T21D-01 INVITED
Extensional Tectonic Events in the Basin and Range of Western North America
Following late Proterozoic rifting and Paleozoic passive-margin development, western North America (W NAm) was dominated by subduction, accretion, retroarc thrust-belt activity, and lateral terrain translations, followed by widespread extension. Extension began with metamorphic core complex (MCC) formation in an Eocene back-arc setting in the NW US and SW Canada, and MCCs rapidly spread south to E Nevada and W Utah, probably triggered by slab collapse following low-angle Laramide subduction. In late Oligocene-early Miocene time, extension occurred in Chihuahua, Mexico, and spread west along with west-migrating arc volcanics, but "jumped" across the unextended Sierra Madre Occidental. MCC extension in the southern Basin and Range (Arizona, New Mexico, and Sonora, Mexico) began at this time and rapidly propagated NW, to SE California and S Nevada, meeting the northern belt by ~20 Ma near Las Vegas, NV. This propagation was probably in response to slab rollback and a migrating slab window. The Rio Grande rift also formed at this time in cratonal crust, so extended crust surrounded the strong Colorado Plateau on the W, S and E. Subsequent extension in Arizona and the Rio Grande Rift was relatively minor but rapid extension in Nevada and E California continued on steeply to gently dipping faults forming half graben basins with tilted footwall ranges and local MCCs. Both WNW- and ESE- striking dextral and sinistral faults, respectively, formed in conjunction. During this phase, subduction ceased, volcanism switched from calcalkaline to bi-modal alkaline, the dextral San Andreas fault system and Gulf of California dextral-oblique rift evolved, and the regional extension direction rotated CW. Lithospheric rupture in W NAm finally occurred during rapid (>300 km in <6 m.y.) transtension in the Gulf of California, along the mid- Miocene volcanic arc, probably in response to basal traction along the continental margin and increased strain rates between the strong Sierra Madre and Peninsular Ranges batholithic belts. This phase of extension left significant lateral variation in Moho depth and degree of continental extension prior to lithospheric rupture.
T21D-02 INVITED
Morphotectonic Development of Western Anatolia
Western Anatolia is characterized by a number of east- west trending grabens and horsts. They have formed under approximately north- south extension according to the analysis of the seismic data. The views on the history of development of the grabens are conflicting. To solve this long lasting controversy new data have been collected from a mapping project revealed that the generation of the present morphology of the region is much more complex than what have been suggested previously; The present east- west trending grabens are very young; post Miocene in age. Prior to their development, the region has suffered different phases of deformation which may be summarized as follows; a. a pregraben stage b. Opening of North-South trending graben stage. c. An earlier stage of North-South extension d. An interrupting stage when the region undervent a regionwide erosion. e. Later stage of North South extension. In this paper, data derived from different areas, based on the mapping conduct, will be documented to support each of these phases, and their timing will be discussed.
T21D-03
Continental Extension in Basin and Range, USA and Western Anatolia Extended Terrane (WAET) in western Turkey: A review
The Basin and Range of western USA and Western Anatolia Extended Terrane (WAET) of western Turkey are two major extended terranes that share many characteristics. They both contain many common structural elements indicative of pervasive continental extension such as well-developed metamorphic core complexes (MCC), detachment surfaces, turtleback faults, extensional folds, supra detachment basins, and extensional shear sense indicators. The Basins and Ranges extension began in Eocene with MCC development in NW USA in a back-arc setting. The extension progressively moved southward into the eastern Great Basin in Late Oligocene-Early Miocene time. Extension in the southern Basin and Range was underway in early-middle Miocene. Well after the onset of extensional faulting in these areas, volcanic activity changed from calc-alkaline to bimodal. Current geological work in the Basin and Range is concentrated on the amount and percentage of extension with respect to extensional faulting, and amount of displacement along major strike-slip fault zones. Continental extension in the WAET initiated in Late Oligocene and progressively developed in three stages. During the first stage, a north-dipping extensional simple-shear zone formed probably due to orogenic collapse. During this stage, erosion and extensional unroofing brought high-grade metamorphic rocks of the central and northern Menderes Massif to the surface by the Early Miocene. The second stage of extension started in Early Miocene, probably caused by slab roll-back of the Hellenic arc. It produced the north-dipping Alasehir and the south-dipping Buyuk Menderes Detachment surfaces which control the Miocene supra detachment sedimentation in the Alasehir and Buyuk Menderes Grabens. The third stage may have started ~5 Ma when the North Anatolian Fault initiated and the westward tectonic escape of the Anatolian Plate started. This stage produced high angle faults within the Alasehir, Buyuk Menderes and Kucuk Menderes Grabens. Current geological work in WAET is concentrated on the age and nature of the metamorphism in the Menderes massif; the exhumation history of the metamorphic rocks along the low angle detachment surfaces; and the amount and percentage of extension.
