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Tectonic deformation can influence spatiotemporal patterns of erosion by changing both base level and the mechanical state of bedrock. Although base-level change and the resulting erosion are well understood, the impact of tectonic damage on bedrock erodibility has rarely been quantified. Eastern Tibet, a tectonically active region with diverse lithologies and multiple active fault zones, provides a suitable field site to understand how tectonic deformation controls erosion and topography. In this study, we quantified erosion coefficients using the relationship between millennial erosion rates and the corresponding channel steepness. Our work shows a twofold increase in erosion coefficients between basins within 15 km of major faults compared to those beyond 15 km, suggesting that tectonic deformation through seismic shaking and rock damage significantly affects eastern Tibet erosion and topography. This work demonstrates a field-based, quantitative relationship between rock erodibility and fault damage, which has important implications for improving landscape evolution models.

The time of origin of Earth’s dynamo is unknown. Detrital zircon crystals containing ferromagnetic inclusions from the Jack Hills of Western Australia have the potential to contain the oldest records of the geodynamo. It has recently been argued that magnetization in these zircons indicates that an active dynamo existed as far back as 4.2 Ga. However, the ages of ferromagnetic inclusions in the zircons are unknown. Here we present the first detailed characterization of the mineralogy and spatial distribution of ferromagnetic minerals in Jack Hills detrital zircons. We demonstrate that ferromagnetic minerals in most Jack Hills zircons are commonly located in cracks and on the zircons’ exteriors. Hematite is observed to dominate the magnetization of many zircons, while other zircons also contain significant quantities of magnetite and goethite. This indicates that the magnetization of most zircons is likely to be dominantly carried by secondary minerals that could be hundreds of millions to billions of years younger than the zircons’ crystallization ages. We conclude that the existence of the geodynamo prior to 3.5 Ga has yet to be established.

Author(s): Haproff, Peter J; Zuza, Andrew V; Yin, An; Harrison, T Mark; Manning, Craig E; Dubey, Chandra S; Ding, Lin; Wu, Chen; Chen, Jianlin

Zircons entrained in mantle-derived magmas offer a unique opportunity to identify cryptic magmatic episodes in the deep crust and thus to image lithospheric thickening and crustal evolution. We investigated zircon xenocrysts from mantle-derived ultrapotassic rocks in southern Tibet to evaluate their potential as a probe of crustal evolution. Similar age (Proterozoic–Paleozoic) distributions of these zircons and those in the Lhasa terrane detrital spectra demonstrate the continental origin of xenocrysts with high U/Yb. Timeprogressive variations in zircon e Hf (t) reveal three major magmatic pulses ca. 90, 50, and 20 Ma, suggesting signifi cant crustal growth in the Lhasa terrane at those times. This is consistent with major mantle inputs previously documented from surface rocks in the Lhasa terrane. Increasing Dy N /Yb N and U/Yb since ca. 55 Ma are interpreted to refl ect progressive crustal thickening in response to the India-Asia convergence. Zircon xenocrysts with varying U-Pb ages and heterogeneous Hf isotopes indicate assimilation of Lhasa terrane crust in the genesis of ultrapotassic magmas.

Our understanding of the geologic evolution of the Himalaya remains incomplete, particularly in regard to structural and geochronologic details of the Proterozoic-Paleozoic Lesser Himalayan Sequence. We conducted an integrated field mapping, geochronological study, and geochemical analysis of the Lesser Himalayan Sequence strata in the Kumaun and Garwhal Himalaya of NW India (78° 00′–80°30′E). Structural observations reveal a systematic change in deformation styles from lower to higher structural levels in the Main Central Thrust footwall. In the south, and at lower structural levels, the Main Central Thrust footwall is characterized by parallel folding and sparse development of axial cleavage. Quartz microstructures indicate that deformation occurred at temperatures below 350 °C. In contrast, in the north and at higher structural levels close to the Main Central Thrust, deformation is characterized by replacement of original bedding, the development of penetrative cleavage, and schistosity locally. Folds are tight, in places isoclinal and overturned. The corresponding quartz microstructures indicate that deformation occurred above 350 °C. Dating of detrital zircons from Lesser Himalayan Sequence metasedimentary units, and igneous zircons from schistose gneisses throughout the Lesser Himalayan Sequence allow refinements to the stratigraphic framework. First, unlike has been observed in western Nepal immediately west of our study area, U-Pb zircon dating suggests the absence of Paleoproterozoic strata in the Ramgarh Thrust hanging wall. This suggests that the thrust cuts upsection laterally along strike. Second, U-Pb detrital zircon dating suggests that metasedimentary strata in the Ramgarh Thrust hanging wall are correlative with basal units of the Outer Lesser Himalayan Sequence, removing the necessity that the Ramgarh Thrust hanging wall was allochthonous with respect to its footwall Lesser Himalayan Sequence units. Schistose gneisses with U-Pb ages of ca. 1850 Ma are older than Lesser Himalayan Sequence units in the area, suggesting that they are thrust-emplaced Lesser Himalayan Sequence basement. We conclude that the Ramgarh Thrust is an out-of-sequence thrust postdating a folding event in its footwall. The earlier proposal that Neoproterozoic-Cambrian Lesser Himalayan Sequence strata are the southern extension of the Tethyan Himalayan Sequence requires that the Tons Thrust, which separates the distal and proximal facies of the Lesser Himalayan Sequence, be a major, south-directed structure with a slip magnitude of >50–100 km.

