactites from the Ilyinets Structure, Ukraine

Oriented hand samples were collected at the Ilyinets impact structure, western Ukraine, for paleomagnetic and petrographic studies. The samples consist of suevites, melt-breccias, impact melt rocks, autochthonous granite breccias, fractured granites, and unfractured gneisses. Three melt-breccias and one impact glass sample were used for laser 40Ar/39Ar dating. The characteristic remanent magnetization (ChRM) of the impact rocks has been acquired during the post-shock cooling, presumably in the presence of hydrothermal fluids. The alternating field demagnetization data indicate that this “impact” ChRM is of dual polarity and distinct from the ca. 1830-Ma-old pre-impact “target” remanence in the unfractured basement rocks. Some fractured autochthonous granites give evidence for impact overprinting, suggesting a fully positive paleomagnetic impact test. Paleomagnetic results suggest that no large-scale post-impact structural tilting of the melt sheets have taken place. The data allow a minor ~5° tilting towards the north. The combined paleomagnetic and 40Ar/39Ar dating results suggest an age of ca. 445 ± 10 Ma for the impact. The petrographic observations show that some secondary hydrothermal alteration took place in the rocks after the impact, which may have been responsible for some of the slightly younger Ar-Ar ages. The results show that the Ukrainian Shield was in contact with Baltica 445 Ma ago.