Edward Farrar

Intercalibration of 40Ar39Ar dating standards

A number of standards used for KAr and 40Ar39Ar dating have been intercalibrated. Multiple splits (∼ 15–40 mg each) of MMhb-1, SB-3 Biotite, LP-6 Biotite 40–60#, GA1550 Biotite, Fish Canyon Tuff-3 Biotite, Taylor Creek Rhyolite Sanidine, Bern 4 Muscovite, Bern 4 Biotite and Pennsylvania State University Orthoclase-1A were irradiated in capsules, fused and analyzed. In replicate analyses, most of these standards proved to be homogeneous in 40Ar★39ArK ratio (age) K, Ca, and 36Ar contents. MMhb-1 is inhomogeneous in age at the ∼ 15-mg level, indicating it is unsuitable for use as an interlaboratory standard using material taken straight from the bottle; the orthoclase sample is also inhomogeneous at the ∼ 30-mg level. Quoted relative to an age of 162.9 Ma for the SB-3 Biotite standard, total fusion ages are: LP-6 Biotite 40–60# = 128.1 Ma; GA1550 Biotite = 97.8 Ma; Fish Canyon Tuff-3 Biotite = 27.95 Ma; Taylor Creek Rhyolite Sanidine = 28.0 Ma; Bern 4 Muscovite = 18.51 Ma; Bern 4 Biotite = 17.21 Ma. These ages are in good agreement with the (absolute) KAr ages of these minerals. Different preparations of biotite and sanidine standards from a welded tuff such as the Fish Canyon Tuff may not be of identical ages. Tests on submilligram splits reveals that SB-3 Biotite, GA1550 Biotite, Fish Canyon Tuff-3 Biotite and Taylor Creek Rhyolite Sanidine are suitable for use as monitors in laser work; the sanidine specimen appears to be the most suitable for use as the primary monitor for 40Ar39Ar dating.

Late Cenozoic deformation in the Central Andes: fault kinematics from the northern Puna, northwestern Argentina and southwestern Bolivia

Abstract Fault kinematics from the northern Puna region of the central Andean Plateau show two phases of deformation. The older deformation involved thrusting on NNE-SSW striking faults prior to about 9 Ma. This produced an average shortening direction (120°±20°) which does not overlap the Miocene-to-Recent convergence direction between the Nazca and South American Plates (82°±4°). Reactivation of normal faults associated with a Cretaceous rift and strike-slip faulting in the weakened magmatic arc can explain the ∼40° discrepancy. Deformation younger than 9 Ma is on normal and strike-slip faults that indicate diverse Plio-Quaternary strain orientations, possibly due to the influence of both gravitational and subduction forces. While the normal faults are minor, five orogen parallel (NS) strike-slip faults are marked by ten- to sixty-kilometer-long scarps. Striae indicate right-lateral strike-slip with minor normal displacement. At least two of these faults reactive older thrusts. By analogy with trench-linked forearc strike-slip faults, the kinematics of these back-arc strike-slip faults can be explained by the decoupling of oblique foreland convergence into two mutually orthogonal components.

Diachronous and independent histories of plutonism and mineralization in the Cornubian Batholith, southwest England

Concordant U-Pb dates of magmatic monazites in the dominant coarse-grained biotite (±muscovite) granites of the late Variscan Cornubian batholith demonstrate that the emplacement of the six main plutons was diachronous, ranging from 293.1 ±1.3 Ma (Carnmenellis) to 274.5 ± 1.4 Ma (Lands End, southern lobe). 40Ar-39Ar age spectra for magmatic muscovites confirm that cooling to about 320 °C at the present level of exposure was accomplished within 4 to 5 Ma in each intrusion. Moreover, age spectra for hydrothermal muscovites show that development of the major Sn (-Cu) lodes of the South Crofty mine commenced before 286 Ma, only 2 to 3 Ma after cooling of the hosting Cam Brea granite, a satellite of the Carnmenellis pluton. The older plutons had therefore approached ambient temperatures, and had even experienced intense hydrothermal activity, before emplacement of the youngest.

Widespread Early Cretaceous flood basalt volcanism in eastern India: geochemical data from the Rajmahal-Bengal-Sylhet Traps

Abstract K-Ar dating of a set of flood basalts from eastern India indicates that volcanism ∼ 115 Ma ago was much more voluminous than heretofore recognised, covering an area of at least 2·105 km2. Separation of India from Antarctica began ∼ 125 Ma ago and in this rift dominated regime, voluminous flood basalts were extruded with the Kerguelen Hotspot perhaps serving as the heat source. We report here on the occurrence of both alkali basalts and olivine tholeiites from the Rajmahal area; preliminary data indicate that more than one type of primary magma was present, one of which resulted from partial melting of a section of the mantle containing metasomatised veins.