An integrated petrologic, thermochronologic, and numerical modeling study constrains the thermal evolution of the Lesser Himalayan Sequence in the Kumaun and Garwhal regions of India. South of the Tons thrust, peak metamorphic temperatures do not exceed 330 °C. On the northern side, a transition toward higher temperatures adjacent to the Main Central thrust hanging wall is documented. In the immediate footwall of the Main Central thrust, peak temperatures average ~550 °C. An inverted thermal field gradient of ~30 °C/km beneath the Vaikrita thrust is calculated. An inverted thermal field gradient of ~20 °C/km is also documented beneath a klippe of the Main Central thrust hanging wall ~25 km south of the Vaikrita thrust, and a peak footwall temperature of 530 °C is recorded close to the shear zone at the base of the klippe. The mechanism for thermal metamorphism in the Lesser Himalayan Sequence is interpreted to be conduction of heat from the Main Central thrust fault and/or hanging wall to the footwall. A localized thermal field gradient is also associated with the Tons thrust. The ^(40)Ar/^(39)Ar thermochronology reveals that the exposed rock in the Kumaun and Garwhal Lesser Himalaya underwent cooling below the closure temperature (Tc) for white mica at different times in the late Tertiary. The proximal footwall of the Munsiari thrust experienced cooling below the white mica Tc at ca. 8.5 Ma. South of the Munsiari thrust, in the proximal footwalls of the Chaukori and Askot klippen, muscovites ages are ca. 11.5 Ma, ~3 Ma older than samples to the north. The age differences in samples from northern and southern exposures are interpreted to document cooling related to thermal relaxation following passage of the Main Central thrust hanging wall atop the Lesser Himalayan Sequence. Results of thermal modeling achieve good fits to the data when scenarios involve an early Miocene phase of overthrusting of a hot hanging wall over a downgoing footwall, followed by the initiation of a duplex within Lesser Himalayan Sequence rocks.

Detailed field mapping, geochronologic and thermochronologic analyses, and petrologic investigations conducted along the southern segment of the late Cenozoic Kongur Shan extensional system provide new information on the Cenozoic tectonic evolution of the eastern Pamir at the western end of the Himalayan-Tibetan orogen. Field relations and cooling-age patterns in the hanging wall and footwall of the active faults show a southward decrease in the magnitude of east-west extension along the southern Kongur Shan extensional system, from 20 km or less along the Muztaghata massif in the north, to l3 km along the Tashkorgan fault in the south. These results, in conjunction with previously published work on the northern segment of the Kongur Shan extensional system, show a general southward decrease in east-west extension along the entire length of the extensional system, consistent with models of extension primarily driven by oroclinal bending or radial thrusting of the Pamir. Petrologic data, 40 Ar/ 39 Ar cooling ages, and monazite Th-Pb ages from schists and gneisses in the footwall of the southern Kongur Shan normal fault along the Muztaghata massif record two tectonic events that immediately preceded late Miocene initiation of east-west extension: (1) high-grade schists and gneisses experienced upper amphibolite facies metamorphic conditions (9–10 kbar, 700–750 °C) dated as late Oligocene to middle Miocene by in situ ion-microprobe analyses of monazite inclusions in garnet; and (2) high-grade schists and gneisses were subsequently rapidly exhumed to shallow crustal levels in the late Miocene with 40 Ar/ 39 Ar biotite cooling ages of 7.5–9 Ma. Rapid exhumation was accommodated in part by the east-west–striking, south-dipping, Shenti normal fault. Field relations and regional geologic correlations indicate that this exhumation event was related to the formation of the Central Pamir gneiss domes, and the antiformal Muztaghata massif is the eastward continuation of the Sares dome of the Central Pamir. These observations suggest that the antiformal gneiss domes of the Central Pamir have not been offset across the Karakorum right-slip fault from the Qiangtang anticlinorium in Tibet. Instead, we propose that the development of the Central Pamir gneiss domes may have been related to Oligocene-Miocene northward underthrusting and thickening of crust beneath the Pamir.

A 500-km-long belt of metamorphic exposures in the Qiangtang block provides an opportunity to study the internal structure of northern Tibetan crust. Metamorphic rocks exposed at two widely separated areas along this belt consist of blueschist-bearing melange and are bounded by Late Triassic–Early Jurassic, domal, low-angle normal faults. We propose that this melange was underplated to the Qiangtang block and was subsequently exhumed by detachment faulting; both the underplating and the exhumation occurred during early Mesozoic southward subduction of oceanic lithosphere along the Jinsha suture. This model predicts that the deeper crust of much of northern Tibet consists of accretionary melange, in contrast to the continental crystalline crust of southern Tibet, and may account for north-south variations of Cenozoic tectonism in Tibet.

