Abstract: Carbonate platforms and reefs are more common in foreland basins than is generally appreciated and may provide a better record of basin evolution and relative sea-level change than siliciclastic strata. Carbonate platform and reefal facies may develop in the proximal foredeep on a variety of topographic highs, in the distal foreland area far from terrigenous influx, or across the entire foreland basin during tectonically quiescent stages of basin development. Basin geometry, dispersal of siliciclastic sediment, subsidence patterns, and deformation structures across the foreland affect carbonate facies distribution and platform morphology during synorogenic stages of foreland basin evolution. Synorogenic foreland carbonate platforms typically have ramp profiles that mimic the flexural profile produced by tectonic loading. During active convergence, the flexural profile is driven toward the foreland by the advancing orogenic wedge. Synorogenic carbonate ramps are forced to onlap and/or backstep cratonward. Basinward parts of some foreland carbonate platforms may be drowned (sensu stricto) during active convergence, especially when: (1) the underlying lithosphere has low rigidity; (2) the orogenic wedge advances rapidly; (3) a eustatic sea-level rise occurs at the same time as migration of the flexural profile; or (4) some other environmental stress affects carbonate-producing benthos. Two-dimensional forward models show that flexural drowning of some Phanerozoic carbonate platforms, even in the absence of a coeval eustatic sea-level rise or other environmental stress, is possible in less than 250,000 yr. In some foreland areas, complex patterns of synorogenic differential subsidence and foreland deformation can affect carbonate facies tracts many hundreds of kilometers cratonward of the proximal foredeep. These patterns of differential subsidence and deformation probably reflect the response of preexisting basement structures or rheological anisotropies in the foreland area to tectonic loading along the plate margin or the response to sublithospheric processes. Quantitative subsidence analyses from some foreland areas suggest that differential subsidence in the distal foreland is related temporally to tectonic loading along the continental margin, but cratonward limits of the differential subsidence are beyond reasonable limits of flexurally produced subsidence. In addition, patterns of differential subsidence in the distal foreland do not have "normal" flexural wavelengths, amplitudes, or orientations with respect to the orogenic wedge. Therefore, while subsidence in the distal foreland is temporally related to convergence along the plate margin, alternative models for lithospheric deformation are necessary to explain the differential subsidence in the distal foreland. Siliciclastic sediment dispersal is another first-order control on carbonate sedimentation in foreland basins. Coarse-grained, siliciclastic sediment may have less affect on suspension-feeding, carbonate-producing benthic organisms than clay and silt. Hence, bedrock geology, paleoclimate, and depositional gradients in the hinterland and foreland sides of the basin indirectly affect carbonate sedimentation. Effects of siliciclastic sedimentation on foreland carbonates also depend on the evolutionary stage of a foredeep. During "underfilled" stages of basin evolution, siliciclastic sediment is trapped in the proximal foredeep and will not affect carbonate-producing benthos on the distal side of the basin. The distal parts of clastic wedges may fill accommodation during "intermediate" stages of basin evolution and provide a substrate for carbonate platforms that prograde from the peripheral bulge. Progradation of siliciclastics during later "overfilled" stages of basin evolution may terminate carbonate platforms even in the distal foreland.

Abstract The gold-rich Superior, Canada, and Yilgarn, Australia, cratons have similar geologic histories dating back to the Mesoarchean and showing strong parallels in the Neoarchean. Orogenesis in each craton is marked by a shift from dominant volcanism to dominant clastic sedimentation above unconformities, followed by granitic plutonism, progressive deformation, and dynamothermal metamorphism. The terminal stages of orogenesis correspond to the intervals of 2660 to 2650 Ma in the Superior craton and 2660 to 2630 Ma in the Yilgarn cra ton. The Yilgarn and Superior cratons contain an estimated 9,200 and 8,500 t Au, respectively. Most of the sig nificant gold deposits (>100 t Au) are concentrated in a few narrow, highly endowed gold belts along which the deposits cluster into camps, commonly spaced every 30 to 50 km. Gold deposits of both cratons show similar tonnages and grades, and their size distributions define a Pareto, rather than log-normal distribution. Large deposits are rare but account for most of the gold endowment of each craton. Three recurring host-rock associations account for a majority of large deposits: iron-rich mafic igneous