T21D-04
Kinematics of Faulting and Structural Evolution of Neogene Supra-detachment Basins on the Menderes Metamorphic Core Complex, Western Anatolia
The Menderes metamorphic massif (MM) in western Anatolia is a classic core complex with exhumed high-grade crustal rocks intruded by granodioritic plutons and overlain by syn-extensional sedimentary rocks. Timing and the mechanism(s) of the initial exhumation of the MM are controversial, and different hypotheses exist in the literature. Major structural grabens (i.e. Alasehir, Buyuk Menderes) within the MM that are bounded by high-angle and seismically active faults are late-stage brittle structures, which characterize the block-faulting phase in the extensional history of the core complex and are filled with Quaternary sediments. On the southern shoulder of the Alasehir graben high-grade metamorphic rocks of the MM are overlain by the Miocene and younger sedimentary rocks above a N-dipping detachment surface. The nearly 100-m-thick cataclastic shear zone beneath this surface contain S-C fabrics, microfaults, Riedel shears, mica-fish structures and shear bands, all consistently indicating top-to-the North shearing. Granodioritic plutons crosscutting the MM and the detachment surface are exposed within this cataclastic zone, displaying extensional ductile and brittle structures. The oldest sedimentary rocks onlapping the cataclastic shear zone of the MM here are the Middle Miocene lacustrine shale and limestone units, unconformably overlain by the Upper Miocene fluvial and alluvial fan deposits. Extensive development of these alluvial fan deposits by the Late Miocene indicates the onset of range-front faulting in the MM by this time, causing a surge of coarse clastic deposition along the northern edge of the core complex. The continued exhumation and uplift of the MM provided the necessary relief and detrital material for the Plio-Pleistocene fluvial systems in the Alasehir supradetachment basin (ASDB). A combination of rotational normal faulting and scissor faulting in the extending ASDB affected the depositional patterns and drainage systems, and produced local unconformities within the basinal stratigraphy. High-angle, oblique-slip scissor faults crosscutting the MM rocks, the detachment surface and the basinal strata offset them for more than few 100 meters and the fault blocks locally show different structural architecture and metamorphic grades, suggesting differential uplift along these scissor faults. This fault kinematics and the distribution of range-parallel and range-perpendicular faults strongly controlled the shape and depth of the accommodation space within the ASDB. At a more regional scale scissor faulting across the MM seems to have controlled the foci of Plio-Pleistocene point-source volcanism in the Aegean extensional province (e.g. Kula area). There are no major interruptions in the syn-extensional depositional history of the ASDB, ruling out the pulsed-extension models suggesting a period of contractional deformation in the late Cenozoic evolution of the MM. The onset of exhumation and extensional tectonics in the MM and western Anatolia was a result of thermal weakening of the orogenic crust, following a widespread episode of post-collisional magmatism in the broader Aegean region during the Eocene through Miocene.
T21D-05
Temporal patterns of detachment faulting along Cycladic extensional metamorphic domes, Aegean region
The Aegean region together with the surrounding mainland areas are known for Miocene to Recent active, notably extensional tectonics that are the result of the retreating Hellenic slab, gravitational collapse of the region and, since the Late Miocene to Early Pliocene, the westward escape of Anatolia relative to Eurasia. Project ACCEL (Aegean Core Complexes along an Extended Lithosphere) has collected an extensive modern structural dataset for the islands of Kea, Kithnos, and Serifos of the western Cyclades. On all three islands, crustal-scale, low-angle frictional-viscous shear zones have been identified and record strikingly consistent SSW-directed extensional kinematics together with a WNE-ESE shortening component. The geology of Kea is dominated by highly-strained, greenschist-facies schists, calc-silicates and marbles; a major, 100's m thick ultramylonite zone defines this northern island as a structural dome. White mica Ar-Ar thermochronometry performed on variably deformed units from different structural levels yield consistent Early Miocene (15-19 Ma) cooling ages across the entire island. Other pervasively deformed shallow crustal regions of the Cyclades (Tinos and Andros; b-type domes of Jolivet et al. 2004) also record similar Early Miocene cooling. Comparable geology and structure is exposed on Serifos although locally deformed under amphibolite-facies conditions and intruded in the south by a Late Miocene granodiorite. A major high strain zone that is present on the island is localized along an earlier (Late Eocene) granitic pluton. White micas from mylonites and gneisses along this shear zone and from rocks in the southern portion of the island yield Late Miocene (8-9 Ma) cooling ages, whereas greenschist-facies units in northern Serifos that are dominated by a different structural record of several phases of folding yield Oligocene (30-34 Ma) mica Ar-Ar cooling ages. Oligocene ages are similarly reported from Evvia and Sifnos, which are noted for their occurrences of the Cycladic Blueschist Unit. We interpret the Late Miocene cooling ages to represent the timing of extension and exhumation during metamorphic core complex genesis, which is coeval with the timing of exhumation the other Cycladic a-type metamorphic domes (Naxos and Paros) that represent deeper parts of the Hellenic accretionary complex. Since Late Miocene extension in the Aegean region has been generally considered to be N-directed, our results suggest a new domain in the Western Cyclades that has been exhumed by S-directed extension.