Cenozoic polyphase landscape and tectonic evolution of the Cordillera Occidental, southernmost Peru

Landforms on the Pacific slope of the Cordillera Occidental (Western Cordillera) in southernmost Peru reflect the episodic epeirogenic uplift of the Central Andes since the earliest Miocene. In the geomorphic history, three major stages are distinguished. Initial uplift followed a mid-Tertiary period of tectonic and magmatic quiescence in which a low-lying, subdued landscape formed through dissection of an Eocene subaerial volcano-plutonic terrain. Uplift and resulting erosional beveling of this landscape, coinciding in their later stages with the explosive reactivation of the magmatic arc, gave rise to the regionally extensive, lower Miocene Altos de Camilaca surface (I), which was mantled soon after its formation by ignimbrites of the Huaylillas Formation. Major relief was achieved in the middle and early-Late Miocene, when a succession of uplift episodes generated a sequence of four discrete pediments, constituting the Multiple Pediment Stage (II). Uplift was accompanied by widespread eruption of ignimbrites of the Chuntacala Formation. Subsequently, in the Pliocene, pedimentation was succeeded as the major erosional process by canyon incision across the lower slopes of the cordillera. During this Valley and Terrace Stage (III), the immediate littoral margin experienced the development of two major terraces, contemporaneous with a transition from ignimbrite eruption to predominantly andesitic volcanism in the High Cordillera in the early Pleistocene. During this time, it is probable that epeirogenic uplift, rather than glacio-eustatic sea-level changes, exerted the dominant control on landform development. In the absence of evidence for large-scale crustal compression in this immediate region since the Cretaceous, it is inferred that Neogene uplift resulted from episodic thickening of the continental crust through “underplating” by basic and/or intermediate magmas. The Neogene chronology of landform development herein defined is remarkably similar to that outlined in other transects of the Central Andes, implying that the continental margin has, on a broad scale, responded uniformly to plate subduction since the early Miocene. However, the low relief evident in the mid-Tertiary in the study area, and not, for instance, in northern Chile (27°S), suggests that crustal thickening through the Neogene was more rapid, and more extensive, in the vicinity of the Arica deflection than in transects to the north and south. The estimated Neogene uplift rate for the transect was in the range 0.06 to 0.19 mm/yr.

Initial 87Sr/86Sr ratios of plutonic and volcanic rocks of the Central Andes between latitudes 26° and 29° south

Initial87Sr/86Sr ratios have been determined for 34 plutonic and volcanic rocks covering the entire age span of magmatic events associated with the Andean orogeny between latitudes 26° and 29° south. The igneous rocks, the majority dated by K/Ar mineral techniques, range in age from Lower Jurassic (190 m.y.) to Quaternary (0.89 m.y.). In addition, initial ratios were determined for three granitoid plutons and one metasediment from the pre-Mesozoic basement which underlies the entire Andean orogen in this transect at shallow depth. The compositions vary from basalt to rhyolite, and from quartz diorite to granodiorite or trondjemite, for the extrusives and intrusives, respectively. Mid-Cretaceous to Quaternary rocks exhibit a systematic west to east increase in mean strontium isotope ratio from 0.7022 to 0.7077, whereas the initial ratios of Jurassic plutons vary from 0.7043 to 0.7059, and do not correlate with age. The existence of unusually low initial ratios (e.g. 0.7022, 0.7023) for several Mesozoic plutonic rocks strongly implies a sub-crustal source for at least some of the Andean magmas. The time-dependent post-Jurassic increase in initial ratio is considered to reflect a systematic change in the composition of partial melts generated in response to the progressive subduction of a lithospheric slab. It is suggested that a systematic change in the locus of melting takes place from along or close to the upper surface of the subduction slab into hanging-wall mantle peridotite as subduction continues.

K-Ar evidence for the post-paleozoic migration of granitic intrusion foci in the Andes of northern Chile

Abstract Twenty new potassium-argon age determinations on mineral separates from granitic rocks of the Andes of northern Chile, between latitudes 26° and 29°South, indicate the existence of at least five major intrusive episodes: Permian, Lower Jurassic, Middle Cretaceous, Lower Paleocene and Upper Eocene. The corresponding intrusive foci appear to have migrated eastwards from the present coastal area in Lower Jurassic times to about 120 km inland in the Upper Eocene. This essentially confirms the suggestions made by previous workers, primarily based on stratigraphic and lead-alpha data.