Neogene sediments belonging to the Siwalik Group crop out in the Himalayan foothills along the length of southern Nepal. Carbon and oxygen isotopic analyses of Siwalik paleosols from four long Siwalik sections record major ecological changes over the past ∼11 m.y. The carbon isotopic composition of both soil carbonate and organic matter shifts dramatically starting ca. 7.0 Ma, marking the displacement of largely C3 vegetation, probably semi-deciduous forest, by C4 grasslands. By the beginning of the Pliocene, all the flood plains of major rivers in this region were dominated by monsoonal grasslands. The floral shift away from woody plants is also reflected by the decline and final disappearance of fossil leaves and the decrease in coal logs in the latest Miocene. A similar carbon isotopic shift has been documented in the paleosol and fossil tooth record of Pakistan, and in terrigenous organic matter from the Bengal Fan, showing that the floral shift was probably continentwide. The latest Miocene also witnessed an average change of ∼4‰ in the oxygen isotopic composition of soil carbonate, as observed previously in Pakistan. The reasons for this are unclear; if not diagenetic, a major environmental change is indicated, perhaps related to that driving the carbon isotopic shift. Recently described pollen and leaf fossils from the Surai Khola section show that evergreen forest was gradually displaced by semi-deciduous and dry deciduous forest between 11 and 6 Ma. The gradual nature of this floral shift, which culminated in the rapid expansion of C4 grasses starting ∼7.0 m.y. ago, is difficult to explain by a decrease in atmospheric pCO2 alone (Cerling et al., 1993) but fits well with a gradual onset of monsoonal conditions in the late Miocene in the northern Indian subcontinent. Himalayan uplift, driving both monsoonal intensification and consumption of CO2 through weathering, may be the common cause behind major late Miocene environmental change globally. However, the decline of effective moisture associated with monsoon development has probably slowed, not increased, the rate of consumption of CO2 by chemical weathering of Himalayan sediments.

Kenny et al.’s (2016) ion microprobe zircon crystallization temperature data for the Sudbury impact crater adds to the existing database (Wielicki et al., 2012) for terrestrial impact melts. They note that zircons from the granophyre layer had not previously been analyzed by ion microprobe commensurate with its volumetric importance (20%–45%; Lightfoot et al., 1997) potentially biasing its comparator value when evaluating possible sources for the Hadean Jack Hills zircon population. However, the authors neglected to quantify the degree to which their data set further constrains this issue. Using data from their GSA Data Repository item 2016143, we tested the hypothesis that variants of the impact zircon record (as determined solely from ion microprobe data; Wielicki et al., 2012; Kenny et al., 2016) represent the same probability distribution (i.e., the null hypothesis) as the Hadean population (Harrison and Schmitt, 2007) through a Kolmogorov-Smirnov test (R Core Team, 2013). We reject the null hypothesis for both the case that the Kenny et al. data alone represent the Hadean population (p = 2 × 10), and that for all reported impact zircons (p < 2 × 10; Wielicki et al., 2012; Kenny et al., 2016). In fact, the hypothesis that the granophyre data alone are equivalent to the Hadean population can be rejected (p = 4 × 10). Taken at face value, these results appear to support the conclusion of Wielicki et al. (2012) that the Hadean Jack Hills zircon temperature distribution was not derived in any significant way from impact-derived zircons. However, Kenny et al. raise the prospect of a selection process preferentially destroying high-temperature Hadean zircons and thus biasing the detrital record to lower temperatures. In fact, nature does tend to bias the detrital zircon record, but that mechanism operates in exactly the opposite sense. Late crystallizing, thus low-temperature, granitoid zircons are known to contain elevated U and Th concentrations which lead to metamictization (Claiborne et al., 2010) and thus their likely removal from the detrital record, resulting in preferential preservation of higher-temperature zircons (Harrison and Schmitt, 2007). Thus, without the benefit of some as yet unknown selection mechanism, the probability of extracting the Hadean Ti-in-zircon temperature distribution from the data reported by Kenny et al., or any published data set of impactproduced zircons, remains vanishingly small.