rocks, iron-rich sedimentary rocks, and felsic to intermediate porphyry stocks and dikes. Most deposits, particularly large ones, occur in greenschist-grade rocks and are associated with shear zones, faults, or folds. Gold miner alization styles include quartz-carbonate veins, sulfidic replacements in banded iron formation (BIF), crusti form carbonate-quartz veins and associated sulfidic replacement lodes, disseminated-stockwork zones, sulfide rich veins and veinlet zones, and massive sulfide lenses. Wall-rock alteration assemblages vary with mineralization style and metamorphic grade. Most deposits consist of a single style of mineralization, but many of the large ones combine two or more of these, and some large deposits are unique in their metal associations. The diversity of styles of mineralization, wall-rock alteration assemblages, and overprinting relationships re quire more than one episode of gold mineralization and more than one ore-forming process. Geologic parage neses, coupled with isotopic age constraints, show that, although the Archean histories of both cratons span >300]m.y., the majority of gold deposits formed during the final 30 to 50 m.y. of that time span, corresponding to the orogenic phase. The majority of gold deposits can thus be regarded as orogenic in timing, but with the available constraints clearly pointing to the existence of more than one mineralizing event and involving dif ferent mineralization types and processes. The best-endowed gold camps (Timmins and Red Lake in the Superior craton; Kalgoorlie, Granny-Wallaby, and Sunrise Dam in the Yilgarn craton) commonly possess an anticlinorial structure, komatiitic and basaltic rocks in the core giving way to stratigraphically higher volcanic and clastic sedimentary rock units. Such camps are further marked by coarse clastic rocks deposited above the metavolcanic rock sequences, by concentrations of shallow-level porphyritic intrusions, by extensive carbonate alteration, by multiple styles and ages of gold mineralization and, in most, by through-going regional faults. However, these characteristics are also shared by a number of less-endowed gold camps. The best-endowed gold belts are distinguished by substantial volumes of komatiite, by a high degree of preservation of supracrustal rocks, by structural highs that juxtapose the lower and uppermost parts of the stratigraphic column, by multiple styles and ages of gold mineralization, and by world-class deposits of other metals. The gold belts commonly are aligned along crustal-scale faults that rep resent long-lived structures, which acted as crustal-scale magma and fluid conduits and also influenced coarse clastic sedimentation. Abundant komatiites may reflect the first tangible connection to the deep crust and mantle, but the nature of the subvolcanic crust, ensimatic in the Timmins-Val d’Or, Superior craton, and ensialic in the Wiluna-Norseman belts, Yilgarn cration, seems unimportant in determining gold prospectivity. Significant uncertainty remains concerning the timing of formation of the deposits, the models that best explain their characteristics, and the fundamental causes of the high concentration of gold in a few areas. Various models have been proposed, invoking volcanic, magmatic, and orogenic (metamorphism and/or deformation) processes. The synorogenic model best accounts for the Au-only quartz-carbonate veins and temporally related mineralization styles. However, synvolcanic and magmatic hydrothermal models are also required to explain the presence of Au base metal deposits and those deposits overprinted by significant deformation and meta morphism. The specific histories of the gold belts and the known constraints on timing of deposits suggest that all of these processes have contributed to the gold endowment, although it is difficult to separate orogenic from magmatic processes because they closely overlap in time and space. Despite the presence of synorogenic quartz-carbonate veins throughout the greenstone belts of both cratons, large deposits of this style are mainly restricted to the gold belts, where they also coexist with large deposits of other styles of gold mineralization. The presence of multiple ages and styles of gold mineralization in the best-endowed gold belts indicate a unique locus of successive formation of gold deposits from various processes operating at different stages of the orogenic phase of the evolution of these belts. This would explain the common overprinting of the early deposit types, potentially of synvolcanic or synplutonic origin, by syn orogenic ones. The concentration of multiple types and ages of significant gold deposits in well-defined gold belts is not a unique feature of the Superior and Yilgarn cratons but is shared by Tertiary gold belts of Nevada, such as the Walker Lane, the Battle Mountain-Eureka trend, and the Carlin trend. This must be a reflection of fundamental crustal structure, and perhaps composition of subcrustal mantle, as much as local ore-forming hydrothermal processes.

