Manfred R. Strecker

Accommodation of transpressional strain in the Arabia‐Eurasia collision zone: new constraints from (U‐Th)/He thermochronology in the Alborz mountains, north Iran

The Alborz range of N Iran provides key information on the spatiotemporal evolution and characteristics of the Arabia-Eurasia continental collision zone. The southwestern Alborz range constitutes a transpressional duplex, which accommodates oblique shortening between Central Iran and the South Caspian Basin. The duplex comprises NW-striking frontal ramps that are kinematically linked to inherited E-W-striking, right-stepping lateral to obliquely oriented ramps. New zircon and apatite (U-Th)/He data provide a high-resolution framework to unravel the evolution of collisional tectonics in this region. Our data record two pulses of fast cooling associated with SW-directed thrusting across the frontal ramps at ~ 18–14 and 9.5-7.5 Ma, resulting in the tectonic repetition of a fossil zircon partial retention zone and a cooling pattern with a half U-shaped geometry. Uniform cooling ages of ~ 7–6 Ma along the southernmost E-W striking oblique ramp and across its associated NW-striking frontal ramps suggests that the ramp was reactivated as a master throughgoing, N-dipping thrust. We interpret this major change in fault kinematics and deformation style to be related to a change in the shortening direction from NE to N/NNE. The reduction in the obliquity of thrusting may indicate the termination of strike-slip faulting (and possibly thrusting) across the Iranian Plateau, which could have been triggered by an increase in elevation. Furthermore, we suggest that ~ 7-6-m.y.-old S-directed thrusting predated inception of the westward motion of the South Caspian Basin.

Late Cenozoic tectonism, collapse caldera and plateau formation in the central Andes

Abstract The evolution of Andean volcanism including the formation of late Miocene to Recent collapse calderas on the Puna plateau is generally interpreted in terms of the kinematic framework of the Nazca and South American Plates. We present evidence that caldera dynamics and associated ignimbrite volcanism are genetically linked to the activity of first-order NW–SE-striking zones of left-lateral transtension on the local and regional scales. Consequently, ages of collapse calderas indicate activity of these fault zones which initiated at about 10 Ma on the Puna plateau. The onset of such faulting points to a change in the deformation regime from dominantly vertical thickening to orogen-parallel stretching upon reaching maximum crustal thickness and critical surface elevation. Horizontal magma sheets that formed at mid-crustal level possibly due to heat advection by volume increase of asthenospheric mantle below thickened crust were tapped by sub-vertical faults. This accounts well for the observed tectono-magmatic phenomena at surface. It follows that formation of collapse calderas and eruption of voluminous ignimbrites appear to be related to the mechanical evolution of the Andean plateau rather than to changes in the geometry of the Wadati–Benioff zone or plate boundary kinematics.

Segmentation of megathrust rupture zones from fore-arc deformation patterns over hundreds to millions of years, Arauco peninsula, Chile

(1) This work explores the control of fore-arc structure on segmentation of megathrust earthquake ruptures using coastal geomorphic markers. The Arauco-Nahuelbuta region at the south-central Chile margin constitutes an anomalous fore-arc sector in terms of topography, geology, and exhumation, located within the overlap between the Concepcion and Valdivia megathrust segments. This boundary, however, is only based on � 500 years of historical records. We integrate deformed marine terraces dated by cosmogenic nuclides, syntectonic sediments, published fission track data, seismic reflection profiles, and microseismicity to analyze this earthquake boundary over 10 2 -10 6 years. Rapid exhumation of Nahuelbutas dome-like core started at 4 ± 1.2 Ma, coeval with inversion of the adjacent Arauco basin resulting in emergence of the Arauco peninsula. Here, similarities between topography, spatiotemporal trends in fission track ages, Pliocene-Pleistocene growth strata, and folded marine terraces suggest that margin-parallel shortening has dominated since Pliocene time. This shortening likely results from translation of a fore-arc sliver or microplate, decoupled from South America by an intra-arc strike-slip fault. Microplate collision against a buttress leads to localized uplift at Arauco accrued by deep-seated reverse faults, as well as incipient oroclinal bending. The extent of the Valdivia segment, which ruptured last in 1960 with an Mw 9.5 event, equals the inferred microplate. We propose that mechanical homogeneity of the fore-arc microplate delimits the Valdivia segment and that a marked discontinuity in the continental basement at Arauco acts as an inhomogeneous barrier controlling nucleation and propagation of 1960-type ruptures. As microplate-related deformation occurs since the Pliocene, we propose that this earthquake boundary and the extent of the Valdivia segment are spatially stable seismotectonic features at million year scale.

