Know (all!) your assumptions, investigate the sensitivities: Towards
more rigorous thermal history modeling practices
Abstract
Thermal history models are interpretive tools that incorporate data from
chronometers and implement the published kinetics in the context of
independent constraints on a sample’s known geologic history, in order
to explore specific gaps in geologic knowledge. Despite their central
role in the interpretation of thermochronologic datasets, our community
has no standards for what characterizes a “robust” thermal history
model result, how a model result’s rigor can and should be demonstrated,
and how to communicate the key layers of interpretation produce a
preferred thermal (and geologic) history. As a result, and through no
fault of any one study or modeling program, published models are a
patchwork of modeling philosophies, assumptions, and auxiliary
hypotheses that are rarely sufficiently explored—to the frustration of
authors, reviewers, and readers. This patchwork can give rise to
conflicting conclusions and generate apparent controversies that
distract from the geologic questions at hand. Therefore, our community
needs to both embrace a diversity of modeling approaches and
collectively discuss and set broad expectations for what constitutes
thermal history modeling best practices. Here, we argue that the
fundamental characteristic of any robust thermal history model result is
that it is accompanied by a clear articulation of the “why”—e.g.,
the reason(s) that a model produces a distinctive history, be it the
power of a geologic constraint, a grain’s age, a spatial relationship
between samples, the choice of kinetic model, etc. We demonstrate this
approach using (U-Th)/He data from basement rocks in the Front Range,
CO, which when modeled require a distinctive Neoproterozoic thermal
history: heating to 235-280°C after ca. 650 Ma and then cooling to
<60°C during Paleozoic time. We demonstrate “why” through a
suite of models that add, modify, and remove geologic constraints and
data from the preferred model. We find that a heating event is required
to produce the observed zircon He age-[eU] trend because (1) there
is no more than ~600 My of radiation damage accumulated
in the zircon crystals, (2) the geologic record places the samples at
the surface prior to 650 Ma, and (3) published Ar ages require that
these rocks were colder than ~250˚C for most of the last
1.5 Gy. By identifying these key factors, our sensitivity test
facilitates comparisons to other studies and directs further discussion
to how confident we are in the parts of our data and model set-up that
produce this distinctive result. More broadly, this exercise
demonstrates one of the challenges of deep-time thermochronology: the
potential to accumulate multiple auxiliary assumptions that control the
model result in ways that are not obvious, even to the experienced model
user, without deliberate exploration of alternative solutions—further
underscoring the need for more open discussion of this topic.