Science, Impermanence, and Aging.

The title “Impermanence” seems appropriate for almost any story involving science. Things change.  Understanding evolves.   But this tale involves a place where the concept of change seems incongruous:  the oldest rocks in the depths of North America’s deepest gorge: Hells Canyon.

The rocks of the Cougar Creek Complex occupy Hells Canyon’s deepest recess. They have been  mashed,  melted, stretched, intruded, and generally treated very badly by Plate Tectonics.  Long ago (in human years, not rock years) legendary geologist Waldemar Lindgren pronounced their relatives near Riggins, Idaho, to be Precambrian. Such highly deformed rocks, Lindgen reasoned, must be ancient. Similar logic applied to the rocks at Cougar Creek.

But by the 1960’s, potassium-argon dating had dispensed with the notion of “ancient” and moved the Riggins Group into the Mesozoic.  Still, there remained that odd snarl of gneiss and contorted greenstones wadded up in the bottom of Hells Canyon.  At the historic Kirkwood Ranch, they appeared as folded but articulate  intrusive screens and dikes. Along the Snake River, candy-striped cliffs, pink and dark greeen, treated the eye. Closer to Pittsburgh Landing, chaos reigned. Augen gneiss and harlequin swirls of rhyolite and basalt dominated.  At Cougar Creek itself,  dark skeins of gabbro embraced scant bits of serpentinite.   In 1986, Nick Walker obtained a date of 309 million years from the Cougar Creek  rocks, and  dates of 280 – 290-something for the rest of this diverse assemblage.  The Cougar Creek Complex was, as logic suggested, the oldest of rocks in Hells Canyon, perhaps a basement upon which everything else rested.

But technology improves, and science marches on.

New datespublished in October, 2011,  reveal that the seemingly ancient rocks of the Cougar Creek Complex are merely the  contemporaries of the rest of Hells’ Canyon’s geology.  In a thorough, eloquent new paper, Gene Kurz and colleagues provide high precision, U-Pb single crystal zircon ages  that reveal two episodes of activity here– the earliest at  “only” 265-248 million years (Middle Permian to Early Triassic), and a younger Late Triassic series about 229 million years in age.  The deformed rocks have been demoted from being original, mysterious “basement”, to simply parts of the arc itself.  The intense deformation, Kurtz notes, came in two episodes: a collision with a spreading ridge, followed by shearing, as the rocks now in Hells Canyon and the Wallowas were faulted along the edge of North America. Why are these rocks so deformed?  They likely represent basement that was warm, and accommodated more plastic deformation–but they are part of the rest of Hells Canyon, not a celebrated and more ancient ancestor.

It’s nice to know that there’s a reprieve from the incessant march of Time.  You can be declared younger than you  thought you were.  And  despite having more wrinkles, you may not be older than your colleagues after all.  It’s just stress. Science is never about absolutes.  It revels in change. Impermanence. Discovering that what we thought we knew is not what we know.  Hooray!