Few events in modern Soviet history have caused as little surprise, either to the world or, one dares to say, to his colleagues, as the death of Konstantin Ustinovich Chernenko. Already seriously ailing at the time of his election as general secretary in February 1984, Chernenko was obviously critically ill for the past few weeks, and it was only a question of time before the 10 oligarchs of the Politburo would be electing a new head of the Soviet regime.

Let us review this rather weird -- not only to a Westerner, but to a Russian man in the street -- sequence. Leonid Brezhnev, although selected general (then called first) secretary in 1964, established his position as more than just first among equals only at the end of the '60s. Under him, the Soviet system congealed into a classical oligarcho-bureaucratic pattern. Members of the ruling elite, unless they were personal rivals of Brezhnev or uncompromising enemies of his policies, could feel secure in their jobs, no matter what their age or, short of complete debility, their physical condition.

Doubtless this situation arose as a reaction to what had been happening under Stalin -- brutal and frequent purges of even the highest officials -- and under Khrushchev, with his highhanded and whimsical ways with his Politburo colleagues.

The Brezhnev pattern thus led inevitably to the top ruling group's becoming a gerontocracy. As some of its members passed away, they tended to be replaced by Brezhnev's old cronies, usually close in age to the man they replaced. When Prime Minister Alexei Kosygin died at 76, he was replaced by Nikolai Tikhonov, one year younger, still hale and hearty in his job and in the Politburo at 80.

It is a mystery how a youngster in his forties like Mikhail Gorbachev managed to get on the Politburo in the late '70s. One possible explanation is that his job then -- secretary of the Central Committee in charge of agriculture -- is one of the most taxing and, usually, most unrewarding in the entire Soviet structure.

With Brezhnev visibly failing during his last two years, collective leadership took on a new meaning: the 13 or so full members of the Politburo became in fact joint proprietors of a vast country. Their own job security became the uppermost consideration, shaping their personnel policies and affecting strongly their overall views on politics and policy. With Brezhnev's death, his elderly colleagues proposed to continue the same pattern: to select as his successor a man who would be in fact chairman of the board, rather than a real boss like the younger Brezhnev, not to mention Khrushchev or (God forbid!) Stalin.

By then it had become almost a rule that a candidate for the post had to meet two preliminary conditions: he had to be a full member of the Politburo and a secretary of the Central Committee. Three people in November of 1982 answered to those qualifications: 71-year-old Chernenko, 52-year-old Gorbachev and 68-year-old Yuri Andropov. Chernenko had long been a close associate of Brezhnev and undoubtedly his personal choice for the succession, something which obviously did not work in his favor. Gorbachev was too young. And so Andropov was chosen, though his colleagues must have known he was already in frail health.

There obviously followed some rather involved maneuvering within the ruling elite, because the office of chairman of the Supreme Presidium -- president of the U.S.S.R. -- remained unfilled until the spring. In itself, the office is mostly ceremonial, but Brezhnev chose to add it to his other duties in 1977. Were there attempts to foist the office on Chernenko or Gorbachev in exchange for their laying down the secretarial job and thus barring themselves from succession?

In any case, the presidency was bestowed on Andropov in the spring of '83, when it already must have been known that he was not merely ailing but mortally ill. At the same time the pool of possible successors was slightly increased by making 60-year-old Grigori Romanov, until then the boss of Leningrad, a secretary of the Central Committee. With Andropov's prolonged "cold" and then death, the logical choice seemed to lie between Gorbachev and Romanov. But the latter, known to be a harsh and authoritarian man even by Soviet standards, evidently did not endear himself to his colleagues in his new position, and the choice fell again on an aged invalid who had previously been bypassed.

This is the background of the elevation of Gorbachev. With Romanov unacceptable, he was the only possible choice. The Soviet Union is now headed by a man who, unlike the rest of his fellow oligarchs, was too young to serve in World War II and who was barely more than a boy when Stalin died in 1953. Other than that, we have little on which to speculate concerning his views and probable policies.

Even as general secretary, it must be remembered, he will remain but the first among equals unless he manages to build up his power base, bring his own people into the Politburo and secretariat, and thus put his own stamp on domestic and foreign policy. Barring that, his colleagues would no doubt try to keep him from becoming a real boss. But in view of the age of most of them, it is quite likely that, in a year or two, we should be able to speak of the real beginning of the Gorbachev era and to see, rather than just guess, what will be its impact on the Soviet economy (the likeliest sphere for a thoroughgoing reform in the near future), on foreign relations and on other areas of policy.

Presumably a younger man could be expected to be impatient with the immobilism that has characterized the Soviet economy and society during the last decade or so and to be inclined to take a fresh look at the risks and costs of the Soviets' expansionism and arms buildup. e must certainly hope so, but let us be cautious when it comes to predictions. The Soviet rulers have a knack for surprising us, and their own people, and not always in a pleasant sense.