Hongcheng Guo

Research Themes

1. Investigation of helium diffusion for better (U-Th)/He dating

Thermochronology takes advanateg of radiogenic production and diffuive loss of nuclides (e.g., helium, argon) to quantify thermal history of minerals, rocks, and timing and magnitudes of geologic processes.

A grand challenge of apatite (U-Th)/He dating is the overdispersion of ages. I have been developing and using continuous rampled heating (CRH), a method that meansures not only bulk He but on-the-fly He fractional release during lab heating. My CRH analyses demonstrated:

Helium diffusion behavior varies on a grain-to-grain basis.
The various fashions of labrotary diffusion behaviors correlate with (U-Th)/He ages.
CRH can screen unkonwn apatites and yield less dispersed ages for geologic applciation, a game changer for (U-Th)/He dating.
Observed abnormal diffusion mechanism (beyond volume diffusion) is temperture dependent.

Most recently, we found that apatite crystals can possess imperfection that act as "diffusion sinks" that can trap helium in both geological time and in lab anlaysis. What is exciting is that the diffusion sinks can poentially record geological thermal history of apatite samples.

Collaborators: Annia Fayon, Paul Fitzgerald, Bruce Idleman, Kalin McDannell, Marissa Tremblay, Peter Zeitler.

2. Mountain buildings at various settings

Intercontinental: Himalayan in 3D

Models that explain the Himalayan tectonic architecture has been largely developed along orogen-perpendicular cross section, assuming uniformity along the length of Himalaya. By compiling the large set of geo- and thermochronologic data sets that constrain timing of major oregnic fault acticities, changes in Hiamlaayan metamorphism, exhumation, post-collision volcanism, and South Asian monsoon intensification, my colleagues and I develope a 3D Himalayan geodynamic model. This model advocates that the slab dynamics has controlled mountain building and impacted monsoon intensifiction via surface topographic changes.

Collaborators: Hui Cao, Peter Clift, Diego Costantino, Laurent Husson, Thomas Muller, Alex Webb, Zhiqin Xu, An Yin .

Intracontinental: Altai's Cenozic Reactivation

As part of the Central Asian Orogenic Belt, located thousands of kilometers from active plate margins, the Altai Mountains have developed impressive topography and serve as the second-largest rain shadow in Asia. The casue of Alati mountain uplift has been enigmatic and been variably attributed to lithospheric stress from the far-field India-Eurasian collision and / or more localized mantle dynamics. We are trying to eastablish a long-term resarch project here, started by answer a first-order question of when did the uplift of the Altai occur. We use bedrock apatite (U-Th)/He dating to pin the timing of the uplift-related rock exhumation, and we use fluvial geomorphologic analysis to learn changes of the uplift rates that were archived in longitudinal profiles. We are able to confirm that the start of uplift should be no later than Mid-Cenzoic, much earlier than previously thought and the the uplift has been unsteady.

Collaborators: Bat Bold, Bruce Idleman, Anne Meltzer, Frank Pazzaglia, Tais Pinto, Janelle Thumma (Myers), Peter Zeitler.

3. Detrital geo- and thermochronology at various timescales

Proof-of-concept: orbital-timescale detrital geochronology

It is generally accepted that climate, erosion, and tectonics are linked in active mountain belts. It remains unclear exactly how this happens and which processes are meaningfully linked to others, partly because models suggest that feedbacks should develop that are part of these links, and the very nature of feedbacks means that it is difficult to tease apart their component parts.

This NSF-funded project, conceptualized by me, aim to apply a novel combination of rock-magnetic cyclostratigraphy and 40Ar/39Ar detrital thermochronology in order to examine the balance of erosion across the central Himalaya, as recorded by Siwalik Group fluvial sediments.

The cyclostratigraphy lets us build an age model with millennial-scale resolution, and thus use detailed detrital white-mica dating to test our hypothesis that astronomical control of monsoon intensity modulates the location and amount of material eroded from the high Himalaya compared to the foothills. By eliminating tectonic deformation as a forcing by focusing on orbital time scales of thousands to tens of thousands of years, we can learn about the nature of the detrital record. This insight into how erosion and climate link together would be important not just to models for how mountain belts evolve but also for how geochemical records and cycles should be interpreted. This work is still undergoing owing to delayed sampling becasue of the COVID pandemic. We have shown that fluvial deposites from Siwalik documented Milankovitch cycles, and 40/Ar/39Ar dating are pending completion.

Collaborators: Marie Genge, Bruce Idleman, Ken Kodama, Peter Zeitler.

Conventional timescales: using ocean sediments to study change of erosion rates of Andes

This study uses detrital apatite thermochronology to learn the changes of rock exhumation rates of the mid-latitude Patagonian Andes during the Pliocene to Pleistocene to test hypothesized driving forces of the rock exhumation in the last a few million years, including glacial erosion and upper crustal deformation.

We are meauring new (U-Th)/He and re-interpreting published fission track cooling ages of detrital apatite grains from legacy core sediment samples from the Ocean Drilling Program (ODP) Leg 141 near the Chile Triple Junction.

We estimated the deposition ages, which are of late Pliocene to Pleistocene, using a suite of age-depth models from modern cyclostratigraphy, published biostratigraphy and magnetostratigraphy. We observe lag times, or the difference between the apatite cooling ages and the deposition ages, that increase towards younger sediments. The increasing lag times, indicative of a slowdown of exhumation, seem to rule out the ridge collision and support the glacial erosion as the major driving force of the regional exhumation. We also find that the mid-Pleistocene decrease of erosion rates temporally coincides with the widely documented Mid-Pleistocene Transition of glacial cycles. We therefore interpret the observed detrital thermochronological data as suggesting a climate-modulated erosion of the mid-latitude Patagonian Andes for the last several million years.

Collaborators:, Bethany Remian, Marissa Tremblay.

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