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The Great Unconformity is a widely distributed surface separating Precambrian rocks from overlying Phanerozoic sedimentary sequences. The causes and implications of this feature, and whether it represents a singular global event, are much debated. Here, we present new apatite (U-Th)/He (AHe) thermochronologic data from the central Canadian Shield that constrain when the Precambrian basement last cooled to near-surface temperatures, likely via exhumation, before deposition of overlying early Paleozoic sedimentary sequences that mark the Great Unconformity. AHe data from 11 samples (n = 57) across a broad region define a similar date-eU pattern, implying a common thermal history. Higher eU (>25 ppm) apatite form distinct flat profiles of reproducible dates at ∼510 ± 49 Ma (mean and 1σ standard deviation), while lower eU (<25 ppm) apatite define a positive date-eU trend with younger dates. The data patterns, geologic context, and thermal history modeling point toward >3 km of erosion across the entire ∼450,000 km2 study area between 650 and 440 Ma, followed by modest reheating during later burial. Plume activity associated with intracratonic basin formation or continental rifting/breakup may have caused this erosion event. The post-650 Ma timing of the last major sub-Great Unconformity exhumation phase in this region implies a late Great Unconformity that is younger than inferred elsewhere in North America. This suggests that this feature is likely the result of multiple temporally distinct erosion events with differing footprints and mechanisms. [ABSTRACT FROM AUTHOR]

Background: Primary care physicians are potentially important sources of interventions aimed at preventing youth smoking. Yet recent surveys suggest that physician smoking prevention practices are less than optimal. Objectives: To document prevention counseling practices and to identify correlates of these activities in a random sample of general practitioners in Montreal, Quebec. Methods: A cross-sectional mail survey. Results: Of 440 eligible general practitioners (GPs), 337 (77%) completed the questionnaire. General practitioners were more likely to ascertain the smoking status of adolescents (70.9%) than preadolescents (35.7%). Although about half of the GPs offered advice to prevent smoking onset in young adults (48.6%) and adolescents (48.3%), fewer did so for preadolescents (34.4%); only 12.1% advised parents to discuss smoking onset with their children. Correlates of ascertaining smoking status included female sex (odds ratio [OR], 1.90; 95% confidence interval [CI], 1.07-3.41), lower proportion of walk-in patients (OR, 2.73; 95% CI, 1.31-5.80), awareness of the "stage of behavior change'' model (OR, 2.17; 95% CI, 1.18-4.04), and higher self-efficacy (OR, 4.12, 95% CI, 2.00-8.69). Correlates of provision of prevention advice included more hours spent in direct patient care (OR, 1.93; 95% CI, 1.13-3.34), favorable beliefs and attitudes (OR, 1.73; 95% CI, 1.06-2.83), and higher self-efficacy (OR, 4.32; 95% CI, 2.25-8.44). Conclusions: Our results point to the need for renewed efforts to enhance preventive efforts in primary care settings. Intervention programs for GPs should emphasize overcoming unfavorable beliefs and attitudes and low self-efficacy. Future research should evaluate the effect of brief prevention counseling adapted to increasingly busy practices. [ABSTRACT FROM AUTHOR]