Subduction complexes along the Andean margin in central and southern Chile yield mid-Paleozoic to lower Mesozoic ages, yet they crop out within 100 km of the modern trench that shows evidence of active accretion along much of its length. The scarcity of uplifted subduction-complex rocks younger than mid-Mesozoic along the Chilean margin and in parts of the Scotia Arc suggests to us that these old, crystalline rocks, uplifted in the Triassic and Jurassic, represent a boundary in the forearc beyond which tectonic erosion does not easily occur. Greenschist-, blueschist-, and amphibolite-facies subduction-complex rocks from the Scotia Arc were originally thought to be a simple continuation of the subduction complexes in Chile. Based on new 40 Ar/ 39 Ar ages, the Scotia Arc subduction complexes reveal a complex history related to distinct local tectonic events and are not a simple continuation of the old accretionary prism in Chile. Structural and metamorphic analysis indicates the earliest and most penetrative deformation in the subduction complexes around the Scotia Arc occurred at some depth in a subduction zone or zones, certainly below the brittle-ductile transition, and in some cases under blueschist-facies conditions. We believe that the early subduction-related deformation and metamorphism in the greenschist- and blueschist-facies rocks of the Scotia Arc to be overprinted by mid-Cretaceous transpression along the South America-Antarctica plate boundary in the case of Elephant Island, transpression and subsequent localized transtension in the earliest Cenozoic in the case of the Darwin Complex, a mid-Cenozoic spreading-rate change in the case of Smith Island, and early Neogene initiation of Drake Passage opening/Shackleton Fracture Zone formation in the case of the Gibbs Island subgroup.

Discrimination of individual tectonometamorphic events in polymetamorphosed terranes requires a comprehensive understanding of the relative timing and conditions of metamorphism and plutonism. We have applied a combination of 40 Ar/ 39 Ar thermochronology, petrology, and thermobarometry to reconstruct the complex Early Proterozoic through early Cenozoic tectonic and metamorphic evolution of continental crust in the Old Woman Mountains area, south-eastern California. Strong Mesozoic thermal events obscure the earlier history in much of the Old Woman Mountains area. In those areas where Early Proterozoic rocks underwent only lower-greenschist-facies metamorphism during the Mesozoic, thermobarometry of pelitic schists indicates that Proterozoic metamorphism occurred at 9 to 11 kbar and ∼700 °C. 40 Ar/ 39 Ar ages of hornblende from samples of interbedded Proterozoic amphibolite indicate that this high-grade metamorphism took place before 1600 Ma. The relatively high-pressure conditions of Early Proterozoic metamorphism in the Old Woman Mountains area contrast with the low-pressure granulite-facies metamorphism that occurred elsewhere in the Mojave Desert at this time. 40 Ar/ 39 Ar analyses of hornblende from Proterozoic rocks within Mesozoic shear zones and hornblende barometry from Jurassic intrusive rocks suggest that tectonism and burial of Paleozoic strata to >10 km began between 170 and 150 Ma. This tectonism resulted in regional greenschist-facies metamorphism. Late-stage mineral assemblages in Proterozoic and Paleozoic pelitic rocks in the Old Woman Mountains area indicate an increase in metamorphic grade from greenschist to upper amphibolite facies toward Late Cretaceous plutons of the 73 Ma Old Woman-Piute batholith. Metamorphic temperatures from garnet-biotite thermometry increase from 650 °C, and 40 Ar/ 39 Ar ages of hornblende from Proterozoic rocks decrease from 1600 to 73 Ma toward these plutons. Barometric calculations from garnet-bearing metamorphic rocks suggest that this Cretaceous metamorphism took place at 3.5 to 5.0 kbar in the Old Woman Mountains. Similar pressures are obtained from hornblende barometry of Late Cretaceous granodiorite. 40 Ar/ 39 Ar and apatite fission-track data from the 73 Ma batholith indicate that the rocks exposed in the Old Woman Mountains cooled rapidly to below 350 to 300 °C by 70 ± 1 Ma and that moderately rapid cooling at rates of 10 to 30 C°/m.y. continued until after 60 Ma. The presence of maximum Mesozoic temperatures on the borders of plutons, hornblende ages of ca. 73 Ma, and similar pressure estimates for plutons and country rocks indicate that the high temperatures needed to cause ductile deformation and amphibolite-facies metamorphism were related to heating from the intrusion of the 73 Ma Old Woman-Piute batholith. Amphibolite-facies metamorphism along pluton contacts was superimposed on the regional greenschist-facies metamorphism related to thrusting and crustal thickening. The extremely rapid cooling in the Old Woman Mountains between 73 and 71 Ma was due largely to conduction of heat into the country rocks, whereas protracted rapid cooling to temperatures below ambient temperatures estimated for the depth of emplacement was due to tectonic denudation at 1 to 2 km/m.y. until about 68 to 65 Ma.

