Lunar and Planetary Science XXX 1838.pdf

A METHOD FOR ESTIMATING 2D HORIZONTAL SHORTENING AT WRINKLE RIDGES FROM REMOTE SENSING DATA: RESULTS FROM THE YAKIMA FOLD BELT (COLUMBIA PLATEAU). D. Mege, ESA CNRS 7072, Departement de Geotectonique, case 129, Universite Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France. E-mail: [email protected].

Summary num ridge could then be higher than 44%. Geologic Wrinkle ridges are observed on many other plane- profiles across 7 among the largest Yakima ridges tary surfaces. Similar structures appear to occur on suggests that in general thrusting accommodated sig- Earth at the Yakima fold belt of the Columbia Plateau. nificantly more compressive strain than folding [6]. Field data available in the literature and length / dis- Assuming that most shortening takes place along placement scaling laws are used to demonstrate a an elliptical thrust fault having maximum displace- method for estimating 2D horizontal shortening of ment perpendicular to the ridge trend d (small axis) large ridged surfaces from systematic ridge length and length (big axis) L beneath the ridge, the hori- measurements on aerial or satellite images. Shorten- zontal displacement at one single ridge will be an el- ing is estimated to be in the range 6.4 – 10.3%, in lipse corresponding to the orthogonal projection of the agreement with previous interpretations that shorten- elliptic fault on a horizontal plane, of small axis dH ing on the Columbia plateau is small compared to and length L. The decrease in surface area due to a shortening in many fold and thrust belts. Application single ridge is the elliptical area of the method to Martian wrinkle ridges is given in

another abstract [1]. A = pLdH (1)

Introduction Horizontal displacement dH is frequently unknown. Yakima ridges on the Columbia Plateau present a However, fault length is proportional to its maximum number of similarities with planetary wrinkle ridges, displacement (e.g.[7]), as expressed in the relation including geomorphology, topography, scale, spacing, preferential formation in flood basalts, and close asso- g = d/L (2) ciation in time with basalt emplacement [2, 3]. Their structure results from a combination of folding and where g is a measure of critical shear strain for fault

thrusting. Based on the structural data available for propagation [8]. Replacing dH by d = dH / cosa in eq. the Yakima ridges, I develop below a simple and rapid (1), where a is the mean dip angle of the thrust plane, method for estimating surface area shortening at a A can be determined for any given g by measuring single ridge using ridge length and displacement / ridge length and using the formula length ratio, and then for a whole ridged surface. I discuss the reliability and limitations of the method. A = pgL2cosa (3)

Surface shortening for one ridge Displacement measurements are available for three I consider that shortening at the Yakima ridge belt sites at the Toppenish Ridge, Umtanum Ridge, and can be approximated to take place on an elliptic fault Saddle Mountains [9, 4, 5]. Ratio g can thus be calcu- plane. This is clearly an oversimplification of real lated at these sites using eq. (2), compared to typical g ridge structures, but it helps estimating the order of values published in the literature, and if it is found magnitude of shortening accommodated by ridges similar, serve as an input parameter in eq. (3), which within a given ridged area. Field work suggests that can be solved for A. thrusting accommodates 44% of the unidimensional Figure 1 shows that assuming realistic mean thrust horizontal shortening across the Umtanum Ridge [4], dips, the g values calculated at the Toppenish Ridge, and 83% of the maximum shortening across the Sad- Umtanum Ridge, and Saddle Mountains are in the dle Mountains Ridge [5]. It is not known however if range obtained for faults of comparable lengths else- the low Umtanum value represents the maximum where on Earth [7]. shortening across the ridge. If it is not the case, and if thrusting is the consequence of exceeding the maxi- Surface shortening for a ridge set mum buckling stress, then across the most strained If the basalt shear strength is constant over the Umtanum profile thrusting should have accommo- ridged area, g must also be constant [8]. In such a dated most of the subsequent deformation induced by case, it is then possible to define a typical g, measure further stress. The amount of shortening by thrusting L, for instance on aerial or satellite images, or on a compared to folding in shortening across the Umta- Lunar and Planetary Science XXX 1838.pdf