The Acambay graben: Active intraarc extension in the trans‐Mexican volcanic belt, Mexico

The trans-Mexican volcanic belt is an active volcanic arc related to subduction along the Middle America trench. The central part of the belt is being deformed by the Chapala-Tula fault zone, an approximately 450-km-long and 50-km-wide zone of active extension. The volcanic arc and the arc-parallel Chapala-Tula fault zone are superposed nearly perpendicularly on the preexisting stress and deformation province of the Mexican Basin and Range. The Acambay graben, about 40 km long and 15 km wide, is located approximately 100 km northwest of Mexico City and is one of the major troughs within the Chapala-Tula fault zone. The border faults of the Acambay graben, Acambay-Tixmadeje in the north and Pastores in the south, are separated in the west by stepovers from range-bounding faults of similar orientation, Epitacio Huerta in the north and Venta de Bravo in the south. The stepovers occur at the intersection of these faults with an older system of Basin and Range faults. An early-stage right-lateral component of motion along the Venta de Bravo and Pastores faults is inferred on a map scale from a left-stepping en echelon array of normal fault segments. The divergence of the en echelon segments from the general fault trend decreases gradually from west to east, suggesting that the early extension was rotational. The present relative displacement along the southern margin of the system, on the other hand, results in a left-lateral strike-slip component. This is documented on a map scale from extension structures at left stepovers and on an outcrop scale from fault striations indicating left-oblique slip. The striations measured at the northern system margin indicate nearly pure extensional dip slip without a consistent lateral displacement component. This is supported on a map scale by the structure of the right stepover between the Acambay-Tixmadeje and Epitacio Huerta faults, which shows no evidence of local extension or shortening. The divergence between the present directions of motion at the southern and northern margins of the extended zone can be explained by a minor rotational deformation component with the pole of rotation being located to the east of the zone of deformation. This could explain why no active extension has been observed to the east of the Chapala-Tula fault zone, in the eastern part of the trans-Mexican volcanic belt. During the Ms = 6.9 Acambay earthquake of November 19, 1912, surface rupture occurred along both margins of the graben at the base of multiple-event scarps. Along the Acambay-Tixmadeje fault, the coseismic rupture is 41 km long. The vertical offset increases gradually from the eastern end of the surface rupture to its center where it is with 50 cm at a maximum. Furthermore, the change in the vertical surface offset along the fault is approximately proportional to the change in height of the Acambay-Tixmadeje multiple-event fault scarp. The easternmost part of the ground rupture passes through a plain and not at the base of a multiple-event scarp as farther west. It may therefore correspond partly to an increase in length of the Acambay-Tixmadeje fault during the 1912 earthquake. The slip rate along the southern border of the Acambay graben can be estimated from the displacement and age of a basalt flow for which we have obtained a 40Ar/39Ar age of 0.4±0.1 Ma. This basalt may be displaced up to 15 m by the Pastores fault, which indicates a middle-late Quaternary slip rate of ≤0.04 mm/yr. Furthermore, based on a coseismic surface rupture of approximately 20 cm along this fault in the 1912 earthquake, we estimate a recurrence interval of ≥5000 years for major earthquakes along the faults of the Acambay graben.

Granite belts in Thailand: evidence from the 40Ar/39Ar geochronological and geological syntheses

Abstract 40 Ar/ 39 Ar radiometric age-dating results together with petrochemical studies of granitoid rocks from nearly all parts of Thailand strongly indicate that the granitoid belts (i.e. Eastern, Central and Western) form in different geological and geotectonic environments and show marked contrast in geochronological evolution. The new 40 Ar/ 39 Ar dating results differ markedly from those previously reported by other methods. The Eastern Granite Belt formed in Early to Late Triassic (245-210 Ma), the Central Belt in Late Triassic (220-180 Ma) to Middle Jurassic, and the Western Belt in Late Cretaceous to Middle Tertiary (80-50 Ma). Granitoid rocks of the S-type which are considered to be related to Sn-W-REE mineralization are widely present in the Western and Central belts as a result of Shan-Thai/Indo-China and Shan-Thai/Western Burma microcontinental plate collisions, respectively. On the other hand, I-type granitoid rocks are inferred to be related to CuFeAuSb mineralization and are largely limited to the Eastern belt, formed as a result of subduction of oceanic lithospheric plates beneath the Shan-Thai and Indo-China microcontinents.

An Integrated Tectono-Magmatic Model for the Evolution of the Southern Peruvian Andes (13-20°S) since 55 Ma

The definition and characterization of the Oligocene-Miocene Crucero Supergroup of the southern Peru Inner Arc domain provide a previously unavailable lithostratigraphic framework for the clarification of the geodynamic context of magmatism across the southern Peruvian transect. We integrate new lithostratigraphic, petrologic, and 40Ar/39Ar geochronologic data for these volcanic and hypabyssal rocks with published information to establish a geotectonic model for the transect since 55 Ma. The early Eocene to Early Miocene tectono-magmatic history of the region was controlled by the evolution of a flat-subduction regime, which was initiated at ∼52 Ma with the onset of rapid convergence, terminating the magmatic and hydrothermal activity of the Toquepala superunit of the Coastal Batholith. Collision of the flat slab with the leading edge of the Brazilian Shield lithospheric mantle at 39-40 Ma resulted in catastrophic failure of the overriding plate and crustal-scale, NE-directed ramping, generating the “Inca...