Page 37 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

36 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
more or less profoundly disturbed in this simple relationship by the major north-south faulting, which has broken the east limb of the structure into long ribbon-like strips, all of which have been dropped in relation to the west limb. Cross faulting has further modified these long strips into numerous faulted blocks, and the terrane has a mosaic pattern with the individual units variously tilted and elevated, one against the other. The San Mateo range is a huge mass of volcanic rocks in which no sedi- mentary beds are known to be present. Numerous faulted blocks occur as small outliers, somewhat radially arranged, to the south in this region, but the fault pattern has no strikingly reg- ular. arrangement such as has been noted elsewhere in the county, and it may be due to shrinking and settling of the central mass of t4 range after the extravasation of the lavas from below.
Along the line of faulting which has elevated the Sierra Cuchillo, Animas Hills and the Lake Valley Hills, these masses appear to be true tilted fault blocks with fault scarp facing the west in each case, and general dip slopes ranging from northeast to southeast in direction and from 20° to 30° in inclination. These mountain masses are separated from the Black Range by local bolson deposits, which have filled a series of enclosed basins and have effectively hidden from view the intervening structural relations. Another small tilted fault block is that of the Mud Springs Mountains west of Hot Springs. Here the fault scarp faces south and the dip of the beds average 20° in a direction of N. 20° E. In the eastern part of the county the easterly dip slopes of the Fra Cristobal Range and the Sierra Caballos and the westerly dip slope of the San Andres Mountains in Socorro County match up in such a way as to give the general impression that the Jornada del Muerto, which lies between them, is a broad detrital-filled synclinal valley. The structure cannot be as simple as this, however, for at several places within the Jornada the tops of low tilted fault blocks project through the surrounding bolson deposits.
The Fra Cristobal Range and the Sierra Caballos, which lie from north to south along the east side of the Rio Grande in the east-central portion of the county, are clearly the remains of two elongated domes. The western limbs of these domes have been faulted down, while the east limbs have remained in approximately their original position and now constitute the mountain masses. The down-faulted east limb of the Black Range and the similarly dislocated west limbs of the Fra Cris- tobal-Caballos masses are believed by the writer to have originally lined up in the formation of a wide, synclinal basin from which the present Cuchillo-Animas Hills-Lake Valley Hills fault blocks have emerged, while the relative sinking of the Rio Grande fault trough was in progress. Faulting along the west limbs of the Fra Cristobal and Caballos domes has been com- plicated and has involved movements of considerable magnitude.