Detrital thermochronology from the Magallanes Basin
SUPPORTING INFORMATION
Methods S1: Zircon U-Pb LA-ICPMS Geochronology
Detrital zircons were extracted from ~5 kg medium-grained sandstone hand-samples
using standard mineral separation techniques, including crushing and grinding, followed by
hydrodynamic sorting on a Gemini water table, fractionation of magnetic minerals with a
Frantz isodynamic magnetic separator, and settling through heavy liquids to exclude phases
with densities less than 3.28 g/cm3. Final zircon separates were mounted in epoxy resin and
lightly polished in order to maximize grain preservation for subsequent (U-Th)/He analysis.
Geographic location and stratigraphic information for samples are shown in Table S1.
U-Pb detrital zircon geochronology was conducted by laser-ablation multicollector
inductively coupled plasma–mass spectrometry (LA–ICPMS) analysis at the University of
Arizona LaserChron Center following the methods of Gehrels and others (2006). Detrital zircons
were randomly analyzed from a linear swath of grains across the sample mount to minimize
sampling bias in characterizing all detrital populations. Zircons were ablated using a New
Wave DUV193 Excimer laser (operating at a wavelength of 193 nm) using a spot diameter of 50
μm. The analysis lasts for 20 s, during which a pit ∼20 μm in depth is excavated. U and Pb
isotopes from the ablated material was measured simultaneously using a Micromass Isoprobe
in static mode, using Faraday detectors for 238U, 232Th, and 208Pb–206Pb and an ion-counting
channel for 204Pb. Common Pb corrections are made by using the measured 204Pb and assuming
initial Pb composition from Stacey and Kramers (1975). In-run analysis of zircon grains of
known isotopic and U-Pb composition (every fifth to sixth measurement) is used to correct for
this fractionation. Concordia diagrams and relative-age-probability diagrams were constructed
using software of Ludwig (2008) (Figure S1). Calculated U-Pb ages use the 206Pb/ 238U ratio for
>1.0 Ga grains and the 206Pb/207Pb ratio for <1.0 Ga grains. Analyses that were >15% discordant
or >15% reverse discordant were excluded from interpretations. Detrital zircon U-Pb analytical
data are summarized in Table S2.
Methods S2: (U-Th)/He Thermochronology
(U-Th)/He analyses were performed in the noble gas laboratory at UC Santa Cruz. For
detrital zircon previously analyzed for U-Pb, grains were hand-selected from the epoxy mounts.
Grains were carefully inspected under high power (112.5X) stereo-zoom with crosspolarization. Crystal morphology and dimensions were obtained from microscope photographs
and used to calculate equivalent spherical radius for a tetragonal or prolate spheroid (Reiners et
al., 2004). Zircon grains previously analyzed for U-Pb were hand-selected from the epoxy
mounts, photographed in three dimensions using a binocular microscope, and packed in
Niobium foil tubes for (U-Th)/He analysis.
1
Detrital thermochronology from the Magallanes Basin
Helium gas extraction on detrital zircons was conducted at UC Santa Cruz using a 908
nm laser diode system. The sample gas was spiked with 1x10-13 moles of 3He, purified with a
SAES ST101 50 l/s getter, and cryogenically transferred to and analyzed by a Pfieffer Vacuum™
Prisma quadrupole mass spectrometer. The extraction process was repeated until >99% of the
He had been liberated from the zircon. Procedural blanks and 4He/3He standard gas were
periodically run to assess backgrounds and 4He/3He fractionation.
Degassed zircons were hydrothermally dissolved at 225°C in 200 μl microcapsules
contained within a 125 mL Parr bomb assembly following an unpublished protocol modified
after Reiners et al. (2006). Parent U and Th isotopes were measured on a Finnigan X-Series ICPMS. Standard analysis of Fish Canyon Tuff zircon (27.8 Ma eruptive age) yielded an alphaejection corrected weighted mean age of 28.9 ± 2.6 Ma (n = 57) for the duration of the sample
analyses. On the basis of these and other laboratory standards, we estimate 7% (2σ standard
error) uncertainty on expected ages. Age calculations were done with an in-house data
reduction package that applies an alpha-ejection correction (Ft) to derive final (U-Th)/He dates
that account for diffusion-domain-dependent loss of the daughter nuclide (Reiners et al., 2004).
