T41C-01
Provenance Constraints on the Mesozoic-Cenozoic Tectonic Evolution of the Queen Charlotte Islands Region
The medial Cretaceous magmatic arc system of western North America was flanked by a series of forearc basins extending from Mexico to Alaska. Cretaceous strata in the Queen Charlotte Islands of northwest British Columbia are unique in this series of basins, as these strata have been displaced from the arc system by formation of the extensional Queen Charlotte basin in Cenozoic time. This displacement complicates reconstruction of the forearc basin, and makes it difficult to evaluate the controls on basin evolution. Sedimentologic, paleontologic, and detrital zircon analyses of forearc strata represented by the Valanginian- Campanian Queen Charlotte Group (QCG) constrain basin evolution and provide a framework for an interpretation of the Mesozoic-Cenozoic tectonic evolution of the Queen Charlotte Islands region. Basin subsidence initiated in Valanginian time with a marine transgression over irregular topography consisting of extensional fault blocks of pre-Cretaceous strata. Locally derived conglomerates at the base of the Longarm Formation are overlain by shallow marine shelf deposits that represent a westward-deepening, fining-upward transgressive succession with an eastern depositional edge that migrated eastward during Valanginian to Aptian time. West-directed paleocurrents and a unimodal detrital zircon population of 120-175 Ma grains provide the first linkage between the Cretaceous QCG and unroofed Jura-Cretaceous plutons of the Coast Plutonic Complex to the east. This initial transgressive sequence is superseded by a second pulse of clastic detritus in early Albian time, characterized by an easterly-derived, fossiliferous shallow-shelf sandstone (Haida Formation), fine-grained, outer shelf to upper slope strata (Bearskin Bay Formation), and mass-sediment gravity flows (Skidegate Formation). The unimodal zircon population (ca 140-175 Ma) in the lower Haida Formation is interpreted to reflect renewed uplift of Jura-Cretaceous arc plutons by contractional tectonism in the arc system to the east. A significant subsidence event in Late Turonian to Coniacian time resulted in accumulation of a thick, westward- prograding fan-delta/submarine fan complex, the Honna Formation, reflecting a tight linkage between orogenic exhumation and basin subsidence. The distinctly bimodal zircon signature of the Honna and upper Skidegate formations strongly suggests rapid exhumation of both syndepositional Late Cretaceous plutons (ca. 90 -110 Ma) and older arc plutons (ca 140-170 Ma), potentially driven by a major magmatic pulse within the Coast Plutonic Complex. This bimodal zircon signature persists through Late Santonian time, represented by fine-grained distal shelf deposits of the Tarundl Formation. This fine-grained succession may represent a decrease in the rate of basin subsidence and final basin infilling resulting from a decrease in exhumation within the arc system associated with the transition from orthogonal convergence to dextral translation in Late Cretaceous to Paleocene time.
T41C-02
Detailed Provenance Analysis Constrains Variations in Basin-Wide Deposition Trends And Source Area Uplift: The Late Cretaceous Nanaimo Basin, Southwest British Columbia, Canada
The Upper Cretaceous Nanaimo Group accumulated in a peripheral foreland basin on the Insular Superterrane in southwestern British Columbia. Detailed facies studies, paleontologic control and regional provenance analysis facilitate reconstruction of basin depositional architecture. Basin subsidence began in Turonian time in response to contractional crustal thickening in the southern Coast Belt and Cascade Range to the east and southeast. Initial sedimentation was diachronous, with non-marine, marginal marine, and shallow marine deposition on a complex paleotopography having local relief >100m. Most clastic detritus was locally derived, but the presence of syndepositional Late Cretaceous zircon indicates rapid unroofing of the arc system on the eastern basin margin. Subsidence dramatically increased in late Santonian -early Campanian time, resulting in progradation of submarine fan lobes west across the basin. Santonian to Maastrichtian sedimentation produced thick successions (>3 km) of complexly intertonguing fan lobes resulting from episodic sediment flux. Predominance of volcanoplutonic debris and paleocurrent data indicate derivation from the Coast Plutonic Complex to the east. Syndepositional detrital zircon throughout the section suggests strong coupling between rapid orogenic exhumation and basin subsidence. By early Campanian time, however, the main continental arc system was locally breached by one or more large rivers, and extraregional sediment from distal back arc regions prograded westward into the basin. Significant (locally >50%) Precambrian detrital zircon from uplift of the Belt Supergroup to the east are mixed with Jura-Cretaceous grains of the Coast Plutonic Complex. Contribution from eastern sources peaked in late Campanian time, reflected by conspicuous quartzite pebbles and cobbles. By Maastrichtian time, the Precambrian component was significantly diluted due to a major pulse of plutonic detritus from final unroofing of the Cretaceous arc system.
