Observaciones geomorfológicas en Ultima Esperanza, Chile, y consideraciones sobre la elasticidad de la corteza terrestre

Abstract

This region can be geologically divided into two belts with an approximate-N-S trend. The eastern one, constituted by sandy and shaly formations belonging to the uppermost Cretaceous and Maastrichtian basal part; geomorphologically this area appears as a half moon shaped composite cuesta slightly concave towards the west. The western belt is constituted by flysch and thick conglomerantes, mainly santonian, deposited by turbidity currents. Tectonics here is more complicated: anticlines and synclines are present as well as numerous faults, inverse the old ones, and normal the recent.

In the westernmost part of the easter belt, three isolated massive hills are present (Jorge Montt, Picana and Otten), with previous sandstones at the top; erosion here is weak, favouring forrests growing. In the same belt two glacial out-spreading hills are observed (Ballena and Solitario), eroded by a glacier in its stage. This galcier proceeded from the Patagonian Rage and directed towards Argentine, depositing a series of varves in a frontal lake; these varves were dragfolded by the same glacier as it advanced towards the east, depositing a moraine over the varves. These hills present an E-W ridge, that is perpendicular to the regional tectonic motive.

In the western belt outstanding tabular hills are observed (Mocho, Manuel Señoret, Ventana, Campanilla; Castillo, Toro, Rotunda and Tetas de Ballena) wich are the result of an apical subborizontal dismembering, constitute by conglomerates. Slopes appear very prominent and much affected by erosion, being the geological series constituted by flysch covered by wildflysch (conglomerates). Forest appears practically where slope-rubbish begin. Conglomerates are impervious and do not  present water reservoires as the sandstones of the isolated massive hills.

Rivers are mature and flowed towards, Argentine at the beginning of the last deglaciacion. The rivers dranage directed towards the Pacific Ocean became more active, may be 5-7 thousand years after the beginning of the last deglaciation (approximately 15 thousand years ago), and capture phenomena, as some rivers courses invertion, were verified. All the rivers now flow into the Pacific Ocean.

Old fluvial terraces of the Don Guillermo River are dipping towards the east. These rivers final part reactivation appears much remarkable. However, the basal marine level is rising at present. Marine terraces of the Seno Skyring and Madre de Dios Archipielago are dipping towards the west.

In the Atacama Desert the easternmost playas (salares) of the Antofagasta Province dipped towards the W; however the westernmost playas of the Puna (a rather 10 km. distance between them) dipped towards the E. The playas location is related to the regional tectonic motive.

It seems that these dipping are related to recent normal faults. After the last glaciation, the earth crust and the mantle uppermost part sunk at the Gutemberg Channel (or of the low velocity) rised isostatically but with "elastic bysteresis"; thus, at least one depression, or groove, was formed to the W, and uplifted with delay. It is presumable, thet here it is not a continous deformation, but that it was formed by slow upward push; so, recent faults were formed, wich slip has a minor value at the groove central part.

These are very small features wich may only measured by levelling. If these tilts are cerefully measured, they might finally give the "elasticity modulus" of the earth crust, still unknown.