Regional Transpression Research Articles

Transpression or polyphase deformation along craton margins: Insights from the Archean–Proterozoic boundary near Sudbury, Canada

Continental margins vary in composition and configuration, resulting in complex structural geometries during deformation. One example is the Archean–Proterozoic boundary near Sudbury, Canada, where the Creighton-Victoria Deformation Zone (CVDZ) contains both reverse dip-slip and dextral strike-slip kinematic indicators. To resolve if these structures formed during a single transpression event or multiple overprinting events, the orientation, relative timing and style of the CVDZ are compared to those of adjacent regional structures. Regional fold axes and their axial planar foliation are subparallel to the steeply-plunging stretching lineation and steeply-dipping foliation, respectively, along the CVDZ. Phase equilibria modelling of syn-tectonic staurolite-chloritoid assemblages indicates that the regional folds and foliation formed at 480–565 °C, similar to temperatures estimated for the formation of dip-slip quartz crystallographic preferred orientations (CPO) along the CVDZ. This suggests that the CVDZ, regional folds and foliation formed during the same regional folding and reverse shearing event. Major lateral lithological changes and inflections in the orientation of the craton margin controlled the formation of structures, and evidence for regional transpression, such as strike-slip kinematic indicators, steeply-plunging stretching lineation, and obliquity of regional folds with respect to major shear zones, resulted from polyphase deformation and transcurrent reactivation of the Superior craton margin.

Neotectonics and paleoseismology of the North Tabriz Fault, Azerbaijan Region, Northwest Iran

Structural characteristics and seismic parameters were investigated to reevaluate seismic hazards along the North Tabriz Fault (NTF). Field surveys and paleoseismology on the central segment of the fault, combined with dating of sedimentary units, document multiple surface-rupturing paleoearthquakes during the past 3.5 ka. The calculated moment magnitudes (Mw) for the identified paleoearthquakes range from 6.3 to 7.5, causing surface ruptures of ca. 50 km with a maximum slip per event of 5.7 ± 0.2 m. Kinematic GPS surveys on displaced geomorphic features, combined with the dating results indicate that the central segment of the NTF slips at a rate of 6.4 ± 0.5 mm.yr-1 over the past 7.5 ka. Fault orientations and scattered volcanic vents in Sahand Volcanic Complex (SVC) suggest that its configuration and evolution have been affected by the regional transpression.

The interplay between magma source and geodynamic evolution in porphyry Cu-Au fertility of the Farallon Negro Volcanic Complex, Argentina

Zircon U-Pb, O, Lu-Hf and trace element systematics are used to track the magmatic-hydrothermal evolution and metallogenic potential of porphyry intrusions and associated Cu-Au deposits. We apply this analytical approach to unveil the poorly constrained geological history of the Farallon Negro Volcanic Complex in the Central Andes of northwest Argentina. This complex contains the world-class Au-rich Cu-Au Bajo de la Alumbrera deposit (here dated at 6.9 ± 0.1 to 6.8 ± 0.2 Ma; U-Pb SHRIMP) and a series of barren to weakly mineralized satellite porphyry intrusions, including El Durazno (8.0 ± 0.3 Ma; U-Pb SHRIMP), San Lucas (7.4 ± 0.1 to 7.3 ± 0.1 Ma; U-Pb SHRIMP) and Las Pampitas (7.2 ± 0.2 to 7.0 ± 0.1 Ma, U-Pb SHRIMP). Zircon Hf and O isotope data record a change in magma source from El Durazno (δ18O = 5.7 ± 0.3‰; εHf = −0.3 ± 1.8) to San Lucas, Las Pampitas and Bajo de la Alumbrera (δ18O > 6.7‰, εHf = − 4.0 ± 0.5 to −2.2 ± 1.0). This reflects a switch from a predominantly mantle-derived magma source to a mixed mantle-derived and supracrustal source that homogenized in a lower-crustal magma assimilation, segregation and homogenization (MASH) zone. All intrusions have a hydrous (Eu/Eu* > 0.5, Dy/Yb < 0.3), oxidized (ΔFMQ >1.0, Ce/Ce* > 100) and, therefore, ‘fertile’ zircon trace element signature, despite having variable Cu and Au grades. Likewise, ore-barren extrusive units in the FNVC have an overlapping ‘fertile’ zircon signature, reinforcing the view that zircon-based proxies for fertility may not be useful for mineralization targeting at a district scale. The magmatic evolution of the FNVC was significantly influenced by the tectonic setting, as the onset of regional transpression at ∼8 Ma after a period of localized extension caused pooling of mantle-derived magmas in a lower-crustal MASH zone and build-up of volatiles and metals. The petrogenetic model proposed for the FNVC emphasizes the role of the geodynamic environment and magma evolution in the Cu-Au endowment of Bajo de la Alumbrera