Thermochronologic, petrologic, and geochemical analyses of epidote-amphibolite-, amphilbolite-, eclogite-, and blueschist-facies blocks in serpentinite-matrix melanges from East San Benito Island and Cedros Island, west-central Baja California, provide constraints on the P-T-t history of this disrupted terrane. Results of {sup 40}Ar/{sup 39}Ar step heating experiments on minerals separated from these blocks vary according to metamorphic grade and indicate different P-T-t histories. Mid-Jurassic (160-170 Ma) epidote-amphibolite- and amphibolite-facies blocks were probably derived from oceanic crust and associated sediments that were metamorphosed during limitation of subduction. Coarse-grained blueschist blocks were likely metamorphosed during continued subduction in late Early Cretaceous time (that is, 115-100 Ma). Some epidote-amphibolite blocks are partially overprinted by blueschist-facies mineral assemblages and apparently record both metamorphic events. Fission-track analyses of apatites indicate that the blocks underwent significantly different post-metamorphic cooling histories. Epidote-amphilbolite and amphibolite blocks cooled below {approximately}100 {degrees}C form mid-Jurassic to Paleocene time; blueschist blocks cooled below {approximately}100 {degrees}C in Oligocene-Miocene time. In general, mafic blocks have trace-element concentrations and REE patterns characteristic of ocean-flood basalts. This study demonstrates that samples with very different P-T-t histories can evidently occur over relatively small length scales (<

We appreciate that [Rasmussen et al. (2011)][1] have characterized a large number of inclusions in Jack Hills (Australia) detrital zircons, but we find two significant flaws in their paper: one of scholarship and one of omission. With regard to the first, their paper implies discovery of reset

A central debate for the evolution of the Himalayan orogen is how the Greater Himalayan Crystalline complex in its core was emplaced during the Cenozoic Indo-Asian collision. Addressing this problem requires knowledge of the structural relationship between the South Tibet detachment fault (STD) and the Main Central thrust (MCT) that bound these rocks from above and below. The fault relationship is exposed in the Himachal Himalaya of northwestern India, where they merge in their updip direction and form a frontal branch line that has been warped by subsequent top-to-the-southwest shear deformation. To elucidate how the two major crustal-scale faults evolved in the western Himalaya, we conducted integrated geologic research employing field mapping, pressure-temperature ( P-T ) analyses, U-Pb zircon geochronology, trace and rare earth element (REE) geochemistry, and thermochronology. Our field study reveals complex geometric relationships among major thrusts with large-magnitude shortening within each thrust sheet. Three successive stages of top-to-the-southwest thrust development are recognized: (1) imbricate stack development, (2) translation of large thrust sheets along low-angle detachments and backthrusting along the STD, and (3) development of duplex systems via underplating. This kinematic process can be quantified by our new analytical data: (1) P-T determinations show 7–9 kbar and 450–630 °C conditions across the STD. The lack of a metamorphic discontinuity across the fault is consistent with a backthrust interpretation. (2) U-Pb zircon geochronology yields ca. 830 Ma and ca. 500 Ma ages of granitoids in the MCT hanging wall, ca. 1.85 Ga ages of granitic gneisses in both the MCT hanging wall and footwall, and 8–6 Ma ages of granitic pegmatites in the MCT footwall. These ages help define regional chronostratigraphy, and the youngest ages reveal a previously unknown intrusion phase. (3) Trace element and REE geochemistry of 1.85 Ga, 830 Ma, and 500 Ma granitoids are characteristic of remelted continental crust, constraining the protolith tectonic setting. (4) U-Pb geochronology of detrital zircon reveals that siliciclastic sedimentary sequences above the STD, below the MCT, and between these two faults have similar age spectra with Neoproterozoic youngest age peaks. This result implies that the STD and MCT each duplicated the same stratigraphic section. (5) Th-Pb geochronology of monazite included in MCT hanging-wall garnet yields Paleozoic and early Tertiary ages, indicating Paleozoic and early Tertiary metamorphism in these rocks. (6) The 40 Ar/ 39 Ar thermochronology of the K-feldspar from southern MCT hanging-wall rocks evinces cooling below 220–230 °C ca. 13–19 Ma or later, constraining the thrust development history. We use these results to derive a tectonic model of crustal shortening across the Himachal Himalaya involving early thickening, tectonic wedging emplacement of the Greater Himalayan Crystalline complex between the MCT and STD, and continued growth of the Himalayan thrust wedge by accretion of thrust horses from the Indian footwall.

Like many granites, the Late Cretaceous intrusives of the eastern Mojave Desert, California, have heretofore provided useful but poorly focused images of their source regions. New studies of lower crustal xenoliths and inherited accessory minerals are sharpening these images. Xenoliths in Tertiary dykes in this region are the residues of an extensive partial melting event. Great diversity in their composition reflects initial heterogeneity (both igneous and sedimentary protoliths) and varying amounts of melt extraction (from 70%)

The three Himalayan lithologic units, the Lesser Himalayan Sequence, the Greater Himalayan Crystalline complex, and the Tethyan Himalayan Sequence, have a specific structural correlation with the Main Central thrust and South Tibet detachment in the central Himalaya. There, the Main Central thrust places the Greater Himalayan Crystalline complex over the Lesser Himalayan Sequence, and the South Tibet detachment places the Tethyan Himalayan Sequence over the Greater Himalayan Crystallines. Although this division has formed the basis for all Himalayan tectonic models, it fails to explain aspects of the geology of the western Himalaya where the Main Central thrust places the Tethyan Himalayan Sequence directly above the Lesser Himalayan Sequence. Our mapping in NW India shows that this relationship results from southward merging of the Main Central thrust and South Tibet detachment. This finding, in conjunction with observed alternating shear senses on the South Tibet detachment, is inconsistent with the wedge-extrusion and erosion-induced channel-flow models (both require only top-to-the-N motion on the South Tibet detachment) but is consistent with a tectonic-wedging model.

