Craig Dietsch

Episodic fluvial incision of rivers and rock uplift in the Himalaya and Transhimalaya

Abstract: Seventeen strath terraces in northern India were dated using 10Be surface exposure methods; ages range from c. 7 to c. 735 ka and provide fluvial incision rates of 0.02 ± 0.003 to 2.6 ± 1.9 mm a−1. On the northern side of the Ladakh Range, incision rates are c. 1 mm a−1; in the northern Zanskar Range they are ≤0.06 ± 0.005 mm a−1. New and published incision rates in southernmost Lahul range from 0.1 ± 0.02 to 13.2 ± 6.2 mm a−1; rates for ages >35 ka are ≤0.4 ± 0.2 mm a−1. Across the Himalaya and Transhimalaya, Holocene fluvial incision rates range from c. 0.02 to c. 26.0 mm a−1, whereas Pleistocene incision rates are ≤5 mm a−1. Many of the Holocene incision rates exceed exhumation rates, whereas Pleistocene incision rates are comparable with exhumation rates. This suggests that long-term fluvial incision is in dynamic steady state with exhumation. The temporal pattern for rates of fluvial incision is probably controlled by episodic incision linked to significant precipitation changes throughout the Quaternary, suggesting that strath terraces with ages >35 ka can be used for assessing long-term rates of rock uplift. In contrast, rates of fluvial incision based on Late Glacial and Holocene strath terraces reflect changes in monsoon intensity and deglaciation events. By determining ages for multiple samples on flights of strath terraces, it is possible to document changes in incision rate, assess whether post-abandonment transient shielding has occurred, and help elucidate tectonic v. climatic controls on their formation. Supplementary material: Tables (DS1–4) for previously published data and recalculated ages for strath terraces, and figures (DS1 and DS2) showing plots of incision rates against time for the Himalaya and Transhimalaya are available at http://www.geolsoc.org.uk/SUP18454.

Geochemistry and tectonic setting of metabasic rocks of the Gneiss Dome Belt, SW New England Appalachians

Abstract Acadian structural domes in the Connecticut Valley Zone in western Connecticut expose polydeformed, amphibolite and upper amphibolite facies pre-Silurian rocks correlative with the Notre Dame subzone (western Dunnage zone) of Williams et al. [Geologic Survey of Canada Paper 88-1B, 1988, pp. 91–98] of the Canadian Appalachians. We have analyzed the geochemistry of 99 metabasic rocks from the Cambrian-Lower Ordovician Rowe Schist (RS), the Arenig or older Taine Mountain Formation (TMF), and the overlying Arenig Collinsville Formation (CF) to identify the tectonic setting of mafic volcanism in the Gneiss Dome Belt (GDB) in western Connecticut and southwestern Massachusetts. The metabasites record epidote-amphibolite to lower amphibolite facies metamorphism and exhibit equilibrium microstructures and only limited retrogression. We used our data to test whether the GDB represents part of a Taconic volcanic arc in southwestern New England. RS and TMF metabasites that can be interpreted as dikes range in composition from subalkaline basalt to basaltic andesite and exhibit overall arc-like signatures. Their REE patterns at 10–25× chondrite are mildly LREE enriched with (La/Yb) N =1.50–1.94. Enriched HFSE from Th to Ce and lack of negative Ta–Nb anomalies indicate that the RS and TMF metabasites experienced some degree of crustal contamination. The majority of CF metabasites range from basaltic andesite to subalkaline basalt. The CF metabasites are geochemically diverse and have a wide range of Ti/V ratios characteristic of boninites and low-Ti arc tholeiites, arc tholeiites, ocean floor basalts, and ocean island and alkali basalts. Based on their geochemical characteristics, CF metabasites have been subdivided into three major types: (1) boninitic and low-Ti IAT metabasites (20% of the analyzed samples), (2) arc-like metabasites (27%), and (3) MORB-like and MORB/WPB transitional metabasites (53%). Boninitic CF metabasites are characterized by extremely depleted HFSE, enriched LILE, and distinctive U-shaped REE patterns below the 10× chondrite level. Arc-like CF metabasites have moderately depleted HFSE from P to Yb and relatively enriched LILE; REE patterns at 10–25× chondrite are mostly mildly LREE enriched with (La/Yb) N =1.68–2.12. HFSE from Th to Ce are relatively enriched indicating a small degree of crustal contamination in these metabasites. MORB-like CF metabasites display signatures characteristic of BABB; they have flat, weakly enriched HFSE and moderately enriched LILE typical of evolved MORBs. REE patterns at 15–35× chondrite are flat to slightly LREE enriched with (La/Yb) N =1.08–1.49. On tectonic discriminant diagrams, the majority of MORB-like CF metabasites define MORB-WPB trend. Four samples with the most elevated REE patterns at 20–100× chondrite and moderately LREE enriched have a transitional MORB/WPB character. The tectonic position and geochemical signatures of the RS and TMF dikes suggest that they represent extension-related magmatism, either in an arc or forearc setting, prior to the establishment of Arenig- to Caradoc-aged magmatism in this part of western New England. The coexistence of boninitic, arc-like, and MORB-like compositions is consistent with a backarc setting as the eruptive environment of the CF metabasites in the GDB in western Connecticut and southwestern Massachusetts as predicted by Crawford et al.s [Earth, and Planetary Science Letters 54, 1981, 346–356] model of boninite generation. Whether the initial rifting and opening of this backarc basin occurred in the forearc or as intra-arc splitting remains unclear. MORB-like compositions among the CF metabasites are well represented and denote a more evolved stage of this backarc basin development. Boninitic and arc-like CF metabasites conclusively prove a supra-subduction zone setting, but the polarity of this subduction remains equivocal. Our results indicate that Taconic accretion in western New England Appalachians was more complex than previously proposed and likely involved forearc, arc, and backarc complexes.