T41C-03
Detrital Zircon Record of Colorado River Incision
The Colorado River is a large, youthful, unequilibrated continental drainage system the base-level for which was established rather abruptly between 5 and 6 million years ago in conjunction with Gulf of California rifting and establishment of the modern river course through the western Grand Canyon and lower Colorado river region. New laser ablation ICPMS detrital zircon U-Pb analyses (~3000) from ~40 samples provide insight into details relating to the cause, timing and consequences of river inception. These samples encompass (1) the modern Colorado River delta, (2) major tributaries including the Green, "Grand", San Juan, Little Colorado and Gila rivers (3) late Miocene to Pliocene sediments along the lower Colorado (4) late Miocene to Pleistocene deltaic and fluvial sediments of the Imperial and Palm Spring Groups in the western Salton Trough, and (5) late Miocene- early Pliocene Bidahochi Formation of eastern Arizona. Data from the western Salton Trough and modern delta yield strata yield remarkably homogeneous age distributions that indicate there was little evolution in Colorado River sediment composition since 5.3 Ma. Detrital zircon is dominated by a mix of local southwest US cratonal basement (1.7 and 1.4 Ga) plus reworked supracrustal sequences of the Colorado Plateau that provide Neoproterozoic, 1.1 Ga, and early Paleozoic zircons. A relative paucity of Grenville-age grains in the earliest part of the delta sequence may reflect an early stage of the modern river prior to deep incision through Colorado Plateau erg deposits. The strong homogeneity of the detrital zircon record from late Miocene to the present is consistent with the ‘lake spillover model' for inception and integration of the modern Colorado River drainage. Abrupt integration of the lower Colorado River after 5.6 Ma is clearly recorded by detrital zircon ages from the laucustrine Bouse Formation and Bullhead alluvium aggradational package. Fluvial-laucustrine deposits of the Bidahochi Formation may represent a lake that overtopped the Kaibab upwarp to initiate western Grand Canyon incision.
T41C-04
Thermo-Kinetic Interpretative Model for Couple Basement-Detrital Thermochronology
Clastic sedimentary sequences shed from orogenic belts provide a fertile, and arguably unique, record of crustal exhumation. However, in spite of the considerable recent analytical advances, interpretive methods for detrital thermochronology have remained relatively basic. There is a clear need for numerical approaches for interpreting detrital thermochronology that are capable of dealing with transient heat flow related to intrusion and faulting and, at the same time, are constructed to appropriately and comprehensively accept input from a variety of thermochronologic constraints. Similarly, there is a need to move beyond simple interpretation of detrital age distributions and employ more informative thermochonometers that are capable of reducing ambiguities in data interpretation that arise in environments with complex thermal histories (such as shallow arc crust). Here we present a general thermo-kinetic model capable of describing rifted arc crust to constrain the initiation time, slip history, and erosion history related to normal faulting by simultaneously fitting detrital and basement thermochronologic data collected within a continental rift setting (Baja California margin of the southern Gulf of California). The San Jose del Cabo (SJDC) normal fault is a major east-dipping normal fault associated the San Jose del Cabo rift basin and mid- to late Cretaceous arc basement (Los Cabos block). Preliminary results indicate that 4-6 of the 10 km total basement denudation is due to Late Miocene-Recent rift-related denudation. Peak exhumation occurred at 7 m.y. at 1 km/m.y. K-feldspar multi-diffusion domain results limit total denudation to 4-6 km and indicate that virtually all slip along the SJDC fault took place after 10 Ma of similar to the age of the oldest known sediments within the SJDC basin.
T41C-05
Exhumation of the Sucking-Dayman Massif, Papua New Guinea
Extension, evident as seafloor spreading in the Woodlark Basin, is actively propagating into the continental crust of Eastern Papua New Guinea, where regional ductile shear zones and brittle faults have recently exhumed metabasite rocks of the Suckling-Dayman Massif (9.5-10°S, 149-150°E). Utilising field, petrographic microscope and electron microprobe analyses, we investigate the structural and kinematic evolution of the Dayman shear zone, as well as the mineral chemistry of key metamorphic assemblages that define the shear zone fabrics. The results of this investigation indicate that the Dayman shear zone is an extensional shear zone with top to the north-northeast transport. Macro- and micro-kinematic indicators in the ductile shear zone include common S-C fabric, mantled porphyroclasts, ‘mica' fish, pressure shadows on pyrite, grain-scale faults, and asymmetric micro-boudinage. Kinematic indicators display a very regular sense of shear consistent with a simple shear dominated shear zone. The shear zone fabric is defined by dominantly greenschist facies metamorphic assemblages including abundant chlorite, epidote, albite and quartz with or without actinolite, titanite and calcite. Low strain pods within the shear zone display relict mafic igneous textures and relict clinopyroxene. The pods may include metamorphic lawsonite and blue amphibole in the mineral assemblage, suggestive of high P/T metamorphism of the metabasite massif prior to exhumation. Sedimentary rocks of the hanging wall include the Gwoira Conglomerate that contains clasts of low-grade (prehnite-pumpellyite facies) sheared metabasite. Metamorphic minerals identified in clasts include prehnite, pumpellyite, chlorite, albite, quartz and epidote. Monomineralic clasts include clinopyroxene, albite and quartz. The majority of clasts are consistent with a low-grade metabasite source that may or may not be sheared. A minor component of the clasts include graphitic intergrowths of quartz and feldspar, consistent with a pegmatitic to felsic source. Rare horizons in the Gwoira Conglomerate contain abundant shallow marine fossils. These observations suggest that the provenance for the Gwoira Conglomerate may include early exhumed low-grade metabasite of the Suckling- Dayman massif and that Gwoira Conglomerate may have been deposited on the flanks of the actively exhuming massif. A lack of metamorphism or ductile deformation in the Gwoira Conglomerate supports this interpretation.