Slide 1

Week 1: Exposure age dating and basic erosion rate studies Kata Juta (the Olgas), NT, Australia Slide 2 Some Processes and Impacts that we dont really have a handle on yet: D = debris flows; G = glacial erosion; B = bedrock failure; R = river inc./dep.; C = climate C G G B B D R inc R dep C C D Slide 3 Form = f(process)? BRK = E > production Incision: K a A m S n ? Cliffs: incision >> production; strength Linear transport: Q s = K d S? Nonlinear Q s ? Near Guge, sw Tibet Slide 4 Bottom line, it still is Slow Down Under 0.9 m/Ma 4.2 & ~10 2.1 m/Ma Outback erosion in brief Slide 5 EROSION OF ROCKY LANDSCAPES - I: FLINDERS RANGES Erosion by slabby blockfall; clasts break down in transit,brief transit time on slope Sampling question: has a surface been recently exposed, or long-exposed ? Slide 6 Quartzite incision: 4.3 m/Ma Quartzite sand: 3.7 m/Ma Blocky quartzite: 7.4 m/Ma Avg. fr. quartzite: 5.5 m/Ma Flinders Ranges examples Slide 7 A proposed conceptual framework for the world Slide 8 Fate of a primary charged galactic cosmic ray particle a. Influence on a proton trajectory of a geomagnetic dipole field that is tilted from Earth's spin axis shown as the western half of the field in a meridian plane e.p.' is equatorial plane. Outer shaded region is forbidden to particles of a given insufficient rigidity. Trajectories 1, 2, and 3, arriving from infinity, must pass through the jaws into inner allowed region and ultimately follow a dipole field line down the horn to atmosphere. Trajectory 4 is impossible due to the opacity of the earth. Distribution of allowed main cone a', Stormer and shadow forbidden cones f ', and the penumbra p' are approximated from for a 10GV positively charged particle at mid-latitude. (Gosse and Phillips, 2001) Slide 9 Major components of a cosmic- ray extensive cascade showing secondary particle production in atmosphere and rock. Numbers in rock refer to in situ cosmogenic nuclide interactions: 1) 35 Cl(n, ) 36 Cl; 2) 16 O(n, 4p3n) 10 Be; 3) 28 Si(n, p2n) 26 Al Slide 10 Slide 11 Slide 12 From Dunne et al., 1999 Slide 13 Slide 14 Slide 15 Slide 16 Simplest applications: Exposure Age dating and erosion Major Assumptions: No inheritance of nuclide concentrations Steady state erosion Simple exposure history (e.g. no shielding) Production rate can be constrained Major Geomorphic Questions tackled: Exposure age of a surface Exposure age of terraces (bedrock and deposits) Erosion rate of exposed bedrock Soil production rates Slide 17 Two-isotope diagrams for interpreting 10 Be and 26 Al measurements. (A) Isotopic trajectory of non-eroding sample exposed continuously at the surface. Numbers to right of curve are exposure ages corresponding to circular symbols. Trajectory ends at saturation where in situ production is equal to decay. In reality, saturation is rarely reached as nuclides are lost by surface erosion. Slide 18 Exposure histories represented in A and B define a banana-shaped window into which measured samples will plot if they have been continually exposed at the surface. Samples that have been shielded will plot in the gray shaded area below the line of steady-state erosion end points shown in B. Abundances of 10 Be normalized to sea level,high latitude. Isotopic trajectories of steadily eroding samples exposed at the surface. Numbers below octagons are rates of erosion in m/Myr. Octagons are end points representing steady-state nuclide abundances resulting from the different rates of erosion. The dashed line represents the continuum of steady-state erosion end points resulting from the spectrum of possible rates of erosion. Erosion acts as an effective decay constant, causing saturation to occur at lower nuclide abundances than in A. Slide 19 cosmic rays rock surface - the simplest application - EXPOSURE AGE DATING Slide 20 cosmic rays eroding surface - And, of key interest to us - long-term EROSION RATES can be inferred