Mesozoic and Cenozoic sedimentary strata exposed throughout southern Alaska contain a rich archive of information on the growth of collisional continental margins through the processes of terrane accretion, magmatism, accretionary prism development, and subduction of oceanic spreading ridges. Two major collisional events define the tectonic growth of southern Alaska: Mesozoic collision of the Wrangellia composite terrane and Cenozoic collision of the Yakutat terrane. The sedimentary record of these two collisional events can be summarized as follows. (1) Middle Jurassic volcaniclastic and sedimentary strata represent shallow-marine deposition in narrow subbasins adjacent to the volcanic edifice of the south-facing, intraoceanic Talkeetna arc. (2) Late Jurassic syndepositional regional shortening resulted in thick sections of conglomerate in proximal parts of both retroarc and forearc basins. In distal retroarc depocenters, fine-grained turbidite sedimentation commenced in a series of basins that presently extend for >2000 km along strike. This time interval also marked cessation of magmatism and rapid exhumation of the Talkeetna oceanic arc. We interpret these observations to reflect initial oblique collision, younging to the northwest, of the Wrangellia composite terrane with the continental margin of western North America. (3) During Early Cretaceous time, Jurassic retroarc basin strata were incorporated into an expanding northverging thrust belt, and sediment was bypassed into more distal collisional retroarc basins located within the suture zone. Compositional data from these collisional basins show that the Wrangellia composite terrane was exhumed to deep stratigraphic levels. Detrital zircon ages from strata in these basins record some sediment derivation from source areas with North American continental margin affinity. Our data clearly show that the Wrangellia composite terrane and the former continental margin were in close proximity by this time. Accretion of this oceanic terrane and associated basinal deposits marked one of the largest additions of juvenile crust to western North America. The collision of the Wrangellia composite terrane also resulted in a change in subduction parameters that eventually prompted development of a new south-facing arc system, the Chisana arc. Construction of this arc was contemporaneous with renewed forearc basin subsidence and sedimentation. (4) Late Early Cretaceous to early Late Cretaceous time was characterized by regional deformation of retroarc collisional basin strata by south-verging thrust faults that are part of a regional thrust belt that extends throughout the northwestern Cordillera. (5) Latest Cretaceous time was characterized by synorogenic sedimentation in forearc and retroarc basins related to regional shortening and exhumation of a coeval continentalmargin arc and older collisional basinal deposits. Forearc depocenters subsided into deep-water settings between the arc and expanding accretionary prism. Nonmarine to marginal-marine strata accumulated in retroarc depocenters influenced by syndepositional thrust-fault deformation. (6) Growth of the southern Alaska continental margin during Paleocene to Early Eocene time is defined by regional nonmarine deposition, magmatism within the suture zone, and expansion of the accretionary prism. Oblique subduction of an oceanic spreading ridge prompted diachronous deformation, synorogenic sedimentation, and magmatism. Subduction of progressively more buoyant, topographically higher lithosphere (the spreading ridge) followed by less buoyant, topographically lower lithosphere prompted coarsegrained alluvial-fluvial sedimentation and slab-window magmatism in remnant forearc basins. (7) Regional transpressive deformation characterized southern Alaska during Middle Eocene to Oligocene time. This deformation generated coarse-grained alluvial-fluvial sedimentation in narrow fault-bound basins along major strike-slip faults, including the Denali and Castle Mountain faults. (8) A second major phase of terrane collision and basin development shaped the southern margin of Alaska during latest Oligocene to Holocene time. Northward translation and collision of the Yakutat terrane, an excised continental fragment of western North America, prompted growth of the largest coastal mountain range on Earth, construction of a new magmatic arc, exhumation of older fore-arc basinal strata, and renewed uplift of the Alaska Range. The sedimentary record of this collisional event is contained in a collisional foreland basin on the north side of the Alaska Range, intraarc basins in the Wrangell Mountains, and a collisional foreland basin within the Yakutat terrane. This phase of collision continues to the present as evidenced by active mountain building, large-magnitude earthquakes, and some of the highest sediment accumulation rates on Earth.

Abstract 40 Ar/ 39 Ar geochronology was carried out on the non-deformed synorogenic granitoid plutons from the Qinling–Dabie Orogen. The new model cooling history (MCH) method is applied to K-feldspar to obtain information about its cooling history. MCH is a quicker and easier method of calculating cooling histories compared with the multi-domain diffusion (MDD) model. Cooling histories indicate that Qinling–Dabie suffered differential uplift and denudation processes since the collision in the Late Triassic. East Qinling uplifted and denuded rapidly from c. 181–187 to c. 150 Ma at a rate of c. 20 °C Ma −1 , three times as fast as west Qinling which was exhumed at a rate of c. 6.6 °C Ma −1 from c. 195–190 to c. 100 Ma. Although west and east Qinling started uplift and denudation at the same time, east Qinling reached the 150 °C geothermal line ( c. 7.5 km deep) at c. 150 Ma, earlier than west Qinling by c. 50 myr, suggesting that earliest collision and strongest compression occurred in east Qinling. Dabie is characterized with a polyphased process of uplift and denudation, implying that the subduction of south China block in this region was multi-staged. The emplacement of the granitoids and uplift pattern along Qinling–Dabie may be due to delamination of the root of the mountains or the slab break-off. Cooling histories suggest that more mass had been removed from the root of east Qinling than west Qinling and Dabie; the final delamination occurred in Dabie at c. 100 Ma but affected the whole Qinling–Dabie orogen.