Ar/ 39 Ar dates on basalts of Grand Canyon provide one of the best records in the world of the interplay among volcanism, differential canyon incision, and neotectonic faulting. Earlier 40 K/ 40 Ar dates indicated that Grand Canyon had been carved to essentially its present depth before 1.2 Ma. But new 40 Ar/ 39 Ar data cut this time frame approximately in half; new ages are all <723 ka, with age probability peaks at 606, 534, 348, 192, and 102 ka. Strategic sampling of basalts provides a semicontinuous record for deciphering late Quaternary incision and faultslip rates and indicates that basalts fl owed into and preserved a record of a progressively deepening bedrock canyon. The Eastern Grand Canyon block (east of Toroweap fault) has bedrock incision rates of 150-175 m/Ma over approximately the last 500 ka; western Grand Canyon block (west of Hurricane fault) has bedrock incision rates of 50-75 m/Ma over approximately the last 720 ka. Fault displacement rates are 97-106 m/Ma on the Toroweap fault (last 500-600 ka) and 70-100 m/Ma on the Hurricane fault (last 200-300 ka). As the river crosses each fault, the apparent incision rate is lowest in the immediate hanging wall, and this rate, plus the displacement rate, is subequal to the incision rate in the footwall. At the reach scale, variation in apparent incision rates delineates ~100 m/Ma of cumulative relative vertical lowering of the western Grand Canyon block relative to the eastern block and 70-100 m of slip accommodated by formation of a hanging-wall anticline. Data from the Lake Mead region indicate that our refi ned fault-dampened incision model has operated over the last 6 Ma. Bedrock incision rate has been 20-30 m/Ma in the lower Colorado River block in the last 5.5 Ma, and displacement on the Wheeler fault has resulted in both lowering of the Lower Colorado River block and formation of a hanging-wall anticline of the 6-Ma Hualapai Limestone. In modeling long-term incision history, extrapolation of Quaternary fault displacement and incision rates linearly back 6 Ma only accounts for approximately two-thirds of eastern and approximately onethird of western Grand Canyon incision. This "incision discrepancy" for carving Grand Canyon is best explained by higher rates during early (5-to 6-Ma) incision in eastern Grand Canyon and the existence of Miocene paleocanyons in western Grand Canyon. Differential incision data provide evidence for relative vertical displacement across Neogene faults of the Colorado Plateau-Basin and Range transition, a key data set for evaluating uplift and incision models. Our data indicate that the Lower Colorado River block has lowered 25-50 m/Ma (150-300 m) relative to the western Grand Canyon block and 125-150 m/Ma (750-900 m) relative to the eastern Grand Canyon block in 6 Ma. The best model explaining the constrained reconstruction of the 5-to 6-Ma Colorado River paleoprofi le, and other geologic data, is that most of the 750-900 m of relative vertical block motion that accompanied canyon 40
Abstract. The 4øAr/39Ar age spectrum analyses of basalt whole rock and partially degassed sanidine xenocrysts for the Lathrop Wells volcanic center, Nevada, yield eruption ages of 77.3 _+ 6.0 and 76.6 _+ 4.9 ka (2o-) for the Q11 and Q12 flows, respectively. Basaltic samples were analyzed in four to six replicate runs and yield normally distributed plateau ages. Miocene ash flow tuff xenoliths were collected from the basaltic units with the majority of samples coming from the Q12. The sanidines from the Q12 xenoliths yielded apparent plateau ages (n = 10) ranging from 69 _+ 17 to 309 _+ 11 ka with a strong cluster (n = 6) at -77 ka. Because the diffusion dimension of sanidine is approximated by the physical crystal size, cracking of the sanidine following the development of an argon concentration profile may result in complex argon age spectra. Experimental studies exploring the natural and laboratory argon systematics of degassed sanidine reveal that crystal breakage and HF acid etching lead to anomalously old apparent ages. These new ages for the QI• and Q12 flows are analytically indistinguishable and agree well with published eruption ages determined by 3He and 36C1 cosmogenic surface exposure dating methods. We conclude that the QI• and Q12 flows are temporally equivalent. Additionally, the agreement between our eruption ages for these units and the published 76 _+ 20 ka [Zreda et al., 1993] 36C1 exposure age for the Lathrop Wells scoria cone (Qs3) suggests that the cone may be temporally equivalent to the QI• and Q12 flows. We believe the simplest and most reasonable interpretation is that the volcanic center is monogenetic.
