Macrogeomorphic evolution of the post‐Triassic Appalachian mountains determined by deconvolution of the offshore basin sedimentary record

Abstract

A perplexing macrogeomorphic problem is the persistence of topography in mountain ranges that were initially formed by orogenic events hundreds of millions of years old. In this paper, we deconvolve the post‐Triassic macrogeomorphic history of a portion of one of these ranges, the central and northern Appalachians, using a well‐documented offshore isopach sedimentary record of the US Atlantic margin. Topography is an important signature of tectonic, eustatic and/or geomorphic processes that produces changes in the erodible thickness of the crust (ETC). We define ETC as the total thickness of crust that would have to be consumed by erosion to reduce the mean elevation of a landscape to sea level. We use the term ‘source flux’, designated by ν˙, to describe the rate of change in ETC attributed to deep‐seated geological processes such as crustal thickening, crustal extension, magmatic intrusions or dynamic flow in the mantle. In a mountain belt, the rate of change of mean elevation with respect to a base level, designated by ż′, can be represented as ż′ = c(ν˙ − kd z′ −; ėc ) −& hairsp;l˙, where kd is a proportionality constant relating the mean mechanical erosion rate to mean elevation, ėc is the mean chemcial erosion rate, c is a compensation ratio (held constant for Airy isostasy at 0.21) and l˙ is the rate of eustatic sea‐level change. This equation describes the sum of constructive source terms, two destructive erosion terms and a eustatic sea‐level term. We use this simple linear equation to develop a landscape evolution model based on the concept of a unit response function. The unit response function is analogous to a unit hydrograph which describes the relationship between input (rainfall) and output (discharge) in a hydrological system. In our case, we solve for the general relationship between the source flux into the mountain belt and the erosional flux out of the belt. Offshore sediment volumes are used to determine the erosional flux. Drainage basin area is treated as either a constant (pinned drainage divide) or as increasing through time (migrating drainage divide). We use a third‐order polynomial fit to a global sea‐level curve to account for long‐term eustatically driven changes in ETC and in drainage basin area. Chemical erosion is treated as a constant fixed at 5 m Myr−1. We consider two end‐member models. The first is a ‘tectonic’ model in which the source flux is allowed to vary while kd is assumed to be constant over geological time and equal to its mean Pleistocene value of about 0.07 Myr−1. The second is an ‘erodibility’ model in which kd is allowed to vary, reflecting changes in climatic, climatic variables and rock‐type erodibility, while the source flux is held constant at zero. The ‘tectonic’ model reveals four important increases in the source flux, ranging from 200 to 2000 m Myr−1 that occur over short (<10 Myr) time spans, followed by a protracted period (>25 Myr) where ν˙ drops below zero to values of −1000 to −6000 m Myr−1. The ‘erodibility’ model produces a topography that decays i...

Late Eocene to middle Miocene (33 to 13 million years ago) vegetation and climate development on the North American Atlantic Coastal Plain (IODP Expedition 313, Site M0027)

“…Tectonic uplift of the Appalachian Highlands during the middle Miocene (e.g. Poag et al, 1989;Pazzaglia and Brandon, 1996;Gallen et al, 2013) increased the altitudinal gradient, promoting the expansion of conifers at higher elevations, as characterises the region today (Delcourt et al, 1984). Oboh et al, 1996;2, 10, 11: e.g.…”

“…Vice versa, an uplift of mountain ranges could cause a shift in vegetation and a temperature decrease. Poag and Sevon (1989), Pazzaglia and Brandon (1996), and Gallen et al (2013) discuss an uplift phase of the Appalachian Highlands (Fig. 1) during the middle Miocene based on sedimentation rates at the U.S. Atlantic continental margin and on analyses from the Cullasaja River basin (Southern Appalachian Highlands).…”

Sedimentation record in the Konkan–Kerala Basin: implications for the evolution of the Western Ghats and the Western Indian passive margin

“…Denudation, sediment supply, and the resultant lithospheric flexural response are intrinsically linked, and the analysis of offshore sedimentation coupled with other methodologies such as AFT thermochronology and drainage basin development are valuable approaches for constraining landscape development on PCMs (e.g., Brown et al, 1990;Pazzaglia & Gardner, 1994;Pazzaglia & Brandon, 1996) . However, most studies of the Western Indian margin have tended to focus on either the onshore post-rift denudational history, or the offshore depositional record, with only a few notable exceptions attempting to integrate both methodologies (Gunnell & Fleitout, 1998Gunnell, 2001b).…”

Measuring middle Pleistocene erosion rates with cosmic‐ray‐produced nuclides in buried alluvial sediment, Fisher Valley, southeastern Utah

“…In the past, the only means of doing this has been by serial sediment mass-balance. For example, Pazzaglia and Brandon (1996) compiled sediment thickness measurements in the western Atlantic Ocean to establish the denudation history of the Appalachians over the last 175 Ma, and Collier et al (2000) took advantage of the closed basin of the Gulf of Corinth in Greece to show that erosion rates in the surrounding uplands were much greater during the last glacial maximum than during the Holocene. Mass-balance measurements of erosion rates such as these are potentially very accurate, but are limited to fortuitous geological circumstances where a closed basin is associated with a fixed drainage area, and there exist accurately dated stratigraphic markers that may be used to assign sediment to different time periods.…”