Structural development and tectonostratigraphic evolution of the Sylhet Trough (northeastern Bengal Basin) in the context of Cenozoic Himalayan Orogeny: Insights from geophysical data interpretation

The Sylhet Trough of the Bengal Basin in the northeastern Indian Plate had started to develop since the Late Mesozoic Gondwana break-up and subsequent onset of the Cenozoic Himalayan Orogeny. However, very little is known about evolution of the Cenozoic Sylhet Trough in response to the latter tectonic event. Therefore, we aim at providing insights into the nature of basement, variations in structural style, kinematics and possible timing of structural activation(s) and tectonostratigraphic evolution of the Sylhet Trough based on geophysical data interpretation. Results show that the Sylhet Trough is mostly underlain by an attenuated continental or oceanic crust, bounded by a NE-SW-trending geophysical feature in the northwest that corresponds to the boundary with the Precambrian continental crust. The overlying Cenozoic sediments show variations in style of deformations and can be divided into three structural domains. Domains 1 and 2 in the southeastern and northeastern parts of the Sylhet Trough, respectively, are characterized by anticlines that are highly deformed to the east and are mostly associated with west- or northwest-dipping reverse faults. However, the anticlinal trends are deviated from a generalized NS-orientation in the Domain 1 to a dominant NE-SW-orientation in the Domain 2. Many of these anticlines resemble positive flower structures, indicating possible impacts of regional transpression associated with the Dauki and Kaladan faults to the north and east; respectively. In contrast, the Domain 3 to the west shows NE-SW-trending extensional structures. A southward thickening of the Oligocene to Miocene pre-kinematic packages indicates an early episode of foreland basin subsidence, although there is no evidence of a major structural activation. Rather, the presence of the Pliocene syn-kinematic packages indicates a major structural reactivation in response to the E-W and N-S crustal shortening since the Pliocene. This younger episode of structural activation can be directly linked with the late Cenozoic subduction related processes of the Indian Plate to the north and to the east, which is also responsible for the transformation of the Sylhet Trough into a remnant ocean basin. This latter (i.e., Pliocene and onward) episode of crustal shortening played a significant role in structural trap formation and preservation of hydrocarbons in the Sylhet Trough.

Multiple sediment incorporation events in a continental magmatic arc: Insight from the metasedimentary rocks of the northern North Cascades, Washington (USA)

Abstract The rheology and composition of arc crust and the overall evolution of continental magmatic arcs can be affected by sediment incorporation events. The exhumed Cretaceous–Eocene North Cascades arc exposes abundant metasedimentary rocks that were incorporated into the arc during multiple events. This study uses field relationships, detrital zircon geochronology, bulk rock geochemistry, geothermometry, and quartz-in-garnet geobarometry to distinguish approximate contacts and emplacement depths for different metasedimentary units to better understand their protolith incorporation history and impact on the arc. The Skagit Gneiss Complex is one of the main deep crustal units of the North Cascades arc. It includes metasedimentary rocks with distinct detrital zircon signatures: Proterozoic–Cretaceous (Group 1) or Triassic–Cretaceous (Group 2) zircon populations. Both metasedimentary groups achieved near-peak metamorphic conditions of 640–800 °C and 5.5–7.9 kbar; several Group 2 samples reveal the higher pressures. A third group of metasedimentary rocks, which was previously interpreted as metamorphosed equivalents of backarc sediments (Group 3), exhibited unimodal Triassic or bimodal Late Jurassic–Early Cretaceous detrital zircon signatures and achieved near-peak conditions of 570–700 °C and 8.7–10.5 kbar. The combined field and analytical data indicate that protoliths of Group 1 and Group 2 metasedimentary rocks were successively deposited in a forearc basin and underthrusted into the arc as a relatively coherent body. Group 3 backarc sediments were incorporated into the arc along a transpressional step-over zone. The incorporation of both forearc and backarc sediments was likely facilitated by arc magmatism that weakened arc crust in combination with regional transpression.