Measured ages were corrected for alpha-ejection for tetragonal and prolate spheroid geometries
(Reiners et al., 2004). Zircon (U-Th)/He results are reported in Table S3.
Methods S3: Forward modeling of He thermochronology data
The following appendix contains detailed input parameters and graphs used in the
numerical thermal modeling of the detrital zircon thermochronology data from the Magallanes
Basin, Southern Andes. Modeling parameters for each sample, described in Table S4 correspond
to the constraints shown conceptually in Figure 9 of the manuscript text. Cumulative
probability distributions of modeled (U-Th)/He dates (He) for acceptable and good fit thermal
models are shown with measured ages in Figure S2.
The K-S test is sensitive to overall differences in both age and magnitude of sample
cumulative distributions, does not assume normal distributions, and is commonly used for
comparing age distributions in detrital studies. Here we use it to evaluate similarity between
measured age distributions and modeled results. We report thermal history results for He date
distributions that fit measured data to with probability values >0.68 (acceptable-fit) and >0.94
(best-fit) numerical significance in the K-S test. Note that this reflects only an ad hoc measure of
goodness-of-fit for the model and measured distributions and does not convey the probability
that the distributions are equivalent. As constructed, the K-S test is only capable of
demonstrating that two distributions are dissimilar if the K-S statistic falls below a specified
confidence level. For example, 95% confidence in the K-S test corresponds to a value of the K-S
statistic of 5%. Values of the statistic higher than 5% are indistinguishable in terms of
probability.
2
Detrital thermochronology from the Magallanes Basin
Methods S4: Estimated Paleogene sedimentation and erosion rates
We estimate plausible sediment accumulation rates during deposition of the
hypothesized ~5 km of stratigraphic overburden above the Maastrichtian Dorotea Formation.
Using the approximation method of Van Hinte (1978) for calculating the original thickness 𝑇𝑜 :
[(1 − Φ𝑛 ) × 𝑇𝑛 ]
𝑇𝑜 =
(1 − Φ𝑜 )
with porosity Φ𝑜 , a compacted overburden thickness 𝑇𝑛 with porosity Φ𝑛 , and coefficient of
porosity 𝑐, where:
Φ𝑛 = Φ𝑜 𝑒(−𝑐𝑧)
For simplicity we do not include a component of water loading and assume that sediments
were deposited in a shallow marine or continental environment. A decompacted sedimentation
rate 𝑆 is calculated by dividing the original thickness 𝑇𝑜 by time since deposition and peak
heating 𝛥𝑡 for two difference scenarios of peak heating at 54 Ma and at 46 Ma:
𝑇𝑜
𝑆=
𝛥𝑡
Estimates of decompacted sedimentation rate range from 0.3- 0.5 mm/yr (Table S5). In a similar
manner, first-order erosion rates from peak heating to deposition of overlying Man Aike/Río
Turbio Formation (Table S5). Estimated erosion rates are between 0.5 and 2.7 mm/yr for the
time between peak burial heating and ca. 44 Ma deposition of the overlying units.
Supplementary Tables and Figures
Figure S1. Tera-Wasserburg Concordia diagrams for zircon U-Pb data for sample 09-208 (A, B),
sample 09-226 (C, D), sample 09-230 (E, F), sample 09-235 (G, H), and sample 09-207 (I, J). All
plots were made with Isoplot (Ludwig, 2008).
Figure S2. Cumulative probability distributions for modeled zircon He dates calculated from
best-fit thermal histories. A) Maastrichtian Dorotea Formation (samples 09-208 and 09-226). B)
Miocene Santa Cruz Formation (for > 65 Ma grains from samples 09-235 and 09-207). Best-fit
distributions are shown for thermal models that pass the Kolmogorov-Smirnoff statistical test
(K-S) with probability values >0.68 and >0.95 compared to cumulative distribution of He dates.
Table S1. Sample location information for detrital zircon thermochronology samples.
Table S2. Detrital zircon U-Pb geochronologic analyses by LA-ICP-MS analysis. The * indicates
radiogenic Pb (corrected for common Pb). All errors are reported at the 1σ level.
Table S3. Combined zircon U-Pb and He data from subset of selected detrital zircons. Ft is the
alpha-ejection correction after Farley (2002). Samples in italics indicate discordant grains that
are not included in probability distribution calculations.
Table S4. Thermal modeling input parameters for calculating forward modeled He dates.
Table S5. Parameters used for decompacted sedimentation and erosion rates.
3