T21D-06
The behavior of monazite in pelitic assemblages from Menderes Massif, western Turkey: Implications for timing and deciphering rates of extension
The Menderes Massif (Western Turkey) is one of several Aegean metamorphic core complexes, and understanding the geochronologic and thermobarometric history of its rocks provides information about the large- scale geodynamic processes that facilitate extension in the Earth's lithosphere. The region experienced compression during the Cambro-Ordovician and Eocene, followed by large-scale extension during the Oligocene. Here, we apply in situ ion microprobe monazite (REEThPO4) dating of garnet-bearing pelites in an attempt to constrain the timing of extension in the Menderes Massif. Rocks exposed in the core complex rarely yield monazite ages consistent with a single population, ranging from the Cambro-Ordovician to Pliocene. As a further complication for extensional settings, the temperature of monazite crystallization in pelites can range from very low diagenetic to ultra-high conditions. These monazites also contain more common Pb than is usually present in the mineral. Monazite may form a variety of reactants, including rare-earth carbonates or oxides, allanite, even garnet. The presence of monazite in pelitic assemblages from the Menderes Massif does not appear related to the major or trace element chemistry of the rocks. As an extreme example, monazite inclusions in garnet in one sample are as much as 400 m.y. different than those in the matrix, suggesting non-equilibrium conditions and problematic P-T calculations. Cathodoluminescence images of the dated pelites show evidence for fluid flow that could have triggered dissolution-reprecipitation reactions for monazites not armored by garnet. Correlating the timing of tectonic events in the Menderes Massif to those occurring in metamorphic core complexes elsewhere in the Aegean is difficult without the use of other dating techniques, and confidence that minerals involved in thermobarometric calculations experienced equilibrium conditions.
T21D-07
From detachment to transtensional faulting: A model for the Lake Mead extensional domain based on new ages and correlation of subbasins
New studies of selected basins in the Miocene extensional belt of the northern Lake Mead domain suggest a new model for the early extensional history of the region (lower Horse Spring Formation and correlative strata). Critical data are from (i) Longwell Ridges area west of Overton Arm and within the Lake Mead fault system, (ii) Salt Spring Wash basin in the hanging wall of the South Virgin-White Hills detachment (SVWHD) fault, and (iii) previously studied subbasins of the south Virgin Mountains in the Gold Butte step-over region. The basins and faulting patterns suggest two stages of basin development related to two distinct faulting episodes, an early period of detachment faulting followed by a switch to faulting mainly along the Lake Mead transtensional fault system while detachment faulting waned. Apatite fission track ages suggest the footwall block of the SVWHD was cooling at 18-17 Ma, but the only evidence for basin deposition at that time is in the Gold Butte step-over where slow rates of sedimentation and facies patterns make faulting on the north side of the Gold Butte block ambiguous. The first basin stage was ca. 16.5 to 15.5 Ma, during which there was slow to moderate faulting and subsidence in a basin along the SVWHD and north of Gold Butte block in the Gold Butte step-over basin; the step- over basin had complex fluvial and lacustrine facies and was synchronous with landslides and debris flows in front of the SVWHD. At ca. 15.5-14.5 Ma, there was a [dramatic] increase in sedimentation rate related to formation of the Gold Butte fault, a change from lacustrine to widespread fluvial, playa, and local landslide facies in the step-over basin, and the peak of exhumation and faulting rates on the SVWHD. The simple step-over basin broke up into numerous subbasins [at[ as initial faults of the Lake Mead fault system formed. From 14.5 to 14.0 Ma, there was completion of a major change from dominantly detachment faulting to dominantly transtensional faulting in the Lake Mead fault system as detachment faulting waned. The Lake Mead fault system began to propagate to the west and faults and subbasins north of Gold Butte died with major progradation of alluvial conglomerates over the step-over basin. The geometry of the SVWHD that dominated the early Lake Mead extension history fundamentally controlled patterns of faulting and magmatism throughout the rest of the extensional history. This process of detachment faulting changing to dominant high-angle faulting and possible lower crustal flow has been suggested previously and may be a general process.