The Sierra de Aguirre Formation, Uruguay: Post-collisional Ediacaran volcanism in the southernmost Dom Feliciano Belt

The Sierra de Aguirre Fm. in southern Uruguay constitutes one of several post-collisional basins of the Dom Feliciano Belt, preserving an important volcanic record associated with this protracted stage of the orogenic system. This study combines new stratigraphic, structural, geochronological and isotopic geochemical results, together with a reassessment of the available geochemical data, to present new insights in the evolution and sources of the basin, as well as a contextualization of it within the post-collisional development of the belt. The opening of the basin was related to a major discontinuity within the belt, with geometrical features suggesting an inverted half-graben structure. Stratigraphic and petrographic studies indicate that the Sierra de Aguirre Fm. preserves the original unconformity with its basement, and represent distal volcanic deposits. U–Pb SHRIMP-IIe zircon ages establish an interval of ca. 10 m.y. for the filling of the basin, between 581.7 ± 5.1 Ma and 571 ± 8 Ma. After the end of deposition, the basin underwent folding associated with the development of axial plane cleavage under lower greenschist facies conditions. This event is interpreted to represent regional transpression recorded also in nearby shear zones between 570 and 560 Ma, as constrained by new K–Ar ages in muscovite and white mica fine fractions. Geochemical and isotopic signature indicate dominant crustal sources for the volcanism, compatible with the basement of the eastern domain of the Dom Feliciano Belt, supporting a trend also observed in the coeval plutonic magmatism.

Zircon and monazite geochronology in the Palmer zone of transpression, south-central New England, USA: Constraints on timing of deformation, high-grade metamorphism, and lithospheric foundering during late Paleozoic oblique collision in the Northern Appalachian orogen

Abstract Middle to late Paleozoic high-angle (45°) oblique convergence between Laurentia and composite Avalon terranes resulted in crustal shortening of ∼5:1 across the Central Maine and Bronson Hill zones of the southern New England Appalachians (USA). The Palmer zone of transpression illustrates the midcrustal expression of magmatism, metamorphism, and ductile deformation that developed in response to oblique convergence in the apparent absence of subduction. Secondary ion mass spectrometry zircon U-Pb and monazite Th-Pb ages, supplemented by zircon laser-ablation–inductively coupled plasma–single-collector mass spectrometry U-Pb ages, expand the geochronology constraining the evolution of the Palmer zone of transpression system and potential models for transcurrent collisional tectonics. Ordovician, Silurian, and Early Devonian plutons, and regional high-grade metapelitic country rocks that comprise the pre-transpressional crustal infrastructure are the same age as lithologic equivalents to the north and northeast along the orogen that did not experience high-angle oblique convergence. All plutons within the Palmer zone of transpression and adjoining areas were deformed (flattened, boudinaged, attenuated, folded, and/or ductilely sheared) by the regional transpression system. The most widespread magmatic event in the study area, which is not observed elsewhere in the Northern Appalachians, is intrusion of a diorite-tonalite suite at 370–360 Ma, the oldest rocks of which contain granulite-facies mineral assemblages and all of which are deformed. Leucopegmatites that intrude pelitic paragneisses, which likely formed in response to high-grade metamorphism and which are all deformed, are 370–355 Ma in age. All metapelitic paragneisses contain foliations and lineations that are synkinematic with retrograde garnet + K-feldspar → sillimanite + biotite assemblages and fabrics that formed during vertical and lateral crustal escape in response to extreme shortening. Contemporaneous diorite-tonalite magmatism and regional high-grade metamorphism are interpreted to reflect regional-scale heating of the crust preceding inception of the transpressional system. Transpressional deformation of both plutons and paragneisses indicates shortening commenced after crustal thermal conditions peaked (ca. 355 Ma). Monazite in paragneiss, the formation of which in most samples is linked texturally to formation of transpressional fabrics, yielded a continuum of Th-Pb ages of ca. 360–330 Ma, indicating transpression continued after peak thermal conditions. The extreme crustal shortening of the Bronson Hill and Central Maine zones from Maine to southern New England, pre-transpressional magmatism, and high-grade metamorphism overprint an Acadian (ca. 400–370 Ma) magmatic-metamorphic infrastructure that occurred in the absence of subduction. The transpressional system exhibits all predicted thermal, magmatic, deformation, metamorphic, and exhumation/erosion characteristics of mantle lithospheric foundering (delamination, detachment, or drip) in response to extreme lithospheric shortening and vertical stretching at ca. 375–370 Ma, leading to advection of heat to the lower continental crust and attendant magmatic and metamorphic responses over the time span 370–355 Ma.

