No abstract

Large-volume travertine deposits in the southeastern Colorado Plateau of New Mexico and Arizona, USA, occur along the Jemez lineament and Rio Grande rift. These groundwater discharge deposits refl ect vent locations for mantle-derived CO 2 , which was conveyed by deeply sourced hydrothermal fl uid input into springs. U-series dating of stratigraphic sections shows that major aggradation and large-volume (2.5 km 3 ) deposition took place across the region episodically at 700-500 ka, 350-200 ka, and 100-40 ka. These pulses of travertine formation coincide with the occurrence of regional basaltic volcanism, which implies an association of travertine deposits with underlying low-velocity mantle that could supply the excess CO 2 . The calculation of landscape denudation rates based on basalt paleosurfaces shows that travertine platforms developed on local topographic highs that required artesian head and fault conduits. Episodic travertine accumulation that led to the formation of the observed travertine platforms represents conditions when fault conduits, high hydraulic head, and high CO 2 fl ux within confi ned aquifer systems were all favorable for facilitating large-volume travertine formation, which was therefore controlled by tectonic activity and paleohydrology. By analogy to the active Springerville-St. Johns CO 2 gas fi eld, the large volumes and similar platform geometries of travertine occurrences in this study are interpreted to represent extinct CO 2 gas reservoirs that were vents for degassing of mantle volatiles into the near-surface system.

Introduction and BackgroundLandslides, whether in rock or soil, occur when slope-parallel shear stresses acting in the downhill direction are greater than or equal to the shear strength resisting sliding within a hillslope. Assuming Coulomb friction, this condition is met when

The causal mechanisms for the onset and patterns of post-Miocene erosion of the western Great Plains remain the subject of an enthusiastic debate concerning the roles of climatically modulated geomorphic parameters and tectonic rock uplift as drivers of long-term erosion. This study distinguishes between these drivers on the plains of New Mexico and Colorado, where post-Miocene erosion and late Cenozoic volcanism of the Jemez lineament have produced distinctive modern landscapes characterized by deep bedrock canyons and inverted, basalt-capped mesas. The 40 Ar/ 39 Ar ages of basalt-capped paleosurfaces defi ne an episodic eruption history in the Raton-Clayton and Ocate volcanic fi elds and help to quantify patterns, amounts, and rates of differential erosion. Several data sets indicate patterns of NE-trending geologic features that require explanation, including: (1) crude volcanic "belts" of similar age, (2) parallel NE-oriented erosional escarpments retreating toward the NW, (3) differential denudation rates increasing systematically NW from a NE-trending hinge line of "low to no" erosion on the Great Plains, (4) a NEtrending zone of broad (50-100 km) convexities in stream profi les identifi ed by an analysis of strath terraces and channel steepness (k sn ), (5) reorganization of stream networks that took advantage of an apparent relative base-level fall in the SE, and (6) an ~150-kmlong, 40 Ar/ 39 Ar-dated composite paleo surface that has been tilted 64 milli degrees/Ma since 3.4 Ma. Our synthesis of new 40 Ar/ 39 Ar geochronology with new calculations of regional surface denudation, channel steepness, and tilt rates shows that post-Miocene patterns of landscape evolution are best interpreted as related to dynamic uplift along the NEtrending Jemez lineament, with second-order impacts from climate-driven geomorphic variables. We interpret this dynamic uplift to be ultimately due to changing mantle structure and buoyancy with associated crustal melt fl ux.