SHORTENING AT TERRESTRIAL WRINKLE RIDGES: D. Mege

detailed structural map of the area, and then deduce There is no evidence for the presence of the shortened area A using eq. (3). The total shortened decollements below the ridges, nor continuation of the surface area S for i ridges is then faults in the rocks underlying the basalts [13]. Despite these uncertainties, the method appears to work

S = SA = S(pgLi²cosai) (4) acceptably well. Two other factors that could potentially affect the shortening estimates are: (1) Other structures on the Columbia Plateau such as the Blue Mountains and the Palouse slope are not included in the calculations. However, their contribution to shortening is probably small compared to the Yakima ridges [6]. (2) More important, contribution of folding, secondary thrusting and minor fracturing has been neglected. Although very few data are available for constraining how much these structures would affect the results, the total shortening may be not exceed the results obtained by perhaps a factor of two. In addition, the uncertainty in measuring ridge length may counterbalance these underestimating factors. Counting one single elliptical thrust where field work would reveal that two mechanically Figure 1 unlinked elliptical thrusts exist results in Length/maximum displacement scaling from various overestimating shortening by a factor of two. As this data sets [7] superimposed on Yakima fold belt data. Light effect is the opposite as the effect resulting from shading: g range expected for small faults; dark shading: g neglecting folding, secondary thrusting and fracturing, range expected for large faults [7]. the overall calculated surface areas are expected to give a reliable appraisal of shortening magnitude. Table 1 gives S for 23 ridges on the Central Similar calculations can be perforned in numerous Columbia River Province structural map [10], wrinkle ridge areas on other planetary bodies, using representative of the whole Yakima structures. Mean data made readily available by orbiting camera - dip angles were taken to be 30° or 60°, and g = 2*10 ², systems. a value found to be representative of the Toppanish, Umtanum, and Saddle Mountains ridges. References [1] Mege D. (1999) LPSC XXX, this volume. [2] Watters Table 1 T.R. (1988) JGR 93, 10,236-10,254. [3] Watters, T.R. Length of Yakima ridges and surface shortening on the (1992) Geology 20, 609-612. [4] Price E. H. (1982) Ph.D. central Columbia Plateau Dissertation, Wash. State Univ. Pullman. [5] Reidel S. P. average L (km) 98 (1984) Am. J. Sci. 284, 942-978. [6] Reidel S. P., Fecht K. SL (km) 2261 R., Hagood M. C., and Tolan T. L. (1989) Geol. Soc. Am. a = 30° a = 60° Sp. Pap. 239, 247-264. [7] Cowie P. A. and Scholz C. H. (1992) J. Struct. Geol. 14, 1149-1156. [8] Cowie P. A. and average A (km²) 732 457 Scholz C. H. (1992) J. Struct. Geol. 14, 1133-1148. S (km²) 16834 10505 [9] Campbell N. P. and Bentley R. D. (1981) Geology 9, 519-524. [10] Tolan T. L. and Reidel S. P. (1989) Geol. Discussion and conclusion Soc. Am. Sp. Pap. 239 (map). [11] Tolan T. L., Reidel S. P., Considering a 163,700 km² current surface area Beeson M. H., Anderson J. L., Fecht K. R. and Swanson D. for the Columbia River Basalt Group [11], the A. (1989) Geol.Soc. Am. Sp. Pap. 239, 1-20. [12] Guellec calculated surface shortening of the Columbia Plateau S., Mugnier J. L., Tardy M. and Roure F. (1990) Mem. Soc. is expected to be in the range 6.4 - 10.3%. These Geol. F. 156, 165-184. [13] Catchings R. D. and Mooney small values are in agreement with previous W. D. (1988) JGR 93, 459-474. interpretation that the Yakima fold belt accommodates minor shortening compared to many fold and thrust belts (e.g. 30% at the Jura Mountains [12]), and is comparable to the 10 - 15% of maximum 1D shortening estimates perpendicular to the ridges [6].