Evolution of the Major Gercino Shear Zone in the Dom Feliciano Belt, South Brazil, and implications for the assembly of southwestern Gondwana

The Dom Feliciano Belt developed during the Brasiliano–Pan-African orogenic cycle due to the tectonic interaction between the Rio de la Plata, Congo and Kalahari cratons, along with the amalgamation of smaller continental fragments. Together with its prolongations to the south, the Major Gercino Shear Zone constitutes one of the main lineaments of the orogenic system, establishing a more than 1000 km long NE-trending tectonic boundary between a granitic batholith and a metavolcano-sedimentary association. Based on combined field, structural, microstructural and textural data, together with new and published geochronological data, a refined model for the geological evolution of the Major Gercino Shear Zone is presented. Regional NW-verging, low-dipping structures were generated between 650 and 615 Ma, preceding the formation of the shear zone. This was followed by the main deformation phase, corresponding to pure-shear dominated dextral strike-slip, interpreted do be controlled by regional transpression during oblique convergence of the continental terranes. This stage lasted until ca. 585 Ma and was coeval with the continuous emplacement of granitic magmatism along the structure. Strain partitioning and localization led to the development of mylonitic belts along the intrusion borders mostly under greenschist facies metamorphic conditions. Late-stage ductile deformation along the structure was active during the cooling of the intrusions until ca. 550 Ma, while the deformation front of the orogenic system migrated to counterpart orogenic belts in the African continent. After tectonic stabilization, the Major Gercino Shear Zone recorded episodic brittle reactivation, possibly related to the Phanerozoic evolution of the active margins and intracratonic basins in Gondwana. © 2018, Springer-Verlag GmbH Germany, part of Springer Nature.

Kinematic evolution of the Mbakop Pan–African granitoids (western Cameroon domain): An integrated AMS and EBSD approach

This study integrates anisotropy of magnetic susceptibility, microstructural and crystallographic preferred orientation (CPO) data from the Mbakop granitic pluton (MGP; Pan-African age) in order to decipher its kinematic evolution. The MGP lies close to NE-SW branch of Central Cameroon Shear Zone (CCSZ) and is emplaced in gneissic basement. High mean magnetic susceptibility and presence of multi–domain magnetite are recorded. Quartz CPO measured using Electron Backscatter diffraction reveals dominance of rhomb <a>, prism <a> and prism <c> slip in different samples, which is consistent with microstructures developed under upper greenschist/amphibolite facies conditions. Quartz CPO along with other kinematic indicators (feldspar porphyroclasts/mineral fish) indicate non-coaxial deformation was important during tectonic evolution of the MGP. Contrasting sense of shear is recorded implying multi-stage mylonitization in the Western Cameroon Domain. Top-towards-south sense of shear is related to regional D2 deformation (613–585 Ma), while top-towards-north is related to D3 (585–540 Ma). The magnetic fabric in MGP records D3. The obliquity between mean orientation of magnetic foliation (N12°E) and the NE-SW CCSZ branch (N48°E) gives kinematic vorticity number of 0.95. This indicates dominantly simple shear with a minor pure shear component. It is concluded that regional transpression was important during MGP emplacement.