A synthesis of the geochronology on basalt fl ows from the southern Uinkaret volcanic fi eld indicates that basalts erupted within and fl owed into Grand Canyon during four major episodes: 725-475 ka, 400-275 ka, 225-150 ka, and 150-75 ka. To extend the usefulness of these dates for understanding volcanic stratigraphy and lava dams in western Grand Canyon, we analyzed light detection and ranging (lidar) data to establish the elevations of the tops and bottoms of basalt-fl ow remnants along the river corridor. When projected onto a longitudinal river profi le, these data show the original extent of now-dissected intracanyon fl ows and aid in correlation of fl ow remnants. Systematic variations in the elevation of fl ow bottoms across the Uinkaret fault block can be used to infer the geometry of a hanging-wall anticline that formed adjacent to the listric Toroweap fault.The 725-475 ka volcanism was most voluminous in the area of the Toroweap fault and produced dike-cored cinder cones on both rims and within the canyon itself. Mapping suggests that a composite volcanic edifi ce was created by numerous fl ows and cinder-cone fragments that intermittently fi lled the canyon. Reliable 40 Ar/ 39 Ar dates were obtained from fl ows associated with this period of volcanism, including Lower Prospect, Upper Prospect, D-Dam, Black Ledge, and Toroweap. Largevolume eruptions helped to drive the far-traveled basalt fl ows (Black Ledge), which fl owed down-canyon over 120 km. A second episode of volcanism, from 400 to 275 ka, was most voluminous along the Hurricane fault at river mile 187.5. This episode produced fl ow stacks that fi lled Whitmore Canyon and produced the 215-m-high Whitmore Dam, which may have also had a composite history. Basaltic river gravels on top of the Whitmore remnants have been interpreted as "outburst-fl ood deposit" but may alternatively represent periods when the river established itself atop the fl ows. Remnants near river level at miles 192 and 195, previously designated as Layered Diabase and Massive Diabase, have been shown by 40 Ar/ 39 Ar dating to be correlative with dated Whitmore fl ow remnants, and they help document the downriver stepped geometry of the Whitmore Dam. The ca. 200 and 100 ka fl ows (previously mapped as Gray Ledge) were smaller fl ows that entered the canyon from the north rim between river mile 181 and Whitmore Canyon (river mile 187.5); they are concordant with dates on the Whitmore Cascade as well as other cascades found along this reach.The combined results suggest a new model for the spatial and temporal distribution of volcanism in Grand Canyon in which composite lava dams and edifi ces, that were generally leaky in proximal areas, were built from 725 to 475 ka near Toroweap fault and around 320 ka near Whitmore Canyon. New data on these and other episodes present a refi ned model for complex interactions of volcanism and fl uvial processes in this classic locality. Available data suggest that the demise of these volcanic edifi ces may have involved either large outburst...
Potential effects of exposure to the synthetic estrogen 17alpha-ethynylestradiol (EE2) were examined in several species of fish from a lake experimentally treated with environmentally relevant concentrations of the contaminant. Ethynylestradiol was added to Lake 260, a small Precambrian shield lake at the Experimental Lakes Area in northwestern Ontario, Canada, from May to October of 2001, 2002, and 2003. Mean concentrations of EE2 in epilimnetic waters ranged between 4.5 and 8.1 ng/L during the three years, with overall means of 6.1 (+/- 2.8), 5.0 (+/- 1.8), and 4.8 (+/- 1.0) ng/L for the three years, respectively. Male and female pearl dace (Margariscus margarita) captured after EE2 additions began contained up to 4,000-fold higher concentrations of the egg yolk precursor vitellogenin than fish captured from the same lake before the EE2 additions or when compared to fish from reference lakes. Edema in the ovaries, inhibited development of testicular tissue, intersex, and histopathological kidney lesions were all evident in fish exposed to EE2. Some indications that EE2 exposure affected in vitro steroidogenic capacity of the ovaries and the testes existed, although results were not always consistent between years. Pearl dace abundance was similar in the lake treated with EE2 and the reference lake. A trend exists toward a reduced overall population of pearl dace from the treated and reference lakes, as do indications that young-of-the-year size classes are less abundant in the EE2-treated lake. Biochemical and histopathological impacts observed in fish exposed to EE2 in this study have not yet been linked to clear population level impacts in pearl dace. Monitoring of these populations is ongoing.
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