East African climate change and orbital forcing during the last 175 kyr BP

Abstract Variations in the temporal and spatial distribution of solar radiation caused by orbital changes provide a partial explanation for the observed long-term fluctuations in African lake levels. The understanding of such relationships is essential for designing climate-prediction models for the tropics. Our assessment of the nature and timing of East African climate change is based on lake-level fluctuations of Lake Naivasha in the Central Kenya Rift (0°55′S 36°20′E), inferred from sediment characteristics, diatoms, authigenic mineral assemblages and 17 single-crystal 40Ar/39Ar age determinations. Assuming that these fluctuations reflect climate changes, the Lake Naivasha record demonstrates that periods of increased humidity in East Africa mainly followed maximum equatorial solar radiation in March or September. Interestingly, the most dramatic change in the Naivasha Basin occurred as early as 146 kyr BP and the highest lake level was recorded at about 139–133 kyr BP. This is consistent with other well-dated low-latitude climate records, but does not correspond to peaks in Northern Hemisphere summer insolation as the trigger for the ice-age cycles. The Naivasha record therefore provides evidence for low-latitude forcing of the ice-age climate cycles.

Low slip rates and long-term preservation of geomorphic features in Central Asia.

In order to understand the dynamics of the India–Asia collision zone, it is important to know the strain distribution in Central Asia, whose determination relies on the slip rates for active faults. Many previous slip-rate estimates of faults in Central Asia were based on the assumption that offset landforms are younger than the Last Glacial Maximum (∼20 kyr ago). In contrast, here we present surface exposure ages of 40 to 170 kyr, obtained using cosmogenic nuclide dating, for a series of terraces near a thrust at the northern margin of the Tibetan Plateau. Combined with the tectonic offset, the ages imply a long-term slip rate of only about 0.35 mm yr-1 for the active thrust, an order of magnitude lower than rates obtained from the assumption that the terraces formed after the Last Glacial Maximum. Our data demonstrate that the preservation potential of geomorphic features in Central Asia is higher than commonly assumed.

Climate change in response to orographic barrier uplift: Paleosol and stable isotope evidence from the late Neogene Santa María basin, northwestern Argentina

The late Cenozoic history of the Santa Maria basin in the northeastern Sierras Pampeanas records the far-reaching climatic consequences of the development of orographic barriers to Atlantic-derived moisture. The evolution of climate and ecosystems is coupled with the uplift of basement blocks west and east of the basin to their present elevations of 4000–5500 m. The climatic and ecological history of the basin for the past 12 m.y. was reconstructed from paleosol characteristics and carbon and oxygen isotopes of pedogenic material. A fluvial environment succeeded the final marine regression after ca. 12 Ma. Paleosols with illuvial clay and calcic nodules indicate a seasonally dry climate. Between ca. 9 and 7 Ma, paleosols formed under conditions of episodic waterlogging, as shown by organic matter preservation and variegated colors. After ca. 7 Ma, pedogenesis occurred under enhanced drainage due to tectonic steepening of the eastern basin margin. This can be related to westward thrusting of the eastern basin-bounding Sierra Aconquija and Cumbres Calchaquies. Calcic and silicic rhizoliths and authigenic clay point to persistent wet-dry seasonality. Carbon isotope values document the spread of C 4 plants. Between ca. 5 and 3 Ma, carbon isotopes show an increase in C 3 plants. This is likely a consequence of uplift of the Sierra de Quilmes to the west, resulting in a transitory episode of orographic precipitation within the Santa Maria basin. Aridification of the valley occurred between ca. 3 and 2.5 Ma and was related to creation of high topography to the east. Development of thick calcretes suggests that the present semiarid conditions were established by the end of Pliocene time. Carbon isotopes show that the Quaternary vegetation was C 4 -plant dominated, and oxygen isotopes attest to intense soil-water evaporation.

Response of intracontinental deformation in the central Andes to late Cenozoic reorganization of South American Plate motions

New stratigraphic, chronologic, and fault kinematic data from the Quebrada del Toro in northwestern Argentina provide a detailed view of late Cenozoic kinematic reorganization in the southern central Andes. The Quebrada del Toro is a thrust-bounded basin filled with 2 km of synorogenic clastic deposits, ranging in age from early Miocene to Holocene. As observed elsewhere in northwestern Argentina, faults in the Quebrada del Toro record an early phase of horizontal NW-SE contraction (beginning between early and late Miocene and ending after 0.98 Ma) and a later phase of horizontal NE-SW contraction (beginning between late Miocene and 4.17 Ma and still active). These results require both kinematic regimes to have been active between 4.17 and 0.98 Ma, compatible with timing and kinematic data from the adjacent central Andes and best explained by a temporal pattern of serial recurrence of the kinematic regimes. A hypothesis that intraplate deformation in the central Andes is driven by the absolute motion of the South American Plate with respect to the hotspot reference frame is tested by comparing Quebrada del Toro kinematics to plate reconstructions and space-based geodetic results. Nazca-South America relative motion and absolute Nazca Plate motion show consistent azimuths throughout the past 20.5 Myr. In contrast, absolute South American Plate motion azimuths mimic the changing contraction directions determined from intraplate Andean faults. Although the timing of the South American Plate motion change is poorly resolved by current data, it is compatible with the Quebrada del Toro data. Absolute upper plate motion might control intraplate kinematics along many of the Earths convergent margins.