Magma‐facilitated transpressional strain partitioning within the Sawtooth metamorphic complex, Idaho: A zone accommodating Cretaceous orogen‐parallel translation in the Idaho batholith

Structural data from metasedimentary rocks and geochronologic data from intrusive rocks in the Idaho batholith provide evidence for the relationship between deformation and magmatism in the northern U.S. Cordillera. The Sawtooth metamorphic complex (SMC), Idaho, is exposed as an inlier in the central Idaho batholith and contains strongly deformed metasedimentary and intrusive rocks. Geologic mapping reveals north-south-striking, alternating contraction- and shear-dominated domains across strike. The contraction-dominated domains consist of centimeter- to tens of meter-scale, shallowly to steeply plunging upright folds with subhorizontal lineations. The shear-dominated domains are characterized by highly strained subvertical foliations, subhorizontal lineations, and syntectonic intrusive sheets. Pervasive S-C structures, winged porphyroclasts, and asymmetric folds indicate dextral strike-slip shearing. The fabrics in the two types of domains are structurally compatible and are interpreted to be broadly synchronous. This work suggests that the SMC structures represent a wrench-dominated transpressional zone, in which the regional strain partitioned into the contraction- and shear-dominated domains and the partitioning was facilitated by emplacement of syntectonic magmas. Zircon U-Pb data of the syntectonic intrusions indicate that the SMC transpressional deformation occurred mainly between ca. 95-92 Ma and ca. 84 Ma, and had ended by ca. 77 Ma. The transpressional deformation in the SMC and the western Idaho shear zone (WISZ) were kinematically compatible and partially coeval. This suggests that the SMC and WISZ represent a regional transpression system and that crustal deformation inboard of the continental margin may have contributed to the northward orogen-parallel translation of accreted terranes during the Late Cretaceous.

Microstructures and magnetic fabrics of the Ngaoundéré granite pluton (Cameroon): Implications to the late-Pan-African evolution of Central Cameroon Shear Zone

The Ngaoundéré granite pluton, in Central-North Cameroon, located near the Central Cameroon Shear zone (CCSZ), and previously studied for its petrography and geochemistry, is characterized by the absence of macroscopic markers of deformation. In this study, we report microstructures and magnetic fabrics (AMS) of this pluton and discuss the relationship with the Pan-African evolution of the CCSZ. The pluton consists of a porphyritic Hbl-Bt-monzogranite at its rim and a porphyritic biotite-granite at its core, a petrographic distribution denoting a normal zoning pattern, i.e. more silicic toward the centre. As expected, magnetic susceptibilities values also exhibit a zoning pattern in agreement with petrographic zonation. Thermomagnetic data indicate that this pluton is dominantly ferromagnetic in behaviour. As indicated by its microstructures, the pluton has suffered a continuum of deformation from the magmatic state to the high temperature solid-state during magma crystallization and solidification. The magnetic foliations dominantly strike NE-SW and dip moderately to steeply and the lineations mostly plunge shallowly to the NE or SW, roughly parallel to NE-to ENE-trending Central Cameroun Shear Zone (CCSZ). The foliation poles define a girdle pattern with a zone axis (52°/11°) rather close to the best line of the lineations (44°/21°). These fabrics correlate with the structures of the country rocks ascribed by several workers to a regional transpression. Toward the margins of the pluton, particularly the northern one, the lineations tend to rotate from NE to N in azimuth. This change is interpreted as due to strain partitioning, simple shearing with NE-SW extension being relayed by compression toward the northern pluton border. This new magnetic fabric study suggests that the Ngaoundéré pluton (poorly dated at c. 575 Ma) was emplaced during the late stages of the CCSZ dextral transpressive movement. It also provides some more constraints on the correlation between the CCSZ system and the shear zone system of NE-Brazil.

The pull apart-type Tardets-Mauléon Basin, a key to understand the formation of the Pyrenees

Contrary to recent conceptual models, we evidence that the Mauléon Basin does not only result from the Pyrenean tectonic inversion of an aborted Albian rift involving a N-S extreme crust thinning, with related detachment and mantle exhumation. It actually corresponds to an element of this rift system where E-W dominant regional sinistral strike slip faulting between its European and Iberian margins generated as early as the Latest Aptian (Clansayesian) an oblique pull apart-type basin: the Tardets-Sorholus Trough. Then, towards the Late to Latest Albian period, the active transverse, SW-NE oriented, Barlanès and Saison listric faults provided the main crustal thinning leading locally (Urdach) to mantle exhumation. Finally, at the beginning of the Late Cretaceous, the trough widened through transtension motion (N-S distension associated with E-W sinistral strike slip) leading to the creation of the Mauléon Basin. This geodynamic evolution gives to the Mauléon Basin its logical place between the western Bilbao Basin where oceanic crust developed through dominant N-S extension process and the central and eastern north Pyrenean basins where dominant E-W left lateral strike slip then transpressive motion preclude mantle denudation. From the Late to Latest Cretaceous, the inverted motion turning to generalized regional transpression led to the closure of the trough, then, by gradual uplift from east to west, to the formation of the Pyrenean range.