The geologic map pattern of the Qiangtang terrane in central Tibet defines a >600-km-long and up to 270-km-wide east-plunging structural culmination. It is characterized by early Mesozoic blueschist-bearing melange and upper Paleozoic strata in the core and mainly Triassic–Jurassic strata along the limbs. In the western Qiangtang terrane (∼84°E), the culmination is unconformably overlain by weakly deformed mid-Cretaceous volcanic flows and tuffs. Along the Bangong suture to the south (32°N, 84°E), mid-Cretaceous nonmarine red beds and volcanic rocks lie unconformably on Jurassic suture zone rocks. These relationships demonstrate that west-central Tibet was above sea level during the mid-Cretaceous and experienced significant denudation prior to mid-Cretaceous time. Growth of the Qiangtang culmination is inferred to have initiated during southward emplacement of a thrust sheet of early Mesozoic melange and upper Paleozoic strata during the Early Cretaceous Lhasa-Qiangtang collision. The north-south width of the inferred thrust sheet provides a minimum slip estimate of ∼150 km at 84°E, decreasing eastward to ∼70 km at 87°E. Paleogene deformation in the Qiangtang terrane is characterized by widely distributed, mainly north-dipping thrust faults that cut Eocene–Oligocene red beds and volcanic rocks in their footwall. Along the Bangong suture, the north-dipping Shiquanhe-Gaize-Amdo thrust system cuts 64 and 43 m.y. old volcanic tuffs in its footwall and accommodated >40 km of post–mid-Cretaceous shortening. The Tertiary south-dipping Gaize–Siling Co backthrust bounds the southern margin of the Bangong suture and marks the northernmost limit of mid-Cretaceous marine strata in central Tibet. Cretaceous deformation and denudation in central Tibet is attributed to northward underthrusting of the Lhasa terrane beneath the Qiangtang terrane along the Bangong suture. This model implies that (1) Cretaceous strata along the Bangong suture and in the northern Lhasa terrane were deposited in a flexural foreland basin system and derived at least in part from the Qiangtang terrane, and (2) the central Tibetan crust was thickened substantially prior to the Indo-Asian collision. Although its magnitude is poorly known, Tertiary shortening in the Qiangtang terrane is more prevalent than in the Lhasa terrane; this difference may be attributed to the presence of underthrust melange in the deeper central Tibetan crust, which would have made it weaker than the Lhasa terrane during the Indo-Asian collision.

The late Cenozoic Kongur Shan extensional system lies along the northeastern margin of the Pamir at the western end of the Himalayan-Tibetan orogen, accommodating east-west extension in the Pamir. At the northern end of the extensional system, the Kongur Shan normal fault juxtaposes medium- to high-grade metamorphic rocks in both its hanging wall and footwall, which record several Mesozoic to Cenozoic tectonic events. Schists within the hanging wall preserve a Buchan metamorphic sequence, dated as Late Triassic to Early Jurassic (230–200 Ma) from monazite inclusions in garnet. Metamorphic ages overlap with U-Pb zircon ages from local granite bodies and are interpreted to be the result of regional arc magmatism created by subduction of the Paleo-Tethys ocean. The northern portion of the footwall of the extensional system exposes an upper-amphibolite-facies unit (~650 °C, 8 kbar), which structurally overlies a lowgrade metagraywacke unit. The high-grade unit records late Early Cretaceous crustal thickening at ca. 125–110 Ma, followed by emplacement over the low-grade metagraywacke along a north-northeast–directed thrust prior to ca. 100 Ma. Together these results indicate signifi cant middle Cretaceous crustal thickening and shortening in the northern Pamir prior to the Indo-Asian collision. A third Late Miocene (ca. 9 Ma) amphibolite-facies metamorphic event (~650–700 °C, 8 kbar) is recorded in footwall gneisses of the Kongur Shan massif. North of the Kongur Shan massif, rapid cooling in the footwall beginning at 7–8 Ma is interpreted to date the initiation of exhumation along the Kongur Shan normal fault. A minimum of 34 km of east-west extension is inferred along the Kongur Shan massif based on the magnitude of exhumation since the Late Miocene (~29 km) and the present dip of the Kongur Shan normal fault (~40°). Field observations and interpretation of satellite images along the southernmost segment of the Kongur Shan extensional system indicate that the magnitude of late Cenozoic east-west extension decreases signifi cantly toward the south. This observation is inconsistent with models in which east-west extension in the Pamir is driven by northward propagation of the right-slip Karakoram fault, suggesting instead that extension is driven by vertical extrusion due to topographic collapse, radial thrusting along the Main Pamir Thrust, or oroclinal bending of the entire Pamir region.

