ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

Northwest of the Cocos Ridge along the Pacific coast, seafloor
bathymetry of the Cocos plate reveals short wavelength
roughness elements related to northeast-trending
seamount chains. Most of these seamounts are aligned parallel
to the Cocos Ridge and produce embayments in the
accrectionary prism as they pass beneath the overriding
plate. Because the linear seamount chains are nearly parallel
to the relative velocity vector between the Cocos plate
and the Panama block, the position of seamount subduction
does not change significantly through time. Thus, forearc
deformation associated with subducting roughness
elements is spatially stationary and cumulative through time.
Locally observed deformation that is not consistent
with the long wavelength bathymetry of the Cocos Ridge
occurs inboard of these linear seamount chains, with the
highest «above background» uplift rates onshore of the linear
seamount chains.
Onland, the Costa Rican forearc is cut by normal, strikeslip
and oblique faults that trend nearly orthogonal to the
coast. These faults cut Mesozoic basement (Nicoya Complex),
Tertiary sediments and volcanics, and Quaternary
marine and fluvial terraces. The Costa Rican forearc is effectively
segmented into discrete tectonic blocks with different
uplift rates. Differences in the distribution and rates
of Quaternary deformation along a -500 km stretch of this
segmented forearc are constrained by offset volcanic rocks
and regional correlation of fluvial and marine terraces.
Terrace chronosequences based on radiometric ages, soil
development, and weathering rind thickness establish absolute
and relative timelines that are correlated across active
faults. Terrace elevations vary significantly along the
coast, with sharp changes across major faults. Higher elevations
typically occur within fault blocks that expose Mesozoic
basement.
This generally well constrained deformation along the coast
allows us to investigate the relationship between uplift
and drainage basin evolution and morphology for fluvial
systems draining the Costa Rican forearc. Along the coast
the first order effect is a broad, regional tilting about an
axis of rotation that is centered on and parallel to the Cocos
Ridge. Inboard of the Cocos Ridge crest the regional
tilting is down-to-the- northeast, away from the coast. This
regional tilting is evident in pronounced drainage basin
asymmetries, where the asymmetry factor (AF) is
AF = 100 (ArlAt)
and Ar is drainage area on the downstream right of the
trunk stream and At is total drainage area. Regional tilting
is further augmented across segmented tectonic blocks in
the forearc where AF values are maximized and show consistent
tilting directions. Additionally, stream length-gradient
index (SL) where
SL = (Lilli&)L
and Lill is change in elevation of a reach, & is reach
length and L is total channel length from the reach of interest
to the drainage divide have values consistent with tilting
directions and asymmetry factors.
These rapidly uplifting blocks generate scarps that deflect
major trunk rivers from coast-parallel, longitudinal valleys
within the active fold and thrust belt into coast-perpendicular,
fault controlled valleys with direct access to the
ocean. These trunk systems tend to migrate down the regional
slope, off the Cocos Ridge crest, further accentuating
basin asymmetries.
ALEXANDR A. GAVRILOV
The largest-scale ring structures of the Earth
(on date of tectonic geomorphology)
Pacific Institute of Geography, Radio st., 7, Vladivostok 690041, Russia
The mid ocean ridges have its orographic continuation on
continent, formed together with mountain chains of land
united closed orogenic systems (Gavrilov, 1989, 1994).
One can distinguish Indo-Pacific (In-P) and Indo-Atlantic
(In-At) global ring structures. Their forms in Mercator
projection are similar to ellipses. The length of big halfaxis
of In-P structure at equator is 10 tho km., the small is 8
tho km. In-At structure has the big half-axis 9,5 tho km., the
small half-axis at 20° W meridian is 7,5 tho km. The geological
and geodynamic asymmetry about equator is characterized
for these structures. There are zones of tectonic
collision and granite magmatism in north within the limit
of continents and, on the contrary, there are rifting processes
and massed basalt eruption in south at ocean districts.
In-P structure can have cosmogenic or endogenic origin,
In-At - only endogenic one. These are the main elements of
global system of ring structures (fig. 1). The anomal high
concentrations of magmatic and metamorphic rocks areas,
epicentres of earthquake and zones of tectonic collisions
indicate that the ring structures are the main reflection
forms of global and regional heat flows.
FIG. 1 - Global System of ring structures
1. boundaries of the land; 2. ocean rifting system; 3. fracture zones; 4.
mountain chains of land; 5. large-scale relief ledge of abyssal trenches
and depressions; 6. geology-geomorphological boundaries of ring structures,
correlated with projection of deep endogenic centre; 7. volcanic
uplifts of ocean bed; 8. abyssal depressions.
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