The sedimentary and tectonic evolution of the Amur River and North Sakhalin Basin: new evidence from seismic stratigraphy and Neogene–Recent sediment budgets

AbstractThe North Sakhalin Basin in the western Sea of Okhotsk has been the main site of sedimentation from the Amur River since the Early Miocene. In this article, we present regional seismic reflection data and a Neogene–Recent sediment budget to constrain the evolution of the basin and its sedimentary fill, and consider the implications for sediment flux from the Amur River, in particular testing models of continental‐scale Neogene drainage capture. The Amur‐derived basin‐fill history can be divided into five distinct stages: the first Amur‐derived sediments (&gt;21–16.5 Ma) were deposited during a period of transtension along the Sakhalin‐Hokkaido Shear Zone, with moderately high sediment flux to the basin (71 Mt year−1). The second stage sequence (16.5–10.4 Ma) was deposited following the cessation of transtension, and was characterised by a significant reduction in sediment flux (24 Mt year−1) and widespread retrogradation of deltaic sediments. The third (10.4–5.3 Ma) and fourth (5.3–2.5 Ma) stages were characterised by progradation of deltaic sediments and an associated increase in sediment flux (48–60 Mt year−1) to the basin. Significant uplift associated with regional transpression started during this time in southeastern Sakhalin, but the north‐eastward propagating strain did not reach the NE shelf of Sakhalin until the Pleistocene (&lt;2.5 Ma). This uplift event, still ongoing today, resulted in recycling of older deltaic sediments from the island of Sakhalin, and contributed to a substantially increased total sediment flux to the adjacent basinal areas (165 Mt year−1). Adjusted rates to discount these local erosional products (117 Mt year−1) imply an Amur catchment‐wide increase in denudation rates during the Late Pliocene–Pleistocene; however, this was likely a result of global climatic and eustatic effects, combined with tectonic processes within the Amur catchment and possibly a smaller drainage capture event by the Sungari tributary, rather than continental‐scale drainage capture involving the entire upper Amur catchment.

Restraining bend tectonics in the Santa Cruz Mountains, California, imaged using 10Be concentrations in river sands

Reverse faults frequently generate large and destructive earthquakes, yet their seismic hazard remains diffi cult to assess with traditional paleoseismic tools because their surface expressions are often complex and subtle. This contribution assesses the utility of millennial- scale denudation rates derived from in-situ cosmogenic 10Be for revealing the spatial patterns and magnitudes of rock uplift produced by slip along reverse faults. We present seventeen basin-averaged denudation rates from rivers draining the Santa Cruz Mountains along the San Andreas fault (California, USA) which closely reproduce known uplift rate patterns associated with a restraining bend along the fault. An additional component of vertical deformation appears to be superposed on the uplift due to the restraining bend; this may result from regional transpression, further irregularities in the fault trace, or interactions with neighboring faults. Our results indicate that 10Be-derived denudation rates can reveal patterns of rock uplift adjacent to reverse faults over length-scales relevant for characterizing their seismic hazard potential.

Geochemistry and mineral chemistry of lode gold mineralisation, SE Egypt: implications for ore genesis and exploration

Orogenic, lode gold mineralisation in the South Eastern Desert of Egypt is related to quartz veins spatially and temporally associated with conjugate NW- and NE-trending brittle–ductile shear zones. These structures are assumed to be linked to a regional transpression deformation which occurred late in the tectonic evolution of the area. In the Betam deposit, gold is confined to quartz(±carbonate) veins cutting through tectonised metagabbro and metasedimentary rocks in the vicinity of small granite intrusions. The ore bodies contain ubiquitous pyrite and arsenopyrite, in addition to minor disseminated chalcopyrite, pyrrhotite, galena, tetrahedrite and rare gold/electrum. New ore microscopy and electron microprobe studies indicate that most free-milling Au is intimately associated with the late-paragenetic galena–tetrahedrite–chalcopyrite assemblage. An early Fe–As sulphide assemblage, however, shows minor traces of refractory gold. New mineralogical and geochemical data are used to better constrain on possible element dispersions for exploration uses. This study indicates that parameters that most consistently define primary dispersion of gold in the mine area include pervasive silicification, sericite and carbonate alteration. The trace element data of gold lodes reflect a systematic dispersion of gold and certain base metals. Low-cost, extensive exploration programs may use elevated concentrations of Ag, Sb, Cu and Pb as tracers for Au ore zones in the Betam mine area and surroundings.