Metapelites collected in central Nepal reveal a discontinuity in metamorphic pressuretemperature-time (P-T-t) paths near the base of the Main Central thrust zone, despite an absence of obvious structural breaks. Garnets in the structurally lowest rocks grew with increasing T and P (loading), whereas garnets 1‐3 km upsection grew with increasing T, but decreasing P (exhumation). Monazite grains in structurally lower rocks yield ionmicroprobe Th-Pb ages of 8‐9 Ma. Structurally higher monazite grains range from 10 to 22 Ma. The P-T-t paths confirm previous interpretations that footwall metamorphism in part resulted from thrust reactivation ca. 8 Ma, but also reflect thermal relaxation following older (20 Ma or older) thrust movement. The Main Central thrust zone formed during pulses of movement that resulted in progressive transfer of material from the lower to upper plate.

The significant capabilities of the ion microprobe for thermochronometric investigations of geologic materials remain largely unexploited. Whereas {sup 208}Pb/{sup 232}Th spot analysis allows {approximately} 10-mm-scale imaging of Pb loss profiles or overgrowths in sectioned monazite grains, the spatial resolution offered by depth profiling into the surface region of natural crystals is more than two orders of magnitude higher. The authors document here the ability of the high-resolution ion microprobe to detect {sup 208}Pb/{sup 232}Th age differences of < 1 m.y. with better than 0.05 {micro}m depth resolution in the outer micron of Tertiary monazites from the hanging wall of the Himalayan Main Central thrust. Age gradients on this scale are inaccessible to ion microprobe spot analysis or conventional thermal ionization mass spectrometry. Interpretation of the near-surface {sup 208}Pb distributions with available monazite Pb diffusion data illustrates the potential of the approach for recovering continuous, high-temperature thermal history information not previously available.

Central to understanding the exhumation history of the Himalaya is knowing the timing of slip and magnitude of displacement on the primary fault systems that bound the range. The widely accepted view that early Miocene deformation in the Himalaya is characterized by simultaneous shortening along the Main Central thrust and extension at shallower crustal levels in part developed on the basis of knowledge of the age of the Rongbuk granite and its apparent crosscutting relationship with the Qomolangma detachment. This key contact, however, has not previously been directly observed. Field mapping of the Qomolangma detachment and its footwall reveals that no leucogranite bodies crosscut the detachment. These observations together with Th-Pb monazite dating of leucogranites exposed in the footwall suggest that slip was occurring across the Qomolangma detachment shear zone ca. 17 Ma. Although there is no evidence that requires simultaneous shortening and extension in the High Himalaya, our observations are consistent with alternating periods of shortening and extension in the Himalaya since the early Miocene.

Geologic mapping and geochronological analysis in southwest (Kailas area) and southeast (Zedong area) Tibet reveal two major episodes of Tertiary crustal shortening along the classic Indus-Tsangpo suture in the Yalu River valley. The older event occurred between ca. 30 and 24 Ma during movement along the north-dipping Gangdese thrust. The development of this thrust caused extensive denudation of the Gangdese batholith in its hanging wall and underthrusting of the Xigaze forearc strata in its footwall. Examination of timing of major tectonic events in central Asia suggests that the initiation of the Gangdese thrust was approximately coeval with the late Oligocene initiation and development of north-south shortening in the eastern Kunlun Shan of northern Tibet, the Nan Shan at the northeastern end of the Altyn Tagh fault, the western Kunlun Shan at the southwestern end of the Altyn Tagh fault, and finally the Tian Shan (north of the Tarim basin). Such regionally synchronous initiation of crustal shortening in and around the plateau may have been related to changes in convergence rate and direction between the Eurasian plate and the Indian and Pacific plates. The younger thrusting event along the Yalu River valley occurred between 19 and 10 Ma along the south-dipping Great Counter thrust system, equivalent to the locally named Renbu-Zedong thrust in southeastern Tibet, the Backthrust system in south- central Tibet, and the South Kailas thrust in southwest Tibet. The coeval development of the Great Counter thrust and the North Himalayan granite-gneiss dome belt is consistent with their development being related to thermal weakening of the north Himalayan and south Tibetan crust, due perhaps to thermal relaxation of an already thickened crust created by the early phase of collision between India and Asia or frictional heating along major thrusts, such as the Main Central thrust, beneath the Himalaya.