From tectonically to erosionally controlled development of the Himalayan orogen

Whether variations in the spatial distribution of erosion influ- ence the location, style, and magnitude of deformation within the Himalayan orogen is a matter of debate. We report new 40 Ar/ 39 Ar white mica and apatite fission-track (AFT) ages that measure the vertical component of exhumation rates along an ;120-km-wide NE-SW transect spanning the greater Sutlej region of northwest India. The 40 Ar/ 39 Ar data indicate that first the High Himalayan Crystalline units cooled below their closing temperature during the early to middle Miocene. Subsequently, Lesser Himalayan Crys- talline nappes cooled rapidly, indicating southward propagation of the orogen during late Miocene to Pliocene time. The AFT data, in contrast, imply synchronous exhumation of a NE-SW-oriented ;80 3 40 km region spanning both crystalline nappes during the Pliocene-Quaternary. The locus of pronounced exhumation de- fined by the AFT data correlates with a region of high precipita- tion, discharge, and sediment flux rates during the Holocene. This correlation suggests that although tectonic processes exerted the dominant control on the denudation pattern before and until the middle Miocene; erosion may have been the most important factor since the Pliocene.

Climatic forcing of asymmetric orogenic evolution in the Eastern Cordillera of Colombia

New apatite fission-track data, paleoelevation estimates from paleobotany, and recently acquired geological data from the Eastern Cordillera of Colombia document the onset of increased exhumation rates in the northeastern Andes at ca. 3 Ma. The Eastern Cordillera forms an efficient orographic barrier that intercepts moisture-laden winds sourced in the Amazon lowlands, leading to high rainfall and erosion gradients across the eastern flank of the range. In contrast, the drier leeward western flank is characterized by lower rates of deformation and exhumation. In light of the geological evolution of the Eastern Cordillera, the combination of these data sets suggests that the orographic barrier reached a critical elevation between ca. 6 and ca. 3 Ma, which ultimately led to protracted, yet more focused erosion along the eastern flank. Sequentially restored structural cross sections across the eastern flank of the Eastern Cordillera indicate that shortening rates also have increased during the past 3 Ma. From fission-track and structural cross-section balancing, we infer that accelerated exhumation led to increasing tectonic rates on the eastern flank, creating a pronounced topographic and structural asymmetry in the Eastern Cordillera. The tectonic and climatic evolution of this orogen thus makes it a prime example of the importance of climatic forcing on tectonic processes.

Coastal deformation and great subduction earthquakes, Isla Santa María, Chile (37°S)

Isla Santa Maria at the active margin of south-central Chile is the result of earthquake-related uplift and deformation in the forearc since at least late Pleistocene time. Field mapping, dating of key depositional horizons, and analysis of seismic-refl ection profi les reveal ongoing deformation in this sector of the Chilean forearc. The 30 km 2 island is located ~12 km above the interplate seismogenic zone and 75 km landward of the trench. It is situated near the southern termination of the Concepcion earthquake rupture segment, where Charles Darwin measured 3 m of coseismic uplift during a M > 8 megathrust earthquake in 1835. Permanent postearthquake deformation from this earthquake and an earlier event in 1751 is registered by emerged, landward-tilted abrasion surfaces. Uplift at ~2 m/k.y. and tilting at ~0.025°/k.y. of the island have been fairly constant throughout the late Quaternary and have resulted in emergence of the island above sea level ~31 k.y. ago. The island is composed of a late Pleistocene upper, tilted surface with two asymmetric tilt domains, and Holocene lowlands characterized by uplifted and tilted strandlines. Industry offshore seismic-refl ection profi les covering an area of ~1800 km 2 and crustal seismicity reveal active reversefault cored anticlines surrounding Isla Santa Maria; the principal fault apparently roots in the plate-interface thrust. These reverse faults in the upper plate result from inversion of late Cretaceous to early Pliocene normal faults and rift structure of the Arauco forearc basin. Positive inversion of these inherited structures started between 3.6 and 2.5 Ma and resulted in continuous shortening rates of ~0.8 mm/yr. The seismic-refl ection profi les show that the asymmetric tilt domains and progressive syntectonic sedimentation are linked to the position of the island in the forelimbs of two converging anticlines, whereas their backlimbs have been removed by cliff retreat. The 2 m uplift contour of the 1835 earthquake is parallel to the strike of active faults and antiforms in the Arauco-Concepcion region. The close relation among the asymmetric uplift and tilt of the island, modern deformation patterns, and reverse faults rooted in the plate interface suggests that slip on the plate interface thrust infl uences, localizes, and segments surface deformation during large interplate earthquakes. Furthermore, the link between positive inversion of pre-existing structures, uplift, and tilt patterns in the forearc emphasizes the importance of inherited structural fabrics in guiding plate-boundary deformation.