Sensitivity of the Southern San Andreas Fault System to Tectonic Boundary Conditions and Fault Configurations

The southern Big Bend of the San Andreas fault (SAF) accommodates transpression along numerous non‐vertical, non‐planar, and intersecting active surfaces. Using three‐dimensional boundary element method (BEM) models, we test the sensitivity of fault slip rates to a range of tectonic boundary conditions constrained by Global Positioning System (GPS) studies of the region (45–50 mm/yr and 320°–325°). Using this approach, our models provide a range of distributed fault slip rates associated with both spatial variations in geometry and present‐day uncertainties in plate motion. Model slip rates match most of the available geologic slip rates, and discrepancies may owe to inaccurate fault geometries. More northerly plate velocity (325°) produces greater transpression along the SAF system associated with greater uplift of the San Bernardino Mountains, greater reverse‐slip rates along range bounding reverse thrust faults, lower strike‐slip rates along the San Andreas and San Jacinto faults, and greater strike‐slip rates along the Eastern California Shear Zone (ECSZ) and Garlock fault. These results suggest that the degree of regional transpression controls the partitioning of deformation between uplift, and slip along both the SAF system and the ECSZ. The insensitivity of modeled slip rates to applied velocity along the southern San Andreas and San Jacinto faults suggests that fault geometry greatly influences slip rates within the San Bernardino Mountains region. A northerly shift in plate velocity orientation could account for the abandonment of the Mill Creek strand of the SAF>95 ka and development of the present‐day active geometry, which accommodates greater uplift.

Emplacement, structural and P-T evolution of the &amp;tilde;346 Ma Miřetín Pluton (eastern Teplá-Barrandian Zone, Bohemian Massif): implications for regional transpressional tectonics

* �Correspondingauthor The calc-alkaline Miřetin Pluton (newly dated at 346 Ma ± 5 Ma; an U-Pb age obtained by laser-ablation ICP MS me- thod on zircons) is a NNE-SSW elongated intrusive body emplaced into the upper- to mid- crustal rocks of the Policka Unit (eastern Tepla-Barrandian Zone; Bohemian Massif). Its composition reveals similarities to other calc-alkaline granitoids, which are mostly interpreted as products of magma mixing between the basic magmas derived from mantle wedge above a subduction zone with crustally-derived acid melts. The conditions of magma crystallization estimated at 653-681 °C and 0.29-0.43 GPa roughly correspond to peak metamorphic evolution of the host volcano-sedimentary rocks of the northwestern part of the Policka and Hlinsko units. The Miřetin Pluton was emplaced into a NNE-SSW oriented transpressional domain, well recognized on a regional scale along the eastern margin of the Tepla-Barrandian Zone. During, or shortly after the magma emplacement, the Miřetin Pluton was affected by pervasive submagmatic to high-T solid-state deformation, reflecting an additional strain increment of regional transpression in a narrow zone of thermal softening. Sharply superimposed low-T solid-state fabric preserved along the western part of the Pluton was connected with normal shearing between the Policka Unit at the bottom and the overlying Hlinsko Unit after 335 Ma.

The 1835 AD fissure eruption at Osorno volcano, Southern Andes: Tectonic control by the intraarc stress field instead of remote megathrust-related dynamic strain

