Exhumation of the Torngat Mountains, northern Labrador, Canada
J. P. Centeno, D. F. Stockli, J. Gosse
The Torngat Mountains are located on Ungava Peninsula in northernmost Labrador and Quebec, Canada, along the western continental margin of the Labrador Sea (Fig. 1). The mountainous landscape is characterized by steep-sided, mostly flat-topped upland surfaces with deeply incised U-shaped valleys, cirques, and fjords. The relief across the Torngat Mountains appears to be distinct, compared to the surrounding Precambrian Canadian Shield, and is characterized by youthful, short wavelength and high amplitude topography ((=~3 km, h0=~1.5 km).
The purpose of this collaborative project with Dalhousie and Montreal Universities is directed towards understanding the geologic and topographic evolution of the Torngat Mountains. There are two main goals associated with this project: (1) to determine the exhumation and incision history and (2) to quantify the tectonic and topographic evolution of the Torngat Mountains using apatite (U-Th)/He thermochronology. These results will allow a better understanding of the underlying causes responsible for the development of the anomalous and distinct topography.
The Torngat Mountains consist of Archean to Paleoproterozoic terranes that were tectonically assembled during the Torngat Orogeny as a result of the oblique collision between the Archean Southeast Rae and Nain cratons in the Paleoproterozoic (Mengel 1988, Hoffman 1990, Van Kranendonk & Ermanovics 1990, Mengel et al. 1991, Bertrand et al. 1993, Van Kranendonk et al. 1993a, Scott & Machado, 1994). The next significant geologic event in the evolution of the Torngat Mountains was the opening of the Labrador Sea that started in the Late Cretaceous (Le Pichon et al. 1971, Gradstein & Srivastava 1980, Roest & Srivastava 1989). Late Mesozoic and Cenozoic marine deposits (~10 km thick) are found offshore Labrador unconformably overlaying continental Precambrian rocks and oceanic crust that formed as a result of the opening of the Labrador Sea (Umpleby 1979, Gradstein & Srivastava 1980, Hall et al. 2002).
There is no agreement yet on the origin of topography along the western margin of the Labrador Sea. Cooke (1929), McMillan (1973), Gradstein & Srivastava (1980), and Chalmers (2000) believe the Labrador Shelf region has undergone renewed tectonic activity since Neogene time. Hall et al. (2002) and Wardle et al. (2002), based on seismic refraction cross sections, believe topography could be the result of isostasy due to a remnant Paleoproterozoic crustal root (>49 km) related to the Torngat Orogeny.
This project will concentrate on the exhumation history and development of topography in the Torngat Mountains. No studies yet have addressed the reason behind the abrupt difference in topography between the rugged Torngat Mountains along the eastern coast of the Ungava Peninsula and the gentle terrain lying west and due south of them. Most of the geologic work done so far has concentrated on the Precambrian crustal evolution and Quaternary glacial history of northern Labrador (e.g., Wardle et al. 2002, Clark 1988).
The principal goal of this project is to determine the low-temperature thermal history of Ungava Peninsula to investigate the exhumational and long-term topographic evolution of the Torngat Mountains, using (U-Th)/He thermochronology. This study will allow us determine both the timing and rates of exhumation (erosion) and the timing of localized incision, allowing the quantification of the tectonic and landscape evolution of this portion of the Labrador Sea margin.
The main hypotheses to be tested are if initial topography resulted from the erosion of the uplifted and exhumed rift shoulder of the Labrador Sea margin, such as Late Cretaceous topography in eastern Australia and southwestern Africa (Bishop and Goldrick 2000, Brown et al. 2000), and/or if it is related to a Neogene uplift event that appears to have affected much of the North Atlantic area, including western Greenland (Japsen and Chalmers 2000, Chalmers 2000).
Thermal histories will be determined using apatite (U-Th)/He thermochronometry, a technique based on the (-decay of 235U, 238U, and 232Th series nuclides. Apatite (U-Th)/He dating was chosen due to its low thermal sensitivity and its low closure temperature (~75ºC) for helium diffusion (Wolf et al. 1996, Farley 2000). He is progressively lost at temperatures between ~45 and ~75°C over geological time; a temperature range termed the He Partial Retention Zone (HePRZ). Vertical sample arrays (like boreholes) allow the reconstruction of progressive cooling and exhumation histories (Stockli et al. 2001, 2002, Farley 2002). Two vertical sample transects (Mt. D'Hiberville, Mt. Inuit) were collected with the purpose of identifying the magnitude, timing, and rate of exhumation (Fig. 2). A ~180 km long east-west transect across Ungava Peninsula (30 samples) was collected to determine exhumational variations across the peninsula and evaluate the amount of rift flank rotation and exhumation (Fig. 2).
Stüwe (1994) and Mancktelow & Grasemann (1997) showed how eroding topography influences the shape of isotherms in the shallow crust. Shallow isotherms (<100°C) tend to mimic topographic features such as deep valleys and high ridges (Fig. 3). The low-temperature sensitivity of apatite (U-Th)/He will be key in detecting isotherm deflections to determine the incision history. For this purpose, I collected two north-south, equal-elevation transects (450 and 750 m, Fig. 2) following the strategy of House et al. (1998).
I expect to find a systematic He age-elevation relationship from the two vertical profiles that will determine a magnitude and rate of exhumation. From the two north-south equal-elevation transects, I expect to find differences in He ages that will help us constrain the timing of incision and development of topography. Older He ages would be expected in samples taken from paleo-valleys and younger He ages would be expected in samples taken from paleo-ridges (House et al. 1998, 2001) (Fig. 3). To investigate the hypotheses that the present rugged topography in the Torngat Mountains is the result of rift margin uplift associated with the opening of the Labrador Sea and/or is the result of a Neogene uplift, the combined data from both vertical transects and the east-west transect across the entire Ungava peninsula will be crucial. He ages across Ungava Peninsula will help us constrain the magnitude of exhumation away from the rift flank and allow the geometric reconstruction of the Labrador Sea rift shoulder. I expect to find Early Paleogene He ages along the Labrador coast (time of peak Labrador Sea spreading rate, therefore peak rift shoulder uplift) and older He ages found farther away from the rift margin along the Ungava Bay coast (Figs. 1 and 2). Corresponding to a possible Neogene uplift, I would expect He ages in the Neogene or younger from the vertical sample transects in the Torngat Mountains (Fig. 2).
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