Compositional zoning reveals multistage growth histories and resorption events in zircon from a high-grade terrane in the eastern Blue Ridge of North Carolina and Georgia. These zoning patterns were used to guide high-resolution ion microprobe dating that places important constraints on the evolution of the southern Appalachian crust. Zircons from granulite facies metapelite have unzoned rims that yield concordant U-Pb ages of 495 ± 14 Ma. We interpret this as the time of rim growth, which occurred during peak metamorphism early in the protracted orogenic history of the region. Detrital cores, characterized by truncated euhedral zoning, are of Grenville age (1.04–1.26 Ga). Zircons from the Whiteside and Rabun plutons have well-defined, rounded, inherited cores and euhedral, oscillatory-zoned magmatic rims. Rims of Rabun zircons record magmatic crystallization at 374 ± 4 Ma, whereas Whiteside rims yield a 466 ± 10 Ma crystallization age. Cores from both plutons include 1.1–1.3 Ga and 2.6–2.7 Ga ages. These data indicate that there was no single, voluminous episode of plutonism in this area, that similar material underpinned the region at least from 370 to 470 Ma, and that previously unrecognized Archean basement or Archean basement–derived sedimentary rock was present in the southern Appalachians. Results of this study verify the value of combining zoning and ion microprobe studies: Using conventional U-Pb methods or ion microprobe dating without knowledge of zoning would have made interpreting the events recorded in these zircons and the ages that they yield difficult or impossible.

The two parallel belts of Miocene granite that extend along much of the Himalaya differ in age, petrogenesis, and emplacement style. We suggest that their origin is linked to shear heating on a continuously active decollement that cuts through previously metamorphosed Indian supracrustal rocks that were transformed into basement during the initial stages of the Indian-Asian collision. Numerical simulations assuming a shear stress of 30 MPa indicate that initiation of slip on the Himalayan thrust at 24 Ma could trigger discontinuous melting reactions leading to formation of the High Himalayan granite chain from 24 to 20 Ma and the North Himalayan belt from 18 to 12 Ma. This model result agrees well with observation as do model predictions regarding emplacement style and location.

Detrital K-feldspar and muscovite samples from Ocean Drilling Program Leg 116 cores have been dated by the {sup 40}Ar/{sup 39}Ar technique and have depositional ages from 0 to 18 Ma. From 4 to 13 individual K-feldspars and 1 to 12 individual muscovites have been dated from 7 stratigraphic levels. In every level at least one K-feldspar and one muscovite yielded a minimum age identical, within uncertainty, to the age of deposition. These results indicate that a significant portion of the material in the Bengal fan is first-cycle detritus derived from the Himalaya. Therefore, the substantial amount of sediment deposited in the distal fan in early to middle Miocene time can be ascribed to a significant pulse of uplift and erosion in the collision zone at this time. Moreover, these data indicate that throughout the Neogene, some part of the Himalayan orogen was undergoing rapid erosion (1 to 10 mm/yr); this erosion must have been less than or equal to uplift relative to sea level. The lack of granulite facies rocks in the eastern Himalaya and Tibetan plateau suggests to us that very rapid uplift must have been distributed in brief pulses over different parts of the mountain belt. These datamore » are incompatible with tectonic models in which the Himalaya and Tibetan plateau are uplifted either uniformly over the past 40 m.y. or mostly within the past 2 to 5 m.y.« less

Geological field investigations and 40Ar/39Ar ages from two basement complexes in southeast Luzon document the first known occurrences of pre-Late Cretaceous age rocks in the eastern Philippines. However, individual components within the two complexes vary in age from Late Jurassic (Caramoan basement complex) to Early Cretaceous and early Miocene (Camarines Norte-Calaguas Islands basement complex). These and other data show that southeast Luzon basement complexes are genetically diverse, and they indicate that the concept of an old, autochthonous basement in the Philippines is open to question. This supports the hypothesis that the Philippine Archipelago is an amalgamation of allochthonous Mesozoic and Cenozoic island-arc, ocean-basin, and continental fragments that were assembled during the Tertiary.

{sup 40}Ar/{sup 39}Ar analyses of muscovite, biotite, and K-feldspar from central New England reveal a remarkable pattern of mineral ages: the ages are progressively younger from central to western New Hampshire and rise sharply near the Vermont border to ages more typical of post-Acadian cooling. This distribution is attributed to differential uplift via isostatic rebound of an anomalously thickened crust at the Bronson Hill anticlinorium. This explanation requires that between 6 and 8 km of normal fault motion has occurred on structures in western New Hampshire, not previously recognized to have accommodated this kind or magnitude of displacement. This hinged, differential uplift occurred from {approximately} 360-170 Ma and is consistent with a time constant for rebound of {approximately} 80 m.y.

{sup 40}Ar/{sup 39}Ar geochronology on hornblendes from central New England reveals a Grenvillian cooling age (1026 {plus minus} 12 Ma) along the eastern margin of the Green Mountain massif, Acadian ages in east-central Vermont (350-397 Ma) and along the Bronson Hill anticlinorium in New Hampshire and Massachusetts (407-280 Ma), and Late Ordovician to Silurian ages (440-451 Ma) along a belt in eastern Vermont and central Massachusetts. These ages indicate a minimal Acadian overprint in eastern Vermont, the highest degree of Acadian overprint being located along the axis of the domes in the Connecticut Valley synclinorium. These data suggest that the formation of the domes occurred in the Acadian. Moreover, the Late Ordovician to Silurian cooling ages in eastern Vermont suggest that much of the metamorphic mineralogy and fabric observed in the Connecticut Valley synclinorium may be pre-Silurian in age.