On January 19th 1835 a strong Strombolian flank eruption at Osorno (41.1°S) volcano was observed by Ch. Darwin, who was at the Chiloé Island at that time. One month later, a large M 8.5 thrust earthquake took place in central Chile with an epicentral area located ca. 480km far away from Osorno volcano and a rupture zone that extended from ca. 35°S to 39°S, fairly similar to rupture zone of the Maule 2010 Mw 8.8 earthquake for what is the predecessor. Despite the dates, both have been considered as a typical earthquake-eruption pair and a case-study of remote tectonic triggering. In order to distinguish between remote and local (intraarc) tectonic control we perform a structural analysis of the 1835 fissure vents and cone alignments comparing the resulting geometry with that expected from both the active regional transpression along the arc and the coseismic strain pattern related with such a remote megathrust earthquake.Although the cone loading effect cannot be neglected, the asymmetric pattern of fissures and cone alignments with a prevailing NE-striking SHmax suggest a dominant arc tectonics control. In turn, volumetric expansion modeled with the inferred remote earthquake parameters is minimal at the Osorno volcano. Thus, the 1835 thrust earthquake would have not had a direct effect on the magma ascent and the final eruptive morphology. This case-study could serve as a threshold for the expected remote triggering related with megathrust earthquakes in the Andean subduction zone where statistically based models suggest possible effects even up to 700km from the epicentral area.

The Cerro Aguas Calientes caldera, NW Argentina: An example of a tectonically controlled, polygenetic collapse caldera, and its regional significance

Polygenetic, silicic collapse calderas are common in the central Andes. Here we describe in detail the Cerro Aguas Calientes caldera in NW Argentina, which comprises two caldera-forming episodes that occurred at 17.15 Ma and 10.3 Ma. We analyse the significance of its structural setting, composition, size and the subsidence style of both caldera episodes. We find that the caldera eruptions had a tectonic trigger. In both cases, an homogeneous dacitic crystal-rich (>60 vol.% of crystals) reservoir of batholithic size became unstable due to the effect of increasing regional transpression, which favoured local dilation through minor strike–slip faults from which ring faults nucleated and permitted caldera collapse. Both calderas are similar in shape, location and products. The 17.15 Ma caldera has an elliptical shape (17 × 14 km) elongated in a N30° trend; both intracaldera and extracaldera ignimbrites covered an area of around 620 km 2 with a minimum volume estimate of 140 km 3 (DRE). The 10.3 Ma episode generated another elliptical caldera (19 × 14 km), with the same orientation as the previous one, from which intracaldera and outflow ignimbrites covered a total area of about 1700 km 2, representing a minimum eruption volume of 350 km 3(DRE). In this paper we discuss the significance of the Cerro Aguas Calientes caldera in comparison with other well known examples from the central Andes in terms of tectonic setting, eruption mechanisms, and volumes of related ignimbrites. We suggest that our kinematic model is a common volcano-tectonic scenario during the Cenozoic in the Puna and Altiplano, which may be applied to explain the origin of other large calderas in the same region.

Response of Alum Rock springs to the October 30, 2007 Alum Rock earthquake and implications for the origin of increased discharge after earthquakes

AbstractThe origins of increased stream flow and spring discharge following earthquakes have been the subject of controversy, in large part because there are many models to explain observations and few measurements suitable for distinguishing between hypotheses. On October 30, 2007 a magnitude 5.5 earthquake occurred near the Alum Rock springs, California, USA. Within a day we documented a several‐fold increase in discharge. Over the following year, we have monitored a gradual return towards pre‐earthquake properties, but for the largest springs there appears to be a permanent increase in discharge. The Alum Rock springs discharge waters that are a mixture between modern (shallow) meteoric water and old (deep) connate waters expelled by regional transpression. After the earthquake, there was a small and temporary decrease in the fraction of connate water in the largest springs. Accompanying this geochemical change was a small (1–2°C) temperature decrease. Combined with the rapid response, this implies that the increased discharge has a shallow origin. Increased discharge at these springs occurs both for earthquakes that cause static volumetric expansion and for those that cause contraction, supporting models in which dynamic strains are responsible for the subsurface changes that cause flow to increase. We make a quantitative comparison between the observed changes and model predictions for three types of models: (i) a permanent increase in permeability; (ii) an increase in permeability followed by a gradual decrease to its pre‐earthquake value; and (iii) an increase of hydraulic head in the groundwater system discharging at the springs. We show that models in which the permeability of the fracture system feeding the springs increases after the earthquake are in general consistent with the changes in discharge. The postseismic decrease in discharge could either reflect the groundwater system adjusting to the new, higher permeability or a gradual return of permeability to pre‐earthquake values; the available data do not allow us to distinguish between these two scenarios. However, the response of these springs to another earthquake will provide critical constraints on the changes that occur in the subsurface and should permit a test of all three types of models.