FROM BOTTOM UP TO TOP DOWN: THE NEED FOR INTEGRATING COMPLEMENTARY THERMOCHRONOMETERS WHEN INVESTIGATING LANDSCAPE EVOLUTION

Low-temperature (low-T) thermochronology provides time-temperature (t-T) paths for rocks and minerals in their ascension from shallow (2-4 km) crustal depths to the earth’s surface. For several decades, apatite fission tracks and their annealing histories provided the only widely available and reliable low-T (<110 °C) thermochronometer. Apatite fission track thermochronometry (AFTT) is now complemented by additional tools (e.g., apatite (U-Th)/He (AHe), apatite 4He/3He) that permits mapping t-T paths for most rocks and minerals from shallow crustal depths to the earth’s surface. Zircon fission track and (U-Th)/He, and feldspar multi-diffusion domain modelling, have been volunteered as low-T thermochronometers, but higher closure temperatures (> 200 °C) make them into middle-to-high-temperature tools. Importantly, thermochronology depends both on measurements and modelling approaches; modelling tools have also evolved rapidly. Unfortunately, modelled thermochronological histories, independently of thermochronometer or modelling tool, are unconstrained at the cold end of the ascension path and often yield unreasonable results.

But the timing of arrival of a rock or mineral at the earth’s surface may be independently determined by geochronology of phases formed by water-rock interaction in shallow environments. In assessing thermal histories, weathering geochronology provides the “25 °C” thermochronometer, closing a t-T path. Timing of arrival at the earth’s surface can also be constrained by cosmogenic isotope systems, particularly stable isotopes such as 3He in hematite and goethite and 21Ne in quartz. These approaches, if used in tandem with AFTT, AHe, or apatite 4He/3He, should produce robust exhumation histories for any rock. However, they are rarely combined. Thus, many t-T paths derived from low-T thermochronometers (bottom-up) without the aid of surface constraints provided by weathering geochronology or cosmogenic isotopes (top-down) produce denudation histories incompatible with geological observations.

I will present three examples where combinations of bottom-up and top-down approaches produce geologically robust exhumation histories. In the first case, the exhumation history of the Urucum BIF, MS, Brazil, complementary bottom-up and top-down thermochronometers were used to generate t-T paths for Mn-horizons in a BIF. A step-by-step description of the processes involved in calibrating a new thermochronometer – 40Ar/39Ar in cryptomelane – illustrates what needs to be measured, calculated, and modelled for a thermochronometer to yield a thermochronological history.

Another example, the complementary use of AFTT, AHe, and 40Ar/39Ar geochronology of Mn oxides in the southern and northern ends of the Serra do Espinhaço, illustrates the risk of ignoring surface constraints when modelling exhumation histories. Without surface controls, AFT and AHe thermal histories often erroneously identify accentuated cooling in the recent past, suggesting much larger eroded columns than that compatible with geological constraints.

A third example is the post-magmatic exhumation history of Paraná Basin sediments and volcanic sequences that crop out in the São Paulo Peripheric Depression (SPPD) and adjacent terranes. Geochronology of weathering profiles in the Alto Paranaíba Igneous Province and in the various elevation steps that characterize the landscape of the SPPD reveals a history of progressive but episodic uplift and denudation. Each uplift event is too small to be resolved by current thermochronometers, but they can be individually mapped and dated by weathering geochronology. 

The regional correlation of the dated weathering profiles shows an episodic rise of the Paraná Basin, overlying volcanics, and Cretaceous sediments decoupled from the uplift history of crystalline basement rocks sustaining the nearby Atlantic Plateau.